diff --git a/.earthignore b/.earthignore
new file mode 100644
index 0000000000..a291dd2885
--- /dev/null
+++ b/.earthignore
@@ -0,0 +1,4 @@
+AUTHORS
+nix
+.envrc
+.gitignore
\ No newline at end of file
diff --git a/.envrc b/.envrc
index 45097a203e..1d953f4bd7 100644
--- a/.envrc
+++ b/.envrc
@@ -1,154 +1 @@
-# Load environment variables from `nix-shell` and export it out.
-#
-# Usage: use_nix [-s <nix-expression>] [-w <path>] [-w <path>] ...
-#   -s nix-expression: The nix expression to use for building the shell environment.
-#   -w path: watch a file for changes. It can be specified multiple times. The
-#      shell specified with -s is automatically watched.
-#
-#   If no nix-expression were given with -s, it will attempt to find and load
-#   the shell using the following files in order: shell.nix and default.nix.
-#
-# Example:
-#   -  use_nix
-#   -  use_nix -s shell.nix -w .nixpkgs-version.json
-#
-# The dependencies pulled by nix-shell are added to Nix's garbage collector
-# roots, such that the environment remains persistent.
-#
-# Nix-shell is invoked only once per environment, and the output is cached for
-# better performance. If any of the watched files change, then the environment
-# is rebuilt.
-#
-# To remove old environments, and allow the GC to collect their dependencies:
-# rm -f .direnv
-#
-use_nix() {
-	if ! validate_version; then
-		echo "This .envrc requires direnv version 2.18.2 or above."
-		exit 1
-	fi
-
-	# define all local variables
-	local shell
-	local files_to_watch=()
-
-	local opt OPTARG OPTIND # define vars used by getopts locally
-	while getopts ":n:s:w:" opt; do
-		case "${opt}" in
-			s)
-				shell="${OPTARG}"
-				files_to_watch=("${files_to_watch[@]}" "${shell}")
-				;;
-			w)
-				files_to_watch=("${files_to_watch[@]}" "${OPTARG}")
-				;;
-			:)
-				fail "Invalid option: $OPTARG requires an argument"
-				;;
-			\?)
-				fail "Invalid option: $OPTARG"
-				;;
-		esac
-	done
-	shift $((OPTIND -1))
-
-	if [[ -z "${shell}" ]]; then
-		if [[ -f shell.nix ]]; then
-			shell=shell.nix
-			files_to_watch=("${files_to_watch[@]}" shell.nix)
-		elif [[ -f default.nix ]]; then
-			shell=default.nix
-			files_to_watch=("${files_to_watch[@]}" default.nix)
-		else
-			fail "ERR: no shell was given"
-		fi
-	fi
-
-	local f
-	for f in "${files_to_watch[@]}"; do
-		if ! [[ -f "${f}" ]]; then
-			fail "cannot watch file ${f} because it does not exist"
-		fi
-	done
-
-	# compute the hash of all the files that makes up the development environment
-	local env_hash="$(hash_contents "${files_to_watch[@]}")"
-
-	# define the paths
-	local dir="$(direnv_layout_dir)"
-	local wd="${dir}/wd-${env_hash}"
-	local drv="${wd}/env.drv"
-	local dump="${wd}/dump.env"
-
-	# Generate the environment if we do not have one generated already.
-	if [[ ! -f "${drv}" ]]; then
-		mkdir -p "${wd}"
-
-		log_status "use nix: deriving new environment"
-		IN_NIX_SHELL=1 nix-instantiate --add-root "${drv}" --indirect "${shell}" > /dev/null
-		nix-store -r $(nix-store --query --references "${drv}") --add-root "${wd}/dep" --indirect > /dev/null
-		if [[ "${?}" -ne 0 ]] || [[ ! -f "${drv}" ]]; then
-			rm -rf "${wd}"
-			fail "use nix: was not able to derive the new environment. Please run 'direnv reload' to try again."
-		fi
-
-		log_status "use nix: updating cache"
-		nix-shell --pure "${drv}" --show-trace --run "$(join_args "$direnv" dump bash)" > "${dump}"
-		if [[ "${?}" -ne 0 ]] || [[ ! -f "${dump}" ]] || ! grep -q IN_NIX_SHELL "${dump}"; then
-			rm -rf "${wd}"
-			fail "use nix: was not able to update the cache of the environment. Please run 'direnv reload' to try again."
-		fi
-	fi
-
-	# evaluate the dump created by nix-shell earlier, but have to merge the PATH
-	# with the current PATH
-	# NOTE: we eval the dump here as opposed to direnv_load it because we don't
-	# want to persist environment variables coming from the shell at the time of
-	# the dump. See https://github.com/direnv/direnv/issues/405 for context.
-	local path_backup="${PATH}"
-	eval $(cat "${dump}")
-	export PATH="${PATH}:${path_backup}"
-
-	# cleanup the environment of variables that are not requried, or are causing issues.
-	unset shellHook  # when shellHook is present, then any nix-shell'd script will execute it!
-
-	# watch all the files we were asked to watch for the environment
-	for f in "${files_to_watch[@]}"; do
-		watch_file "${f}"
-	done
-}
-
-fail() {
-	log_error "${@}"
-	exit 1
-}
-
-hash_contents() {
-	if has md5sum; then
-		cat "${@}" | md5sum | cut -c -32
-	elif has md5; then
-		cat "${@}" | md5 -q
-	fi
-}
-
-hash_file() {
-	if has md5sum; then
-		md5sum "${@}" | cut -c -32
-	elif has md5; then
-		md5 -q "${@}"
-	fi
-}
-
-validate_version() {
-	local version="$("${direnv}" version)"
-	local major="$(echo "${version}" | cut -d. -f1)"
-	local minor="$(echo "${version}" | cut -d. -f2)"
-	local patch="$(echo "${version}" | cut -d. -f3)"
-
-	if [[ "${major}" -gt 2 ]]; then return 0; fi
-	if [[ "${major}" -eq 2 ]] && [[ "${minor}" -gt 18 ]]; then return 0; fi
-	if [[ "${major}" -eq 2 ]] && [[ "${minor}" -eq 18 ]] && [[ "${patch}" -ge 2 ]]; then return 0; fi
-	return 1
-}
-
-use_nix -s shell.nix
+use nix
diff --git a/.github/workflows/benchmarks.yml b/.github/workflows/benchmarks.yml
new file mode 100644
index 0000000000..3549a505f8
--- /dev/null
+++ b/.github/workflows/benchmarks.yml
@@ -0,0 +1,38 @@
+on:
+  pull_request:
+    paths-ignore:
+          - '**.md'
+
+name: Benchmarks
+
+env:
+  RUST_BACKTRACE: 1
+
+jobs:
+  prep-dependency-container:
+    name: benchmark roc programs
+    runs-on: [self-hosted, i7-6700K]
+    timeout-minutes: 60
+    env:
+      FORCE_COLOR: 1
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          ref: "trunk"
+          clean: "true" 
+
+      - name: Earthly version
+        run: earthly --version
+
+      - name: on trunk; prepare a self-contained benchmark folder
+        run: ./ci/safe-earthly.sh --build-arg BENCH_SUFFIX=trunk +prep-bench-folder 
+
+      - uses: actions/checkout@v2
+        with:
+          clean: "false" # we want to keep the benchmark folder
+
+      - name: on current branch; prepare a self-contained benchmark folder
+        run: ./ci/safe-earthly.sh +prep-bench-folder
+
+      - name: execute benchmarks with regression check
+        run: ./ci/bench-runner.sh
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 5c83ca27c3..f3e6f6051a 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -1,4 +1,7 @@
-on: [pull_request]
+on:
+  pull_request:
+    paths-ignore:
+          - '**.md'
 
 name: CI
 
@@ -21,5 +24,4 @@ jobs:
         run: earthly --version
 
       - name: install dependencies, build, run zig tests, rustfmt, clippy, cargo test --release
-        run: ./ci/safe-earthly-test-all.sh
-
+        run: ./ci/safe-earthly.sh +test-all
diff --git a/.github/workflows/nightly.yml b/.github/workflows/nightly.yml
index 934baf2e8f..9f3791e4ee 100644
--- a/.github/workflows/nightly.yml
+++ b/.github/workflows/nightly.yml
@@ -1,4 +1,4 @@
-on:  
+on:
   schedule:
     - cron:  '0 0 * * *'
 
diff --git a/.github/workflows/spellcheck.yml b/.github/workflows/spellcheck.yml
new file mode 100644
index 0000000000..fd319f4e65
--- /dev/null
+++ b/.github/workflows/spellcheck.yml
@@ -0,0 +1,24 @@
+on: [pull_request]
+
+name: SpellCheck
+
+env:
+  RUST_BACKTRACE: 1
+
+jobs:
+  spell-check:
+    name: spell check
+    runs-on: [self-hosted]
+    timeout-minutes: 10
+    env:
+      FORCE_COLOR: 1
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          clean: "true"
+
+      - name: Earthly version
+        run: earthly --version
+
+      - name: install spell checker, do spell check
+        run: ./ci/safe-earthly.sh +check-typos
diff --git a/.github/workflows/www.yml b/.github/workflows/www.yml
new file mode 100644
index 0000000000..308cc4dd0e
--- /dev/null
+++ b/.github/workflows/www.yml
@@ -0,0 +1,22 @@
+name: deploy www.roc-lang.org
+
+# Whenever a commit lands on trunk, deploy the site
+on:
+  push:
+    branches:
+      - deploy-www # TODO change to trunk
+
+jobs:
+  deploy:
+    name: 'Deploy to Netlify'
+    runs-on: [self-hosted]
+    steps:
+      - uses: jsmrcaga/action-netlify-deploy@v1.6.0
+        with:
+          install_command: 'pwd; cd ../../www'
+          build_command: 'bash build.sh'
+          build_directory: 'build'
+          NETLIFY_AUTH_TOKEN: ${{ secrets.NETLIFY_AUTH_TOKEN }}
+          NETLIFY_SITE_ID: ${{ secrets.NETLIFY_SITE_ID }}
+          NETLIFY_DEPLOY_MESSAGE: "Deploy git ref ${{ github.ref }}"
+          NETLIFY_DEPLOY_TO_PROD: true
diff --git a/.gitignore b/.gitignore
index 10d627c503..d0ccfa7943 100644
--- a/.gitignore
+++ b/.gitignore
@@ -14,6 +14,7 @@ vgcore.*
 #editors
 .idea/
 .vscode/
+.ignore
 
 #files too big to track in git
 editor/benches/resources/100000_lines.roc
@@ -27,3 +28,5 @@ editor/benches/resources/500_lines.roc
 # rust cache (sccache folder)
 sccache_dir
 
+# self-contained benchmark folder
+bench-folder*
diff --git a/.llvmenv b/.llvmenv
index 95c4e8d271..4044f90867 100644
--- a/.llvmenv
+++ b/.llvmenv
@@ -1 +1 @@
-10.0.0
\ No newline at end of file
+12.0.0
diff --git a/BUILDING_FROM_SOURCE.md b/BUILDING_FROM_SOURCE.md
index b2f92c10cf..c40a02e40d 100644
--- a/BUILDING_FROM_SOURCE.md
+++ b/BUILDING_FROM_SOURCE.md
@@ -20,7 +20,7 @@ For debugging LLVM IR, we use [DebugIR](https://github.com/vaivaswatha/debugir).
 
 ### libunwind & libc++-dev
 
-MacOS systems should already have `libunwind`, but other systems will need to install it (On Ubuntu, this can be donw with `sudo apt-get install libunwind-dev`).
+MacOS systems should already have `libunwind`, but other systems will need to install it (On Ubuntu, this can be done with `sudo apt-get install libunwind-dev`).
 Some systems may already have `libc++-dev` on them, but if not, you may need to install it. (On Ubuntu, this can be done with `sudo apt-get install libc++-dev`.)
 
 ### libcxb libraries
@@ -40,23 +40,43 @@ sudo apt-get install libxcb-render0-dev libxcb-shape0-dev libxcb-xfixes0-dev
 ```
 
 ### Zig
-**version: 0.7.x**
+**version: 0.8.0**
 
-If you're on MacOS, you can install with `brew install zig`
-If you're on Ubuntu and use Snap, you can install with `snap install zig --classic --beta`
-For any other OS, checkout the [Zig installation page](https://github.com/ziglang/zig/wiki/Install-Zig-from-a-Package-Manager)
+For any OS, you can use [`zigup`](https://github.com/marler8997/zigup) to manage zig installations.
+
+If you prefer a package manager, you can try the following:
+- For MacOS, you can install with `brew install zig`
+- For, Ubuntu, you can use Snap, you can install with `snap install zig --classic --beta`
+- For other systems, checkout this [page](https://github.com/ziglang/zig/wiki/Install-Zig-from-a-Package-Manager)
+
+If you want to install it manually, you can also download Zig directly [here](https://ziglang.org/download/). Just make sure you download the right version, the bleeding edge master build is the first download link on this page.
 
 ### LLVM
-**version: 10.0.x**
+**version: 12.0.x**
+
+For macOS, you can install LLVM 12 using `brew install llvm@12` and then adding
+`/usr/local/opt/llvm/bin` to your `PATH`. You can confirm this worked by
+running `llc --version` - it should mention "LLVM version 12.0.0" at the top.
 
 For Ubuntu and Debian, you can use the `Automatic installation script` at [apt.llvm.org](https://apt.llvm.org):
 ```
 sudo bash -c "$(wget -O - https://apt.llvm.org/llvm.sh)"
 ```
 
-For macOS, check the troubleshooting section below.
+If you use this script, you'll need to add `clang` and `llvm-as` to your `PATH`.
+By default, the script installs them as `llvm-as-12` and `clang-12`,
+respectively. You can address this with symlinks like so:
 
-There are also plenty of alternative options at http://releases.llvm.org/download.html
+```
+sudo ln -s /usr/bin/clang-12 /usr/bin/clang
+```
+```
+sudo ln -s /usr/bin/llvm-as-12 /usr/bin/llvm-as
+````
+
+There are also alternative installation options at http://releases.llvm.org/download.html
+
+[Troubleshooting](#troubleshooting)
 
 ## Using Nix
 
@@ -94,6 +114,10 @@ You should be in a shell with everything needed to build already installed. Next
 
 You should be in a repl now. Have fun!
 
+### Extra tips
+
+If you plan on using `nix-shell` regularly, check out [direnv](https://direnv.net/) and [lorri](https://github.com/target/lorri). Whenever you `cd` into `roc/`, they will automatically load the Nix dependecies into your current shell, so you never have to run nix-shell directly!
+
 ### Editor
 
 When you want to run the editor from Ubuntu inside nix you need to install [nixGL](https://github.com/guibou/nixGL) as well:
@@ -132,33 +156,24 @@ On Ubuntu, running `sudo apt install pkg-config cmake libx11-dev` fixed this.
 
 If you encounter `cannot find -lz` run `sudo apt install zlib1g-dev`.
 
+If you encounter:
+```
+error: No suitable version of LLVM was found system-wide or pointed
+       to by LLVM_SYS_120_PREFIX.
+```
+Add `export LLVM_SYS_120_PREFIX=/usr/lib/llvm-12` to your `~/.bashrc` or equivalent file for your shell.
+
 ### LLVM installation on macOS
 
-By default homebrew will try to install llvm 11, which is currently
-unsupported. You need to install an older version (10.0.0_3) by doing:
-
-```
-$ brew edit llvm
-
-# Replace the contents of the file with https://raw.githubusercontent.com/Homebrew/homebrew-core/6616d50fb0b24dbe30f5e975210bdad63257f517/Formula/llvm.rb
-
-# we expect llvm-as-10 to be present
-$ ln -s /usr/local/opt/llvm/bin/{llvm-as,llvm-as-10}
-
-# "pinning" ensures that homebrew doesn't update it automatically
-$ brew pin llvm
-```
+If installing LLVM fails, it might help to run `sudo xcode-select -r` before installing again.
 
 It might also be useful to add these exports to your shell:
 
 ```
-export PATH="/usr/local/opt/llvm/bin:$PATH"
 export LDFLAGS="-L/usr/local/opt/llvm/lib -Wl,-rpath,/usr/local/opt/llvm/lib"
 export CPPFLAGS="-I/usr/local/opt/llvm/include"
 ```
 
-If installing LLVM still fails, it might help to run `sudo xcode-select -r` before installing again.
-
 ### LLVM installation on Windows
 
 Installing LLVM's prebuilt binaries doesn't seem to be enough for the `llvm-sys` crate that Roc depends on, so I had to build LLVM from source
diff --git a/Cargo.lock b/Cargo.lock
index 17ae544bde..f0b1a3fe97 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2,9 +2,9 @@
 # It is not intended for manual editing.
 [[package]]
 name = "ab_glyph"
-version = "0.2.10"
+version = "0.2.11"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7a933731feda8b460bdad9a9e43bb386baba6ec593d2bc19716ef3c75c09085c"
+checksum = "af0ac006645f86f20f6c6fa4dcaef920bf803df819123626f9440e35835e7d80"
 dependencies = [
  "ab_glyph_rasterizer",
  "owned_ttf_parser 0.12.0",
@@ -18,9 +18,9 @@ checksum = "d9fe5e32de01730eb1f6b7f5b51c17e03e2325bf40a74f754f04f130043affff"
 
 [[package]]
 name = "addr2line"
-version = "0.14.1"
+version = "0.15.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a55f82cfe485775d02112886f4169bde0c5894d75e79ead7eafe7e40a25e45f7"
+checksum = "e7a2e47a1fbe209ee101dd6d61285226744c6c8d3c21c8dc878ba6cb9f467f3a"
 dependencies = [
  "gimli",
 ]
@@ -39,20 +39,20 @@ checksum = "739f4a8db6605981345c5654f3a85b056ce52f37a39d34da03f25bf2151ea16e"
 
 [[package]]
 name = "ahash"
-version = "0.7.2"
+version = "0.7.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7f200cbb1e856866d9eade941cf3aa0c5d7dd36f74311c4273b494f4ef036957"
+checksum = "43bb833f0bf979d8475d38fbf09ed3b8a55e1885fe93ad3f93239fc6a4f17b98"
 dependencies = [
- "getrandom 0.2.2",
+ "getrandom 0.2.3",
  "once_cell",
  "version_check",
 ]
 
 [[package]]
 name = "aho-corasick"
-version = "0.7.15"
+version = "0.7.18"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7404febffaa47dac81aa44dba71523c9d069b1bdc50a77db41195149e17f68e5"
+checksum = "1e37cfd5e7657ada45f742d6e99ca5788580b5c529dc78faf11ece6dc702656f"
 dependencies = [
  "memchr",
 ]
@@ -97,6 +97,15 @@ dependencies = [
  "num-traits",
 ]
 
+[[package]]
+name = "approx"
+version = "0.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "072df7202e63b127ab55acfe16ce97013d5b97bf160489336d3f1840fd78e99e"
+dependencies = [
+ "num-traits",
+]
+
 [[package]]
 name = "arena-pool"
 version = "0.1.0"
@@ -153,15 +162,16 @@ checksum = "cdb031dd78e28731d87d56cc8ffef4a8f36ca26c38fe2de700543e627f8a464a"
 
 [[package]]
 name = "backtrace"
-version = "0.3.56"
+version = "0.3.60"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9d117600f438b1707d4e4ae15d3595657288f8235a0eb593e80ecc98ab34e1bc"
+checksum = "b7815ea54e4d821e791162e078acbebfd6d8c8939cd559c9335dceb1c8ca7282"
 dependencies = [
  "addr2line",
+ "cc",
  "cfg-if 1.0.0",
  "libc",
  "miniz_oxide",
- "object 0.23.0",
+ "object 0.25.2",
  "rustc-demangle",
 ]
 
@@ -233,9 +243,9 @@ dependencies = [
 
 [[package]]
 name = "bstr"
-version = "0.2.15"
+version = "0.2.16"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a40b47ad93e1a5404e6c18dec46b628214fee441c70f4ab5d6942142cc268a3d"
+checksum = "90682c8d613ad3373e66de8c6411e0ae2ab2571e879d2efbf73558cc66f21279"
 dependencies = [
  "lazy_static",
  "memchr",
@@ -245,9 +255,9 @@ dependencies = [
 
 [[package]]
 name = "bumpalo"
-version = "3.6.1"
+version = "3.7.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "63396b8a4b9de3f4fdfb320ab6080762242f66a8ef174c49d8e19b674db4cdbe"
+checksum = "9c59e7af012c713f529e7a3ee57ce9b31ddd858d4b512923602f74608b009631"
 
 [[package]]
 name = "byte-tools"
@@ -270,7 +280,7 @@ version = "1.0.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8e215f8c2f9f79cb53c8335e687ffd07d5bfcb6fe5fc80723762d0be46e7cc54"
 dependencies = [
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "syn 1.0.72",
 ]
@@ -299,18 +309,18 @@ dependencies = [
 
 [[package]]
 name = "cast"
-version = "0.2.5"
+version = "0.2.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "cc38c385bfd7e444464011bb24820f40dd1c76bcdfa1b78611cb7c2e5cafab75"
+checksum = "57cdfa5d50aad6cb4d44dcab6101a7f79925bd59d82ca42f38a9856a28865374"
 dependencies = [
  "rustc_version",
 ]
 
 [[package]]
 name = "cc"
-version = "1.0.67"
+version = "1.0.68"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e3c69b077ad434294d3ce9f1f6143a2a4b89a8a2d54ef813d85003a4fd1137fd"
+checksum = "4a72c244c1ff497a746a7e1fb3d14bd08420ecda70c8f25c7112f2781652d787"
 dependencies = [
  "jobserver",
 ]
@@ -378,11 +388,28 @@ source = "git+https://github.com/rtfeldman/clap?branch=master#e1d83a78804a271b05
 dependencies = [
  "heck",
  "proc-macro-error",
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "syn 1.0.72",
 ]
 
+[[package]]
+name = "cli_utils"
+version = "0.1.0"
+dependencies = [
+ "bumpalo",
+ "criterion",
+ "inlinable_string",
+ "roc_cli",
+ "roc_collections",
+ "roc_load",
+ "roc_module",
+ "serde",
+ "serde-xml-rs",
+ "strip-ansi-escapes",
+ "tempfile",
+]
+
 [[package]]
 name = "clipboard-win"
 version = "3.1.1"
@@ -457,7 +484,7 @@ dependencies = [
 [[package]]
 name = "confy"
 version = "0.4.0"
-source = "git+https://github.com/rust-cli/confy#0f0c452d652e85674348d9ad6e54bbdebbbd848a"
+source = "git+https://github.com/rust-cli/confy#6ae700bb0e6e2f9f7138d0c1871f604013c8f59f"
 dependencies = [
  "directories-next",
  "serde",
@@ -479,9 +506,9 @@ version = "0.2.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "29c36c619c422113552db4eb28cddba8faa757e33f758cc3415bd2885977b591"
 dependencies = [
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
- "unicode-xid 0.2.1",
+ "unicode-xid 0.2.2",
 ]
 
 [[package]]
@@ -588,9 +615,9 @@ dependencies = [
 
 [[package]]
 name = "cpufeatures"
-version = "0.1.1"
+version = "0.1.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "dec1028182c380cc45a2e2c5ec841134f2dfd0f8f5f0a5bcd68004f81b5efdf4"
+checksum = "ed00c67cb5d0a7d64a44f6ad2668db7e7530311dd53ea79bcd4fb022c64911c8"
 dependencies = [
  "libc",
 ]
@@ -606,9 +633,8 @@ dependencies = [
 
 [[package]]
 name = "criterion"
-version = "0.3.4"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ab327ed7354547cc2ef43cbe20ef68b988e70b4b593cbd66a2a61733123a3d23"
+version = "0.3.5"
+source = "git+https://github.com/Anton-4/criterion.rs#9bd532e35486a3b321d4012534d3e97751a53f85"
 dependencies = [
  "atty",
  "cast",
@@ -633,8 +659,7 @@ dependencies = [
 [[package]]
 name = "criterion-plot"
 version = "0.4.3"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e022feadec601fba1649cfa83586381a4ad31c6bf3a9ab7d408118b05dd9889d"
+source = "git+https://github.com/Anton-4/criterion.rs#9bd532e35486a3b321d4012534d3e97751a53f85"
 dependencies = [
  "cast",
  "itertools 0.9.0",
@@ -671,7 +696,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "06ed27e177f16d65f0f0c22a213e17c696ace5dd64b14258b52f9417ccb52db4"
 dependencies = [
  "cfg-if 1.0.0",
- "crossbeam-utils 0.8.3",
+ "crossbeam-utils 0.8.5",
 ]
 
 [[package]]
@@ -692,8 +717,8 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "94af6efb46fef72616855b036a624cf27ba656ffc9be1b9a3c931cfc7749a9a9"
 dependencies = [
  "cfg-if 1.0.0",
- "crossbeam-epoch 0.9.3",
- "crossbeam-utils 0.8.3",
+ "crossbeam-epoch 0.9.5",
+ "crossbeam-utils 0.8.5",
 ]
 
 [[package]]
@@ -713,14 +738,14 @@ dependencies = [
 
 [[package]]
 name = "crossbeam-epoch"
-version = "0.9.3"
+version = "0.9.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2584f639eb95fea8c798496315b297cf81b9b58b6d30ab066a75455333cf4b12"
+checksum = "4ec02e091aa634e2c3ada4a392989e7c3116673ef0ac5b72232439094d73b7fd"
 dependencies = [
  "cfg-if 1.0.0",
- "crossbeam-utils 0.8.3",
+ "crossbeam-utils 0.8.5",
  "lazy_static",
- "memoffset 0.6.3",
+ "memoffset 0.6.4",
  "scopeguard",
 ]
 
@@ -748,11 +773,10 @@ dependencies = [
 
 [[package]]
 name = "crossbeam-utils"
-version = "0.8.3"
+version = "0.8.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e7e9d99fa91428effe99c5c6d4634cdeba32b8cf784fc428a2a687f61a952c49"
+checksum = "d82cfc11ce7f2c3faef78d8a684447b40d503d9681acebed6cb728d45940c4db"
 dependencies = [
- "autocfg 1.0.1",
  "cfg-if 1.0.0",
  "lazy_static",
 ]
@@ -818,7 +842,7 @@ checksum = "f0c960ae2da4de88a91b2d920c2a7233b400bc33cb28453a2987822d8392519b"
 dependencies = [
  "fnv",
  "ident_case",
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "strsim",
  "syn 1.0.72",
@@ -841,7 +865,7 @@ version = "2.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fcc3dd5e9e9c0b295d6e1e4d811fb6f157d5ffd784b8d202fc62eac8035a770b"
 dependencies = [
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "syn 1.0.72",
 ]
@@ -943,6 +967,15 @@ version = "1.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9ea835d29036a4087793836fa931b08837ad5e957da9e23886b29586fb9b6650"
 
+[[package]]
+name = "drm-fourcc"
+version = "2.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ebbf3a5ed4671aabffefce172ff43d69c1f27dd2c6aea28e5212a70f32ada0cf"
+dependencies = [
+ "serde",
+]
+
 [[package]]
 name = "dtoa"
 version = "0.4.8"
@@ -984,6 +1017,16 @@ dependencies = [
  "termcolor",
 ]
 
+[[package]]
+name = "external-memory"
+version = "0.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e4dfe8d292b014422776a8c516862d2bff8a81b223a4461dfdc45f3862dc9d39"
+dependencies = [
+ "bitflags",
+ "drm-fourcc",
+]
+
 [[package]]
 name = "fake-simd"
 version = "0.1.2"
@@ -1053,9 +1096,9 @@ checksum = "3dcaa9ae7725d12cdb85b3ad99a434db70b468c09ded17e012d86b5c1010f7a7"
 
 [[package]]
 name = "futures"
-version = "0.3.14"
+version = "0.3.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a9d5813545e459ad3ca1bff9915e9ad7f1a47dc6a91b627ce321d5863b7dd253"
+checksum = "0e7e43a803dae2fa37c1f6a8fe121e1f7bf9548b4dfc0522a42f34145dadfc27"
 dependencies = [
  "futures-channel",
  "futures-core",
@@ -1068,9 +1111,9 @@ dependencies = [
 
 [[package]]
 name = "futures-channel"
-version = "0.3.14"
+version = "0.3.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ce79c6a52a299137a6013061e0cf0e688fce5d7f1bc60125f520912fdb29ec25"
+checksum = "e682a68b29a882df0545c143dc3646daefe80ba479bcdede94d5a703de2871e2"
 dependencies = [
  "futures-core",
  "futures-sink",
@@ -1078,15 +1121,15 @@ dependencies = [
 
 [[package]]
 name = "futures-core"
-version = "0.3.14"
+version = "0.3.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "098cd1c6dda6ca01650f1a37a794245eb73181d0d4d4e955e2f3c37db7af1815"
+checksum = "0402f765d8a89a26043b889b26ce3c4679d268fa6bb22cd7c6aad98340e179d1"
 
 [[package]]
 name = "futures-executor"
-version = "0.3.14"
+version = "0.3.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "10f6cb7042eda00f0049b1d2080aa4b93442997ee507eb3828e8bd7577f94c9d"
+checksum = "badaa6a909fac9e7236d0620a2f57f7664640c56575b71a7552fbd68deafab79"
 dependencies = [
  "futures-core",
  "futures-task",
@@ -1095,40 +1138,42 @@ dependencies = [
 
 [[package]]
 name = "futures-io"
-version = "0.3.14"
+version = "0.3.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "365a1a1fb30ea1c03a830fdb2158f5236833ac81fa0ad12fe35b29cddc35cb04"
+checksum = "acc499defb3b348f8d8f3f66415835a9131856ff7714bf10dadfc4ec4bdb29a1"
 
 [[package]]
 name = "futures-macro"
-version = "0.3.14"
+version = "0.3.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "668c6733a182cd7deb4f1de7ba3bf2120823835b3bcfbeacf7d2c4a773c1bb8b"
+checksum = "a4c40298486cdf52cc00cd6d6987892ba502c7656a16a4192a9992b1ccedd121"
 dependencies = [
+ "autocfg 1.0.1",
  "proc-macro-hack",
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "syn 1.0.72",
 ]
 
 [[package]]
 name = "futures-sink"
-version = "0.3.14"
+version = "0.3.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5c5629433c555de3d82861a7a4e3794a4c40040390907cfbfd7143a92a426c23"
+checksum = "a57bead0ceff0d6dde8f465ecd96c9338121bb7717d3e7b108059531870c4282"
 
 [[package]]
 name = "futures-task"
-version = "0.3.14"
+version = "0.3.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ba7aa51095076f3ba6d9a1f702f74bd05ec65f555d70d2033d55ba8d69f581bc"
+checksum = "8a16bef9fc1a4dddb5bee51c989e3fbba26569cbb0e31f5b303c184e3dd33dae"
 
 [[package]]
 name = "futures-util"
-version = "0.3.14"
+version = "0.3.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3c144ad54d60f23927f0a6b6d816e4271278b64f005ad65e4e35291d2de9c025"
+checksum = "feb5c238d27e2bf94ffdfd27b2c29e3df4a68c4193bb6427384259e2bf191967"
 dependencies = [
+ "autocfg 1.0.1",
  "futures-channel",
  "futures-core",
  "futures-io",
@@ -1184,9 +1229,9 @@ dependencies = [
 
 [[package]]
 name = "getrandom"
-version = "0.2.2"
+version = "0.2.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c9495705279e7140bf035dde1f6e750c162df8b625267cd52cc44e0b156732c8"
+checksum = "7fcd999463524c52659517fe2cea98493cfe485d10565e7b0fb07dbba7ad2753"
 dependencies = [
  "cfg-if 1.0.0",
  "libc",
@@ -1195,9 +1240,9 @@ dependencies = [
 
 [[package]]
 name = "gfx-auxil"
-version = "0.9.0"
+version = "0.10.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9ccf8711c9994dfa34337466bee3ae1462e172874c432ce4eb120ab2e98d39cf"
+checksum = "1694991b11d642680e82075a75c7c2bd75556b805efa7660b705689f05b1ab1c"
 dependencies = [
  "fxhash",
  "gfx-hal",
@@ -1206,14 +1251,15 @@ dependencies = [
 
 [[package]]
 name = "gfx-backend-dx11"
-version = "0.8.0"
+version = "0.9.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6f839f27f8c8a6dc553ccca7f5b35a42009432bc25db9688bba7061cd394161f"
+checksum = "8f9e453baf3aaef2b0c354ce0b3d63d76402e406a59b64b7182d123cfa6635ae"
 dependencies = [
  "arrayvec",
  "bitflags",
  "gfx-auxil",
  "gfx-hal",
+ "gfx-renderdoc",
  "libloading 0.7.0",
  "log",
  "parking_lot",
@@ -1228,9 +1274,9 @@ dependencies = [
 
 [[package]]
 name = "gfx-backend-dx12"
-version = "0.8.0"
+version = "0.9.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3937738b0da5839bba4e33980d29f9a06dbce184d04a3a08c9a949e7953700e3"
+checksum = "61f09e9d8c2aa69e9a21eb83c0f5d1a286c6d37da011f796e550d180b08090ce"
 dependencies = [
  "arrayvec",
  "bit-set",
@@ -1238,6 +1284,7 @@ dependencies = [
  "d3d12",
  "gfx-auxil",
  "gfx-hal",
+ "gfx-renderdoc",
  "log",
  "parking_lot",
  "range-alloc",
@@ -1250,9 +1297,9 @@ dependencies = [
 
 [[package]]
 name = "gfx-backend-empty"
-version = "0.8.0"
+version = "0.9.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2ac55ada4bfcd35479b3421eea324d36d7da5f724e2f66ecb36d4efdb7041a5e"
+checksum = "29c8f813c47791918aa00dc9c9ddf961d23fa8c2a5d869e6cb8ea84f944820f4"
 dependencies = [
  "gfx-hal",
  "log",
@@ -1261,9 +1308,9 @@ dependencies = [
 
 [[package]]
 name = "gfx-backend-gl"
-version = "0.8.1"
+version = "0.9.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "0caa03d6e0b7b4f202aea1f20c3f3288cfa06d92d24cea9d69c9a7627967244a"
+checksum = "6bae057fc3a0ab23ecf97ae51d4017d27d5ddf0aab16ee6dcb58981af88c3152"
 dependencies = [
  "arrayvec",
  "bitflags",
@@ -1283,15 +1330,16 @@ dependencies = [
 
 [[package]]
 name = "gfx-backend-metal"
-version = "0.8.2"
+version = "0.9.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "340895ad544ba46433acb3bdabece0ef16f2dbedc030adbd7c9eaf2839fbed41"
+checksum = "0de85808e2a98994c6af925253f8a9593bc57180ef1ea137deab6d35cc949517"
 dependencies = [
  "arrayvec",
  "bitflags",
  "block",
  "cocoa-foundation",
  "copyless",
+ "core-graphics-types",
  "foreign-types",
  "fxhash",
  "gfx-hal",
@@ -1308,15 +1356,16 @@ dependencies = [
 
 [[package]]
 name = "gfx-backend-vulkan"
-version = "0.8.0"
+version = "0.9.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a353fc6fdb42ec646de49bbb74e4870e37a7e680caf33f3ac0615c30b1146d94"
+checksum = "a9861ec855acbbc65c0e4f966d761224886e811dc2c6d413a4776e9293d0e5c0"
 dependencies = [
  "arrayvec",
  "ash",
  "byteorder",
  "core-graphics-types",
  "gfx-hal",
+ "gfx-renderdoc",
  "inplace_it",
  "log",
  "naga",
@@ -1329,21 +1378,33 @@ dependencies = [
 
 [[package]]
 name = "gfx-hal"
-version = "0.8.0"
+version = "0.9.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6d285bfd566f6b9134af908446ca350c0a1047495dfb9bbd826e701e8ee1d259"
+checksum = "7fbb575ea793dd0507b3082f4f2cde62dc9f3cebd98f5cd49ba2a4da97a976fd"
 dependencies = [
  "bitflags",
+ "external-memory",
  "naga",
  "raw-window-handle",
  "thiserror",
 ]
 
 [[package]]
-name = "gimli"
-version = "0.23.0"
+name = "gfx-renderdoc"
+version = "0.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f6503fe142514ca4799d4c26297c4248239fe8838d827db6bd6065c6ed29a6ce"
+checksum = "c8027995e247e2426d3a00d13f5191dd56c314bff02dc4b54cbf727f1ba9c40a"
+dependencies = [
+ "libloading 0.7.0",
+ "log",
+ "renderdoc-sys",
+]
+
+[[package]]
+name = "gimli"
+version = "0.24.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0e4075386626662786ddb0ec9081e7c7eeb1ba31951f447ca780ef9f5d568189"
 
 [[package]]
 name = "glow"
@@ -1387,20 +1448,20 @@ dependencies = [
 
 [[package]]
 name = "glyph_brush_layout"
-version = "0.2.1"
+version = "0.2.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "10bc06d530bf20c1902f1b02799ab7372ff43f6119770c49b0bc3f21bd148820"
+checksum = "15cf18cf985bd942f05e14552b63c9d08f7d0ed1ec79a977eb9747c9e065f497"
 dependencies = [
  "ab_glyph",
- "approx 0.4.0",
+ "approx 0.5.0",
  "xi-unicode",
 ]
 
 [[package]]
 name = "gpu-alloc"
-version = "0.4.5"
+version = "0.4.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "bc76088804bb65a6f3b880bea9306fdaeffb25ebb453105fafa691282ee9fdba"
+checksum = "cbc1b6ca374e81862526786d9cb42357ce03706ed1b8761730caafd02ab91f3a"
 dependencies = [
  "bitflags",
  "gpu-alloc-types",
@@ -1456,15 +1517,15 @@ version = "0.11.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ab5ef0d4909ef3724cc8cce6ccc8572c5c817592e9285f5464f8e86f8bd3726e"
 dependencies = [
- "ahash 0.7.2",
+ "ahash 0.7.4",
  "bumpalo",
 ]
 
 [[package]]
 name = "heck"
-version = "0.3.2"
+version = "0.3.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "87cbf45460356b7deeb5e3415b5563308c0a9b057c85e12b06ad551f98d0a6ac"
+checksum = "6d621efb26863f0e9924c6ac577e8275e5e6b77455db64ffa6c65c904e9e132c"
 dependencies = [
  "unicode-segmentation",
 ]
@@ -1513,7 +1574,7 @@ dependencies = [
  "bitmaps",
  "rand_core 0.5.1",
  "rand_xoshiro",
- "sized-chunks 0.6.4",
+ "sized-chunks 0.6.5",
  "typenum",
  "version_check",
 ]
@@ -1541,7 +1602,7 @@ dependencies = [
  "bitmaps",
  "rand_core 0.5.1",
  "rand_xoshiro",
- "sized-chunks 0.6.4",
+ "sized-chunks 0.6.5",
  "typenum",
  "version_check",
 ]
@@ -1582,7 +1643,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ce046d161f000fffde5f432a0d034d0341dc152643b2598ed5bfce44c4f3a8f0"
 dependencies = [
  "proc-macro-hack",
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "syn 1.0.72",
  "unindent",
@@ -1591,7 +1652,14 @@ dependencies = [
 [[package]]
 name = "inkwell"
 version = "0.1.0"
-source = "git+https://github.com/rtfeldman/inkwell?tag=llvm10-0.release4#9ae45f072645165885b2f347a4c0cc5ce9e22c80"
+dependencies = [
+ "inkwell 0.1.0 (git+https://github.com/rtfeldman/inkwell?tag=llvm12-0.release5)",
+]
+
+[[package]]
+name = "inkwell"
+version = "0.1.0"
+source = "git+https://github.com/rtfeldman/inkwell?tag=llvm12-0.release5#b5aa51313d4cfa4e1b52ad630cb786140671478b"
 dependencies = [
  "either",
  "inkwell_internals",
@@ -1605,9 +1673,9 @@ dependencies = [
 [[package]]
 name = "inkwell_internals"
 version = "0.3.0"
-source = "git+https://github.com/rtfeldman/inkwell?tag=llvm10-0.release4#9ae45f072645165885b2f347a4c0cc5ce9e22c80"
+source = "git+https://github.com/rtfeldman/inkwell?tag=llvm12-0.release5#b5aa51313d4cfa4e1b52ad630cb786140671478b"
 dependencies = [
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "syn 1.0.72",
 ]
@@ -1674,9 +1742,9 @@ checksum = "8eaf4bc02d17cbdd7ff4c7438cafcdf7fb9a4613313ad11b4f8fefe7d3fa0130"
 
 [[package]]
 name = "jobserver"
-version = "0.1.21"
+version = "0.1.22"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5c71313ebb9439f74b00d9d2dcec36440beaf57a6aa0623068441dd7cd81a7f2"
+checksum = "972f5ae5d1cb9c6ae417789196c803205313edde988685da5e3aae0827b9e7fd"
 dependencies = [
  "libc",
 ]
@@ -1730,9 +1798,9 @@ checksum = "830d08ce1d1d941e6b30645f1a0eb5643013d835ce3779a5fc208261dbe10f55"
 
 [[package]]
 name = "libc"
-version = "0.2.93"
+version = "0.2.96"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9385f66bf6105b241aa65a61cb923ef20efc665cb9f9bb50ac2f0c4b7f378d41"
+checksum = "5600b4e6efc5421841a2138a6b082e07fe12f9aaa12783d50e5d13325b26b4fc"
 
 [[package]]
 name = "libloading"
@@ -1762,9 +1830,9 @@ checksum = "7fb9b38af92608140b86b693604b9ffcc5824240a484d1ecd4795bacb2fe88f3"
 
 [[package]]
 name = "llvm-sys"
-version = "100.2.1"
+version = "120.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "15d9c00ce56221b2150e2d4d51887ff139fce5a0e50346c744861d1e66d2f7c4"
+checksum = "7b883556196140c6b6e7a18b19236b9a699c8611aad2e48a0a6403cf1123945a"
 dependencies = [
  "cc",
  "lazy_static",
@@ -1775,9 +1843,9 @@ dependencies = [
 
 [[package]]
 name = "lock_api"
-version = "0.4.3"
+version = "0.4.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5a3c91c24eae6777794bb1997ad98bbb87daf92890acab859f7eaa4320333176"
+checksum = "0382880606dff6d15c9476c416d18690b72742aa7b605bb6dd6ec9030fbf07eb"
 dependencies = [
  "scopeguard",
 ]
@@ -1814,9 +1882,9 @@ checksum = "60302e4db3a61da70c0cb7991976248362f30319e88850c487b9b95bbf059e00"
 
 [[package]]
 name = "memchr"
-version = "2.3.4"
+version = "2.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "0ee1c47aaa256ecabcaea351eae4a9b01ef39ed810004e298d2511ed284b1525"
+checksum = "b16bd47d9e329435e309c58469fe0791c2d0d1ba96ec0954152a5ae2b04387dc"
 
 [[package]]
 name = "memmap2"
@@ -1838,22 +1906,22 @@ dependencies = [
 
 [[package]]
 name = "memoffset"
-version = "0.6.3"
+version = "0.6.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f83fb6581e8ed1f85fd45c116db8405483899489e38406156c25eb743554361d"
+checksum = "59accc507f1338036a0477ef61afdae33cde60840f4dfe481319ce3ad116ddf9"
 dependencies = [
  "autocfg 1.0.1",
 ]
 
 [[package]]
 name = "metal"
-version = "0.22.0"
+version = "0.23.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1c12e48c737ee9a55e8bb2352bcde588f79ae308d3529ee888f7cc0f469b5777"
+checksum = "79d7d769f1c104b8388294d6594d491d2e21240636f5f94d37f8a0f3d7904450"
 dependencies = [
  "bitflags",
  "block",
- "cocoa-foundation",
+ "core-graphics-types",
  "foreign-types",
  "log",
  "objc",
@@ -1917,14 +1985,15 @@ name = "morphic_lib"
 version = "0.1.0"
 dependencies = [
  "sha2",
+ "smallvec",
  "thiserror",
 ]
 
 [[package]]
 name = "naga"
-version = "0.4.1"
+version = "0.5.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f470a97eafcdd0dbea43d5e1a8ef3557aa31f49ba643d9430dbbf911c162b24c"
+checksum = "ef670817eef03d356d5a509ea275e7dd3a78ea9e24261ea3cb2dfed1abb08f64"
 dependencies = [
  "bit-set",
  "bitflags",
@@ -1933,6 +2002,7 @@ dependencies = [
  "log",
  "num-traits",
  "petgraph",
+ "rose_tree",
  "spirv_headers",
  "thiserror",
 ]
@@ -1971,7 +2041,7 @@ checksum = "05d1c6307dc424d0f65b9b06e94f88248e6305726b14729fd67a5e47b2dc481d"
 dependencies = [
  "darling",
  "proc-macro-crate",
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "syn 1.0.72",
 ]
@@ -2082,7 +2152,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ffa5a33ddddfee04c0283a7653987d634e880347e96b5b2ed64de07efb59db9d"
 dependencies = [
  "proc-macro-crate",
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "syn 1.0.72",
 ]
@@ -2126,12 +2196,6 @@ dependencies = [
  "objc",
 ]
 
-[[package]]
-name = "object"
-version = "0.23.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a9a7ab5d64814df0fe4a4b5ead45ed6c5f181ee3ff04ba344313a6c80446c5d4"
-
 [[package]]
 name = "object"
 version = "0.24.0"
@@ -2143,6 +2207,15 @@ dependencies = [
  "indexmap",
 ]
 
+[[package]]
+name = "object"
+version = "0.25.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f8bc1d42047cf336f0f939c99e97183cf31551bf0f2865a2ec9c8d91fd4ffb5e"
+dependencies = [
+ "memchr",
+]
+
 [[package]]
 name = "once_cell"
 version = "1.7.2"
@@ -2169,9 +2242,9 @@ checksum = "624a8340c38c1b80fd549087862da4ba43e08858af025b236e509b6649fc13d5"
 
 [[package]]
 name = "ordered-float"
-version = "2.1.1"
+version = "2.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "766f840da25490628d8e63e529cd21c014f6600c6b8517add12a6fa6167a6218"
+checksum = "f100fcfb41e5385e0991f74981732049f9b896821542a219420491046baafdc2"
 dependencies = [
  "num-traits",
 ]
@@ -2200,7 +2273,7 @@ version = "0.12.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "3a3c7a20e3f122223e68eef6ca58e39bc1ea8a1d83418ba4c2c1ba189d2ee355"
 dependencies = [
- "ttf-parser 0.12.0",
+ "ttf-parser 0.12.1",
 ]
 
 [[package]]
@@ -2232,7 +2305,7 @@ version = "0.5.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0b4b5f600e60dd3a147fb57b4547033d382d1979eb087af310e91cb45a63b1f4"
 dependencies = [
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "syn 1.0.72",
 ]
@@ -2259,7 +2332,7 @@ dependencies = [
  "instant",
  "libc",
  "petgraph",
- "redox_syscall 0.2.5",
+ "redox_syscall 0.2.8",
  "smallvec",
  "thread-id",
  "winapi 0.3.9",
@@ -2298,7 +2371,7 @@ checksum = "99b8db626e31e5b81787b9783425769681b347011cc59471e33ea46d2ea0cf55"
 dependencies = [
  "pest",
  "pest_meta",
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "syn 1.0.72",
 ]
@@ -2388,9 +2461,8 @@ checksum = "3831453b3449ceb48b6d9c7ad7c96d5ea673e9b470a1dc578c2ce6521230884c"
 
 [[package]]
 name = "plotters"
-version = "0.3.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "45ca0ae5f169d0917a7c7f5a9c1a3d3d9598f18f529dd2b8373ed988efea307a"
+version = "0.3.1"
+source = "git+https://github.com/Anton-4/plotters#d043988179b61db714ad60f678637ee145e363d3"
 dependencies = [
  "num-traits",
  "plotters-backend",
@@ -2458,7 +2530,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "18f33027081eba0a6d8aba6d1b1c3a3be58cbb12106341c2d5759fcd9b5277e7"
 dependencies = [
  "proc-macro-error-attr",
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "syn 1.0.72",
  "version_check",
@@ -2470,7 +2542,7 @@ version = "0.4.12"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8a5b4b77fdb63c1eca72173d68d24501c54ab1269409f6b672c85deb18af69de"
 dependencies = [
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "syn 1.0.72",
  "syn-mid",
@@ -2500,18 +2572,18 @@ dependencies = [
 
 [[package]]
 name = "proc-macro2"
-version = "1.0.26"
+version = "1.0.27"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a152013215dca273577e18d2bf00fa862b89b24169fb78c4c95aeb07992c9cec"
+checksum = "f0d8caf72986c1a598726adc988bb5984792ef84f5ee5aa50209145ee8077038"
 dependencies = [
- "unicode-xid 0.2.1",
+ "unicode-xid 0.2.2",
 ]
 
 [[package]]
 name = "profiling"
-version = "1.0.1"
+version = "1.0.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c3a66d5e88679f2720126c11ee29da07a08f094eac52306b066edd7d393752d6"
+checksum = "0a7c000c0ce9d9bb94c0fbacdf20e5087fbe652c556ffb2c9387d980e17d51fb"
 
 [[package]]
 name = "pulldown-cmark"
@@ -2564,7 +2636,7 @@ version = "1.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b22a693222d716a9587786f37ac3f6b4faedb5b80c23914e7303ff5a1d8016e9"
 dependencies = [
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "syn 1.0.72",
 ]
@@ -2584,7 +2656,7 @@ version = "1.0.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c3d0b9745dc2debf507c8422de05d7226cc1f0644216dfdfead988f9b1ab32a7"
 dependencies = [
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
 ]
 
 [[package]]
@@ -2692,7 +2764,7 @@ version = "0.6.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "34cf66eb183df1c5876e2dcf6b13d57340741e8dc255b48e40a26de954d06ae7"
 dependencies = [
- "getrandom 0.2.2",
+ "getrandom 0.2.3",
 ]
 
 [[package]]
@@ -2810,9 +2882,9 @@ dependencies = [
 
 [[package]]
 name = "rayon"
-version = "1.5.0"
+version = "1.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8b0d8e0819fadc20c74ea8373106ead0600e3a67ef1fe8da56e39b9ae7275674"
+checksum = "c06aca804d41dbc8ba42dfd964f0d01334eceb64314b9ecf7c5fad5188a06d90"
 dependencies = [
  "autocfg 1.0.1",
  "crossbeam-deque 0.8.0",
@@ -2822,13 +2894,13 @@ dependencies = [
 
 [[package]]
 name = "rayon-core"
-version = "1.9.0"
+version = "1.9.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9ab346ac5921dc62ffa9f89b7a773907511cdfa5490c572ae9be1be33e8afa4a"
+checksum = "d78120e2c850279833f1dd3582f730c4ab53ed95aeaaaa862a2a5c71b1656d8e"
 dependencies = [
  "crossbeam-channel 0.5.1",
  "crossbeam-deque 0.8.0",
- "crossbeam-utils 0.8.3",
+ "crossbeam-utils 0.8.5",
  "lazy_static",
  "num_cpus",
 ]
@@ -2850,9 +2922,9 @@ checksum = "41cc0f7e4d5d4544e8861606a285bb08d3e70712ccc7d2b84d7c0ccfaf4b05ce"
 
 [[package]]
 name = "redox_syscall"
-version = "0.2.5"
+version = "0.2.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "94341e4e44e24f6b591b59e47a8a027df12e008d73fd5672dbea9cc22f4507d9"
+checksum = "742739e41cd49414de871ea5e549afb7e2a3ac77b589bcbebe8c82fab37147fc"
 dependencies = [
  "bitflags",
 ]
@@ -2863,15 +2935,15 @@ version = "0.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "528532f3d801c87aec9def2add9ca802fe569e44a544afe633765267840abe64"
 dependencies = [
- "getrandom 0.2.2",
- "redox_syscall 0.2.5",
+ "getrandom 0.2.3",
+ "redox_syscall 0.2.8",
 ]
 
 [[package]]
 name = "regex"
-version = "1.4.5"
+version = "1.5.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "957056ecddbeba1b26965114e191d2e8589ce74db242b6ea25fc4062427a5c19"
+checksum = "d07a8629359eb56f1e2fb1652bb04212c072a87ba68546a04065d525673ac461"
 dependencies = [
  "aho-corasick",
  "memchr",
@@ -2880,18 +2952,15 @@ dependencies = [
 
 [[package]]
 name = "regex-automata"
-version = "0.1.9"
+version = "0.1.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ae1ded71d66a4a97f5e961fd0cb25a5f366a42a41570d16a763a69c092c26ae4"
-dependencies = [
- "byteorder",
-]
+checksum = "6c230d73fb8d8c1b9c0b3135c5142a8acee3a0558fb8db5cf1cb65f8d7862132"
 
 [[package]]
 name = "regex-syntax"
-version = "0.6.23"
+version = "0.6.25"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "24d5f089152e60f62d28b835fbff2cd2e8dc0baf1ac13343bef92ab7eed84548"
+checksum = "f497285884f3fcff424ffc933e56d7cbca511def0c9831a7f9b5f6153e3cc89b"
 
 [[package]]
 name = "remove_dir_all"
@@ -2902,6 +2971,12 @@ dependencies = [
  "winapi 0.3.9",
 ]
 
+[[package]]
+name = "renderdoc-sys"
+version = "0.7.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f1382d1f0a252c4bf97dc20d979a2fdd05b024acd7c2ed0f7595d7817666a157"
+
 [[package]]
 name = "roc_build"
 version = "0.1.0"
@@ -2910,7 +2985,7 @@ dependencies = [
  "im 14.3.0",
  "im-rc 14.3.0",
  "indoc 0.3.6",
- "inkwell",
+ "inkwell 0.1.0",
  "inlinable_string",
  "libloading 0.6.7",
  "maplit",
@@ -2921,7 +2996,7 @@ dependencies = [
  "roc_can",
  "roc_collections",
  "roc_constrain",
- "roc_gen",
+ "roc_gen_llvm",
  "roc_load",
  "roc_module",
  "roc_mono",
@@ -2981,11 +3056,13 @@ version = "0.1.0"
 dependencies = [
  "bumpalo",
  "clap 3.0.0-beta.1",
+ "cli_utils",
  "const_format",
+ "criterion",
  "im 14.3.0",
  "im-rc 14.3.0",
  "indoc 0.3.6",
- "inkwell",
+ "inkwell 0.1.0",
  "inlinable_string",
  "libc",
  "libloading 0.6.7",
@@ -3001,7 +3078,7 @@ dependencies = [
  "roc_docs",
  "roc_editor",
  "roc_fmt",
- "roc_gen",
+ "roc_gen_llvm",
  "roc_load",
  "roc_module",
  "roc_mono",
@@ -3014,10 +3091,7 @@ dependencies = [
  "roc_unify",
  "rustyline",
  "rustyline-derive",
- "serde",
- "serde-xml-rs",
  "serial_test",
- "strip-ansi-escapes",
  "target-lexicon",
  "tempfile",
 ]
@@ -3065,6 +3139,7 @@ dependencies = [
  "roc_load",
  "roc_module",
  "roc_region",
+ "roc_types",
 ]
 
 [[package]]
@@ -3078,7 +3153,6 @@ dependencies = [
  "colored",
  "confy",
  "copypasta",
- "criterion",
  "env_logger 0.8.3",
  "futures",
  "glyph_brush",
@@ -3138,41 +3212,6 @@ dependencies = [
  "roc_region",
 ]
 
-[[package]]
-name = "roc_gen"
-version = "0.1.0"
-dependencies = [
- "bumpalo",
- "either",
- "im 14.3.0",
- "im-rc 14.3.0",
- "indoc 0.3.6",
- "inkwell",
- "inlinable_string",
- "libc",
- "maplit",
- "pretty_assertions 0.5.1",
- "quickcheck 0.8.5",
- "quickcheck_macros 0.8.0",
- "roc_build",
- "roc_builtins",
- "roc_can",
- "roc_collections",
- "roc_load",
- "roc_module",
- "roc_mono",
- "roc_parse",
- "roc_problem",
- "roc_region",
- "roc_reporting",
- "roc_solve",
- "roc_std",
- "roc_types",
- "roc_unify",
- "target-lexicon",
- "tokio",
-]
-
 [[package]]
 name = "roc_gen_dev"
 version = "0.1.0"
@@ -3211,6 +3250,41 @@ dependencies = [
  "tokio",
 ]
 
+[[package]]
+name = "roc_gen_llvm"
+version = "0.1.0"
+dependencies = [
+ "bumpalo",
+ "im 14.3.0",
+ "im-rc 14.3.0",
+ "indoc 0.3.6",
+ "inkwell 0.1.0",
+ "inlinable_string",
+ "libc",
+ "maplit",
+ "morphic_lib",
+ "pretty_assertions 0.5.1",
+ "quickcheck 0.8.5",
+ "quickcheck_macros 0.8.0",
+ "roc_build",
+ "roc_builtins",
+ "roc_can",
+ "roc_collections",
+ "roc_load",
+ "roc_module",
+ "roc_mono",
+ "roc_parse",
+ "roc_problem",
+ "roc_region",
+ "roc_reporting",
+ "roc_solve",
+ "roc_std",
+ "roc_types",
+ "roc_unify",
+ "target-lexicon",
+ "tokio",
+]
+
 [[package]]
 name = "roc_load"
 version = "0.1.0"
@@ -3220,6 +3294,7 @@ dependencies = [
  "indoc 0.3.6",
  "inlinable_string",
  "maplit",
+ "morphic_lib",
  "num_cpus",
  "parking_lot",
  "pretty_assertions 0.5.1",
@@ -3419,10 +3494,19 @@ dependencies = [
 ]
 
 [[package]]
-name = "rustc-demangle"
-version = "0.1.18"
+name = "rose_tree"
+version = "0.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6e3bad0ee36814ca07d7968269dd4b7ec89ec2da10c4bb613928d3077083c232"
+checksum = "284de9dae38774e2813aaabd7e947b4a6fe9b8c58c2309f754a487cdd50de1c2"
+dependencies = [
+ "petgraph",
+]
+
+[[package]]
+name = "rustc-demangle"
+version = "0.1.19"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "410f7acf3cb3a44527c5d9546bad4bf4e6c460915d5f9f2fc524498bfe8f70ce"
 
 [[package]]
 name = "rustc-hash"
@@ -3432,9 +3516,9 @@ checksum = "08d43f7aa6b08d49f382cde6a7982047c3426db949b1424bc4b7ec9ae12c6ce2"
 
 [[package]]
 name = "rustc_version"
-version = "0.2.3"
+version = "0.3.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "138e3e0acb6c9fb258b19b67cb8abd63c00679d2851805ea151465464fe9030a"
+checksum = "f0dfe2087c51c460008730de8b57e6a320782fbfb312e1f4d520e6c6fae155ee"
 dependencies = [
  "semver",
 ]
@@ -3505,24 +3589,27 @@ checksum = "d29ab0c6d3fc0ee92fe66e2d99f700eab17a8d57d1c1d3b748380fb20baa78cd"
 
 [[package]]
 name = "semver"
-version = "0.9.0"
+version = "0.11.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1d7eb9ef2c18661902cc47e535f9bc51b78acd254da71d375c2f6720d9a40403"
+checksum = "f301af10236f6df4160f7c3f04eec6dbc70ace82d23326abad5edee88801c6b6"
 dependencies = [
  "semver-parser",
 ]
 
 [[package]]
 name = "semver-parser"
-version = "0.7.0"
+version = "0.10.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "388a1df253eca08550bef6c72392cfe7c30914bf41df5269b68cbd6ff8f570a3"
+checksum = "00b0bef5b7f9e0df16536d3961cfb6e84331c065b4066afb39768d0e319411f7"
+dependencies = [
+ "pest",
+]
 
 [[package]]
 name = "serde"
-version = "1.0.125"
+version = "1.0.126"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "558dc50e1a5a5fa7112ca2ce4effcb321b0300c0d4ccf0776a9f60cd89031171"
+checksum = "ec7505abeacaec74ae4778d9d9328fe5a5d04253220a85c4ee022239fc996d03"
 dependencies = [
  "serde_derive",
 ]
@@ -3551,11 +3638,11 @@ dependencies = [
 
 [[package]]
 name = "serde_derive"
-version = "1.0.125"
+version = "1.0.126"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b093b7a2bb58203b5da3056c05b4ec1fed827dcfdb37347a8841695263b3d06d"
+checksum = "963a7dbc9895aeac7ac90e74f34a5d5261828f79df35cbed41e10189d3804d43"
 dependencies = [
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "syn 1.0.72",
 ]
@@ -3600,7 +3687,7 @@ version = "0.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b2acd6defeddb41eb60bb468f8825d0cfd0c2a76bc03bfd235b6a1dc4f6a1ad5"
 dependencies = [
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "syn 1.0.72",
 ]
@@ -3619,9 +3706,9 @@ dependencies = [
 
 [[package]]
 name = "sha2"
-version = "0.9.4"
+version = "0.9.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d8f6b75b17576b792bef0db1bcc4b8b8bcdf9506744cf34b974195487af6cff2"
+checksum = "b362ae5752fd2137731f9fa25fd4d9058af34666ca1966fb969119cc35719f12"
 dependencies = [
  "block-buffer 0.9.0",
  "cfg-if 1.0.0",
@@ -3648,9 +3735,9 @@ dependencies = [
 
 [[package]]
 name = "sized-chunks"
-version = "0.6.4"
+version = "0.6.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "65e65d6a9f13cd78f361ea5a2cf53a45d67cdda421ba0316b9be101560f3d207"
+checksum = "16d69225bde7a69b235da73377861095455d298f2b970996eec25ddbb42b3d1e"
 dependencies = [
  "bitmaps",
  "typenum",
@@ -3658,9 +3745,9 @@ dependencies = [
 
 [[package]]
 name = "slab"
-version = "0.4.2"
+version = "0.4.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c111b5bd5695e56cffe5129854aa230b39c93a305372fdbb2668ca2394eea9f8"
+checksum = "f173ac3d1a7e3b28003f40de0b5ce7fe2710f9b9dc3fc38664cebee46b3b6527"
 
 [[package]]
 name = "slotmap"
@@ -3720,7 +3807,7 @@ version = "0.6.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1508efa03c362e23817f96cde18abed596a25219a8b2c66e8db33c03543d315b"
 dependencies = [
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "syn 1.0.72",
 ]
@@ -3793,9 +3880,9 @@ version = "1.0.72"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a1e8cdbefb79a9a5a65e0db8b47b723ee907b7c7f8496c76a1770b5c310bab82"
 dependencies = [
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
- "unicode-xid 0.2.1",
+ "unicode-xid 0.2.2",
 ]
 
 [[package]]
@@ -3804,7 +3891,7 @@ version = "0.5.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "baa8e7560a164edb1621a55d18a0c59abf49d360f47aa7b821061dd7eea7fac9"
 dependencies = [
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "syn 1.0.72",
 ]
@@ -3815,10 +3902,10 @@ version = "0.12.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b834f2d66f734cb897113e34aaff2f1ab4719ca946f9a7358dba8f8064148701"
 dependencies = [
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "syn 1.0.72",
- "unicode-xid 0.2.1",
+ "unicode-xid 0.2.2",
 ]
 
 [[package]]
@@ -3836,7 +3923,7 @@ dependencies = [
  "cfg-if 1.0.0",
  "libc",
  "rand 0.8.3",
- "redox_syscall 0.2.5",
+ "redox_syscall 0.2.8",
  "remove_dir_all",
  "winapi 0.3.9",
 ]
@@ -3859,7 +3946,7 @@ dependencies = [
  "im 14.3.0",
  "im-rc 14.3.0",
  "indoc 0.3.6",
- "inkwell",
+ "inkwell 0.1.0",
  "inlinable_string",
  "libc",
  "libloading 0.6.7",
@@ -3871,7 +3958,7 @@ dependencies = [
  "roc_can",
  "roc_collections",
  "roc_constrain",
- "roc_gen",
+ "roc_gen_llvm",
  "roc_load",
  "roc_module",
  "roc_mono",
@@ -3887,6 +3974,50 @@ dependencies = [
  "tokio",
 ]
 
+[[package]]
+name = "test_mono"
+version = "0.1.0"
+dependencies = [
+ "bumpalo",
+ "either",
+ "im 14.3.0",
+ "im-rc 14.3.0",
+ "indoc 0.3.6",
+ "inlinable_string",
+ "libc",
+ "libloading 0.6.7",
+ "pretty_assertions 0.5.1",
+ "quickcheck 0.8.5",
+ "quickcheck_macros 0.8.0",
+ "roc_build",
+ "roc_builtins",
+ "roc_can",
+ "roc_collections",
+ "roc_constrain",
+ "roc_load",
+ "roc_module",
+ "roc_mono",
+ "roc_parse",
+ "roc_problem",
+ "roc_region",
+ "roc_reporting",
+ "roc_solve",
+ "roc_types",
+ "roc_unify",
+ "target-lexicon",
+ "test_mono_macros",
+]
+
+[[package]]
+name = "test_mono_macros"
+version = "0.1.0"
+dependencies = [
+ "darling",
+ "proc-macro2 1.0.27",
+ "quote 1.0.9",
+ "syn 1.0.72",
+]
+
 [[package]]
 name = "textwrap"
 version = "0.11.0"
@@ -3898,20 +4029,20 @@ dependencies = [
 
 [[package]]
 name = "thiserror"
-version = "1.0.24"
+version = "1.0.25"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e0f4a65597094d4483ddaed134f409b2cb7c1beccf25201a9f73c719254fa98e"
+checksum = "fa6f76457f59514c7eeb4e59d891395fab0b2fd1d40723ae737d64153392e9c6"
 dependencies = [
  "thiserror-impl",
 ]
 
 [[package]]
 name = "thiserror-impl"
-version = "1.0.24"
+version = "1.0.25"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7765189610d8241a44529806d6fd1f2e0a08734313a35d5b3a556f92b381f3c0"
+checksum = "8a36768c0fbf1bb15eca10defa29526bda730a2376c2ab4393ccfa16fb1a318d"
 dependencies = [
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "syn 1.0.72",
 ]
@@ -3974,9 +4105,9 @@ checksum = "3e5d7cd7ab3e47dda6e56542f4bbf3824c15234958c6e1bd6aaa347e93499fdc"
 
 [[package]]
 name = "ttf-parser"
-version = "0.12.0"
+version = "0.12.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "85e00391c1f3d171490a3f8bd79999b0002ae38d3da0d6a3a306c754b053d71b"
+checksum = "2fc71742ead70703a55d184f82087302f2f9ffa3793e64db46a78bf75dd723f4"
 
 [[package]]
 name = "twox-hash"
@@ -4036,9 +4167,9 @@ checksum = "fc72304796d0818e357ead4e000d19c9c174ab23dc11093ac919054d20a6a7fc"
 
 [[package]]
 name = "unicode-xid"
-version = "0.2.1"
+version = "0.2.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f7fe0bb3479651439c9112f72b6c505038574c9fbb575ed1bf3b797fa39dd564"
+checksum = "8ccb82d61f80a663efe1f787a51b16b5a51e3314d6ac365b08639f52387b33f3"
 
 [[package]]
 name = "unindent"
@@ -4153,7 +4284,7 @@ dependencies = [
  "bumpalo",
  "lazy_static",
  "log",
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "syn 1.0.72",
  "wasm-bindgen-shared",
@@ -4187,7 +4318,7 @@ version = "0.2.74"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "be2241542ff3d9f241f5e2cb6dd09b37efe786df8851c54957683a49f0987a97"
 dependencies = [
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "syn 1.0.72",
  "wasm-bindgen-backend",
@@ -4257,7 +4388,7 @@ version = "0.28.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "389d680d7bd67512dc9c37f39560224327038deb0f0e8d33f870900441b68720"
 dependencies = [
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "quote 1.0.9",
  "xml-rs",
 ]
@@ -4285,9 +4416,9 @@ dependencies = [
 
 [[package]]
 name = "wgpu"
-version = "0.8.1"
+version = "0.9.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "215fd50e66f794bd16683e7e0e0b9b53be265eb10fdf02276caf5de3e5743fcf"
+checksum = "bd247f8b26fd3d42ef2f320d378025cd6e84d782ef749fab45cc3b981fbe3275"
 dependencies = [
  "arrayvec",
  "js-sys",
@@ -4305,9 +4436,9 @@ dependencies = [
 
 [[package]]
 name = "wgpu-core"
-version = "0.8.1"
+version = "0.9.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1d56c368fc0e6f3927c711d2b55a51ad4321218efc0239c4acf69e456ab70399"
+checksum = "2af5c8acd3ae5781a277cdf65a17f3a7135de5ae782775620e74ea16c9d47770"
 dependencies = [
  "arrayvec",
  "bitflags",
@@ -4335,18 +4466,18 @@ dependencies = [
 
 [[package]]
 name = "wgpu-types"
-version = "0.8.0"
+version = "0.9.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "aa248d90c8e6832269b8955bf800e8241f942c25e18a235b7752226804d21556"
+checksum = "4f5c9678cd533558e28b416d66947b099742df1939307478db54f867137f1b60"
 dependencies = [
  "bitflags",
 ]
 
 [[package]]
 name = "wgpu_glyph"
-version = "0.12.0"
+version = "0.13.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "634570b440f4c24c2e6049ed01ec832c23d338dea3eca654d5760838017a1c8b"
+checksum = "0fee8c96eda18195a7ad9989737183e0a357f14b15e98838c76abbcf56a5f970"
 dependencies = [
  "bytemuck",
  "glyph_brush",
@@ -4458,9 +4589,9 @@ dependencies = [
 
 [[package]]
 name = "x11-clipboard"
-version = "0.5.1"
+version = "0.5.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e5e937afd03b64b7be4f959cc044e09260a47241b71e56933f37db097bf7859d"
+checksum = "b397ace6e980510de59a4fe3d4c758dffab231d6d747ce9fa1aba6b6035d5f32"
 dependencies = [
  "xcb",
 ]
@@ -4539,7 +4670,7 @@ version = "0.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d498dbd1fd7beb83c86709ae1c33ca50942889473473d287d56ce4770a18edfb"
 dependencies = [
- "proc-macro2 1.0.26",
+ "proc-macro2 1.0.27",
  "syn 1.0.72",
  "synstructure",
 ]
diff --git a/Cargo.toml b/Cargo.toml
index 8343f413ed..06f10ff1fd 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -14,20 +14,30 @@ members = [
     "compiler/reporting",
     "compiler/fmt",
     "compiler/mono",
+    "compiler/test_mono_macros",
+    "compiler/test_mono",
     "compiler/load",
-    "compiler/gen",
+    "compiler/gen_llvm",
     "compiler/gen_dev",
     "compiler/build",
     "compiler/arena_pool",
     "compiler/test_gen",
     "vendor/ena",
+    "vendor/inkwell",
     "vendor/pathfinding",
     "vendor/pretty",
     "editor",
     "cli",
+    "cli/cli_utils",
     "roc_std",
-    "docs"
+    "docs",
 ]
+# Needed to be able to run `cargo run -p roc_cli --no-default-features` -
+# see www/build.sh for more.
+#
+# Without the `-p` flag, cargo ignores `--no-default-features` when you have a
+# workspace, and without `resolver = "2"` here, you can't use `-p` like this.
+resolver = "2"
 
 # Optimizations based on https://deterministic.space/high-performance-rust.html
 [profile.release]
diff --git a/Earthfile b/Earthfile
index 8d7be65f9e..3415c05a3b 100644
--- a/Earthfile
+++ b/Earthfile
@@ -1,4 +1,4 @@
-FROM rust:1.52-slim-buster
+FROM rust:1.53-slim-buster
 WORKDIR /earthbuild
 
 prep-debian:
@@ -13,17 +13,18 @@ install-other-libs:
 install-zig-llvm-valgrind-clippy-rustfmt:
     FROM +install-other-libs
     # zig
-    RUN wget -c https://ziglang.org/download/0.7.1/zig-linux-x86_64-0.7.1.tar.xz --no-check-certificate
-    RUN tar -xf zig-linux-x86_64-0.7.1.tar.xz
-    RUN ln -s /earthbuild/zig-linux-x86_64-0.7.1/zig /usr/bin/zig
+    RUN wget -c https://ziglang.org/download/0.8.0/zig-linux-x86_64-0.8.0.tar.xz --no-check-certificate
+    RUN tar -xf zig-linux-x86_64-0.8.0.tar.xz
+    RUN ln -s /earthbuild/zig-linux-x86_64-0.8.0/zig /usr/bin/zig
     # llvm
     RUN apt -y install lsb-release software-properties-common gnupg
     RUN wget https://apt.llvm.org/llvm.sh
     RUN chmod +x llvm.sh
-    RUN ./llvm.sh 10
-    RUN ln -s /usr/bin/clang-10 /usr/bin/clang
+    RUN ./llvm.sh 12
+    RUN ln -s /usr/bin/clang-12 /usr/bin/clang
+    RUN ln -s /usr/bin/llvm-as-12 /usr/bin/llvm-as
     # use lld as linker
-    RUN ln -s /usr/bin/lld-10 /usr/bin/ld.lld
+    RUN ln -s /usr/bin/lld-12 /usr/bin/ld.lld
     ENV RUSTFLAGS="-C link-arg=-fuse-ld=lld -C target-cpu=native"
     # valgrind
     RUN apt -y install autotools-dev cmake automake libc6-dbg
@@ -38,6 +39,8 @@ install-zig-llvm-valgrind-clippy-rustfmt:
     RUN rustup component add clippy
     # rustfmt
     RUN rustup component add rustfmt
+    # criterion
+    RUN cargo install --git https://github.com/Anton-4/cargo-criterion --branch main
     # sccache
     RUN apt -y install libssl-dev
     RUN cargo install sccache
@@ -97,15 +100,49 @@ check-rustfmt:
     RUN cargo fmt --version
     RUN cargo fmt --all -- --check
 
+check-typos:
+    RUN cargo install typos-cli
+    COPY --dir .github ci cli compiler docs editor examples nightly_benches packages roc_std www *.md LEGAL_DETAILS shell.nix ./
+    RUN typos
+
 test-rust:
     FROM +copy-dirs-and-cache
     ENV RUST_BACKTRACE=1
     RUN --mount=type=cache,target=$SCCACHE_DIR \
         cargo test --release && sccache --show-stats
 
+verify-no-git-changes:
+    FROM +test-rust
+    # If running tests caused anything to be changed or added (without being
+    # included in a .gitignore somewhere), fail the build!
+    #
+    # How it works: the `git ls-files` command lists all the modified or
+    # uncommitted files in the working tree, the `| grep -E .` command returns a
+    # zero exit code if it listed any files and nonzero otherwise (which is the
+    # opposite of what we want), and the `!` at the start inverts the exit code.
+    RUN ! git ls-files --deleted --modified --others --exclude-standard | grep -E .
+
 test-all:
     BUILD +test-zig
     BUILD +check-rustfmt
     BUILD +check-clippy
+    BUILD +check-typos
     BUILD +test-rust
-    
+    BUILD +verify-no-git-changes
+
+# compile everything needed for benchmarks and output a self-contained folder
+prep-bench-folder:
+    FROM +copy-dirs-and-cache
+    ARG BENCH_SUFFIX=branch
+    RUN cargo criterion -V
+    RUN --mount=type=cache,target=$SCCACHE_DIR cd cli && cargo criterion --no-run
+    RUN mkdir -p bench-folder/compiler/builtins/bitcode/src
+    RUN mkdir -p bench-folder/target/release/deps
+    RUN mkdir -p bench-folder/examples/benchmarks
+    RUN cp examples/benchmarks/*.roc bench-folder/examples/benchmarks/
+    RUN cp -r examples/benchmarks/platform bench-folder/examples/benchmarks/
+    RUN cp compiler/builtins/bitcode/src/str.zig bench-folder/compiler/builtins/bitcode/src
+    RUN cp target/release/roc bench-folder/target/release
+    # copy the most recent time bench to bench-folder
+    RUN cp target/release/deps/`ls -t target/release/deps/ | grep time_bench | head -n 1` bench-folder/target/release/deps/time_bench
+    SAVE ARTIFACT bench-folder AS LOCAL bench-folder-$BENCH_SUFFIX
diff --git a/README.md b/README.md
index 925ff3944e..001b4fbe66 100644
--- a/README.md
+++ b/README.md
@@ -46,7 +46,7 @@ By using systems-level programming languages like C and C++, platform authors sa
 Roc is designed to make the "systems-level platform, higher-level application" experience as nice as possible.
 
 * **Application** authors code exclusively in Roc. It's a language designed for nice ergonomics. The syntax resembles Ruby or CoffeeScript, and it has a fast compiler with full type inference.
-* **Platform** authors code almost exclusively in a systems-level langauge like C, C++, Rust, or [Zig](https://ziglang.org/), except for the thin Roc API they expose to application authors. Roc application code compiles to machine code, and production builds of Roc apps benefit from the same [LLVM](https://llvm.org/) optimizations that C++, Rust, and Zig do. Roc application authors do not need to know this lower-level code exists; all they have to interact with is the platform's API, which is exposed as an ordinary Roc API.
+* **Platform** authors code almost exclusively in a systems-level language like C, C++, Rust, or [Zig](https://ziglang.org/), except for the thin Roc API they expose to application authors. Roc application code compiles to machine code, and production builds of Roc apps benefit from the same [LLVM](https://llvm.org/) optimizations that C++, Rust, and Zig do. Roc application authors do not need to know this lower-level code exists; all they have to interact with is the platform's API, which is exposed as an ordinary Roc API.
 
 Every Roc application is built on top of exactly one Roc platform. There is no such thing as a Roc application that runs without a platform, and there is no default platform. You must choose one!
 
diff --git a/ci/bench-runner.sh b/ci/bench-runner.sh
new file mode 100755
index 0000000000..22ce98d951
--- /dev/null
+++ b/ci/bench-runner.sh
@@ -0,0 +1,84 @@
+#!/usr/bin/env bash
+
+# script to return exit code 1 if benchmarks have regressed
+
+# benchmark trunk
+ulimit -s unlimited
+cd bench-folder-trunk
+./target/release/deps/time_bench --bench
+cd ..
+
+# move benchmark results so they can be compared later
+cp -r bench-folder-trunk/target/criterion bench-folder-branch/target/
+
+cd bench-folder-branch
+
+LOG_FILE="bench_log.txt"
+touch $LOG_FILE
+
+FULL_CMD=" ./target/release/deps/time_bench --bench"
+echo $FULL_CMD
+
+script -efq $LOG_FILE -c "$FULL_CMD"
+EXIT_CODE=$?
+
+if cat $LOG_FILE | grep -q "regressed"; then
+
+    grep -B3 "regressed" $LOG_FILE | sed 's/\x1B\[[0-9;]\{1,\}[A-Za-z]//g' | grep -o "\".*\"" | rev | cut -d' ' -f1 | rev > slow_benches_1.txt
+    echo "regression(s) detected in:"
+    cat slow_benches_1.txt
+    echo ""
+    echo ""
+    echo "------<<<<<<>>>>>>------"
+    echo "Benchmark detected regression. Running benchmark again to confirm..."
+    echo "------<<<<<<>>>>>>------"
+    echo ""
+    echo ""
+
+    # delete criterion folder to remove old benchmark data
+    rm -rf ./target/criterion
+
+    # benchmark trunk again
+    cd ../bench-folder-trunk
+    rm -rf target/criterion
+    ./target/release/deps/time_bench --bench
+
+    cd ../bench-folder-branch
+    cp -r ../bench-folder-trunk/target/criterion ./target
+
+    rm $LOG_FILE
+    touch $LOG_FILE
+
+    script -efq $LOG_FILE -c "$FULL_CMD"
+    EXIT_CODE=$?
+
+    if cat $LOG_FILE | grep -q "regressed"; then
+
+        grep -B3 "regressed" $LOG_FILE | sed 's/\x1B\[[0-9;]\{1,\}[A-Za-z]//g' | grep -o "\".*\"" | rev | cut -d' ' -f1 | rev > slow_benches_2.txt
+        echo "regression(s) detected in:"
+        cat slow_benches_2.txt
+
+        if [[ $(grep -Fxf slow_benches_1.txt slow_benches_2.txt | wc -l) -gt 0 ]]; then
+            echo ""
+            echo ""
+            echo "------<<<<<<!!!!!!>>>>>>------"
+            echo "Benchmarks were run twice and a regression was detected both times for the following benchmarks:"
+            grep -Fxf slow_benches_1.txt slow_benches_2.txt
+            echo "------<<<<<<!!!!!!>>>>>>------"
+            echo ""
+            echo ""
+            exit 1
+        else
+            echo "Benchmarks were run twice and a regression was detected on one run. We assume this was a fluke."
+            exit 0
+        fi
+    else
+        echo "Benchmarks were run twice and a regression was detected on one run. We assume this was a fluke."
+        exit 0
+    fi
+else
+    echo ""
+    echo "Benchmark execution finished with exit code: $EXIT_CODE."
+    echo ""
+    exit $EXIT_CODE
+fi
\ No newline at end of file
diff --git a/ci/safe-earthly-test-all.sh b/ci/safe-earthly.sh
similarity index 62%
rename from ci/safe-earthly-test-all.sh
rename to ci/safe-earthly.sh
index dbd03d6c79..0ddc13c5c0 100755
--- a/ci/safe-earthly-test-all.sh
+++ b/ci/safe-earthly.sh
@@ -1,9 +1,12 @@
 #!/usr/bin/env bash
-
 LOG_FILE="earthly_log.txt"
 touch $LOG_FILE
 
-script -efq $LOG_FILE -c "earthly --config ci/earthly-conf.yml +test-all"
+# first arg + everything after
+ARGS=${@:1}
+FULL_CMD="earthly --config ci/earthly-conf.yml $ARGS"
+echo $FULL_CMD
+script -efq $LOG_FILE -c "$FULL_CMD"
 EXIT_CODE=$?
 
 if grep -q "failed to mount" "$LOG_FILE"; then
@@ -14,7 +17,7 @@ if grep -q "failed to mount" "$LOG_FILE"; then
   echo "------<<<<<<!!!!!!>>>>>>------"
   echo ""
   echo ""
-  earthly --config ci/earthly-conf.yml --no-cache +test-all
+  earthly --config ci/earthly-conf.yml --no-cache $ARGS
 else
   exit $EXIT_CODE
 fi
diff --git a/cli/Cargo.toml b/cli/Cargo.toml
index 007d67b699..531e9a76ee 100644
--- a/cli/Cargo.toml
+++ b/cli/Cargo.toml
@@ -15,7 +15,12 @@ test = false
 bench = false
 
 [features]
-default = ["target-x86"]
+default = ["target-x86", "llvm", "editor"]
+
+# This is a separate feature because when we generate docs on Netlify,
+# it doesn't have LLVM installed. (Also, it doesn't need to do code gen.)
+llvm = ["inkwell", "roc_gen_llvm", "roc_build/llvm"]
+editor = ["roc_editor"]
 
 target-x86 = []
 
@@ -45,11 +50,11 @@ roc_unify = { path = "../compiler/unify" }
 roc_solve = { path = "../compiler/solve" }
 roc_mono = { path = "../compiler/mono" }
 roc_load = { path = "../compiler/load" }
-roc_gen = { path = "../compiler/gen" }
-roc_build = { path = "../compiler/build" }
+roc_gen_llvm = { path = "../compiler/gen_llvm", optional = true }
+roc_build = { path = "../compiler/build", default-features = false }
 roc_fmt = { path = "../compiler/fmt" }
 roc_reporting = { path = "../compiler/reporting" }
-roc_editor = { path = "../editor" }
+roc_editor = { path = "../editor", optional = true }
 # TODO switch to clap 3.0.0 once it's out. Tried adding clap = "~3.0.0-beta.1" and cargo wouldn't accept it
 clap = { git = "https://github.com/rtfeldman/clap", branch = "master" }
 const_format = "0.2.8"
@@ -62,24 +67,7 @@ inlinable_string = "0.1"
 libc = "0.2"
 libloading = "0.6"
 
-# NOTE: rtfeldman/inkwell is a fork of TheDan64/inkwell which does not change anything.
-#
-# The reason for this fork is that the way Inkwell is designed, you have to use
-# a particular branch (e.g. "llvm8-0") in Cargo.toml. That would be fine, except that
-# breaking changes get pushed directly to that branch, which breaks our build
-# without warning.
-#
-# We tried referencing a specific rev on TheDan64/inkwell directly (instead of branch),
-# but although that worked locally, it did not work on GitHub Actions. (After a few
-# hours of investigation, gave up trying to figure out why.) So this is the workaround:
-# having an immutable tag on the rtfeldman/inkwell fork which points to
-# a particular "release" of Inkwell.
-#
-# When we want to update Inkwell, we can sync up rtfeldman/inkwell to the latest
-# commit of TheDan64/inkwell, push a new tag which points to the latest commit,
-# change the tag value in this Cargo.toml to point to that tag, and `cargo update`.
-# This way, GitHub Actions works and nobody's builds get broken.
-inkwell = { git = "https://github.com/rtfeldman/inkwell", tag = "llvm10-0.release4", features = [ "llvm10-0" ] }
+inkwell = { path = "../vendor/inkwell", optional = true }
 target-lexicon = "0.10"
 tempfile = "3.1.0"
 
@@ -89,8 +77,12 @@ maplit = "1.0.1"
 indoc = "0.3.3"
 quickcheck = "0.8"
 quickcheck_macros = "0.8"
-strip-ansi-escapes = "0.1"
-serde = { version = "1.0", features = ["derive"] }
-serde-xml-rs = "0.4"
 serial_test = "0.5"
 tempfile = "3.1.0"
+criterion = { git = "https://github.com/Anton-4/criterion.rs"}
+cli_utils = { path = "cli_utils" }
+
+[[bench]]
+name = "time_bench"
+harness = false
+
diff --git a/cli/benches/README.md b/cli/benches/README.md
new file mode 100644
index 0000000000..0c91904cfa
--- /dev/null
+++ b/cli/benches/README.md
@@ -0,0 +1,17 @@
+
+# Running the benchmarks
+
+Install cargo criterion:
+```
+cargo install cargo-criterion
+```
+
+To prevent stack overflow on the `CFold` benchmark:
+```
+ulimit -s unlimited
+```
+
+In the `cli` folder execute:
+```
+cargo criterion
+```
\ No newline at end of file
diff --git a/cli/benches/time_bench.rs b/cli/benches/time_bench.rs
new file mode 100644
index 0000000000..d86230a669
--- /dev/null
+++ b/cli/benches/time_bench.rs
@@ -0,0 +1,31 @@
+use cli_utils::bench_utils::{
+    bench_cfold, bench_deriv, bench_nqueens, bench_quicksort, bench_rbtree_ck, bench_rbtree_delete,
+};
+use criterion::{
+    criterion_group, criterion_main, measurement::WallTime, BenchmarkGroup, Criterion, SamplingMode,
+};
+
+fn bench_group_wall_time(c: &mut Criterion) {
+    let mut group = c.benchmark_group("bench-group_wall-time");
+    // calculate statistics based on a fixed(flat) 300 runs
+    group.sampling_mode(SamplingMode::Flat);
+    group.sample_size(300);
+
+    let bench_funcs: Vec<fn(Option<&mut BenchmarkGroup<WallTime>>) -> ()> = vec![
+        bench_nqueens,       // queens 11
+        bench_cfold,         // e = mkExpr 17 1
+        bench_deriv,         // nest deriv 8 f
+        bench_rbtree_ck,     // ms = makeMap 5 80000
+        bench_rbtree_delete, // m = makeMap 100000
+        bench_quicksort,     // list size 10000
+    ];
+
+    for bench_func in bench_funcs.iter() {
+        bench_func(Some(&mut group))
+    }
+
+    group.finish();
+}
+
+criterion_group!(benches, bench_group_wall_time);
+criterion_main!(benches);
diff --git a/cli/cli_utils/Cargo.toml b/cli/cli_utils/Cargo.toml
new file mode 100644
index 0000000000..d80d17e3c8
--- /dev/null
+++ b/cli/cli_utils/Cargo.toml
@@ -0,0 +1,23 @@
+[package]
+name = "cli_utils"
+version = "0.1.0"
+authors = ["The Roc Contributors"]
+license = "UPL-1.0"
+repository = "https://github.com/rtfeldman/roc"
+edition = "2018"
+description = "Shared code for cli tests and benchmarks"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+roc_cli = { path = "../../cli" }
+roc_collections = { path = "../../compiler/collections" }
+roc_load = { path = "../../compiler/load" }
+roc_module = { path = "../../compiler/module" }
+bumpalo = { version = "3.6.1", features = ["collections"] }
+criterion = { git = "https://github.com/Anton-4/criterion.rs"} 
+inlinable_string = "0.1"
+serde = { version = "1.0", features = ["derive"] }
+serde-xml-rs = "0.4"
+strip-ansi-escapes = "0.1"
+tempfile = "3.1.0"
diff --git a/cli/cli_utils/src/bench_utils.rs b/cli/cli_utils/src/bench_utils.rs
new file mode 100644
index 0000000000..f60fc89a3e
--- /dev/null
+++ b/cli/cli_utils/src/bench_utils.rs
@@ -0,0 +1,135 @@
+use crate::helpers::{example_file, run_cmd, run_roc};
+use criterion::{black_box, measurement::Measurement, BenchmarkGroup};
+use std::path::Path;
+
+fn exec_bench_w_input<T: Measurement>(
+    file: &Path,
+    stdin_str: &str,
+    executable_filename: &str,
+    expected_ending: &str,
+    bench_group_opt: Option<&mut BenchmarkGroup<T>>,
+) {
+    let flags: &[&str] = &["--optimize"];
+
+    let compile_out = run_roc(&[&["build", file.to_str().unwrap()], flags].concat());
+
+    if !compile_out.stderr.is_empty() {
+        panic!("{}", compile_out.stderr);
+    }
+
+    assert!(
+        compile_out.status.success(),
+        "build ended with bad status {:?}",
+        compile_out
+    );
+
+    check_cmd_output(file, stdin_str, executable_filename, expected_ending);
+
+    bench_cmd(file, stdin_str, executable_filename, bench_group_opt);
+}
+
+fn check_cmd_output(
+    file: &Path,
+    stdin_str: &str,
+    executable_filename: &str,
+    expected_ending: &str,
+) {
+    let out = run_cmd(
+        file.with_file_name(executable_filename).to_str().unwrap(),
+        &[stdin_str],
+        &[],
+    );
+
+    if !&out.stdout.ends_with(expected_ending) {
+        panic!(
+            "expected output to end with {:?} but instead got {:#?}",
+            expected_ending, out
+        );
+    }
+    assert!(out.status.success());
+}
+
+fn bench_cmd<T: Measurement>(
+    file: &Path,
+    stdin_str: &str,
+    executable_filename: &str,
+    bench_group_opt: Option<&mut BenchmarkGroup<T>>,
+) {
+    if let Some(bench_group) = bench_group_opt {
+        bench_group.bench_function(&format!("Benchmarking {:?}", executable_filename), |b| {
+            b.iter(|| {
+                run_cmd(
+                    black_box(file.with_file_name(executable_filename).to_str().unwrap()),
+                    black_box(&[stdin_str]),
+                    &[],
+                )
+            })
+        });
+    } else {
+        run_cmd(
+            black_box(file.with_file_name(executable_filename).to_str().unwrap()),
+            black_box(&[stdin_str]),
+            &[],
+        );
+    }
+}
+
+pub fn bench_nqueens<T: Measurement>(bench_group_opt: Option<&mut BenchmarkGroup<T>>) {
+    exec_bench_w_input(
+        &example_file("benchmarks", "NQueens.roc"),
+        "11",
+        "nqueens",
+        "2680\n",
+        bench_group_opt,
+    );
+}
+
+pub fn bench_cfold<T: Measurement>(bench_group_opt: Option<&mut BenchmarkGroup<T>>) {
+    exec_bench_w_input(
+        &example_file("benchmarks", "CFold.roc"),
+        "17",
+        "cfold",
+        "396354 & 396354\n",
+        bench_group_opt,
+    );
+}
+
+pub fn bench_deriv<T: Measurement>(bench_group_opt: Option<&mut BenchmarkGroup<T>>) {
+    exec_bench_w_input(
+        &example_file("benchmarks", "Deriv.roc"),
+        "8",
+        "deriv",
+        "1 count: 6\n2 count: 22\n3 count: 90\n4 count: 420\n5 count: 2202\n6 count: 12886\n7 count: 83648\n8 count: 598592\n",
+        bench_group_opt,
+    );
+}
+
+pub fn bench_rbtree_ck<T: Measurement>(bench_group_opt: Option<&mut BenchmarkGroup<T>>) {
+    exec_bench_w_input(
+        &example_file("benchmarks", "RBTreeCk.roc"),
+        "80000",
+        "rbtree-ck",
+        "8000\n",
+        bench_group_opt,
+    );
+}
+
+pub fn bench_rbtree_delete<T: Measurement>(bench_group_opt: Option<&mut BenchmarkGroup<T>>) {
+    exec_bench_w_input(
+        &example_file("benchmarks", "RBTreeDel.roc"),
+        "100000",
+        "rbtree-del",
+        "7000\n",
+        bench_group_opt,
+    );
+}
+
+pub fn bench_quicksort<T: Measurement>(bench_group_opt: Option<&mut BenchmarkGroup<T>>) {
+    exec_bench_w_input(
+        &example_file("benchmarks", "QuicksortApp.roc"),
+        "1", // 1 for sorting large list, 0 for a small list
+        "quicksortapp",
+        "[ 0, 0, 1, 1, 2, 2, 2, 3, 3, 4, 6, 6, 7, 7, 8, 8, 8, 8, 8, 8, 9, 10, 10, 11, 11, 12, 13, 13, 14, 14, 14, 15, 15, 15, 16, 16, 18, 18, 19, 19, 19, 20, 21, 21, 21, 21, 22, 23, 23, 23, 25, 26, 27, 27, 28, 28, 28, 29, 29, 29, 29, 30, 31, 32, 33, 34, 35, 35, 35, 36, 36, 36, 37, 38, 38, 39, 39, 39, 39, 39, 39, 40, 40, 41, 42, 42, 42, 42, 42, 43, 43, 44, 46, 47, 47, 47, 48, 50, 51, 51, 52, 52, 52, 53, 54, 54, 55, 55, 55, 56, 57, 57, 58, 58, 58, 58, 58, 59, 59, 60, 60, 61, 62, 63, 63, 63, 63, 64, 65, 65, 65, 66, 66, 66, 66, 67, 67, 68, 69, 69, 70, 70, 71, 71, 71, 72, 72, 73, 73, 73, 74, 75, 75, 75, 76, 78, 79, 79, 80, 81, 81, 82, 82, 83, 83, 84, 84, 86, 86, 87, 87, 88, 88, 88, 89, 89, 90, 90, 90, 91, 92, 92, 92, 93, 93, 93, 94, 95, 95, 96, 97, 98, 99, 100, 100, 101, 102, 102, 102, 104, 104, 105, 106, 106, 106, 106, 106, 106, 107, 107, 108, 108, 108, 109, 109, 109, 109, 110, 112, 112, 112, 113, 113, 113, 113, 113, 114, 115, 117, 117, 117, 118, 119, 119, 119, 120, 120, 121, 123, 124, 125, 125, 126, 126, 126, 126, 127, 129, 131, 131, 131, 131, 131, 131, 131, 132, 133, 133, 134, 134, 134, 135, 135, 135, 135, 135, 137, 138, 138, 138, 139, 139, 140, 141, 142, 142, 142, 144, 144, 145, 145, 145, 147, 147, 147, 147, 148, 149, 149, 149, 150, 150, 151, 151, 151, 151, 153, 155, 156, 159, 160, 160, 160, 161, 161, 162, 162, 162, 162, 162, 162, 163, 163, 163, 163, 163, 163, 164, 164, 164, 164, 164, 165, 165, 165, 165, 165, 166, 166, 166, 166, 166, 167, 167, 167, 167, 168, 169, 170, 170, 170, 170, 172, 172, 172, 173, 173, 173, 174, 175, 176, 177, 177, 178, 178, 178, 178, 179, 179, 180, 180, 181, 181, 182, 183, 183, 185, 186, 186, 186, 186, 186, 186, 186, 187, 187, 187, 188, 188, 188, 190, 190, 190, 190, 190, 192, 193, 194, 194, 194, 195, 195, 196, 197, 198, 198, 198, 199, 199, 199, 200, 200, 201, 201, 201, 204, 205, 205, 205, 207, 207, 207, 208, 208, 208, 208, 210, 210, 210, 210, 211, 211, 213, 214, 214, 214, 218, 218, 218, 218, 218, 218, 219, 221, 222, 223, 223, 223, 224, 224, 224, 224, 224, 224, 224, 225, 226, 226, 226, 226, 226, 227, 227, 228, 228, 229, 229, 229, 229, 230, 230, 230, 230, 232, 233, 233, 234, 235, 236, 236, 237, 237, 238, 240, 240, 242, 242, 243, 244, 246, 247, 247, 247, 247, 248, 248, 248, 249, 249, 249, 249, 249, 250, 250, 250, 251, 251, 252, 252, 253, 255, 255, 256, 256, 256, 257, 257, 257, 258, 258, 258, 258, 258, 259, 259, 260, 260, 260, 261, 261, 261, 262, 263, 265, 265, 266, 267, 267, 267, 268, 268, 268, 270, 270, 270, 271, 271, 273, 274, 274, 274, 275, 277, 277, 279, 279, 280, 281, 281, 282, 283, 283, 285, 286, 288, 288, 289, 289, 290, 290, 290, 290, 290, 291, 291, 291, 291, 292, 292, 292, 293, 294, 294, 295, 295, 295, 295, 295, 298, 298, 301, 301, 301, 302, 302, 303, 304, 305, 305, 306, 307, 307, 308, 308, 309, 309, 309, 309, 310, 310, 311, 311, 311, 312, 313, 313, 313, 314, 315, 316, 316, 316, 316, 317, 318, 318, 319, 319, 319, 320, 321, 321, 322, 322, 322, 322, 323, 323, 323, 324, 324, 324, 325, 326, 326, 328, 329, 329, 330, 330, 330, 331, 331, 331, 331, 332, 332, 333, 333, 333, 333, 334, 334, 334, 335, 336, 336, 337, 337, 337, 337, 339, 339, 340, 341, 341, 343, 344, 344, 345, 345, 345, 346, 346, 347, 348, 348, 348, 349, 350, 351, 351, 351, 352, 353, 354, 354, 354, 355, 356, 356, 357, 358, 358, 358, 359, 359, 360, 361, 361, 362, 362, 363, 364, 364, 365, 365, 365, 366, 366, 367, 367, 368, 368, 369, 369, 369, 370, 370, 370, 370, 370, 371, 372, 373, 373, 374, 374, 375, 375, 376, 377, 377, 378, 379, 381, 381, 383, 384, 385, 385, 385, 385, 386, 386, 387, 388, 388, 388, 389, 389, 390, 391, 391, 391, 392, 392, 393, 393, 394, 394, 394, 395, 395, 396, 396, 397, 397, 398, 399, 400, 400, 401, 401, 402, 402, 403, 404, 404, 405, 406, 406, 407, 407, 407, 408, 408, 408, 408, 408, 409, 409, 409, 411, 411, 412, 412, 413, 413, 413, 413, 413, 414, 414, 414, 415, 416, 416, 416, 416, 417, 417, 418, 418, 418, 418, 419, 420, 420, 420, 421, 421, 422, 422, 423, 423, 423, 424, 424, 424, 424, 425, 425, 425, 426, 426, 427, 427, 427, 428, 428, 429, 429, 429, 430, 430, 431, 432, 433, 433, 433, 434, 434, 434, 434, 437, 438, 438, 438, 438, 438, 439, 440, 441, 441, 442, 442, 443, 444, 444, 444, 445, 445, 445, 447, 447, 447, 448, 448, 449, 449, 450, 450, 450, 451, 452, 453, 453, 453, 453, 455, 455, 456, 456, 457, 458, 459, 459, 460, 460, 461, 461, 464, 465, 465, 465, 466, 466, 467, 467, 467, 467, 468, 469, 469, 470, 470, 471, 471, 471, 472, 473, 473, 473, 473, 474, 475, 475, 475, 476, 476, 476, 477, 477, 477, 478, 478, 479, 481, 481, 481, 482, 482, 482, 483, 483, 483, 484, 484, 485, 488, 488, 488, 488, 489, 490, 491, 491, 491, 492, 492, 493, 493, 493, 493, 493, 495, 495, 496, 496, 496, 496, 496, 496, 497, 497, 498, 498, 498, 498, 498, 499, 500, 500, 501, 501, 501, 502, 502, 502, 502, 503, 503, 503, 505, 505, 506, 507, 507, 507, 507, 508, 508, 510, 510, 510, 511, 511, 512, 512, 513, 513, 513, 513, 514, 514, 515, 516, 517, 518, 519, 519, 519, 520, 521, 521, 522, 522, 523, 523, 523, 525, 525, 526, 527, 527, 527, 528, 528, 528, 530, 531, 532, 532, 532, 532, 532, 535, 535, 537, 538, 538, 538, 540, 540, 540, 541, 541, 541, 541, 541, 542, 543, 543, 543, 543, 544, 544, 545, 545, 545, 546, 547, 547, 547, 548, 549, 549, 551, 552, 552, 553, 553, 553, 554, 554, 554, 555, 556, 557, 557, 557, 558, 558, 558, 559, 559, 559, 560, 560, 560, 561, 561, 561, 561, 562, 562, 562, 563, 563, 565, 566, 566, 567, 568, 569, 570, 570, 571, 571, 571, 571, 572, 572, 572, 574, 575, 576, 576, 577, 580, 581, 581, 582, 582, 582, 583, 583, 584, 585, 585, 585, 586, 587, 587, 588, 588, 588, 589, 591, 591, 591, 592, 592, 592, 593, 593, 593, 594, 594, 594, 594, 595, 595, 595, 596, 596, 596, 596, 596, 597, 597, 599, 599, 600, 600, 601, 601, 601, 602, 602, 603, 603, 604, 605, 605, 605, 606, 607, 608, 610, 612, 612, 613, 613, 614, 614, 615, 615, 615, 616, 616, 616, 617, 617, 619, 619, 619, 619, 620, 621, 621, 622, 624, 624, 624, 624, 625, 625, 628, 628, 628, 629, 629, 630, 630, 630, 630, 632, 633, 633, 634, 635, 638, 638, 639, 640, 641, 641, 643, 643, 644, 644, 644, 645, 645, 645, 646, 646, 646, 647, 647, 647, 647, 648, 648, 649, 650, 650, 650, 650, 650, 650, 651, 652, 652, 652, 653, 653, 653, 653, 654, 655, 655, 655, 655, 656, 657, 657, 657, 658, 658, 659, 659, 659, 659, 659, 660, 660, 661, 662, 663, 664, 665, 666, 666, 666, 667, 667, 667, 667, 667, 668, 668, 669, 670, 670, 670, 671, 672, 672, 672, 672, 672, 673, 673, 674, 674, 674, 675, 676, 676, 677, 678, 678, 679, 679, 680, 681, 681, 682, 683, 683, 684, 684, 685, 686, 686, 686, 686, 687, 687, 688, 690, 690, 691, 691, 693, 693, 694, 694, 697, 697, 698, 700, 701, 702, 702, 703, 703, 703, 704, 705, 706, 706, 707, 708, 708, 709, 709, 710, 710, 711, 712, 712, 712, 712, 712, 712, 713, 713, 714, 714, 716, 716, 716, 717, 717, 717, 718, 718, 718, 718, 719, 719, 719, 720, 720, 721, 721, 722, 723, 724, 725, 726, 726, 727, 729, 729, 729, 730, 730, 731, 731, 732, 732, 734, 734, 734, 735, 735, 736, 736, 736, 737, 737, 738, 739, 740, 740, 740, 741, 741, 742, 742, 742, 742, 744, 744, 744, 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4338, 4338, 4338, 4338, 4339, 4339, 4339, 4340, 4340, 4340, 4340, 4340, 4340, 4341, 4341, 4341, 4343, 4343, 4344, 4344, 4344, 4345, 4346, 4347, 4347, 4348, 4348, 4348, 4352, 4353, 4355, 4356, 4356, 4357, 4357, 4358, 4358, 4358, 4358, 4359, 4360, 4360, 4360, 4361, 4361, 4361, 4361, 4362, 4362, 4363, 4363, 4364, 4365, 4365, 4365, 4368, 4370, 4370, 4370, 4371, 4371, 4371, 4372, 4372, 4372, 4372, 4374, 4374, 4374, 4375, 4375, 4376, 4376, 4377, 4377, 4380, 4380, 4381, 4381, 4381, 4382, 4382, 4384, 4386, 4386, 4387, 4387, 4389, 4389, 4389, 4389, 4389, 4389, 4390, 4391, 4391, 4392, 4393, 4393, 4394, 4394, 4394, 4394, 4395, 4395, 4396, 4398, 4398, 4398, 4399, 4400, 4400, 4400, 4401, 4401, 4402, 4403, 4403, 4403, 4404, 4405, 4405, 4405, 4407, 4408, 4408, 4408, 4409, 4410, 4410, 4410, 4410, 4411, 4411, 4412, 4412, 4413, 4413, 4414, 4414, 4414, 4414, 4414, 4415, 4415, 4419, 4419, 4419, 4419, 4420, 4420, 4420, 4421, 4421, 4421, 4421, 4423, 4424, 4425, 4426, 4427, 4427, 4428, 4429, 4429, 4430, 4430, 4430, 4431, 4431, 4431, 4431, 4432, 4432, 4432, 4432, 4432, 4432, 4433, 4433, 4434, 4434, 4435, 4435, 4435, 4435, 4436, 4436, 4436, 4436, 4437, 4437, 4438, 4438, 4438, 4438, 4438, 4439, 4439, 4440, 4440, 4441, 4441, 4442, 4443, 4444, 4444, 4446, 4446, 4447, 4447, 4447, 4448, 4448, 4448, 4449, 4450, 4451, 4452, 4453, 4453, 4454, 4454, 4455, 4455, 4455, 4456, 4456, 4456, 4457, 4457, 4457, 4457, 4457, 4457, 4457, 4457, 4459, 4460, 4460, 4461, 4462, 4462, 4462, 4462, 4465, 4465, 4466, 4467, 4468, 4468, 4469, 4470, 4470, 4471, 4471, 4471, 4471, 4471, 4471, 4471, 4471, 4472, 4472, 4472, 4473, 4473, 4474, 4474, 4474, 4475, 4475, 4476, 4477, 4479, 4479, 4479, 4479, 4481, 4481, 4481, 4481, 4482, 4482, 4482, 4483, 4483, 4484, 4484, 4486, 4487, 4487, 4487, 4487, 4487, 4488, 4488, 4488, 4489, 4491, 4491, 4492, 4492, 4493, 4493, 4494, 4494, 4494, 4494, 4494, 4495, 4495, 4495, 4496, 4496, 4496, 4497, 4498, 4499, 4500, 4500, 4501, 4501, 4503, 4503, 4503, 4504, 4504, 4506, 4509, 4509, 4509, 4509, 4510, 4510, 4511, 4511, 4511, 4512, 4513, 4513, 4514, 4514, 4514, 4515, 4517, 4518, 4521, 4521, 4521, 4521, 4522, 4523, 4523, 4524, 4524, 4525, 4525, 4525, 4525, 4525, 4526, 4526, 4527, 4527, 4528, 4528, 4528, 4529, 4529, 4529, 4529, 4530, 4531, 4532, 4533, 4533, 4534, 4535, 4536, 4536, 4536, 4536, 4537, 4537, 4538, 4539, 4539, 4542, 4542, 4542, 4543, 4543, 4543, 4544, 4544, 4546, 4547, 4547, 4548, 4548, 4549, 4549, 4550, 4550, 4551, 4552, 4552, 4552, 4553, 4553, 4554, 4554, 4554, 4554, 4554, 4555, 4555, 4556, 4556, 4557, 4557, 4558, 4558, 4559, 4559, 4559, 4560, 4560, 4562, 4563, 4563, 4564, 4565, 4566, 4566, 4566, 4567, 4567, 4567, 4567, 4567, 4568, 4568, 4568, 4569, 4569, 4570, 4571, 4572, 4572, 4572, 4572, 4573, 4574, 4574, 4574, 4575, 4575, 4575, 4575, 4575, 4575, 4576, 4576, 4577, 4577, 4578, 4578, 4578, 4579, 4579, 4579, 4579, 4580, 4580, 4580, 4580, 4580, 4581, 4581, 4582, 4583, 4584, 4584, 4586, 4586, 4587, 4588, 4589, 4590, 4590, 4592, 4592, 4592, 4593, 4594, 4594, 4594, 4595, 4595, 4595, 4596, 4597, 4597, 4597, 4598, 4598, 4600, 4600, 4600, 4600, 4601, 4601, 4602, 4602, 4602, 4603, 4604, 4604, 4605, 4605, 4605, 4606, 4607, 4608, 4608, 4608, 4609, 4609, 4609, 4610, 4611, 4611, 4612, 4612, 4614, 4614, 4614, 4614, 4615, 4615, 4616, 4616, 4616, 4616, 4617, 4617, 4617, 4617, 4618, 4618, 4618, 4618, 4620, 4621, 4621, 4621, 4622, 4623, 4623, 4623, 4624, 4624, 4625, 4625, 4626, 4626, 4627, 4627, 4627, 4629, 4629, 4630, 4630, 4631, 4631, 4631, 4631, 4631, 4631, 4632, 4633, 4634, 4634, 4634, 4635, 4635, 4635, 4635, 4636, 4636, 4636, 4636, 4637, 4637, 4638, 4639, 4639, 4640, 4640, 4640, 4641, 4641, 4642, 4643, 4643, 4643, 4644, 4644, 4645, 4646, 4646, 4647, 4648, 4649, 4649, 4649, 4649, 4651, 4651, 4653, 4654, 4655, 4655, 4656, 4656, 4657, 4658, 4658, 4658, 4659, 4659, 4659, 4659, 4659, 4660, 4661, 4662, 4662, 4663, 4663, 4664, 4664, 4665, 4665, 4666, 4666, 4666, 4667, 4667, 4668, 4669, 4669, 4669, 4669, 4670, 4670, 4670, 4671, 4673, 4673, 4674, 4674, 4674, 4674, 4675, 4675, 4675, 4676, 4677, 4678, 4678, 4679, 4679, 4679, 4679, 4680, 4680, 4681, 4681, 4683, 4683, 4683, 4683, 4684, 4684, 4685, 4685, 4686, 4686, 4687, 4687, 4688, 4690, 4690, 4690, 4690, 4691, 4691, 4693, 4693, 4693, 4693, 4693, 4695, 4695, 4697, 4697, 4698, 4699, 4699, 4700, 4700, 4700, 4701, 4701, 4701, 4702, 4703, 4703, 4704, 4704, 4704, 4705, 4705, 4705, 4706, 4707, 4707, 4707, 4708, 4708, 4709, 4709, 4710, 4710, 4710, 4711, 4711, 4712, 4712, 4714, 4715, 4716, 4716, 4717, 4718, 4718, 4718, 4718, 4719, 4719, 4720, 4720, 4720, 4720, 4721, 4721, 4721, 4722, 4722, 4725, 4725, 4726, 4726, 4727, 4728, 4728, 4728, 4728, 4728, 4729, 4729, 4730, 4730, 4731, 4731, 4732, 4732, 4733, 4733, 4733, 4733, 4733, 4734, 4734, 4734, 4734, 4735, 4735, 4735, 4736, 4737, 4738, 4738, 4738, 4738, 4738, 4739, 4739, 4740, 4740, 4741, 4741, 4743, 4743, 4743, 4744, 4744, 4744, 4744, 4744, 4746, 4746, 4746, 4746, 4747, 4747, 4747, 4748, 4748, 4748, 4749, 4749, 4749, 4750, 4751, 4751, 4751, 4752, 4753, 4753, 4753, 4755, 4755, 4756, 4756, 4757, 4757, 4757, 4758, 4758, 4760, 4760, 4760, 4761, 4761, 4762, 4762, 4762, 4763, 4764, 4764, 4765, 4766, 4767, 4767, 4768, 4768, 4769, 4770, 4771, 4771, 4771, 4773, 4774, 4774, 4774, 4774, 4775, 4778, 4779, 4780, 4780, 4780, 4781, 4781, 4782, 4782, 4782, 4783, 4785, 4785, 4786, 4787, 4787, 4787, 4787, 4788, 4788, 4788, 4788, 4788, 4789, 4790, 4790, 4790, 4791, 4791, 4791, 4792, 4792, 4792, 4792, 4792, 4792, 4793, 4794, 4794, 4796, 4796, 4796, 4796, 4797, 4798, 4798, 4799, 4799, 4799, 4799, 4801, 4801, 4802, 4802, 4802, 4803, 4805, 4805, 4808, 4808, 4808, 4810, 4810, 4810, 4811, 4811, 4811, 4811, 4812, 4812, 4813, 4814, 4815, 4815, 4816, 4816, 4816, 4816, 4816, 4817, 4817, 4817, 4818, 4818, 4818, 4819, 4819, 4820, 4822, 4822, 4822, 4822, 4822, 4822, 4823, 4823, 4823, 4824, 4824, 4825, 4826, 4826, 4827, 4827, 4828, 4828, 4828, 4829, 4829, 4830, 4830, 4830, 4831, 4831, 4831, 4832, 4832, 4833, 4834, 4834, 4834, 4834, 4835, 4835, 4835, 4836, 4837, 4838, 4838, 4838, 4838, 4838, 4838, 4839, 4839, 4839, 4839, 4840, 4840, 4841, 4842, 4842, 4842, 4843, 4843, 4843, 4843, 4843, 4844, 4844, 4845, 4846, 4846, 4847, 4847, 4847, 4847, 4847, 4848, 4848, 4849, 4849, 4849, 4849, 4849, 4850, 4850, 4851, 4853, 4853, 4853, 4854, 4854, 4856, 4856, 4857, 4857, 4857, 4858, 4858, 4859, 4859, 4859, 4859, 4860, 4860, 4861, 4862, 4862, 4863, 4863, 4863, 4863, 4864, 4864, 4864, 4864, 4865, 4865, 4866, 4866, 4867, 4867, 4869, 4870, 4870, 4870, 4870, 4870, 4870, 4871, 4871, 4871, 4872, 4873, 4873, 4874, 4874, 4875, 4875, 4876, 4876, 4876, 4876, 4877, 4879, 4879, 4879, 4881, 4882, 4882, 4883, 4883, 4883, 4884, 4884, 4886, 4888, 4888, 4888, 4889, 4890, 4890, 4890, 4891, 4891, 4892, 4892, 4892, 4892, 4893, 4893, 4893, 4894, 4894, 4894, 4894, 4894, 4894, 4895, 4898, 4899, 4899, 4900, 4901, 4901, 4901, 4901, 4902, 4902, 4903, 4904, 4904, 4904, 4904, 4904, 4905, 4906, 4908, 4908, 4909, 4910, 4910, 4911, 4911, 4912, 4912, 4913, 4913, 4914, 4914, 4914, 4915, 4915, 4916, 4917, 4917, 4918, 4918, 4920, 4921, 4921, 4921, 4921, 4922, 4922, 4922, 4922, 4923, 4923, 4924, 4924, 4924, 4925, 4926, 4926, 4926, 4927, 4928, 4928, 4928, 4928, 4928, 4928, 4929, 4930, 4930, 4931, 4932, 4934, 4934, 4935, 4935, 4936, 4936, 4937, 4937, 4937, 4937, 4937, 4938, 4939, 4939, 4939, 4939, 4939, 4939, 4942, 4943, 4943, 4944, 4944, 4944, 4944, 4944, 4945, 4946, 4946, 4946, 4947, 4948, 4948, 4948, 4949, 4949, 4950, 4950, 4950, 4950, 4951, 4951, 4952, 4956, 4956, 4957, 4957, 4957, 4957, 4958, 4960, 4960, 4960, 4961, 4961, 4961, 4962, 4962, 4962, 4963, 4963, 4964, 4964, 4964, 4965, 4966, 4967, 4967, 4968, 4968, 4968, 4971, 4971, 4972, 4972, 4974, 4975, 4975, 4975, 4976, 4976, 4977, 4978, 4978, 4979, 4979, 4979, 4980, 4980, 4980, 4981, 4981, 4981, 4981, 4982, 4982, 4982, 4982, 4982, 4983, 4983, 4983, 4983, 4984, 4985, 4985, 4986, 4986, 4986, 4986, 4987, 4987, 4988, 4989, 4989, 4989, 4990, 4991, 4992, 4992, 4993, 4993, 4994, 4995, 4995, 4996, 4997, 4997, 4997, 4997, 4997, 4998, 4998, 4999, 4999 ]\n",
+        bench_group_opt,
+    );
+}
diff --git a/cli/tests/helpers.rs b/cli/cli_utils/src/helpers.rs
similarity index 100%
rename from cli/tests/helpers.rs
rename to cli/cli_utils/src/helpers.rs
diff --git a/cli/cli_utils/src/lib.rs b/cli/cli_utils/src/lib.rs
new file mode 100644
index 0000000000..b5cded1f52
--- /dev/null
+++ b/cli/cli_utils/src/lib.rs
@@ -0,0 +1,2 @@
+pub mod bench_utils;
+pub mod helpers;
diff --git a/cli/src/build.rs b/cli/src/build.rs
index e6f753ec11..d75bf0361b 100644
--- a/cli/src/build.rs
+++ b/cli/src/build.rs
@@ -5,8 +5,8 @@ use roc_build::{
 };
 use roc_can::builtins::builtin_defs_map;
 use roc_collections::all::MutMap;
-use roc_gen::llvm::build::OptLevel;
 use roc_load::file::LoadingProblem;
+use roc_mono::ir::OptLevel;
 use std::path::PathBuf;
 use std::time::{Duration, SystemTime};
 use target_lexicon::Triple;
@@ -26,12 +26,23 @@ pub enum BuildOutcome {
     Errors,
 }
 
+impl BuildOutcome {
+    pub fn status_code(&self) -> i32 {
+        match self {
+            Self::NoProblems => 0,
+            Self::OnlyWarnings => 1,
+            Self::Errors => 2,
+        }
+    }
+}
+
 pub struct BuiltFile {
     pub binary_path: PathBuf,
     pub outcome: BuildOutcome,
     pub total_time: Duration,
 }
 
+#[cfg(feature = "llvm")]
 pub fn build_file<'a>(
     arena: &'a Bump,
     target: &Triple,
@@ -204,10 +215,14 @@ pub fn build_file<'a>(
     let total_time = compilation_start.elapsed().unwrap();
 
     // If the cmd errored out, return the Err.
-    cmd_result?;
+    let exit_status = cmd_result?;
 
     // TODO change this to report whether there were errors or warnings!
-    let outcome = BuildOutcome::NoProblems;
+    let outcome = if exit_status.success() {
+        BuildOutcome::NoProblems
+    } else {
+        BuildOutcome::Errors
+    };
 
     Ok(BuiltFile {
         binary_path,
diff --git a/cli/src/lib.rs b/cli/src/lib.rs
index 91948044c2..c7efdbc1a5 100644
--- a/cli/src/lib.rs
+++ b/cli/src/lib.rs
@@ -1,12 +1,12 @@
 #[macro_use]
 extern crate clap;
 
-use build::{build_file, BuildOutcome, BuiltFile};
+use build::{BuildOutcome, BuiltFile};
 use bumpalo::Bump;
 use clap::{App, AppSettings, Arg, ArgMatches};
 use roc_build::link::LinkType;
-use roc_gen::llvm::build::OptLevel;
 use roc_load::file::LoadingProblem;
+use roc_mono::ir::OptLevel;
 use std::env;
 use std::io;
 use std::path::{Path, PathBuf};
@@ -25,12 +25,13 @@ pub const CMD_DOCS: &str = "docs";
 
 pub const FLAG_DEBUG: &str = "debug";
 pub const FLAG_OPTIMIZE: &str = "optimize";
+pub const FLAG_LIB: &str = "lib";
 pub const ROC_FILE: &str = "ROC_FILE";
 pub const DIRECTORY_OR_FILES: &str = "DIRECTORY_OR_FILES";
 pub const ARGS_FOR_APP: &str = "ARGS_FOR_APP";
 
 pub fn build_app<'a>() -> App<'a> {
-    App::new("roc")
+    let app = App::new("roc")
         .version(crate_version!())
         .subcommand(App::new(CMD_BUILD)
             .about("Build a program")
@@ -45,6 +46,12 @@ pub fn build_app<'a>() -> App<'a> {
                     .help("Optimize the compiled program to run faster. (Optimization takes time to complete.)")
                     .required(false),
             )
+            .arg(
+                Arg::with_name(FLAG_LIB)
+                    .long(FLAG_LIB)
+                    .help("Build a C library instead of an executable.")
+                    .required(false),
+            )
             .arg(
                 Arg::with_name(FLAG_DEBUG)
                     .long(FLAG_DEBUG)
@@ -81,26 +88,31 @@ pub fn build_app<'a>() -> App<'a> {
         .subcommand(App::new(CMD_REPL)
             .about("Launch the interactive Read Eval Print Loop (REPL)")
         )
-        .subcommand(App::new(CMD_EDIT)
-            .about("Launch the Roc editor")
-            .arg(Arg::with_name(DIRECTORY_OR_FILES)
-                .index(1)
-                .multiple(true)
-                .required(false)
-                .help("(optional) The directory or files to open on launch.")
-            )
-        )
         .subcommand(
             App::new(CMD_DOCS)
                 .about("Generate documentation for Roc modules")
                 .arg(Arg::with_name(DIRECTORY_OR_FILES)
                     .index(1)
                     .multiple(true)
-                    .required(true)
+                    .required(false)
                     .help("The directory or files to build documentation for")
 
                 )
+        );
+
+    if cfg!(feature = "editor") {
+        app.subcommand(
+            App::new(CMD_EDIT).about("Launch the Roc editor").arg(
+                Arg::with_name(DIRECTORY_OR_FILES)
+                    .index(1)
+                    .multiple(true)
+                    .required(false)
+                    .help("(optional) The directory or files to open on launch."),
+            ),
         )
+    } else {
+        app
+    }
 }
 
 pub fn docs(files: Vec<PathBuf>) {
@@ -111,12 +123,15 @@ pub fn docs(files: Vec<PathBuf>) {
     )
 }
 
+#[derive(Debug, PartialEq, Eq)]
 pub enum BuildConfig {
     BuildOnly,
     BuildAndRun { roc_file_arg_index: usize },
 }
 
+#[cfg(feature = "llvm")]
 pub fn build(target: &Triple, matches: &ArgMatches, config: BuildConfig) -> io::Result<i32> {
+    use build::build_file;
     use BuildConfig::*;
 
     let arena = Bump::new();
@@ -130,6 +145,12 @@ pub fn build(target: &Triple, matches: &ArgMatches, config: BuildConfig) -> io::
     };
     let emit_debug_info = matches.is_present(FLAG_DEBUG);
 
+    let link_type = if matches.is_present(FLAG_LIB) {
+        LinkType::Dylib
+    } else {
+        LinkType::Executable
+    };
+
     let path = Path::new(filename).canonicalize().unwrap();
     let src_dir = path.parent().unwrap().canonicalize().unwrap();
 
@@ -159,7 +180,7 @@ pub fn build(target: &Triple, matches: &ArgMatches, config: BuildConfig) -> io::
         path,
         opt_level,
         emit_debug_info,
-        LinkType::Executable,
+        link_type,
     );
 
     match res_binary_path {
@@ -175,13 +196,6 @@ pub fn build(target: &Triple, matches: &ArgMatches, config: BuildConfig) -> io::
                         .strip_prefix(env::current_dir().unwrap())
                         .unwrap_or(&binary_path);
 
-                    // Return a nonzero exit code if there were problems
-                    let status_code = match outcome {
-                        BuildOutcome::NoProblems => 0,
-                        BuildOutcome::OnlyWarnings => 1,
-                        BuildOutcome::Errors => 2,
-                    };
-
                     // No need to waste time freeing this memory,
                     // since the process is about to exit anyway.
                     std::mem::forget(arena);
@@ -192,7 +206,8 @@ pub fn build(target: &Triple, matches: &ArgMatches, config: BuildConfig) -> io::
                         total_time.as_millis()
                     );
 
-                    Ok(status_code)
+                    // Return a nonzero exit code if there were problems
+                    Ok(outcome.status_code())
                 }
                 BuildAndRun { roc_file_arg_index } => {
                     let mut cmd = Command::new(binary_path);
@@ -210,23 +225,9 @@ pub fn build(target: &Triple, matches: &ArgMatches, config: BuildConfig) -> io::
                         }
                     }
 
-                    // Run the compiled app
-                    let exit_status = cmd
-                        .current_dir(original_cwd)
-                        .spawn()
-                        .unwrap_or_else(|err| panic!("Failed to run app after building it: {:?}", err))
-                        .wait()
-                        .expect("TODO gracefully handle block_on failing when roc run spawns a subprocess for the compiled app");
-
-                    // `roc run` exits with the same status code as the app it ran.
-                    //
-                    // If you want to know whether there were compilation problems
-                    // via status code, use either `roc build` or `roc check` instead!
-                    match exit_status.code() {
-                        Some(code) => Ok(code),
-                        None => {
-                            todo!("TODO gracefully handle the roc run subprocess terminating with a signal.");
-                        }
+                    match outcome {
+                        BuildOutcome::Errors => Ok(outcome.status_code()),
+                        _ => roc_run(cmd.current_dir(original_cwd)),
                     }
                 }
             }
@@ -241,3 +242,37 @@ pub fn build(target: &Triple, matches: &ArgMatches, config: BuildConfig) -> io::
         }
     }
 }
+
+#[cfg(target_family = "unix")]
+fn roc_run(cmd: &mut Command) -> io::Result<i32> {
+    use std::os::unix::process::CommandExt;
+
+    // This is much faster than spawning a subprocess if we're on a UNIX system!
+    let err = cmd.exec();
+
+    // If exec actually returned, it was definitely an error! (Otherwise,
+    // this process would have been replaced by the other one, and we'd
+    // never actually reach this line of code.)
+    Err(err)
+}
+
+#[cfg(not(target_family = "unix"))]
+fn roc_run(cmd: &mut Command) -> io::Result<i32> {
+    // Run the compiled app
+    let exit_status = cmd
+                        .spawn()
+                        .unwrap_or_else(|err| panic!("Failed to run app after building it: {:?}", err))
+                        .wait()
+                        .expect("TODO gracefully handle block_on failing when roc run spawns a subprocess for the compiled app");
+
+    // `roc run` exits with the same status code as the app it ran.
+    //
+    // If you want to know whether there were compilation problems
+    // via status code, use either `roc build` or `roc check` instead!
+    match exit_status.code() {
+        Some(code) => Ok(code),
+        None => {
+            todo!("TODO gracefully handle the roc run subprocess terminating with a signal.");
+        }
+    }
+}
diff --git a/cli/src/main.rs b/cli/src/main.rs
index 191e8409b0..c4d659a5be 100644
--- a/cli/src/main.rs
+++ b/cli/src/main.rs
@@ -1,17 +1,27 @@
 use roc_cli::{
-    build, build_app, docs, repl, BuildConfig, CMD_BUILD, CMD_DOCS, CMD_EDIT, CMD_REPL, CMD_RUN,
+    build_app, docs, repl, BuildConfig, CMD_BUILD, CMD_DOCS, CMD_EDIT, CMD_REPL, CMD_RUN,
     DIRECTORY_OR_FILES, ROC_FILE,
 };
+use std::fs::{self, FileType};
 use std::io;
 use std::path::{Path, PathBuf};
 use target_lexicon::Triple;
 
+#[cfg(feature = "llvm")]
+use roc_cli::build;
+use std::ffi::{OsStr, OsString};
+
+#[cfg(not(feature = "llvm"))]
+fn build(_target: &Triple, _matches: &clap::ArgMatches, _config: BuildConfig) -> io::Result<i32> {
+    panic!("Building without LLVM is not currently supported.");
+}
+
 fn main() -> io::Result<()> {
     let matches = build_app().get_matches();
 
     let exit_code = match matches.subcommand_name() {
         None => {
-            roc_editor::launch(&[])?;
+            launch_editor(&[])?;
 
             // rustc couldn't infer the error type here
             Result::<i32, io::Error>::Ok(0)
@@ -44,14 +54,14 @@ fn main() -> io::Result<()> {
                 .values_of_os(DIRECTORY_OR_FILES)
             {
                 None => {
-                    roc_editor::launch(&[])?;
+                    launch_editor(&[])?;
                 }
                 Some(values) => {
                     let paths = values
                         .map(|os_str| Path::new(os_str))
                         .collect::<Vec<&Path>>();
 
-                    roc_editor::launch(&paths)?;
+                    launch_editor(&paths)?;
                 }
             }
 
@@ -59,17 +69,37 @@ fn main() -> io::Result<()> {
             Ok(0)
         }
         Some(CMD_DOCS) => {
-            let values = matches
+            let maybe_values = matches
                 .subcommand_matches(CMD_DOCS)
                 .unwrap()
-                .values_of_os(DIRECTORY_OR_FILES)
-                .unwrap();
+                .values_of_os(DIRECTORY_OR_FILES);
 
-            let paths = values
-                .map(|os_str| Path::new(os_str).to_path_buf())
-                .collect::<Vec<PathBuf>>();
+            let mut values: Vec<OsString> = Vec::new();
 
-            docs(paths);
+            match maybe_values {
+                None => {
+                    let mut os_string_values: Vec<OsString> = Vec::new();
+                    read_all_roc_files(&OsStr::new("./").to_os_string(), &mut os_string_values)?;
+                    for os_string in os_string_values {
+                        values.push(os_string);
+                    }
+                }
+                Some(os_values) => {
+                    for os_str in os_values {
+                        values.push(os_str.to_os_string());
+                    }
+                }
+            }
+
+            let mut roc_files = Vec::new();
+
+            // Populate roc_files
+            for os_str in values {
+                let metadata = fs::metadata(os_str.clone())?;
+                roc_files_recursive(os_str.as_os_str(), metadata.file_type(), &mut roc_files)?;
+            }
+
+            docs(roc_files);
 
             Ok(0)
         }
@@ -78,3 +108,51 @@ fn main() -> io::Result<()> {
 
     std::process::exit(exit_code);
 }
+
+fn read_all_roc_files(
+    dir: &OsString,
+    mut roc_file_paths: &mut Vec<OsString>,
+) -> Result<(), std::io::Error> {
+    let entries = fs::read_dir(dir)?;
+
+    for entry in entries {
+        let path = entry?.path();
+
+        if path.is_dir() {
+            read_all_roc_files(&path.into_os_string(), &mut roc_file_paths)?;
+        } else if path.extension().and_then(OsStr::to_str) == Some("roc") {
+            let file_path = path.into_os_string();
+            roc_file_paths.push(file_path);
+        }
+    }
+
+    Ok(())
+}
+
+fn roc_files_recursive<P: AsRef<Path>>(
+    path: P,
+    file_type: FileType,
+    roc_files: &mut Vec<PathBuf>,
+) -> io::Result<()> {
+    if file_type.is_dir() {
+        for entry_res in fs::read_dir(path)? {
+            let entry = entry_res?;
+
+            roc_files_recursive(entry.path(), entry.file_type()?, roc_files)?;
+        }
+    } else {
+        roc_files.push(path.as_ref().to_path_buf());
+    }
+
+    Ok(())
+}
+
+#[cfg(feature = "editor")]
+fn launch_editor(filepaths: &[&Path]) -> io::Result<()> {
+    roc_editor::launch(filepaths)
+}
+
+#[cfg(not(feature = "editor"))]
+fn launch_editor(_filepaths: &[&Path]) -> io::Result<()> {
+    panic!("Cannot launch the editor because this build of roc did not include `feature = \"editor\"`!");
+}
diff --git a/cli/src/repl.rs b/cli/src/repl.rs
index c0d7e03917..4de48bb3df 100644
--- a/cli/src/repl.rs
+++ b/cli/src/repl.rs
@@ -1,15 +1,12 @@
 use const_format::concatcp;
+#[cfg(feature = "llvm")]
 use gen::{gen_and_eval, ReplOutput};
-use roc_gen::llvm::build::OptLevel;
 use roc_parse::parser::{EExpr, SyntaxError};
-use rustyline::error::ReadlineError;
 use rustyline::highlight::{Highlighter, PromptInfo};
 use rustyline::validate::{self, ValidationContext, ValidationResult, Validator};
-use rustyline::Editor;
 use rustyline_derive::{Completer, Helper, Hinter};
 use std::borrow::Cow;
 use std::io;
-use target_lexicon::Triple;
 
 const BLUE: &str = "\u{001b}[36m";
 const PINK: &str = "\u{001b}[35m";
@@ -30,7 +27,9 @@ pub const INSTRUCTIONS: &str = "Enter an expression, or :help, or :exit/:q.\n";
 pub const PROMPT: &str = concatcp!("\n", BLUE, "»", END_COL, " ");
 pub const CONT_PROMPT: &str = concatcp!(BLUE, "…", END_COL, " ");
 
+#[cfg(feature = "llvm")]
 mod eval;
+#[cfg(feature = "llvm")]
 mod gen;
 
 #[derive(Completer, Helper, Hinter)]
@@ -107,7 +106,16 @@ impl Validator for InputValidator {
     }
 }
 
+#[cfg(not(feature = "llvm"))]
 pub fn main() -> io::Result<()> {
+    panic!("The REPL currently requires being built with LLVM.");
+}
+
+#[cfg(feature = "llvm")]
+pub fn main() -> io::Result<()> {
+    use rustyline::error::ReadlineError;
+    use rustyline::Editor;
+
     // To debug rustyline:
     // <UNCOMMENT> env_logger::init();
     // <RUN WITH:> RUST_LOG=rustyline=debug cargo run repl 2> debug.log
@@ -226,7 +234,11 @@ fn report_parse_error(fail: SyntaxError) {
     println!("TODO Gracefully report parse error in repl: {:?}", fail);
 }
 
+#[cfg(feature = "llvm")]
 fn eval_and_format<'a>(src: &str) -> Result<String, SyntaxError<'a>> {
+    use roc_mono::ir::OptLevel;
+    use target_lexicon::Triple;
+
     gen_and_eval(src.as_bytes(), Triple::host(), OptLevel::Normal).map(|output| match output {
         ReplOutput::NoProblems { expr, expr_type } => {
             format!("\n{} {}:{} {}", expr, PINK, END_COL, expr_type)
diff --git a/cli/src/repl/eval.rs b/cli/src/repl/eval.rs
index 94f51ec6cf..7d150a4756 100644
--- a/cli/src/repl/eval.rs
+++ b/cli/src/repl/eval.rs
@@ -2,10 +2,11 @@ use bumpalo::collections::Vec;
 use bumpalo::Bump;
 use libloading::Library;
 use roc_collections::all::MutMap;
-use roc_gen::{run_jit_function, run_jit_function_dynamic_type};
+use roc_gen_llvm::{run_jit_function, run_jit_function_dynamic_type};
 use roc_module::ident::{Lowercase, TagName};
 use roc_module::operator::CalledVia;
 use roc_module::symbol::{Interns, ModuleId, Symbol};
+use roc_mono::ir::ProcLayout;
 use roc_mono::layout::{union_sorted_tags_help, Builtin, Layout, UnionLayout, UnionVariant};
 use roc_parse::ast::{AssignedField, Expr, StrLiteral};
 use roc_region::all::{Located, Region};
@@ -37,7 +38,7 @@ pub unsafe fn jit_to_ast<'a>(
     arena: &'a Bump,
     lib: Library,
     main_fn_name: &str,
-    layout: &Layout<'a>,
+    layout: ProcLayout<'a>,
     content: &Content,
     interns: &Interns,
     home: ModuleId,
@@ -53,11 +54,14 @@ pub unsafe fn jit_to_ast<'a>(
     };
 
     match layout {
-        Layout::FunctionPointer(&[], result) => {
+        ProcLayout {
+            arguments: [],
+            result,
+        } => {
             // this is a thunk
-            jit_to_ast_help(&env, lib, main_fn_name, result, content)
+            jit_to_ast_help(&env, lib, main_fn_name, &result, content)
         }
-        _ => jit_to_ast_help(&env, lib, main_fn_name, layout, content),
+        _ => Err(ToAstProblem::FunctionLayout),
     }
 }
 
@@ -79,12 +83,26 @@ fn jit_to_ast_help<'a>(
             )
         }
         Layout::Builtin(Builtin::Usize) => Ok(run_jit_function!(lib, main_fn_name, usize, |num| {
-            num_to_ast(env, nat_to_ast(env.arena, num), content)
+            num_to_ast(env, number_literal_to_ast(env.arena, num), content)
         })),
+        Layout::Builtin(Builtin::Int16) => {
+            Ok(run_jit_function!(lib, main_fn_name, i16, |num| num_to_ast(
+                env,
+                number_literal_to_ast(env.arena, num),
+                content
+            )))
+        }
+        Layout::Builtin(Builtin::Int32) => {
+            Ok(run_jit_function!(lib, main_fn_name, i32, |num| num_to_ast(
+                env,
+                number_literal_to_ast(env.arena, num),
+                content
+            )))
+        }
         Layout::Builtin(Builtin::Int64) => {
             Ok(run_jit_function!(lib, main_fn_name, i64, |num| num_to_ast(
                 env,
-                i64_to_ast(env.arena, num),
+                number_literal_to_ast(env.arena, num),
                 content
             )))
         }
@@ -93,13 +111,20 @@ fn jit_to_ast_help<'a>(
                 lib,
                 main_fn_name,
                 i128,
-                |num| num_to_ast(env, i128_to_ast(env.arena, num), content)
+                |num| num_to_ast(env, number_literal_to_ast(env.arena, num), content)
             ))
         }
+        Layout::Builtin(Builtin::Float32) => {
+            Ok(run_jit_function!(lib, main_fn_name, f32, |num| num_to_ast(
+                env,
+                number_literal_to_ast(env.arena, num),
+                content
+            )))
+        }
         Layout::Builtin(Builtin::Float64) => {
             Ok(run_jit_function!(lib, main_fn_name, f64, |num| num_to_ast(
                 env,
-                f64_to_ast(env.arena, num),
+                number_literal_to_ast(env.arena, num),
                 content
             )))
         }
@@ -116,7 +141,7 @@ fn jit_to_ast_help<'a>(
                 }
             }))
         }
-        Layout::Builtin(Builtin::List(_, elem_layout)) => Ok(run_jit_function!(
+        Layout::Builtin(Builtin::List(elem_layout)) => Ok(run_jit_function!(
             lib,
             main_fn_name,
             (*const u8, usize),
@@ -125,29 +150,33 @@ fn jit_to_ast_help<'a>(
         Layout::Builtin(other) => {
             todo!("add support for rendering builtin {:?} to the REPL", other)
         }
-        Layout::PhantomEmptyStruct => Ok(run_jit_function!(lib, main_fn_name, &u8, |_| {
-            Expr::Record {
-                fields: &[],
-                final_comments: env.arena.alloc([]),
-            }
-        })),
         Layout::Struct(field_layouts) => {
             let ptr_to_ast = |ptr: *const u8| match content {
                 Content::Structure(FlatType::Record(fields, _)) => {
-                    struct_to_ast(env, ptr, field_layouts, fields)
+                    Ok(struct_to_ast(env, ptr, field_layouts, fields))
                 }
                 Content::Structure(FlatType::EmptyRecord) => {
-                    struct_to_ast(env, ptr, field_layouts, &MutMap::default())
+                    Ok(struct_to_ast(env, ptr, field_layouts, &MutMap::default()))
                 }
                 Content::Structure(FlatType::TagUnion(tags, _)) => {
                     debug_assert_eq!(tags.len(), 1);
 
                     let (tag_name, payload_vars) = tags.iter().next().unwrap();
 
-                    single_tag_union_to_ast(env, ptr, field_layouts, tag_name.clone(), payload_vars)
+                    Ok(single_tag_union_to_ast(
+                        env,
+                        ptr,
+                        field_layouts,
+                        tag_name.clone(),
+                        payload_vars,
+                    ))
                 }
-                Content::Structure(FlatType::FunctionOrTagUnion(tag_name, _, _)) => {
-                    single_tag_union_to_ast(env, ptr, field_layouts, tag_name.clone(), &[])
+                Content::Structure(FlatType::FunctionOrTagUnion(tag_name, _, _)) => Ok(
+                    single_tag_union_to_ast(env, ptr, field_layouts, tag_name.clone(), &[]),
+                ),
+                Content::Structure(FlatType::Func(_, _, _)) => {
+                    // a function with a struct as the closure environment
+                    Err(ToAstProblem::FunctionLayout)
                 }
                 other => {
                     unreachable!(
@@ -162,81 +191,141 @@ fn jit_to_ast_help<'a>(
 
             let result_stack_size = layout.stack_size(env.ptr_bytes);
 
-            Ok(run_jit_function_dynamic_type!(
+            run_jit_function_dynamic_type!(
                 lib,
                 main_fn_name,
                 result_stack_size as usize,
                 |bytes: *const u8| { ptr_to_ast(bytes as *const u8) }
-            ))
+            )
         }
-        Layout::Union(UnionLayout::NonRecursive(union_layouts)) => match content {
-            Content::Structure(FlatType::TagUnion(tags, _)) => {
-                debug_assert_eq!(union_layouts.len(), tags.len());
+        Layout::Union(UnionLayout::NonRecursive(union_layouts)) => {
+            let union_layout = UnionLayout::NonRecursive(union_layouts);
 
-                let tags_vec: std::vec::Vec<(TagName, std::vec::Vec<Variable>)> =
-                    tags.iter().map(|(a, b)| (a.clone(), b.clone())).collect();
+            match content {
+                Content::Structure(FlatType::TagUnion(tags, _)) => {
+                    debug_assert_eq!(union_layouts.len(), tags.len());
 
-                let union_variant = union_sorted_tags_help(env.arena, tags_vec, None, env.subs);
+                    let tags_vec: std::vec::Vec<(TagName, std::vec::Vec<Variable>)> =
+                        tags.iter().map(|(a, b)| (a.clone(), b.clone())).collect();
 
-                let size = layout.stack_size(env.ptr_bytes);
-                use roc_mono::layout::WrappedVariant::*;
-                match union_variant {
-                    UnionVariant::Wrapped(variant) => {
-                        match variant {
-                            NonRecursive {
-                                sorted_tag_layouts: tags_and_layouts,
+                    let union_variant = union_sorted_tags_help(env.arena, tags_vec, None, env.subs);
+
+                    let size = layout.stack_size(env.ptr_bytes);
+                    use roc_mono::layout::WrappedVariant::*;
+                    match union_variant {
+                        UnionVariant::Wrapped(variant) => {
+                            match variant {
+                                NonRecursive {
+                                    sorted_tag_layouts: tags_and_layouts,
+                                } => {
+                                    Ok(run_jit_function_dynamic_type!(
+                                        lib,
+                                        main_fn_name,
+                                        size as usize,
+                                        |ptr: *const u8| {
+                                            // Because this is a `Wrapped`, the first 8 bytes encode the tag ID
+                                            let offset = tags_and_layouts
+                                                .iter()
+                                                .map(|(_, fields)| {
+                                                    fields
+                                                        .iter()
+                                                        .map(|l| l.stack_size(env.ptr_bytes))
+                                                        .sum()
+                                                })
+                                                .max()
+                                                .unwrap_or(0);
+
+                                            let tag_id = match union_layout.tag_id_builtin() {
+                                                Builtin::Int1 => {
+                                                    *(ptr.add(offset as usize) as *const i8) as i64
+                                                }
+                                                Builtin::Int8 => {
+                                                    *(ptr.add(offset as usize) as *const i8) as i64
+                                                }
+                                                Builtin::Int16 => {
+                                                    *(ptr.add(offset as usize) as *const i16) as i64
+                                                }
+                                                _ => unreachable!("invalid tag id layout"),
+                                            };
+
+                                            // use the tag ID as an index, to get its name and layout of any arguments
+                                            let (tag_name, arg_layouts) =
+                                                &tags_and_layouts[tag_id as usize];
+
+                                            let tag_expr = tag_name_to_expr(env, tag_name);
+                                            let loc_tag_expr =
+                                                &*env.arena.alloc(Located::at_zero(tag_expr));
+
+                                            let variables = &tags[tag_name];
+
+                                            debug_assert_eq!(arg_layouts.len(), variables.len());
+
+                                            // NOTE assumes the data bytes are the first bytes
+                                            let it =
+                                                variables.iter().copied().zip(arg_layouts.iter());
+                                            let output = sequence_of_expr(env, ptr, it);
+                                            let output = output.into_bump_slice();
+
+                                            Expr::Apply(loc_tag_expr, output, CalledVia::Space)
+                                        }
+                                    ))
+                                }
+                                Recursive {
+                                    sorted_tag_layouts: tags_and_layouts,
+                                } => {
+                                    Ok(run_jit_function_dynamic_type!(
+                                        lib,
+                                        main_fn_name,
+                                        size as usize,
+                                        |ptr: *const u8| {
+                                            // Because this is a `Wrapped`, the first 8 bytes encode the tag ID
+                                            let tag_id = *(ptr as *const i64);
+
+                                            // use the tag ID as an index, to get its name and layout of any arguments
+                                            let (tag_name, arg_layouts) =
+                                                &tags_and_layouts[tag_id as usize];
+
+                                            let tag_expr = tag_name_to_expr(env, tag_name);
+                                            let loc_tag_expr =
+                                                &*env.arena.alloc(Located::at_zero(tag_expr));
+
+                                            let variables = &tags[tag_name];
+
+                                            // because the arg_layouts include the tag ID, it is one longer
+                                            debug_assert_eq!(
+                                                arg_layouts.len() - 1,
+                                                variables.len()
+                                            );
+
+                                            // skip forward to the start of the first element, ignoring the tag id
+                                            let ptr = ptr.offset(8);
+
+                                            let it =
+                                                variables.iter().copied().zip(&arg_layouts[1..]);
+                                            let output = sequence_of_expr(env, ptr, it);
+                                            let output = output.into_bump_slice();
+
+                                            Expr::Apply(loc_tag_expr, output, CalledVia::Space)
+                                        }
+                                    ))
+                                }
+                                _ => todo!(),
                             }
-                            | Recursive {
-                                sorted_tag_layouts: tags_and_layouts,
-                            } => {
-                                Ok(run_jit_function_dynamic_type!(
-                                    lib,
-                                    main_fn_name,
-                                    size as usize,
-                                    |ptr: *const u8| {
-                                        // Because this is a `Wrapped`, the first 8 bytes encode the tag ID
-                                        let tag_id = *(ptr as *const i64);
-
-                                        // use the tag ID as an index, to get its name and layout of any arguments
-                                        let (tag_name, arg_layouts) =
-                                            &tags_and_layouts[tag_id as usize];
-
-                                        let tag_expr = tag_name_to_expr(env, tag_name);
-                                        let loc_tag_expr =
-                                            &*env.arena.alloc(Located::at_zero(tag_expr));
-
-                                        let variables = &tags[tag_name];
-
-                                        // because the arg_layouts include the tag ID, it is one longer
-                                        debug_assert_eq!(arg_layouts.len() - 1, variables.len());
-
-                                        // skip forward to the start of the first element, ignoring the tag id
-                                        let ptr = ptr.offset(8);
-
-                                        let it = variables.iter().copied().zip(&arg_layouts[1..]);
-                                        let output = sequence_of_expr(env, ptr, it);
-                                        let output = output.into_bump_slice();
-
-                                        Expr::Apply(loc_tag_expr, output, CalledVia::Space)
-                                    }
-                                ))
-                            }
-                            _ => todo!(),
                         }
+                        _ => unreachable!("any other variant would have a different layout"),
                     }
-                    _ => unreachable!("any other variant would have a different layout"),
                 }
-            }
-            Content::Structure(FlatType::RecursiveTagUnion(_, _, _)) => {
-                todo!("print recursive tag unions in the REPL")
-            }
-            Content::Alias(_, _, actual) => {
-                let content = env.subs.get_without_compacting(*actual).content;
+                Content::Structure(FlatType::RecursiveTagUnion(_, _, _)) => {
+                    todo!("print recursive tag unions in the REPL")
+                }
+                Content::Alias(_, _, actual) => {
+                    let content = env.subs.get_without_compacting(*actual).content;
 
-                jit_to_ast_help(env, lib, main_fn_name, layout, &content)
+                    jit_to_ast_help(env, lib, main_fn_name, layout, &content)
+                }
+                other => unreachable!("Weird content for Union layout: {:?}", other),
             }
-            other => unreachable!("Weird content for Union layout: {:?}", other),
-        },
+        }
         Layout::Union(UnionLayout::Recursive(_))
         | Layout::Union(UnionLayout::NullableWrapped { .. })
         | Layout::Union(UnionLayout::NullableUnwrapped { .. })
@@ -246,8 +335,6 @@ fn jit_to_ast_help<'a>(
         }
 
         Layout::Closure(_, _, _) => Err(ToAstProblem::FunctionLayout),
-        Layout::FunctionPointer(_, _) => Err(ToAstProblem::FunctionLayout),
-        Layout::Pointer(_) => todo!("add support for rendering pointers in the REPL"),
     }
 }
 
@@ -272,15 +359,30 @@ fn ptr_to_ast<'a>(
     content: &Content,
 ) -> Expr<'a> {
     match layout {
+        Layout::Builtin(Builtin::Int128) => {
+            let num = unsafe { *(ptr as *const i128) };
+
+            num_to_ast(env, number_literal_to_ast(env.arena, num), content)
+        }
         Layout::Builtin(Builtin::Int64) => {
             let num = unsafe { *(ptr as *const i64) };
 
-            num_to_ast(env, i64_to_ast(env.arena, num), content)
+            num_to_ast(env, number_literal_to_ast(env.arena, num), content)
+        }
+        Layout::Builtin(Builtin::Int32) => {
+            let num = unsafe { *(ptr as *const i32) };
+
+            num_to_ast(env, number_literal_to_ast(env.arena, num), content)
+        }
+        Layout::Builtin(Builtin::Int16) => {
+            let num = unsafe { *(ptr as *const i16) };
+
+            num_to_ast(env, number_literal_to_ast(env.arena, num), content)
         }
         Layout::Builtin(Builtin::Usize) => {
             let num = unsafe { *(ptr as *const usize) };
 
-            num_to_ast(env, nat_to_ast(env.arena, num), content)
+            num_to_ast(env, number_literal_to_ast(env.arena, num), content)
         }
         Layout::Builtin(Builtin::Int1) => {
             // TODO: bits are not as expected here.
@@ -292,13 +394,18 @@ fn ptr_to_ast<'a>(
         Layout::Builtin(Builtin::Float64) => {
             let num = unsafe { *(ptr as *const f64) };
 
-            num_to_ast(env, f64_to_ast(env.arena, num), content)
+            num_to_ast(env, number_literal_to_ast(env.arena, num), content)
+        }
+        Layout::Builtin(Builtin::Float32) => {
+            let num = unsafe { *(ptr as *const f32) };
+
+            num_to_ast(env, number_literal_to_ast(env.arena, num), content)
         }
         Layout::Builtin(Builtin::EmptyList) => Expr::List {
             items: &[],
             final_comments: &[],
         },
-        Layout::Builtin(Builtin::List(_, elem_layout)) => {
+        Layout::Builtin(Builtin::List(elem_layout)) => {
             // Turn the (ptr, len) wrapper struct into actual ptr and len values.
             let len = unsafe { *(ptr.offset(env.ptr_bytes as isize) as *const usize) };
             let ptr = unsafe { *(ptr as *const *const u8) };
@@ -855,25 +962,7 @@ fn num_to_ast<'a>(env: &Env<'a, '_>, num_expr: Expr<'a>, content: &Content) -> E
 
 /// This is centralized in case we want to format it differently later,
 /// e.g. adding underscores for large numbers
-fn nat_to_ast(arena: &Bump, num: usize) -> Expr<'_> {
-    Expr::Num(arena.alloc(format!("{}", num)))
-}
-
-/// This is centralized in case we want to format it differently later,
-/// e.g. adding underscores for large numbers
-fn i64_to_ast(arena: &Bump, num: i64) -> Expr<'_> {
-    Expr::Num(arena.alloc(format!("{}", num)))
-}
-
-/// This is centralized in case we want to format it differently later,
-/// e.g. adding underscores for large numbers
-fn i128_to_ast(arena: &Bump, num: i128) -> Expr<'_> {
-    Expr::Num(arena.alloc(format!("{}", num)))
-}
-
-/// This is centralized in case we want to format it differently later,
-/// e.g. adding underscores for large numbers
-fn f64_to_ast(arena: &Bump, num: f64) -> Expr<'_> {
+fn number_literal_to_ast<T: std::fmt::Display>(arena: &Bump, num: T) -> Expr<'_> {
     Expr::Num(arena.alloc(format!("{}", num)))
 }
 
diff --git a/cli/src/repl/gen.rs b/cli/src/repl/gen.rs
index 5a30b723fe..797fdba999 100644
--- a/cli/src/repl/gen.rs
+++ b/cli/src/repl/gen.rs
@@ -8,9 +8,9 @@ use roc_can::builtins::builtin_defs_map;
 use roc_collections::all::{MutMap, MutSet};
 use roc_fmt::annotation::Formattable;
 use roc_fmt::annotation::{Newlines, Parens};
-use roc_gen::llvm::build::{build_proc, build_proc_header, OptLevel};
-use roc_gen::llvm::externs::add_default_roc_externs;
+use roc_gen_llvm::llvm::externs::add_default_roc_externs;
 use roc_load::file::LoadingProblem;
+use roc_mono::ir::OptLevel;
 use roc_parse::parser::SyntaxError;
 use roc_types::pretty_print::{content_to_string, name_all_type_vars};
 use std::path::{Path, PathBuf};
@@ -68,7 +68,8 @@ pub fn gen_and_eval<'a>(
 
     use roc_load::file::MonomorphizedModule;
     let MonomorphizedModule {
-        mut procedures,
+        procedures,
+        entry_point,
         interns,
         exposed_to_host,
         mut subs,
@@ -130,7 +131,9 @@ pub fn gen_and_eval<'a>(
         let context = Context::create();
         let builder = context.create_builder();
         let ptr_bytes = target.pointer_width().unwrap().bytes() as u32;
-        let module = arena.alloc(roc_gen::llvm::build::module_from_builtins(&context, ""));
+        let module = arena.alloc(roc_gen_llvm::llvm::build::module_from_builtins(
+            &context, "",
+        ));
 
         // Add roc_alloc, roc_realloc, and roc_dealloc, since the repl has no
         // platform to provide them.
@@ -166,12 +169,12 @@ pub fn gen_and_eval<'a>(
 
         let module = arena.alloc(module);
         let (module_pass, function_pass) =
-            roc_gen::llvm::build::construct_optimization_passes(module, opt_level);
+            roc_gen_llvm::llvm::build::construct_optimization_passes(module, opt_level);
 
-        let (dibuilder, compile_unit) = roc_gen::llvm::build::Env::new_debug_info(module);
+        let (dibuilder, compile_unit) = roc_gen_llvm::llvm::build::Env::new_debug_info(module);
 
         // Compile and add all the Procs before adding main
-        let env = roc_gen::llvm::build::Env {
+        let env = roc_gen_llvm::llvm::build::Env {
             arena: &arena,
             builder: &builder,
             dibuilder: &dibuilder,
@@ -185,65 +188,11 @@ pub fn gen_and_eval<'a>(
             exposed_to_host: MutSet::default(),
         };
 
-        let mut layout_ids = roc_mono::layout::LayoutIds::default();
-        let mut headers = Vec::with_capacity(procedures.len());
-
-        // Add all the Proc headers to the module.
-        // We have to do this in a separate pass first,
-        // because their bodies may reference each other.
-        let mut scope = roc_gen::llvm::build::Scope::default();
-        for ((symbol, layout), proc) in procedures.drain() {
-            let fn_val = build_proc_header(&env, &mut layout_ids, symbol, layout, &proc);
-
-            if proc.args.is_empty() {
-                // this is a 0-argument thunk, i.e. a top-level constant definition
-                // it must be in-scope everywhere in the module!
-                scope.insert_top_level_thunk(symbol, arena.alloc(layout), fn_val);
-            }
-
-            headers.push((proc, fn_val));
-        }
-
-        // Build each proc using its header info.
-        for (proc, fn_val) in headers {
-            let mut current_scope = scope.clone();
-
-            // only have top-level thunks for this proc's module in scope
-            // this retain is not needed for correctness, but will cause less confusion when debugging
-            let home = proc.name.module_id();
-            current_scope.retain_top_level_thunks_for_module(home);
-
-            build_proc(&env, &mut layout_ids, scope.clone(), proc, fn_val);
-
-            // call finalize() before any code generation/verification
-            env.dibuilder.finalize();
-
-            if fn_val.verify(true) {
-                function_pass.run_on(&fn_val);
-            } else {
-                let mode = "NON-OPTIMIZED";
-
-                eprintln!(
-                    "\n\nFunction {:?} failed LLVM verification in {} build. Its content was:\n",
-                    fn_val.get_name().to_str().unwrap(),
-                    mode,
-                );
-
-                fn_val.print_to_stderr();
-
-                panic!(
-                    "The preceding code was from {:?}, which failed LLVM verification in {} build.",
-                    fn_val.get_name().to_str().unwrap(),
-                    mode,
-                );
-            }
-        }
-
-        let (main_fn_name, main_fn) = roc_gen::llvm::build::promote_to_main_function(
+        let (main_fn_name, main_fn) = roc_gen_llvm::llvm::build::build_procedures_return_main(
             &env,
-            &mut layout_ids,
-            main_fn_symbol,
-            main_fn_layout,
+            opt_level,
+            procedures,
+            entry_point,
         );
 
         env.dibuilder.finalize();
@@ -277,7 +226,7 @@ pub fn gen_and_eval<'a>(
                 &arena,
                 lib,
                 main_fn_name,
-                &arena.alloc(main_fn_layout).full(),
+                main_fn_layout,
                 &content,
                 &env.interns,
                 home,
diff --git a/cli/tests/cli_run.rs b/cli/tests/cli_run.rs
index 2c55633f16..0ca7fb114e 100644
--- a/cli/tests/cli_run.rs
+++ b/cli/tests/cli_run.rs
@@ -7,14 +7,12 @@ extern crate roc_collections;
 extern crate roc_load;
 extern crate roc_module;
 
-mod helpers;
-
 #[macro_use]
 extern crate maplit;
 
 #[cfg(test)]
 mod cli_run {
-    use crate::helpers::{
+    use cli_utils::helpers::{
         example_file, extract_valgrind_errors, run_cmd, run_roc, run_with_valgrind, ValgrindError,
         ValgrindErrorXWhat,
     };
@@ -304,4 +302,162 @@ mod cli_run {
 
         assert_eq!(all_examples, std::collections::HashMap::default());
     }
+
+    #[serial(hello_world)]
+    fn run_hello_world() {
+        check_output(
+            &example_file("hello-world", "Hello.roc"),
+            "hello-world",
+            &[],
+            "Hello, World!\n",
+            true,
+        );
+    }
+
+    #[test]
+    #[serial(hello_world)]
+    fn run_hello_world_optimized() {
+        check_output(
+            &example_file("hello-world", "Hello.roc"),
+            "hello-world",
+            &[],
+            "Hello, World!\n",
+            true,
+        );
+    }
+
+    #[test]
+    #[serial(quicksort)]
+    fn run_quicksort_not_optimized() {
+        check_output(
+            &example_file("quicksort", "Quicksort.roc"),
+            "quicksort",
+            &[],
+            "[0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2]\n",
+            true,
+        );
+    }
+
+    #[test]
+    #[serial(quicksort)]
+    fn run_quicksort_optimized() {
+        check_output(
+            &example_file("quicksort", "Quicksort.roc"),
+            "quicksort",
+            &["--optimize"],
+            "[0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2]\n",
+            true,
+        );
+    }
+
+    #[test]
+    #[serial(quicksort)]
+    fn run_quicksort_optimized_valgrind() {
+        check_output(
+            &example_file("quicksort", "Quicksort.roc"),
+            "quicksort",
+            &["--optimize"],
+            "[0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2]\n",
+            true,
+        );
+    }
+
+    #[test]
+    #[serial(nqueens)]
+    fn run_nqueens_not_optimized() {
+        check_output_with_stdin(
+            &example_file("benchmarks", "NQueens.roc"),
+            "6",
+            "nqueens",
+            &[],
+            "4\n",
+            true,
+        );
+    }
+
+    #[test]
+    #[serial(cfold)]
+    fn run_cfold_not_optimized() {
+        check_output_with_stdin(
+            &example_file("benchmarks", "CFold.roc"),
+            "3",
+            "cfold",
+            &[],
+            "11 & 11\n",
+            true,
+        );
+    }
+
+    #[test]
+    #[serial(deriv)]
+    fn run_deriv_not_optimized() {
+        check_output_with_stdin(
+            &example_file("benchmarks", "Deriv.roc"),
+            "2",
+            "deriv",
+            &[],
+            "1 count: 6\n2 count: 22\n",
+            true,
+        );
+    }
+
+    #[test]
+    #[serial(deriv)]
+    fn run_rbtree_insert_not_optimized() {
+        check_output(
+            &example_file("benchmarks", "RBTreeInsert.roc"),
+            "rbtree-insert",
+            &[],
+            "Node Black 0 {} Empty Empty\n",
+            true,
+        );
+    }
+
+    #[test]
+    #[serial(deriv)]
+    fn run_rbtree_delete_not_optimized() {
+        check_output_with_stdin(
+            &example_file("benchmarks", "RBTreeDel.roc"),
+            "420",
+            "rbtree-del",
+            &[],
+            "30\n",
+            true,
+        );
+    }
+    #[test]
+    #[serial(astar)]
+    fn run_astar_optimized_1() {
+        check_output(
+            &example_file("benchmarks", "TestAStar.roc"),
+            "test-astar",
+            &[],
+            "True\n",
+            false,
+        );
+    }
+
+    #[test]
+    #[serial(base64)]
+    fn base64() {
+        check_output(
+            &example_file("benchmarks", "TestBase64.roc"),
+            "test-base64",
+            &[],
+            "encoded: SGVsbG8gV29ybGQ=\ndecoded: Hello World\n",
+            true,
+        );
+    }
+
+    #[test]
+    #[serial(closure)]
+    fn closure() {
+        check_output(
+            &example_file("benchmarks", "Closure.roc"),
+            "closure",
+            &[],
+            "",
+            true,
+        );
+    }
 }
diff --git a/cli/tests/fixtures/multi-dep-str/platform/host.zig b/cli/tests/fixtures/multi-dep-str/platform/host.zig
index 2d335ec0f7..e60688a037 100644
--- a/cli/tests/fixtures/multi-dep-str/platform/host.zig
+++ b/cli/tests/fixtures/multi-dep-str/platform/host.zig
@@ -59,7 +59,7 @@ pub export fn main() i32 {
     roc__mainForHost_1_exposed(&callresult);
 
     // stdout the result
-    stdout.print("{}\n", .{callresult.content.asSlice()}) catch unreachable;
+    stdout.print("{s}\n", .{callresult.content.asSlice()}) catch unreachable;
 
     callresult.content.deinit();
 
diff --git a/cli/tests/fixtures/multi-dep-thunk/platform/host.zig b/cli/tests/fixtures/multi-dep-thunk/platform/host.zig
index 2d335ec0f7..e60688a037 100644
--- a/cli/tests/fixtures/multi-dep-thunk/platform/host.zig
+++ b/cli/tests/fixtures/multi-dep-thunk/platform/host.zig
@@ -59,7 +59,7 @@ pub export fn main() i32 {
     roc__mainForHost_1_exposed(&callresult);
 
     // stdout the result
-    stdout.print("{}\n", .{callresult.content.asSlice()}) catch unreachable;
+    stdout.print("{s}\n", .{callresult.content.asSlice()}) catch unreachable;
 
     callresult.content.deinit();
 
diff --git a/cli/tests/repl_eval.rs b/cli/tests/repl_eval.rs
index 62efefc23d..87d53837a8 100644
--- a/cli/tests/repl_eval.rs
+++ b/cli/tests/repl_eval.rs
@@ -4,12 +4,10 @@ extern crate pretty_assertions;
 #[macro_use]
 extern crate indoc;
 
-mod helpers;
-
 #[cfg(test)]
 mod repl_eval {
-    use crate::helpers;
-    use roc_gen::run_roc::RocCallResult;
+    use cli_utils::helpers;
+    use roc_gen_llvm::run_roc::RocCallResult;
 
     #[test]
     fn check_discriminant_size() {
@@ -18,7 +16,8 @@ mod repl_eval {
         let value: i64 = 1234;
         assert_eq!(
             std::mem::size_of_val(&RocCallResult::Success(value)),
-            roc_gen::run_roc::ROC_CALL_RESULT_DISCRIMINANT_SIZE + std::mem::size_of_val(&value)
+            roc_gen_llvm::run_roc::ROC_CALL_RESULT_DISCRIMINANT_SIZE
+                + std::mem::size_of_val(&value)
         )
     }
 
@@ -500,15 +499,11 @@ mod repl_eval {
 
     #[test]
     fn identity_lambda() {
-        // Even though this gets unwrapped at runtime, the repl should still
-        // report it as a record
         expect_success("\\x -> x", "<function> : a -> a");
     }
 
     #[test]
     fn stdlib_function() {
-        // Even though this gets unwrapped at runtime, the repl should still
-        // report it as a record
         expect_success("Num.abs", "<function> : Num a -> Num a");
     }
 
diff --git a/compiler/build/Cargo.toml b/compiler/build/Cargo.toml
index a7f173d2e2..77dcf1d8a6 100644
--- a/compiler/build/Cargo.toml
+++ b/compiler/build/Cargo.toml
@@ -19,7 +19,7 @@ roc_unify = { path = "../unify" }
 roc_solve = { path = "../solve" }
 roc_mono = { path = "../mono" }
 roc_load = { path = "../load" }
-roc_gen = { path = "../gen" }
+roc_gen_llvm = { path = "../gen_llvm", optional = true }
 roc_reporting = { path = "../reporting" }
 im    = "14" # im and im-rc should always have the same version!
 im-rc = "14" # im and im-rc should always have the same version!
@@ -28,24 +28,7 @@ inlinable_string = "0.1.0"
 libloading = "0.6"
 tempfile = "3.1.0"
 serde_json = "1.0"
-# NOTE: rtfeldman/inkwell is a fork of TheDan64/inkwell which does not change anything.
-#
-# The reason for this fork is that the way Inkwell is designed, you have to use
-# a particular branch (e.g. "llvm8-0") in Cargo.toml. That would be fine, except that
-# breaking changes get pushed directly to that branch, which breaks our build
-# without warning.
-#
-# We tried referencing a specific rev on TheDan64/inkwell directly (instead of branch),
-# but although that worked locally, it did not work on GitHub Actions. (After a few
-# hours of investigation, gave up trying to figure out why.) So this is the workaround:
-# having an immutable tag on the rtfeldman/inkwell fork which points to
-# a particular "release" of Inkwell.
-#
-# When we want to update Inkwell, we can sync up rtfeldman/inkwell to the latest
-# commit of TheDan64/inkwell, push a new tag which points to the latest commit,
-# change the tag value in this Cargo.toml to point to that tag, and `cargo update`.
-# This way, GitHub Actions works and nobody's builds get broken.
-inkwell = { git = "https://github.com/rtfeldman/inkwell", tag = "llvm10-0.release4", features = [ "llvm10-0" ] }
+inkwell = { path = "../../vendor/inkwell", optional = true }
 target-lexicon = "0.10"
 
 [dev-dependencies]
@@ -56,6 +39,10 @@ quickcheck = "0.8"
 quickcheck_macros = "0.8"
 
 [features]
+default = ["llvm"]
 target-arm = []
 target-aarch64 = []
 target-webassembly = []
+# This is a separate feature because when we generate docs on Netlify,
+# it doesn't have LLVM installed. (Also, it doesn't need to do code gen.)
+llvm = ["inkwell", "roc_gen_llvm"]
diff --git a/compiler/build/src/link.rs b/compiler/build/src/link.rs
index 95983e1a8d..ee1124c905 100644
--- a/compiler/build/src/link.rs
+++ b/compiler/build/src/link.rs
@@ -1,21 +1,21 @@
-use crate::target;
 use crate::target::arch_str;
-use inkwell::module::Module;
-use inkwell::targets::{CodeModel, FileType, RelocMode};
+#[cfg(feature = "llvm")]
 use libloading::{Error, Library};
-use roc_gen::llvm::build::OptLevel;
+#[cfg(feature = "llvm")]
+use roc_mono::ir::OptLevel;
 use std::collections::HashMap;
 use std::env;
 use std::io;
 use std::path::{Path, PathBuf};
 use std::process::{Child, Command, Output};
 use target_lexicon::{Architecture, OperatingSystem, Triple};
-use tempfile::tempdir;
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
 pub enum LinkType {
-    Executable,
-    Dylib,
+    // These numbers correspond to the --lib flag; if it's present
+    // (e.g. is_present returns `1 as bool`), this will be 1 as well.
+    Executable = 0,
+    Dylib = 1,
 }
 
 /// input_paths can include the host as well as the app. e.g. &["host.o", "roc_app.o"]
@@ -360,6 +360,9 @@ fn link_linux(
     };
 
     let env_path = env::var("PATH").unwrap_or_else(|_| "".to_string());
+
+    init_arch(target);
+
     // NOTE: order of arguments to `ld` matters here!
     // The `-l` flags should go after the `.o` arguments
     Ok((
@@ -466,8 +469,8 @@ fn link_macos(
                 "-lc++",
                 // "-lc++abi",
                 // "-lunwind", // TODO will eventually need this, see https://github.com/rtfeldman/roc/pull/554#discussion_r496370840
-                "-framework",
-                "Security", // This "-framework Security" arg is needed for the `rand` crate in examples/cli
+                // "-framework", // Uncomment this line & the following ro run the `rand` crate in examples/cli
+                // "Security",
                 // Output
                 "-o",
                 output_path.to_str().unwrap(), // app
@@ -477,12 +480,16 @@ fn link_macos(
     ))
 }
 
+#[cfg(feature = "llvm")]
 pub fn module_to_dylib(
-    module: &Module,
+    module: &inkwell::module::Module,
     target: &Triple,
     opt_level: OptLevel,
 ) -> Result<Library, Error> {
-    let dir = tempdir().unwrap();
+    use crate::target::{self, convert_opt_level};
+    use inkwell::targets::{CodeModel, FileType, RelocMode};
+
+    let dir = tempfile::tempdir().unwrap();
     let filename = PathBuf::from("Test.roc");
     let file_path = dir.path().join(filename);
     let mut app_o_file = file_path;
@@ -492,7 +499,8 @@ pub fn module_to_dylib(
     // Emit the .o file using position-indepedent code (PIC) - needed for dylibs
     let reloc = RelocMode::PIC;
     let model = CodeModel::Default;
-    let target_machine = target::target_machine(target, opt_level.into(), reloc, model).unwrap();
+    let target_machine =
+        target::target_machine(target, convert_opt_level(opt_level), reloc, model).unwrap();
 
     target_machine
         .write_to_file(module, FileType::Object, &app_o_file)
@@ -529,3 +537,13 @@ fn validate_output(file_name: &str, cmd_name: &str, output: Output) {
         }
     }
 }
+
+#[cfg(feature = "llvm")]
+fn init_arch(target: &Triple) {
+    crate::target::init_arch(target);
+}
+
+#[cfg(not(feature = "llvm"))]
+fn init_arch(_target: &Triple) {
+    panic!("Tried to initialize LLVM when crate was not built with `feature = \"llvm\"` enabled");
+}
diff --git a/compiler/build/src/program.rs b/compiler/build/src/program.rs
index 3fad005f6e..82d7261950 100644
--- a/compiler/build/src/program.rs
+++ b/compiler/build/src/program.rs
@@ -1,14 +1,14 @@
-use crate::target;
-use bumpalo::Bump;
-use inkwell::context::Context;
-use inkwell::targets::{CodeModel, FileType, RelocMode};
-pub use roc_gen::llvm::build::FunctionIterator;
-use roc_gen::llvm::build::{build_proc, build_proc_header, module_from_builtins, OptLevel, Scope};
+#[cfg(feature = "llvm")]
+use roc_gen_llvm::llvm::build::module_from_builtins;
+#[cfg(feature = "llvm")]
+pub use roc_gen_llvm::llvm::build::FunctionIterator;
+#[cfg(feature = "llvm")]
 use roc_load::file::MonomorphizedModule;
-use roc_mono::layout::LayoutIds;
+#[cfg(feature = "llvm")]
+use roc_mono::ir::OptLevel;
+#[cfg(feature = "llvm")]
 use std::path::{Path, PathBuf};
-use std::time::{Duration, SystemTime};
-use target_lexicon::Triple;
+use std::time::Duration;
 
 #[derive(Debug, Clone, Copy, Default)]
 pub struct CodeGenTiming {
@@ -19,16 +19,24 @@ pub struct CodeGenTiming {
 // TODO how should imported modules factor into this? What if those use builtins too?
 // TODO this should probably use more helper functions
 // TODO make this polymorphic in the llvm functions so it can be reused for another backend.
+#[cfg(feature = "llvm")]
 #[allow(clippy::cognitive_complexity)]
 pub fn gen_from_mono_module(
-    arena: &Bump,
+    arena: &bumpalo::Bump,
     mut loaded: MonomorphizedModule,
     roc_file_path: &Path,
-    target: Triple,
+    target: target_lexicon::Triple,
     app_o_file: &Path,
     opt_level: OptLevel,
     emit_debug_info: bool,
 ) -> CodeGenTiming {
+    use crate::target::{self, convert_opt_level};
+    use inkwell::attributes::{Attribute, AttributeLoc};
+    use inkwell::context::Context;
+    use inkwell::module::Linkage;
+    use inkwell::targets::{CodeModel, FileType, RelocMode};
+    use std::time::SystemTime;
+
     use roc_reporting::report::{
         can_problem, mono_problem, type_problem, RocDocAllocator, DEFAULT_PALETTE,
     };
@@ -88,9 +96,6 @@ pub fn gen_from_mono_module(
     // module.strip_debug_info();
 
     // mark our zig-defined builtins as internal
-    use inkwell::attributes::{Attribute, AttributeLoc};
-    use inkwell::module::Linkage;
-
     let app_ll_file = {
         let mut temp = PathBuf::from(roc_file_path);
         temp.set_extension("ll");
@@ -100,30 +105,32 @@ pub fn gen_from_mono_module(
 
     let kind_id = Attribute::get_named_enum_kind_id("alwaysinline");
     debug_assert!(kind_id > 0);
-    let attr = context.create_enum_attribute(kind_id, 1);
+    let enum_attr = context.create_enum_attribute(kind_id, 1);
 
     for function in FunctionIterator::from_module(module) {
         let name = function.get_name().to_str().unwrap();
+
+        // mark our zig-defined builtins as internal
         if name.starts_with("roc_builtins") {
             function.set_linkage(Linkage::Internal);
         }
 
-        if name.starts_with("roc_builtins.dict") || name.starts_with("dict.RocDict") {
-            function.add_attribute(AttributeLoc::Function, attr);
-        }
-
-        if name.starts_with("roc_builtins.list") || name.starts_with("list.RocList") {
-            function.add_attribute(AttributeLoc::Function, attr);
+        if name.starts_with("roc_builtins.dict")
+            || name.starts_with("dict.RocDict")
+            || name.starts_with("roc_builtins.list")
+            || name.starts_with("list.RocList")
+        {
+            function.add_attribute(AttributeLoc::Function, enum_attr);
         }
     }
 
     let builder = context.create_builder();
-    let (dibuilder, compile_unit) = roc_gen::llvm::build::Env::new_debug_info(module);
-    let (mpm, fpm) = roc_gen::llvm::build::construct_optimization_passes(module, opt_level);
+    let (dibuilder, compile_unit) = roc_gen_llvm::llvm::build::Env::new_debug_info(module);
+    let (mpm, _fpm) = roc_gen_llvm::llvm::build::construct_optimization_passes(module, opt_level);
 
     // Compile and add all the Procs before adding main
     let ptr_bytes = target.pointer_width().unwrap().bytes() as u32;
-    let env = roc_gen::llvm::build::Env {
+    let env = roc_gen_llvm::llvm::build::Env {
         arena: &arena,
         builder: &builder,
         dibuilder: &dibuilder,
@@ -136,55 +143,13 @@ pub fn gen_from_mono_module(
         exposed_to_host: loaded.exposed_to_host.keys().copied().collect(),
     };
 
-    // Populate Procs further and get the low-level Expr from the canonical Expr
-    let mut headers = Vec::with_capacity(loaded.procedures.len());
-
-    // Add all the Proc headers to the module.
-    // We have to do this in a separate pass first,
-    // because their bodies may reference each other.
-    let mut layout_ids = LayoutIds::default();
-
-    let mut scope = Scope::default();
-    for ((symbol, layout), proc) in loaded.procedures {
-        let fn_val = build_proc_header(&env, &mut layout_ids, symbol, layout, &proc);
-
-        if proc.args.is_empty() {
-            // this is a 0-argument thunk, i.e. a top-level constant definition
-            // it must be in-scope everywhere in the module!
-            scope.insert_top_level_thunk(symbol, arena.alloc(layout), fn_val);
-        }
-
-        headers.push((proc, fn_val));
-    }
-
-    // Build each proc using its header info.
-    for (proc, fn_val) in headers {
-        // NOTE: This is here to be uncommented in case verification fails.
-        // (This approach means we don't have to defensively clone name here.)
-        //
-        // println!("\n\nBuilding and then verifying function {:?}\n\n", proc);
-        build_proc(&env, &mut layout_ids, scope.clone(), proc, fn_val);
-
-        // call finalize() before any code generation/verification
-        env.dibuilder.finalize();
-
-        if fn_val.verify(true) {
-            fpm.run_on(&fn_val);
-        } else {
-            fn_val.print_to_stderr();
-
-            // write the ll code to a file, so we can modify it
-            env.module.print_to_file(&app_ll_file).unwrap();
-
-            // env.module.print_to_stderr();
-            // NOTE: If this fails, uncomment the above println to debug.
-            panic!(
-                r"Non-main function {:?} failed LLVM verification. I wrote the full LLVM IR to {:?}",
-                fn_val.get_name(),
-                app_ll_file,
-            );
-        }
-    }
+    roc_gen_llvm::llvm::build::build_procedures(
+        &env,
+        opt_level,
+        loaded.procedures,
+        loaded.entry_point,
+        Some(&app_ll_file),
+    );
 
     env.dibuilder.finalize();
 
@@ -199,8 +164,9 @@ pub fn gen_from_mono_module(
         env.module.print_to_file(&app_ll_file).unwrap();
 
         panic!(
-            "😱 LLVM errors when defining module; I wrote the full LLVM IR to {:?}\n\n {:?}",
-            app_ll_file, errors,
+            "😱 LLVM errors when defining module; I wrote the full LLVM IR to {:?}\n\n {}",
+            app_ll_file,
+            errors.to_string(),
         );
     }
 
@@ -229,7 +195,7 @@ pub fn gen_from_mono_module(
         // run the debugir https://github.com/vaivaswatha/debugir tool
         match Command::new("debugir")
             .env_clear()
-            .args(&[app_ll_file.to_str().unwrap()])
+            .args(&["-instnamer", app_ll_file.to_str().unwrap()])
             .output()
         {
             Ok(_) => {}
@@ -245,7 +211,7 @@ pub fn gen_from_mono_module(
         }
 
         // assemble the .ll into a .bc
-        let _ = Command::new("llvm-as-10")
+        let _ = Command::new("llvm-as")
             .env_clear()
             .args(&[
                 app_ll_dbg_file.to_str().unwrap(),
@@ -257,7 +223,7 @@ pub fn gen_from_mono_module(
 
         // write the .o file. Note that this builds the .o for the local machine,
         // and ignores the `target_machine` entirely.
-        let _ = Command::new("llc-10")
+        let _ = Command::new("llc-12")
             .env_clear()
             .args(&[
                 "-filetype=obj",
@@ -273,7 +239,7 @@ pub fn gen_from_mono_module(
         let reloc = RelocMode::Default;
         let model = CodeModel::Default;
         let target_machine =
-            target::target_machine(&target, opt_level.into(), reloc, model).unwrap();
+            target::target_machine(&target, convert_opt_level(opt_level), reloc, model).unwrap();
 
         target_machine
             .write_to_file(&env.module, FileType::Object, &app_o_file)
diff --git a/compiler/build/src/target.rs b/compiler/build/src/target.rs
index 8137357377..4e171359d0 100644
--- a/compiler/build/src/target.rs
+++ b/compiler/build/src/target.rs
@@ -1,7 +1,10 @@
-use inkwell::targets::{
-    CodeModel, InitializationConfig, RelocMode, Target, TargetMachine, TargetTriple,
+#[cfg(feature = "llvm")]
+use inkwell::{
+    targets::{CodeModel, InitializationConfig, RelocMode, Target, TargetMachine, TargetTriple},
+    OptimizationLevel,
 };
-use inkwell::OptimizationLevel;
+#[cfg(feature = "llvm")]
+use roc_mono::ir::OptLevel;
 use target_lexicon::{Architecture, OperatingSystem, Triple};
 
 pub fn target_triple_str(target: &Triple) -> &'static str {
@@ -28,36 +31,20 @@ pub fn target_triple_str(target: &Triple) -> &'static str {
     }
 }
 
-/// NOTE: arch_str is *not* the same as the beginning of the magic target triple
-/// string! For example, if it's "x86-64" here, the magic target triple string
-/// will begin with "x86_64" (with an underscore) instead.
-pub fn arch_str(target: &Triple) -> &'static str {
-    // Best guide I've found on how to determine these magic strings:
-    //
-    // https://stackoverflow.com/questions/15036909/clang-how-to-list-supported-target-architectures
+#[cfg(feature = "llvm")]
+pub fn init_arch(target: &Triple) {
     match target.architecture {
         Architecture::X86_64 => {
             Target::initialize_x86(&InitializationConfig::default());
-
-            "x86-64"
         }
         Architecture::Aarch64(_) if cfg!(feature = "target-aarch64") => {
             Target::initialize_aarch64(&InitializationConfig::default());
-            "aarch64"
         }
         Architecture::Arm(_) if cfg!(feature = "target-arm") => {
-            // NOTE: why not enable arm and wasm by default?
-            //
-            // We had some trouble getting them to link properly. This may be resolved in the
-            // future, or maybe it was just some weird configuration on one machine.
             Target::initialize_arm(&InitializationConfig::default());
-
-            "arm"
         }
         Architecture::Wasm32 if cfg!(feature = "target-webassembly") => {
             Target::initialize_webassembly(&InitializationConfig::default());
-
-            "wasm32"
         }
         _ => panic!(
             "TODO gracefully handle unsupported target architecture: {:?}",
@@ -66,6 +53,26 @@ pub fn arch_str(target: &Triple) -> &'static str {
     }
 }
 
+/// NOTE: arch_str is *not* the same as the beginning of the magic target triple
+/// string! For example, if it's "x86-64" here, the magic target triple string
+/// will begin with "x86_64" (with an underscore) instead.
+pub fn arch_str(target: &Triple) -> &'static str {
+    // Best guide I've found on how to determine these magic strings:
+    //
+    // https://stackoverflow.com/questions/15036909/clang-how-to-list-supported-target-architectures
+    match target.architecture {
+        Architecture::X86_64 => "x86-64",
+        Architecture::Aarch64(_) if cfg!(feature = "target-aarch64") => "aarch64",
+        Architecture::Arm(_) if cfg!(feature = "target-arm") => "arm",
+        Architecture::Wasm32 if cfg!(feature = "target-webassembly") => "wasm32",
+        _ => panic!(
+            "TODO gracefully handle unsupported target architecture: {:?}",
+            target.architecture
+        ),
+    }
+}
+
+#[cfg(feature = "llvm")]
 pub fn target_machine(
     target: &Triple,
     opt: OptimizationLevel,
@@ -74,6 +81,8 @@ pub fn target_machine(
 ) -> Option<TargetMachine> {
     let arch = arch_str(target);
 
+    init_arch(target);
+
     Target::from_name(arch).unwrap().create_target_machine(
         &TargetTriple::create(target_triple_str(target)),
         "generic",
@@ -83,3 +92,11 @@ pub fn target_machine(
         model,
     )
 }
+
+#[cfg(feature = "llvm")]
+pub fn convert_opt_level(level: OptLevel) -> OptimizationLevel {
+    match level {
+        OptLevel::Normal => OptimizationLevel::None,
+        OptLevel::Optimize => OptimizationLevel::Aggressive,
+    }
+}
diff --git a/compiler/builtins/README.md b/compiler/builtins/README.md
index fdb5bd793b..8e1eb0e291 100644
--- a/compiler/builtins/README.md
+++ b/compiler/builtins/README.md
@@ -9,15 +9,15 @@ Towards the bottom of `symbol.rs` there is a `define_builtins!` macro being used
 Some of these have `#` inside their name (`first#list`, `#lt` ..). This is a trick we are doing to hide implementation details from Roc programmers. To a Roc programmer, a name with `#` in it is invalid, because `#` means everything after it is parsed to a comment. We are constructing these functions manually, so we are circumventing the parsing step and dont have such restrictions. We get to make functions and values with `#` which as a consequence are not accessible to Roc programmers. Roc programmers simply cannot reference them.
 
 But we can use these values and some of these are necessary for implementing builtins. For example, `List.get` returns tags, and it is not easy for us to create tags when composing LLVM. What is easier however, is:
-- ..writing `List.#getUnsafe` that has the dangerous signature of `List elem, Int -> elem` in LLVM
-- ..writing `List elem, Int -> Result elem [ OutOfBounds ]*` in a type safe way that uses `getUnsafe` internally, only after it checks if the `elem` at `Int` index exists.
+- ..writing `List.#getUnsafe` that has the dangerous signature of `List elem, Nat -> elem` in LLVM
+- ..writing `List elem, Nat -> Result elem [ OutOfBounds ]*` in a type safe way that uses `getUnsafe` internally, only after it checks if the `elem` at `Nat` index exists.
 
 
 ### can/src/builtins.rs
 
 Right at the top of this module is a function called `builtin_defs`. All this is doing is mapping the `Symbol` defined in `module/src/symbol.rs` to its implementation. Some of the builtins are quite complex, such as `list_get`. What makes `list_get` is that it returns tags, and in order to return tags it first has to  defer to lower-level functions via an if statement.
 
-Lets look at `List.repeat : elem, Int -> List elem`, which is more straight-forward, and points directly to its lower level implementation:
+Lets look at `List.repeat : elem, Nat -> List elem`, which is more straight-forward, and points directly to its lower level implementation:
 ```
 fn list_repeat(symbol: Symbol, var_store: &mut VarStore) -> Def {
     let elem_var = var_store.fresh();
@@ -42,7 +42,7 @@ fn list_repeat(symbol: Symbol, var_store: &mut VarStore) -> Def {
     )
 }
 ```
-In these builtin definitions you will need to allocate for and list the arguments. For `List.repeat`, the arguments are the `elem_var` and the `len_var`. So in both the `body` and `defn` we list these arguments in a vector, with the `Symobl::ARG_1` adn` Symvol::ARG_2` designating which argument is which.
+In these builtin definitions you will need to allocate for and list the arguments. For `List.repeat`, the arguments are the `elem_var` and the `len_var`. So in both the `body` and `defn` we list these arguments in a vector, with the `Symobl::ARG_1` and` Symvol::ARG_2` designating which argument is which.
 
 Since `List.repeat` is implemented entirely as low level functions, its `body` is a `RunLowLevel`, and the `op` is `LowLevel::ListRepeat`. Lets talk about `LowLevel` in the next section.
 
@@ -60,7 +60,7 @@ Its one thing to actually write these functions, its _another_ thing to let the
 
 ## Specifying how we pass args to the function
 ### builtins/mono/src/borrow.rs
-After we have all of this, we need to specify if the arguements we're passing are owned, borrowed or irrelvant. Towards the bottom of this file, add a new case for you builtin and specify each arg. Be sure to read the comment, as it explains this in more detail.
+After we have all of this, we need to specify if the arguments we're passing are owned, borrowed or irrelvant. Towards the bottom of this file, add a new case for you builtin and specify each arg. Be sure to read the comment, as it explains this in more detail.
 
 ## Specifying the uniqueness of a function
 ### builtins/src/unique.rs
diff --git a/compiler/builtins/bitcode/.gitignore b/compiler/builtins/bitcode/.gitignore
index cd473dd7c4..18b2c682d4 100644
--- a/compiler/builtins/bitcode/.gitignore
+++ b/compiler/builtins/bitcode/.gitignore
@@ -1,5 +1,7 @@
 zig-cache
 src/zig-cache
+benchmark/zig-cache
 builtins.ll
 builtins.bc
 builtins.o
+dec
diff --git a/compiler/builtins/bitcode/README.md b/compiler/builtins/bitcode/README.md
index 1cbef7b727..1d9fbb225b 100644
--- a/compiler/builtins/bitcode/README.md
+++ b/compiler/builtins/bitcode/README.md
@@ -3,10 +3,10 @@
 ## Adding a bitcode builtin
 
 To add a builtin:
-1. Add the function to the relevent module. For `Num` builtin use it in `src/num.zig`, for `Str` builtins use `src/str.zig`, and so on. **For anything you add, you must add tests for it!** Not only does to make the builtins more maintainable, it's the the easiest way to test these functions on Zig. To run the test, run: `zig build test`
+1. Add the function to the relevant module. For `Num` builtin use it in `src/num.zig`, for `Str` builtins use `src/str.zig`, and so on. **For anything you add, you must add tests for it!** Not only does to make the builtins more maintainable, it's the the easiest way to test these functions on Zig. To run the test, run: `zig build test`
 2. Make sure the function is public with the `pub` keyword and uses the C calling convention. This is really easy, just add `pub` and `callconv(.C)` to the function declaration like so: `pub fn atan(num: f64) callconv(.C) f64 { ... }`
-3. In `src/main.zig`, export the function. This is also organized by module. For example, for a `Num` function find the `Num` section and add: `comptime { exportNumFn(num.atan, "atan"); }`. The first arguement is the function, the second is the name of it in LLVM.
-4. In `compiler/builtins/src/bitcode.rs`, add a constant for the new function. This is how we use it in Rust. Once again, this is organized by module, so just find the relevent area and add your new function.
+3. In `src/main.zig`, export the function. This is also organized by module. For example, for a `Num` function find the `Num` section and add: `comptime { exportNumFn(num.atan, "atan"); }`. The first argument is the function, the second is the name of it in LLVM.
+4. In `compiler/builtins/src/bitcode.rs`, add a constant for the new function. This is how we use it in Rust. Once again, this is organized by module, so just find the relevant area and add your new function.
 5. You can now  your function in Rust using `call_bitcode_fn` in `llvm/src/build.rs`!
 
 ## How it works
@@ -32,4 +32,4 @@ There will be two directories like `roc_builtins-[some random characters]`, look
 
 ## Calling bitcode functions
 
-use the `call_bitcode_fn` function defined in `llvm/src/build.rs` to call bitcode funcitons.
+use the `call_bitcode_fn` function defined in `llvm/src/build.rs` to call bitcode functions.
diff --git a/compiler/builtins/bitcode/benchmark.sh b/compiler/builtins/bitcode/benchmark.sh
new file mode 100755
index 0000000000..ef7b3d6b35
--- /dev/null
+++ b/compiler/builtins/bitcode/benchmark.sh
@@ -0,0 +1,6 @@
+#!/bin/bash
+
+set -euxo pipefail
+
+zig build-exe benchmark/dec.zig -O ReleaseFast --main-pkg-path .
+./dec
diff --git a/compiler/builtins/bitcode/benchmark/dec.zig b/compiler/builtins/bitcode/benchmark/dec.zig
new file mode 100644
index 0000000000..96d35a7af1
--- /dev/null
+++ b/compiler/builtins/bitcode/benchmark/dec.zig
@@ -0,0 +1,174 @@
+const std = @import("std");
+const time = std.time;
+const Timer = time.Timer;
+
+const RocStr = @import("../src/str.zig").RocStr;
+const RocDec = @import("../src/dec.zig").RocDec;
+
+var timer: Timer = undefined;
+
+pub fn main() !void {
+    const stdout = std.io.getStdOut().writer();
+    timer = try Timer.start();
+
+    try stdout.print("7 additions took ", .{});
+    try avg_runs(add7);
+
+    try stdout.print("7 subtractions took ", .{});
+    try avg_runs(sub7);
+
+    try stdout.print("7 multiplications took ", .{});
+    try avg_runs(mul7);
+
+    try stdout.print("7 divisions took ", .{});
+    try avg_runs(div7);
+}
+
+fn avg_runs(func: fn() u64) !void {
+    const stdout = std.io.getStdOut().writer();
+    var first_run = func();
+    var lowest = first_run;
+    var highest = first_run;
+    var sum = first_run;
+
+    // 31 runs
+    var runs = [_]u64{ first_run, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
+    var next_run: usize = 1; // we already did first_run
+
+    while (next_run < runs.len) {
+        const run = func();
+
+        lowest = std.math.min(lowest, run);
+        highest = std.math.max(highest, run);
+
+        runs[next_run] = run;
+
+        next_run += 1;
+    }
+
+    std.sort.sort(u64, &runs, {}, comptime std.sort.asc(u64));
+
+    const median = runs[runs.len / 2];
+
+    try stdout.print("{}ns (lowest: {}ns, highest: {}ns)\n", .{median, lowest, highest});
+}
+
+fn add7() u64 {
+    var str1 = RocStr.init("1.2", 3);
+    const dec1 = RocDec.fromStr(str1).?;
+
+    var str2 = RocStr.init("3.4", 3);
+    const dec2 = RocDec.fromStr(str2).?;
+
+    timer.reset();
+
+    var a = dec1.add(dec2);
+    a = a.add(dec1);
+    a = a.add(dec2);
+    a = a.add(dec1);
+    a = a.add(dec2);
+    a = a.add(dec1);
+    a = a.add(dec2);
+    a = a.add(dec1);
+    a = a.add(dec2);
+    a = a.add(dec1);
+    a = a.add(dec2);
+    a = a.add(dec1);
+    a = a.add(dec2);
+    a = a.add(dec1);
+    a = a.add(dec2);
+    a = a.add(dec1);
+    a = a.add(dec2);
+
+    return timer.read();
+}
+
+fn sub7() u64 {
+    var str1 = RocStr.init("1.2", 3);
+    const dec1 = RocDec.fromStr(str1).?;
+
+    var str2 = RocStr.init("3.4", 3);
+    const dec2 = RocDec.fromStr(str2).?;
+
+    timer.reset();
+
+    var a = dec1.sub(dec2);
+    a = a.sub(dec1);
+    a = a.sub(dec2);
+    a = a.sub(dec1);
+    a = a.sub(dec2);
+    a = a.sub(dec1);
+    a = a.sub(dec2);
+    a = a.sub(dec1);
+    a = a.sub(dec2);
+    a = a.sub(dec1);
+    a = a.sub(dec2);
+    a = a.sub(dec1);
+    a = a.sub(dec2);
+    a = a.sub(dec1);
+    a = a.sub(dec2);
+    a = a.sub(dec1);
+    a = a.sub(dec2);
+
+    return timer.read();
+}
+
+fn mul7() u64 {
+    var str1 = RocStr.init("1.2", 3);
+    const dec1 = RocDec.fromStr(str1).?;
+
+    var str2 = RocStr.init("3.4", 3);
+    const dec2 = RocDec.fromStr(str2).?;
+
+    timer.reset();
+
+    var a = dec1.mul(dec2);
+    a = a.mul(dec1);
+    a = a.mul(dec2);
+    a = a.mul(dec1);
+    a = a.mul(dec2);
+    a = a.mul(dec1);
+    a = a.mul(dec2);
+    a = a.mul(dec1);
+    a = a.mul(dec2);
+    a = a.mul(dec1);
+    a = a.mul(dec2);
+    a = a.mul(dec1);
+    a = a.mul(dec2);
+    a = a.mul(dec1);
+    a = a.mul(dec2);
+    a = a.mul(dec1);
+    a = a.mul(dec2);
+
+    return timer.read();
+}
+
+fn div7() u64 {
+    var str1 = RocStr.init("1.2", 3);
+    const dec1 = RocDec.fromStr(str1).?;
+
+    var str2 = RocStr.init("3.4", 3);
+    const dec2 = RocDec.fromStr(str2).?;
+
+    timer.reset();
+
+    var a = dec1.div(dec2);
+    a = a.div(dec1);
+    a = a.div(dec2);
+    a = a.div(dec1);
+    a = a.div(dec2);
+    a = a.div(dec1);
+    a = a.div(dec2);
+    a = a.div(dec1);
+    a = a.div(dec2);
+    a = a.div(dec1);
+    a = a.div(dec2);
+    a = a.div(dec1);
+    a = a.div(dec2);
+    a = a.div(dec1);
+    a = a.div(dec2);
+    a = a.div(dec1);
+    a = a.div(dec2);
+
+    return timer.read();
+}
diff --git a/compiler/builtins/bitcode/build.sh b/compiler/builtins/bitcode/build.sh
new file mode 100755
index 0000000000..e863770c8e
--- /dev/null
+++ b/compiler/builtins/bitcode/build.sh
@@ -0,0 +1,5 @@
+#!/bin/bash
+
+set -euxo pipefail
+
+zig build-obj src/main.zig -O ReleaseFast -femit-llvm-ir=builtins.ll -femit-bin=builtins.o --strip
diff --git a/compiler/builtins/bitcode/build.zig b/compiler/builtins/bitcode/build.zig
index 9d3b0456a7..e9835e0db5 100644
--- a/compiler/builtins/bitcode/build.zig
+++ b/compiler/builtins/bitcode/build.zig
@@ -1,16 +1,15 @@
-const builtin = @import("builtin");
 const std = @import("std");
 const mem = std.mem;
 const Builder = std.build.Builder;
 
 pub fn build(b: *Builder) void {
-    // b.setPreferredReleaseMode(builtin.Mode.Debug);
-    b.setPreferredReleaseMode(builtin.Mode.ReleaseFast);
+    // b.setPreferredReleaseMode(builtin.Mode.Debug
+    b.setPreferredReleaseMode(.ReleaseFast);
     const mode = b.standardReleaseOptions();
 
     // Options
     const fallback_main_path = "./src/main.zig";
-    const main_path_desc = b.fmt("Override path to main.zig. Used by \"ir\" and \"test\". Defaults to \"{}\". ", .{fallback_main_path});
+    const main_path_desc = b.fmt("Override path to main.zig. Used by \"ir\" and \"test\". Defaults to \"{s}\". ", .{fallback_main_path});
     const main_path = b.option([]const u8, "main-path", main_path_desc) orelse fallback_main_path;
 
     // Tests
diff --git a/compiler/builtins/bitcode/run-tests.sh b/compiler/builtins/bitcode/run-tests.sh
index bf13fccb8f..92bb9fae6d 100755
--- a/compiler/builtins/bitcode/run-tests.sh
+++ b/compiler/builtins/bitcode/run-tests.sh
@@ -6,4 +6,4 @@ set -euxo pipefail
 zig build test
 
 # fmt every zig
-find src/*.zig -type f -print0 | xargs -n 1 -0 zig fmt --check
+find src/*.zig -type f -print0 | xargs -n 1 -0 zig fmt --check || (echo "zig fmt --check FAILED! Check the previuous lines to see which files were improperly formatted." && exit 1)
diff --git a/compiler/builtins/bitcode/src/dec.zig b/compiler/builtins/bitcode/src/dec.zig
index 189347780f..011e37cac1 100644
--- a/compiler/builtins/bitcode/src/dec.zig
+++ b/compiler/builtins/bitcode/src/dec.zig
@@ -1,16 +1,199 @@
 const std = @import("std");
+const str = @import("str.zig");
 
 const math = std.math;
+const RocStr = str.RocStr;
 
 pub const RocDec = struct {
     num: i128,
 
-    pub const decimal_places: u32 = 18;
+    pub const decimal_places: u5 = 18;
+    pub const whole_number_places: u5 = 21;
+    const max_digits: u6 = 39;
+    const leading_zeros: [17]u8 = "00000000000000000".*;
 
     pub const min: RocDec = .{ .num = math.minInt(i128) };
     pub const max: RocDec = .{ .num = math.maxInt(i128) };
 
-    pub const one_point_zero: RocDec = .{ .num = comptime math.pow(i128, 10, RocDec.decimal_places) };
+    pub const one_point_zero_i128: i128 = math.pow(i128, 10, RocDec.decimal_places);
+    pub const one_point_zero: RocDec = .{ .num = one_point_zero_i128 };
+
+    pub fn fromU64(num: u64) RocDec {
+        return .{ .num = num * one_point_zero_i128 };
+    }
+
+    pub fn fromStr(roc_str: RocStr) ?RocDec {
+        if (roc_str.isEmpty()) {
+            return null;
+        }
+
+        const length = roc_str.len();
+
+        const roc_str_slice = roc_str.asSlice();
+
+        var is_negative: bool = roc_str_slice[0] == '-';
+        var initial_index: usize = if (is_negative) 1 else 0;
+
+        var point_index: ?usize = null;
+        var index: usize = initial_index;
+        while (index < length) {
+            var byte: u8 = roc_str_slice[index];
+            if (byte == '.' and point_index == null) {
+                point_index = index;
+                index += 1;
+                continue;
+            }
+
+            if (!isDigit(byte)) {
+                return null;
+            }
+            index += 1;
+        }
+
+        var before_str_length = length;
+        var after_val_i128: ?i128 = null;
+        if (point_index) |pi| {
+            before_str_length = pi;
+
+            var after_str_len = (length - 1) - pi;
+            if (after_str_len > decimal_places) {
+                std.debug.panic("TODO runtime exception for too many decimal places!", .{});
+            }
+            var diff_decimal_places = decimal_places - after_str_len;
+
+            var after_str = roc_str_slice[pi + 1 .. length];
+            var after_u64 = std.fmt.parseUnsigned(u64, after_str, 10) catch null;
+            after_val_i128 = if (after_u64) |f| @intCast(i128, f) * math.pow(i128, 10, diff_decimal_places) else null;
+        }
+
+        var before_str = roc_str_slice[initial_index..before_str_length];
+        var before_val_not_adjusted = std.fmt.parseUnsigned(i128, before_str, 10) catch null;
+
+        var before_val_i128: ?i128 = null;
+        if (before_val_not_adjusted) |before| {
+            var result: i128 = undefined;
+            var overflowed = @mulWithOverflow(i128, before, one_point_zero_i128, &result);
+            if (overflowed) {
+                std.debug.panic("TODO runtime exception for overflow!", .{});
+            }
+            before_val_i128 = result;
+        }
+
+        var dec: ?RocDec = null;
+        if (before_val_i128) |before| {
+            if (after_val_i128) |after| {
+                var result: i128 = undefined;
+                var overflowed = @addWithOverflow(i128, before, after, &result);
+                if (overflowed) {
+                    std.debug.panic("TODO runtime exception for overflow!", .{});
+                }
+                dec = .{ .num = result };
+            } else {
+                dec = .{ .num = before };
+            }
+        } else if (after_val_i128) |after| {
+            dec = .{ .num = after };
+        }
+
+        if (dec) |d| {
+            if (is_negative) {
+                dec = d.negate();
+            }
+        }
+
+        return dec;
+    }
+
+    fn isDigit(c: u8) bool {
+        return (c -% 48) <= 9;
+    }
+
+    pub fn toStr(self: RocDec) ?RocStr {
+        // Special case
+        if (self.num == 0) {
+            return RocStr.init("0.0", 3);
+        }
+
+        // Check if this Dec is negative, and if so convert to positive
+        // We will handle adding the '-' later
+        const is_negative = self.num < 0;
+        const num = if (is_negative) std.math.negate(self.num) catch {
+            std.debug.panic("TODO runtime exception failing to negate", .{});
+        } else self.num;
+
+        // Format the backing i128 into an array of digits (u8s)
+        var digit_bytes: [max_digits + 1]u8 = undefined;
+        var num_digits = std.fmt.formatIntBuf(digit_bytes[0..], num, 10, false, .{});
+
+        // Get the slice for before the decimal point
+        var before_digits_slice: []const u8 = undefined;
+        var before_digits_offset: usize = 0;
+        var before_digits_adjust: u6 = 0;
+        if (num_digits > decimal_places) {
+            before_digits_offset = num_digits - decimal_places;
+            before_digits_slice = digit_bytes[0..before_digits_offset];
+        } else {
+            before_digits_adjust = @intCast(u6, math.absInt(@intCast(i7, num_digits) - decimal_places) catch {
+                std.debug.panic("TODO runtime exception for overflow when getting abs", .{});
+            });
+            before_digits_slice = "0";
+        }
+
+        // Figure out how many trailing zeros there are
+        // I tried to use https://ziglang.org/documentation/0.8.0/#ctz and it mostly worked,
+        // but was giving seemingly incorrect values for certain numbers. So instead we use
+        //  a while loop and figure it out that way.
+        //
+        // const trailing_zeros = @ctz(u6, num);
+        //
+        var trailing_zeros: u6 = 0;
+        var index = decimal_places - 1 - before_digits_adjust;
+        var is_consecutive_zero = true;
+        while (index != 0) {
+            var digit = digit_bytes[before_digits_offset + index];
+            if (digit == '0' and is_consecutive_zero) {
+                trailing_zeros += 1;
+            } else {
+                is_consecutive_zero = false;
+            }
+            index -= 1;
+        }
+
+        // Figure out if we need to prepend any zeros to the after decimal point
+        // For example, for the number 0.000123 we need to prepend 3 zeros after the decimal point
+        // This will only be needed for numbers less 0.01, otherwise after_digits_slice will handle this
+        const after_zeros_num = if (num_digits < decimal_places) decimal_places - num_digits else 0;
+        const after_zeros_slice: []const u8 = leading_zeros[0..after_zeros_num];
+
+        // Get the slice for after the decimal point
+        var after_digits_slice: []const u8 = undefined;
+        if ((num_digits - before_digits_offset) == trailing_zeros) {
+            after_digits_slice = "0";
+        } else {
+            after_digits_slice = digit_bytes[before_digits_offset .. num_digits - trailing_zeros];
+        }
+
+        // Get the slice for the sign
+        const sign_slice: []const u8 = if (is_negative) "-" else leading_zeros[0..0];
+
+        // Hardcode adding a `1` for the '.' character
+        const str_len: usize = sign_slice.len + before_digits_slice.len + 1 + after_zeros_slice.len + after_digits_slice.len;
+
+        // Join the slices together
+        // We do `max_digits + 2` here because we need to account for a possible sign ('-') and the dot ('.').
+        // Ideally, we'd use str_len here
+        var str_bytes: [max_digits + 2]u8 = undefined;
+        _ = std.fmt.bufPrint(str_bytes[0..str_len], "{s}{s}.{s}{s}", .{ sign_slice, before_digits_slice, after_zeros_slice, after_digits_slice }) catch {
+            std.debug.panic("TODO runtime exception failing to print slices", .{});
+        };
+
+        return RocStr.init(&str_bytes, str_len);
+    }
+
+    pub fn negate(self: RocDec) ?RocDec {
+        var negated = math.negate(self.num) catch null;
+        return if (negated) |n| .{ .num = n } else null;
+    }
 
     pub fn add(self: RocDec, other: RocDec) RocDec {
         var answer: i128 = undefined;
@@ -23,6 +206,17 @@ pub const RocDec = struct {
         }
     }
 
+    pub fn sub(self: RocDec, other: RocDec) RocDec {
+        var answer: i128 = undefined;
+        const overflowed = @subWithOverflow(i128, self.num, other.num, &answer);
+
+        if (!overflowed) {
+            return RocDec{ .num = answer };
+        } else {
+            std.debug.panic("TODO runtime exception for overflow!", .{});
+        }
+    }
+
     pub fn mul(self: RocDec, other: RocDec) RocDec {
         const self_i128 = self.num;
         const other_i128 = other.num;
@@ -58,6 +252,76 @@ pub const RocDec = struct {
             return .{ .num = unsigned_answer };
         }
     }
+
+    pub fn div(self: RocDec, other: RocDec) RocDec {
+        const numerator_i128 = self.num;
+        const denominator_i128 = other.num;
+
+        // (0 / n) is always 0
+        if (numerator_i128 == 0) {
+            return RocDec{ .num = 0 };
+        }
+
+        // (n / 0) is an error
+        if (denominator_i128 == 0) {
+            std.debug.panic("TODO runtime exception for divide by 0!", .{});
+        }
+
+        // If they're both negative, or if neither is negative, the final answer
+        // is positive or zero. If one is negative and the denominator isn't, the
+        // final answer is negative (or zero, in which case final sign won't matter).
+        //
+        // It's important that we do this in terms of negatives, because doing
+        // it in terms of positives can cause bugs when one is zero.
+        const is_answer_negative = (numerator_i128 < 0) != (denominator_i128 < 0);
+
+        // Break the two i128s into two { hi: u64, lo: u64 } tuples, discarding
+        // the sign for now.
+        //
+        // We'll multiply all 4 combinations of these (hi1 x lo1, hi2 x lo2,
+        // hi1 x lo2, hi2 x lo1) and add them as appropriate, then apply the
+        // appropriate sign at the very end.
+        //
+        // We do checked_abs because if we had -i128::MAX before, this will overflow.
+
+        const numerator_abs_i128 = math.absInt(numerator_i128) catch {
+            // Currently, if you try to do multiplication on i64::MIN, panic
+            // unless you're specifically multiplying by 0 or 1.
+            //
+            // Maybe we could support more cases in the future
+            if (denominator_i128 == one_point_zero_i128) {
+                return self;
+            } else {
+                std.debug.panic("TODO runtime exception for overflow when dividing!", .{});
+            }
+        };
+        const numerator_u128 = @intCast(u128, numerator_abs_i128);
+
+        const denominator_abs_i128 = math.absInt(denominator_i128) catch {
+            // Currently, if you try to do multiplication on i64::MIN, panic
+            // unless you're specifically multiplying by 0 or 1.
+            //
+            // Maybe we could support more cases in the future
+            if (numerator_i128 == one_point_zero_i128) {
+                return other;
+            } else {
+                std.debug.panic("TODO runtime exception for overflow when dividing!", .{});
+            }
+        };
+        const denominator_u128 = @intCast(u128, denominator_abs_i128);
+
+        const numerator_u256: U256 = mul_u128(numerator_u128, math.pow(u128, 10, decimal_places));
+        const answer = div_u256_by_u128(numerator_u256, denominator_u128);
+
+        var unsigned_answer: i128 = undefined;
+        if (answer.hi == 0 and answer.lo <= math.maxInt(i128)) {
+            unsigned_answer = @intCast(i128, answer.lo);
+        } else {
+            std.debug.panic("TODO runtime exception for overflow when dividing!", .{});
+        }
+
+        return RocDec{ .num = if (is_answer_negative) -unsigned_answer else unsigned_answer };
+    }
 };
 
 const U256 = struct {
@@ -75,7 +339,7 @@ fn mul_and_decimalize(a: u128, b: u128) i128 {
     // floor(2^315/10^18) is 66749594872528440074844428317798503581334516323645399060845050244444366430645
 
     // Add 1.
-    // This can't overflow because the intial numbers are only 127bit due to removing the sign bit.
+    // This can't overflow because the initial numbers are only 127bit due to removing the sign bit.
     var overflowed = @addWithOverflow(u128, lhs_lo, 1, &lhs_lo);
     lhs_hi = blk: {
         if (overflowed) {
@@ -98,7 +362,7 @@ fn mul_and_decimalize(a: u128, b: u128) i128 {
     const lk = mul_u128(lhs_hi, rhs_hi);
 
     const e = ea.hi;
-    const _a = ea.lo;
+    // const _a = ea.lo;
 
     const g = gf.hi;
     const f = gf.lo;
@@ -216,18 +480,476 @@ fn mul_u128(a: u128, b: u128) U256 {
     return .{ .hi = hi, .lo = lo };
 }
 
-const one_e20: i256 = 100000000000000000000;
+// Multiply two 128-bit ints and divide the result by 10^DECIMAL_PLACES
+//
+// Adapted from https://github.com/nlordell/ethnum-rs
+// Copyright (c) 2020 Nicholas Rodrigues Lordello
+// Licensed under the Apache License version 2.0
+//
+// When translating this to Zig, we often have to use math.shr/shl instead of >>/<<
+// This is because casting to the right types for Zig can be kind of tricky.
+// See https://github.com/ziglang/zig/issues/7605
+fn div_u256_by_u128(numer: U256, denom: u128) U256 {
+    const N_UDWORD_BITS: u8 = 128;
+    const N_UTWORD_BITS: u9 = 256;
 
-const expectEqual = std.testing.expectEqual;
+    var q: U256 = undefined;
+    var r: U256 = undefined;
+    var sr: u8 = undefined;
 
-test "add" {
-    const dec: RocDec = .{ .num = 0 };
+    // special case
+    if (numer.hi == 0) {
+        // 0 X
+        // ---
+        // 0 X
+        return .{
+            .hi = 0,
+            .lo = numer.lo / denom,
+        };
+    }
 
-    expectEqual(RocDec{ .num = 0 }, dec.add(.{ .num = 0 }));
+    // numer.hi != 0
+    if (denom == 0) {
+        // K X
+        // ---
+        // 0 0
+        return .{
+            .hi = 0,
+            .lo = numer.hi / denom,
+        };
+    } else {
+        // K X
+        // ---
+        // 0 K
+        // NOTE: Modified from `if (d.low() & (d.low() - 1)) == 0`.
+        if (math.isPowerOfTwo(denom)) {
+            // if d is a power of 2
+            if (denom == 1) {
+                return numer;
+            }
+
+            sr = @ctz(u128, denom);
+
+            return .{
+                .hi = math.shr(u128, numer.hi, sr),
+                .lo = math.shl(u128, numer.hi, N_UDWORD_BITS - sr) | math.shr(u128, numer.lo, sr),
+            };
+        }
+
+        // K X
+        // ---
+        // 0 K
+        var denom_leading_zeros = @clz(u128, denom);
+        var numer_hi_leading_zeros = @clz(u128, numer.hi);
+        sr = 1 + N_UDWORD_BITS + denom_leading_zeros - numer_hi_leading_zeros;
+        // 2 <= sr <= N_UTWORD_BITS - 1
+        // q.all = n.all << (N_UTWORD_BITS - sr);
+        // r.all = n.all >> sr;
+        // #[allow(clippy::comparison_chain)]
+        if (sr == N_UDWORD_BITS) {
+            q = .{
+                .hi = numer.lo,
+                .lo = 0,
+            };
+            r = .{
+                .hi = 0,
+                .lo = numer.hi,
+            };
+        } else if (sr < N_UDWORD_BITS) {
+            // 2 <= sr <= N_UDWORD_BITS - 1
+            q = .{
+                .hi = math.shl(u128, numer.lo, N_UDWORD_BITS - sr),
+                .lo = 0,
+            };
+            r = .{
+                .hi = math.shr(u128, numer.hi, sr),
+                .lo = math.shl(u128, numer.hi, N_UDWORD_BITS - sr) | math.shr(u128, numer.lo, sr),
+            };
+        } else {
+            // N_UDWORD_BITS + 1 <= sr <= N_UTWORD_BITS - 1
+            q = .{
+                .hi = math.shl(u128, numer.hi, N_UTWORD_BITS - sr) | math.shr(u128, numer.lo, sr - N_UDWORD_BITS),
+                .lo = math.shl(u128, numer.lo, N_UTWORD_BITS - sr),
+            };
+            r = .{
+                .hi = 0,
+                .lo = math.shr(u128, numer.hi, sr - N_UDWORD_BITS),
+            };
+        }
+    }
+
+    // Not a special case
+    // q and r are initialized with:
+    // q.all = n.all << (N_UTWORD_BITS - sr);
+    // r.all = n.all >> sr;
+    // 1 <= sr <= N_UTWORD_BITS - 1
+    var carry: u128 = 0;
+
+    while (sr > 0) {
+        // r:q = ((r:q)  << 1) | carry
+        r.hi = (r.hi << 1) | (r.lo >> (N_UDWORD_BITS - 1));
+        r.lo = (r.lo << 1) | (q.hi >> (N_UDWORD_BITS - 1));
+        q.hi = (q.hi << 1) | (q.lo >> (N_UDWORD_BITS - 1));
+        q.lo = (q.lo << 1) | carry;
+
+        // carry = 0;
+        // if (r.all >= d.all)
+        // {
+        //     r.all -= d.all;
+        //      carry = 1;
+        // }
+        // NOTE: Modified from `(d - r - 1) >> (N_UTWORD_BITS - 1)` to be an
+        // **arithmetic** shift.
+
+        var lo: u128 = undefined;
+        var lo_overflowed: bool = undefined;
+        var hi: u128 = undefined;
+
+        lo_overflowed = @subWithOverflow(u128, denom, r.lo, &lo);
+        hi = 0 -% @intCast(u128, @bitCast(u1, lo_overflowed)) -% r.hi;
+
+        lo_overflowed = @subWithOverflow(u128, lo, 1, &lo);
+        hi = hi -% @intCast(u128, @bitCast(u1, lo_overflowed));
+
+        // TODO this U256 was originally created by:
+        //
+        // ((hi as i128) >> 127).as_u256()
+        //
+        // ...however, I can't figure out where that function is defined.
+        // Maybe it's defined using a macro or something. Anyway, hopefully
+        // this is what it would do in this scenario.
+        var s = .{
+            .hi = 0,
+            .lo = math.shr(u128, hi, 127),
+        };
+
+        carry = s.lo & 1;
+
+        // var (lo, carry) = r.lo.overflowing_sub(denom & s.lo);
+        lo_overflowed = @subWithOverflow(u128, r.lo, (denom & s.lo), &lo);
+        hi = r.hi -% @intCast(u128, @bitCast(u1, lo_overflowed));
+
+        r = .{ .hi = hi, .lo = lo };
+
+        sr -= 1;
+    }
+
+    var hi = (q.hi << 1) | (q.lo >> (127));
+    var lo = (q.lo << 1) | carry;
+
+    return .{ .hi = hi, .lo = lo };
 }
 
-test "mul" {
-    var dec1: RocDec = .{ .num = 0 };
+const testing = std.testing;
+const expectEqual = testing.expectEqual;
+const expectError = testing.expectError;
+const expectEqualSlices = testing.expectEqualSlices;
+const expect = testing.expect;
 
-    expectEqual(RocDec{ .num = 0 }, dec1.mul(.{ .num = 0 }));
+test "fromU64" {
+    var dec = RocDec.fromU64(25);
+
+    try expectEqual(RocDec{ .num = 25000000000000000000 }, dec);
+}
+
+test "fromStr: empty" {
+    var roc_str = RocStr.init("", 0);
+    var dec = RocDec.fromStr(roc_str);
+
+    try expectEqual(dec, null);
+}
+
+test "fromStr: 0" {
+    var roc_str = RocStr.init("0", 1);
+    var dec = RocDec.fromStr(roc_str);
+
+    try expectEqual(RocDec{ .num = 0 }, dec.?);
+}
+
+test "fromStr: 1" {
+    var roc_str = RocStr.init("1", 1);
+    var dec = RocDec.fromStr(roc_str);
+
+    try expectEqual(RocDec.one_point_zero, dec.?);
+}
+
+test "fromStr: 123.45" {
+    var roc_str = RocStr.init("123.45", 6);
+    var dec = RocDec.fromStr(roc_str);
+
+    try expectEqual(RocDec{ .num = 123450000000000000000 }, dec.?);
+}
+
+test "fromStr: .45" {
+    var roc_str = RocStr.init(".45", 3);
+    var dec = RocDec.fromStr(roc_str);
+
+    try expectEqual(RocDec{ .num = 450000000000000000 }, dec.?);
+}
+
+test "fromStr: 0.45" {
+    var roc_str = RocStr.init("0.45", 4);
+    var dec = RocDec.fromStr(roc_str);
+
+    try expectEqual(RocDec{ .num = 450000000000000000 }, dec.?);
+}
+
+test "fromStr: 123" {
+    var roc_str = RocStr.init("123", 3);
+    var dec = RocDec.fromStr(roc_str);
+
+    try expectEqual(RocDec{ .num = 123000000000000000000 }, dec.?);
+}
+
+test "fromStr: -.45" {
+    var roc_str = RocStr.init("-.45", 4);
+    var dec = RocDec.fromStr(roc_str);
+
+    try expectEqual(RocDec{ .num = -450000000000000000 }, dec.?);
+}
+
+test "fromStr: -0.45" {
+    var roc_str = RocStr.init("-0.45", 5);
+    var dec = RocDec.fromStr(roc_str);
+
+    try expectEqual(RocDec{ .num = -450000000000000000 }, dec.?);
+}
+
+test "fromStr: -123" {
+    var roc_str = RocStr.init("-123", 4);
+    var dec = RocDec.fromStr(roc_str);
+
+    try expectEqual(RocDec{ .num = -123000000000000000000 }, dec.?);
+}
+
+test "fromStr: -123.45" {
+    var roc_str = RocStr.init("-123.45", 7);
+    var dec = RocDec.fromStr(roc_str);
+
+    try expectEqual(RocDec{ .num = -123450000000000000000 }, dec.?);
+}
+
+test "fromStr: abc" {
+    var roc_str = RocStr.init("abc", 3);
+    var dec = RocDec.fromStr(roc_str);
+
+    try expectEqual(dec, null);
+}
+
+test "fromStr: 123.abc" {
+    var roc_str = RocStr.init("123.abc", 7);
+    var dec = RocDec.fromStr(roc_str);
+
+    try expectEqual(dec, null);
+}
+
+test "fromStr: abc.123" {
+    var roc_str = RocStr.init("abc.123", 7);
+    var dec = RocDec.fromStr(roc_str);
+
+    try expectEqual(dec, null);
+}
+
+test "fromStr: .123.1" {
+    var roc_str = RocStr.init(".123.1", 6);
+    var dec = RocDec.fromStr(roc_str);
+
+    try expectEqual(dec, null);
+}
+
+test "toStr: 123.45" {
+    var dec: RocDec = .{ .num = 123450000000000000000 };
+    var res_roc_str = dec.toStr();
+
+    const res_slice: []const u8 = "123.45"[0..];
+    try expectEqualSlices(u8, res_slice, res_roc_str.?.asSlice());
+}
+
+test "toStr: -123.45" {
+    var dec: RocDec = .{ .num = -123450000000000000000 };
+    var res_roc_str = dec.toStr();
+
+    const res_slice: []const u8 = "-123.45"[0..];
+    try expectEqualSlices(u8, res_slice, res_roc_str.?.asSlice());
+}
+
+test "toStr: 123.0" {
+    var dec: RocDec = .{ .num = 123000000000000000000 };
+    var res_roc_str = dec.toStr();
+
+    const res_slice: []const u8 = "123.0"[0..];
+    try expectEqualSlices(u8, res_slice, res_roc_str.?.asSlice());
+}
+
+test "toStr: -123.0" {
+    var dec: RocDec = .{ .num = -123000000000000000000 };
+    var res_roc_str = dec.toStr();
+
+    const res_slice: []const u8 = "-123.0"[0..];
+    try expectEqualSlices(u8, res_slice, res_roc_str.?.asSlice());
+}
+
+test "toStr: 0.45" {
+    var dec: RocDec = .{ .num = 450000000000000000 };
+    var res_roc_str = dec.toStr();
+
+    const res_slice: []const u8 = "0.45"[0..];
+    try expectEqualSlices(u8, res_slice, res_roc_str.?.asSlice());
+}
+
+test "toStr: -0.45" {
+    var dec: RocDec = .{ .num = -450000000000000000 };
+    var res_roc_str = dec.toStr();
+
+    const res_slice: []const u8 = "-0.45"[0..];
+    try expectEqualSlices(u8, res_slice, res_roc_str.?.asSlice());
+}
+
+test "toStr: 0.00045" {
+    var dec: RocDec = .{ .num = 000450000000000000 };
+    var res_roc_str = dec.toStr();
+
+    const res_slice: []const u8 = "0.00045"[0..];
+    try expectEqualSlices(u8, res_slice, res_roc_str.?.asSlice());
+}
+
+test "toStr: -0.00045" {
+    var dec: RocDec = .{ .num = -000450000000000000 };
+    var res_roc_str = dec.toStr();
+
+    const res_slice: []const u8 = "-0.00045"[0..];
+    try expectEqualSlices(u8, res_slice, res_roc_str.?.asSlice());
+}
+
+test "toStr: -111.123456789" {
+    var dec: RocDec = .{ .num = -111123456789000000000 };
+    var res_roc_str = dec.toStr();
+
+    const res_slice: []const u8 = "-111.123456789"[0..];
+    try expectEqualSlices(u8, res_slice, res_roc_str.?.asSlice());
+}
+
+test "toStr: 123.1111111" {
+    var dec: RocDec = .{ .num = 123111111100000000000 };
+    var res_roc_str = dec.toStr();
+
+    const res_slice: []const u8 = "123.1111111"[0..];
+    try expectEqualSlices(u8, res_slice, res_roc_str.?.asSlice());
+}
+
+test "toStr: 123.1111111111111 (big str)" {
+    var dec: RocDec = .{ .num = 123111111111111000000 };
+    var res_roc_str = dec.toStr();
+    errdefer res_roc_str.?.deinit();
+    defer res_roc_str.?.deinit();
+
+    const res_slice: []const u8 = "123.111111111111"[0..];
+    try expectEqualSlices(u8, res_slice, res_roc_str.?.asSlice());
+}
+
+test "toStr: 123.111111111111444444 (max number of decimal places)" {
+    var dec: RocDec = .{ .num = 123111111111111444444 };
+    var res_roc_str = dec.toStr();
+    errdefer res_roc_str.?.deinit();
+    defer res_roc_str.?.deinit();
+
+    const res_slice: []const u8 = "123.111111111111444444"[0..];
+    try expectEqualSlices(u8, res_slice, res_roc_str.?.asSlice());
+}
+
+test "toStr: 12345678912345678912.111111111111111111 (max number of digits)" {
+    var dec: RocDec = .{ .num = 12345678912345678912111111111111111111 };
+    var res_roc_str = dec.toStr();
+    errdefer res_roc_str.?.deinit();
+    defer res_roc_str.?.deinit();
+
+    const res_slice: []const u8 = "12345678912345678912.111111111111111111"[0..];
+    try expectEqualSlices(u8, res_slice, res_roc_str.?.asSlice());
+}
+
+test "toStr: 0" {
+    var dec: RocDec = .{ .num = 0 };
+    var res_roc_str = dec.toStr();
+
+    const res_slice: []const u8 = "0.0"[0..];
+    try expectEqualSlices(u8, res_slice, res_roc_str.?.asSlice());
+}
+
+test "add: 0" {
+    var dec: RocDec = .{ .num = 0 };
+
+    try expectEqual(RocDec{ .num = 0 }, dec.add(.{ .num = 0 }));
+}
+
+test "add: 1" {
+    var dec: RocDec = .{ .num = 0 };
+
+    try expectEqual(RocDec{ .num = 1 }, dec.add(.{ .num = 1 }));
+}
+
+test "sub: 0" {
+    var dec: RocDec = .{ .num = 1 };
+
+    try expectEqual(RocDec{ .num = 1 }, dec.sub(.{ .num = 0 }));
+}
+
+test "sub: 1" {
+    var dec: RocDec = .{ .num = 1 };
+
+    try expectEqual(RocDec{ .num = 0 }, dec.sub(.{ .num = 1 }));
+}
+
+test "mul: by 0" {
+    var dec: RocDec = .{ .num = 0 };
+
+    try expectEqual(RocDec{ .num = 0 }, dec.mul(.{ .num = 0 }));
+}
+
+test "mul: by 1" {
+    var dec: RocDec = RocDec.fromU64(15);
+
+    try expectEqual(RocDec.fromU64(15), dec.mul(RocDec.fromU64(1)));
+}
+
+test "mul: by 2" {
+    var dec: RocDec = RocDec.fromU64(15);
+
+    try expectEqual(RocDec.fromU64(30), dec.mul(RocDec.fromU64(2)));
+}
+
+test "div: 0 / 2" {
+    var dec: RocDec = RocDec.fromU64(0);
+
+    try expectEqual(RocDec.fromU64(0), dec.div(RocDec.fromU64(2)));
+}
+
+test "div: 2 / 2" {
+    var dec: RocDec = RocDec.fromU64(2);
+
+    try expectEqual(RocDec.fromU64(1), dec.div(RocDec.fromU64(2)));
+}
+
+test "div: 20 / 2" {
+    var dec: RocDec = RocDec.fromU64(20);
+
+    try expectEqual(RocDec.fromU64(10), dec.div(RocDec.fromU64(2)));
+}
+
+test "div: 8 / 5" {
+    var dec: RocDec = RocDec.fromU64(8);
+    var res: RocDec = RocDec.fromStr(RocStr.init("1.6", 3)).?;
+    try expectEqual(res, dec.div(RocDec.fromU64(5)));
+}
+
+test "div: 10 / 3" {
+    var numer: RocDec = RocDec.fromU64(10);
+    var denom: RocDec = RocDec.fromU64(3);
+
+    var roc_str = RocStr.init("3.333333333333333333", 20);
+    errdefer roc_str.deinit();
+    defer roc_str.deinit();
+
+    var res: RocDec = RocDec.fromStr(roc_str).?;
+
+    try expectEqual(res, numer.div(denom));
 }
diff --git a/compiler/builtins/bitcode/src/dict.zig b/compiler/builtins/bitcode/src/dict.zig
index 84f7bf6980..ea9ea768fc 100644
--- a/compiler/builtins/bitcode/src/dict.zig
+++ b/compiler/builtins/bitcode/src/dict.zig
@@ -20,13 +20,9 @@ const Slot = packed enum(u8) {
     PreviouslyFilled,
 };
 
-const MaybeIndexTag = enum {
-    index, not_found
-};
+const MaybeIndexTag = enum { index, not_found };
 
-const MaybeIndex = union(MaybeIndexTag) {
-    index: usize, not_found: void
-};
+const MaybeIndex = union(MaybeIndexTag) { index: usize, not_found: void };
 
 fn nextSeed(seed: u64) u64 {
     // TODO is this a valid way to get a new seed? are there better ways?
@@ -572,14 +568,6 @@ pub fn dictKeys(dict: RocDict, alignment: Alignment, key_width: usize, value_wid
     const data_bytes = length * key_width;
     var ptr = allocateWithRefcount(data_bytes, alignment);
 
-    var offset = blk: {
-        if (alignment.keyFirst()) {
-            break :blk 0;
-        } else {
-            break :blk (dict.capacity() * value_width);
-        }
-    };
-
     i = 0;
     var copied: usize = 0;
     while (i < size) : (i += 1) {
@@ -621,14 +609,6 @@ pub fn dictValues(dict: RocDict, alignment: Alignment, key_width: usize, value_w
     const data_bytes = length * value_width;
     var ptr = allocateWithRefcount(data_bytes, alignment);
 
-    var offset = blk: {
-        if (alignment.keyFirst()) {
-            break :blk (dict.capacity() * key_width);
-        } else {
-            break :blk 0;
-        }
-    };
-
     i = 0;
     var copied: usize = 0;
     while (i < size) : (i += 1) {
@@ -648,7 +628,7 @@ pub fn dictValues(dict: RocDict, alignment: Alignment, key_width: usize, value_w
     output.* = RocList{ .bytes = ptr, .length = length };
 }
 
-fn doNothing(ptr: Opaque) callconv(.C) void {
+fn doNothing(_: Opaque) callconv(.C) void {
     return;
 }
 
@@ -768,8 +748,6 @@ pub fn dictWalk(
     key_width: usize,
     value_width: usize,
     accum_width: usize,
-    inc_key: Inc,
-    inc_value: Inc,
     output: Opaque,
 ) callconv(.C) void {
     const alignment_u32 = alignment.toU32();
@@ -795,9 +773,7 @@ pub fn dictWalk(
 
                 caller(data, key, value, b2, b1);
 
-                const temp = b1;
-                b2 = b1;
-                b1 = temp;
+                std.mem.swap([*]u8, &b1, &b2);
             },
             else => {},
         }
diff --git a/compiler/builtins/bitcode/src/hash.zig b/compiler/builtins/bitcode/src/hash.zig
index 45414e68f3..10687a3231 100644
--- a/compiler/builtins/bitcode/src/hash.zig
+++ b/compiler/builtins/bitcode/src/hash.zig
@@ -8,10 +8,9 @@ const str = @import("str.zig");
 const mem = std.mem;
 
 pub fn wyhash(seed: u64, bytes: ?[*]const u8, length: usize) callconv(.C) u64 {
-    const stdout = std.io.getStdOut().writer();
-
     if (bytes) |nonnull| {
-        return wyhash_hash(seed, nonnull[0..length]);
+        const slice = nonnull[0..length];
+        return wyhash_hash(seed, slice);
     } else {
         return 42;
     }
@@ -202,13 +201,13 @@ const expectEqual = std.testing.expectEqual;
 test "test vectors" {
     const hash = Wyhash.hash;
 
-    expectEqual(hash(0, ""), 0x0);
-    expectEqual(hash(1, "a"), 0xbed235177f41d328);
-    expectEqual(hash(2, "abc"), 0xbe348debe59b27c3);
-    expectEqual(hash(3, "message digest"), 0x37320f657213a290);
-    expectEqual(hash(4, "abcdefghijklmnopqrstuvwxyz"), 0xd0b270e1d8a7019c);
-    expectEqual(hash(5, "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"), 0x602a1894d3bbfe7f);
-    expectEqual(hash(6, "12345678901234567890123456789012345678901234567890123456789012345678901234567890"), 0x829e9c148b75970e);
+    try expectEqual(hash(0, ""), 0x0);
+    try expectEqual(hash(1, "a"), 0xbed235177f41d328);
+    try expectEqual(hash(2, "abc"), 0xbe348debe59b27c3);
+    try expectEqual(hash(3, "message digest"), 0x37320f657213a290);
+    try expectEqual(hash(4, "abcdefghijklmnopqrstuvwxyz"), 0xd0b270e1d8a7019c);
+    try expectEqual(hash(5, "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"), 0x602a1894d3bbfe7f);
+    try expectEqual(hash(6, "12345678901234567890123456789012345678901234567890123456789012345678901234567890"), 0x829e9c148b75970e);
 }
 
 test "test vectors streaming" {
@@ -216,19 +215,19 @@ test "test vectors streaming" {
     for ("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789") |e| {
         wh.update(mem.asBytes(&e));
     }
-    expectEqual(wh.final(), 0x602a1894d3bbfe7f);
+    try expectEqual(wh.final(), 0x602a1894d3bbfe7f);
 
     const pattern = "1234567890";
     const count = 8;
     const result = 0x829e9c148b75970e;
-    expectEqual(Wyhash.hash(6, pattern ** 8), result);
+    try expectEqual(Wyhash.hash(6, pattern ** 8), result);
 
     wh = Wyhash.init(6);
     var i: u32 = 0;
     while (i < count) : (i += 1) {
         wh.update(pattern);
     }
-    expectEqual(wh.final(), result);
+    try expectEqual(wh.final(), result);
 }
 
 test "iterative non-divisible update" {
@@ -250,6 +249,6 @@ test "iterative non-divisible update" {
         }
         const iterative_hash = wy.final();
 
-        std.testing.expectEqual(iterative_hash, non_iterative_hash);
+        try std.testing.expectEqual(iterative_hash, non_iterative_hash);
     }
 }
diff --git a/compiler/builtins/bitcode/src/helpers/grapheme.zig b/compiler/builtins/bitcode/src/helpers/grapheme.zig
index 252873cb4e..36cd2c4418 100644
--- a/compiler/builtins/bitcode/src/helpers/grapheme.zig
+++ b/compiler/builtins/bitcode/src/helpers/grapheme.zig
@@ -42,7 +42,7 @@ pub const BoundClass = enum(u8) {
 };
 
 test "Bound Class" {
-    expectEqual(0, @enumToInt(BoundClass.START));
+    try expectEqual(0, @enumToInt(BoundClass.START));
 }
 
 // https://github.com/JuliaStrings/utf8proc/blob/master/utf8proc.c#L261
@@ -112,7 +112,7 @@ fn unsafeCodepointToBoundClass(codepoint: u21) *const BoundClass {
 }
 
 test "unsafeCodepointToBoundClass: valid" {
-    expectEqual(BoundClass.CONTROL, unsafeCodepointToBoundClass(8).*);
+    try expectEqual(BoundClass.CONTROL, unsafeCodepointToBoundClass(8).*);
 }
 
 // https://github.com/JuliaStrings/utf8proc/blob/master/utf8proc.c#L242
@@ -125,11 +125,11 @@ fn codepointToBoundClass(codepoint: u21) *const BoundClass {
 }
 
 test "codepointToBoundClass: valid" {
-    expectEqual(BoundClass.CONTROL, codepointToBoundClass(8).*);
+    try expectEqual(BoundClass.CONTROL, codepointToBoundClass(8).*);
 }
 
 test "codepointToBoundClass: invalid" {
-    expectEqual(BoundClass.OTHER, codepointToBoundClass(codepoint_max + 5).*);
+    try expectEqual(BoundClass.OTHER, codepointToBoundClass(codepoint_max + 5).*);
 }
 
 // https://github.com/JuliaStrings/utf8proc/blob/master/utf8proc.c#L319
diff --git a/compiler/builtins/bitcode/src/list.zig b/compiler/builtins/bitcode/src/list.zig
index f5dbea5dc1..602b680d21 100644
--- a/compiler/builtins/bitcode/src/list.zig
+++ b/compiler/builtins/bitcode/src/list.zig
@@ -12,6 +12,7 @@ const Opaque = ?[*]u8;
 const Inc = fn (?[*]u8) callconv(.C) void;
 const IncN = fn (?[*]u8, usize) callconv(.C) void;
 const Dec = fn (?[*]u8) callconv(.C) void;
+const HasTagId = fn (u16, ?[*]u8) callconv(.C) extern struct { matched: bool, data: ?[*]u8 };
 
 pub const RocList = extern struct {
     bytes: ?[*]u8,
@@ -405,11 +406,14 @@ pub fn listKeepOks(
     before_width: usize,
     result_width: usize,
     after_width: usize,
+    has_tag_id: HasTagId,
     dec_result: Dec,
 ) callconv(.C) RocList {
+    const good_constructor: u16 = 1;
+
     return listKeepResult(
         list,
-        RocResult.isOk,
+        good_constructor,
         caller,
         data,
         inc_n_data,
@@ -418,6 +422,7 @@ pub fn listKeepOks(
         before_width,
         result_width,
         after_width,
+        has_tag_id,
         dec_result,
     );
 }
@@ -432,11 +437,14 @@ pub fn listKeepErrs(
     before_width: usize,
     result_width: usize,
     after_width: usize,
+    has_tag_id: HasTagId,
     dec_result: Dec,
 ) callconv(.C) RocList {
+    const good_constructor: u16 = 0;
+
     return listKeepResult(
         list,
-        RocResult.isErr,
+        good_constructor,
         caller,
         data,
         inc_n_data,
@@ -445,13 +453,14 @@ pub fn listKeepErrs(
         before_width,
         result_width,
         after_width,
+        has_tag_id,
         dec_result,
     );
 }
 
 pub fn listKeepResult(
     list: RocList,
-    is_good_constructor: fn (RocResult) bool,
+    good_constructor: u16,
     caller: Caller1,
     data: Opaque,
     inc_n_data: IncN,
@@ -460,6 +469,7 @@ pub fn listKeepResult(
     before_width: usize,
     result_width: usize,
     after_width: usize,
+    has_tag_id: HasTagId,
     dec_result: Dec,
 ) RocList {
     if (list.bytes) |source_ptr| {
@@ -479,11 +489,14 @@ pub fn listKeepResult(
             const before_element = source_ptr + (i * before_width);
             caller(data, before_element, temporary);
 
-            const result = utils.RocResult{ .bytes = temporary };
-
-            const after_element = temporary + @sizeOf(i64);
-            if (is_good_constructor(result)) {
-                @memcpy(target_ptr + (kept * after_width), after_element, after_width);
+            // a record { matched: bool, data: ?[*]u8 }
+            // for now, that data pointer is just the input `temporary` pointer
+            // this will change in the future to only return a pointer to the
+            // payload of the tag
+            const answer = has_tag_id(good_constructor, temporary);
+            if (answer.matched) {
+                const contents = (answer.data orelse unreachable);
+                @memcpy(target_ptr + (kept * after_width), contents, after_width);
                 kept += 1;
             } else {
                 dec_result(temporary);
@@ -542,9 +555,7 @@ pub fn listWalk(
             const element = source_ptr + i * element_width;
             caller(data, element, b2, b1);
 
-            const temp = b1;
-            b2 = b1;
-            b1 = temp;
+            std.mem.swap([*]u8, &b1, &b2);
         }
     }
 
@@ -591,9 +602,7 @@ pub fn listWalkBackwards(
             const element = source_ptr + i * element_width;
             caller(data, element, b2, b1);
 
-            const temp = b1;
-            b2 = b1;
-            b1 = temp;
+            std.mem.swap([*]u8, &b1, &b2);
         }
     }
 
@@ -610,12 +619,13 @@ pub fn listWalkUntil(
     accum: Opaque,
     alignment: u32,
     element_width: usize,
+    continue_stop_width: usize,
     accum_width: usize,
+    has_tag_id: HasTagId,
     dec: Dec,
     output: Opaque,
 ) callconv(.C) void {
     // [ Continue a, Stop a ]
-    const CONTINUE: usize = 0;
 
     if (accum_width == 0) {
         return;
@@ -626,9 +636,10 @@ pub fn listWalkUntil(
         return;
     }
 
-    const bytes_ptr: [*]u8 = utils.alloc(TAG_WIDTH + accum_width, alignment);
+    const bytes_ptr: [*]u8 = utils.alloc(continue_stop_width, alignment);
 
-    @memcpy(bytes_ptr + TAG_WIDTH, accum orelse unreachable, accum_width);
+    // NOTE: assumes data bytes are the first bytes in a tag
+    @memcpy(bytes_ptr, accum orelse unreachable, accum_width);
 
     if (list.bytes) |source_ptr| {
         var i: usize = 0;
@@ -640,10 +651,12 @@ pub fn listWalkUntil(
                 inc_n_data(data, 1);
             }
 
-            caller(data, element, bytes_ptr + TAG_WIDTH, bytes_ptr);
+            caller(data, element, bytes_ptr, bytes_ptr);
 
-            const usizes: [*]usize = @ptrCast([*]usize, @alignCast(8, bytes_ptr));
-            if (usizes[0] != 0) {
+            // [ Continue ..., Stop ]
+            const tag_id = has_tag_id(0, bytes_ptr);
+
+            if (!tag_id.matched) {
                 // decrement refcount of the remaining items
                 i += 1;
                 while (i < size) : (i += 1) {
@@ -654,7 +667,7 @@ pub fn listWalkUntil(
         }
     }
 
-    @memcpy(output orelse unreachable, bytes_ptr + TAG_WIDTH, accum_width);
+    @memcpy(output orelse unreachable, bytes_ptr, accum_width);
     utils.dealloc(bytes_ptr, alignment);
 }
 
@@ -722,6 +735,28 @@ pub fn listAppend(list: RocList, alignment: u32, element: Opaque, element_width:
     return output;
 }
 
+pub fn listSwap(
+    list: RocList,
+    alignment: u32,
+    element_width: usize,
+    index_1: usize,
+    index_2: usize,
+) callconv(.C) RocList {
+    const size = list.len();
+    if (index_1 >= size or index_2 >= size) {
+        // Either index out of bounds so we just return
+        return list;
+    }
+
+    const newList = list.makeUnique(alignment, element_width);
+
+    if (newList.bytes) |source_ptr| {
+        swapElements(source_ptr, element_width, index_1, index_2);
+    }
+
+    return newList;
+}
+
 pub fn listDrop(
     list: RocList,
     alignment: u32,
@@ -759,43 +794,19 @@ pub fn listDrop(
 }
 
 pub fn listRange(width: utils.IntWidth, low: Opaque, high: Opaque) callconv(.C) RocList {
-    const IntWidth = utils.IntWidth;
-
-    switch (width) {
-        IntWidth.U8 => {
-            return helper1(u8, low, high);
-        },
-        IntWidth.U16 => {
-            return helper1(u16, low, high);
-        },
-        IntWidth.U32 => {
-            return helper1(u32, low, high);
-        },
-        IntWidth.U64 => {
-            return helper1(u64, low, high);
-        },
-        IntWidth.U128 => {
-            return helper1(u128, low, high);
-        },
-        IntWidth.I8 => {
-            return helper1(i8, low, high);
-        },
-        IntWidth.I16 => {
-            return helper1(i16, low, high);
-        },
-        IntWidth.I32 => {
-            return helper1(i32, low, high);
-        },
-        IntWidth.I64 => {
-            return helper1(i64, low, high);
-        },
-        IntWidth.I128 => {
-            return helper1(i128, low, high);
-        },
-        IntWidth.Usize => {
-            return helper1(usize, low, high);
-        },
-    }
+    return switch (width) {
+        .U8 => helper1(u8, low, high),
+        .U16 => helper1(u16, low, high),
+        .U32 => helper1(u32, low, high),
+        .U64 => helper1(u64, low, high),
+        .U128 => helper1(u128, low, high),
+        .I8 => helper1(i8, low, high),
+        .I16 => helper1(i16, low, high),
+        .I32 => helper1(i32, low, high),
+        .I64 => helper1(i64, low, high),
+        .I128 => helper1(i128, low, high),
+        .Usize => helper1(usize, low, high),
+    };
 }
 
 fn helper1(comptime T: type, low: Opaque, high: Opaque) RocList {
@@ -910,7 +921,7 @@ inline fn swapHelp(width: usize, temporary: [*]u8, ptr1: [*]u8, ptr2: [*]u8) voi
 }
 
 fn swap(width_initial: usize, p1: [*]u8, p2: [*]u8) void {
-    const threshold: comptime usize = 64;
+    const threshold: usize = 64;
 
     var width = width_initial;
 
@@ -936,11 +947,6 @@ fn swap(width_initial: usize, p1: [*]u8, p2: [*]u8) void {
 }
 
 fn swapElements(source_ptr: [*]u8, element_width: usize, index_1: usize, index_2: usize) void {
-    const threshold: comptime usize = 64;
-
-    var buffer_actual: [threshold]u8 = undefined;
-    var buffer: [*]u8 = buffer_actual[0..];
-
     var element_at_i = source_ptr + (index_1 * element_width);
     var element_at_j = source_ptr + (index_2 * element_width);
 
@@ -1015,7 +1021,23 @@ pub fn listConcat(list_a: RocList, list_b: RocList, alignment: u32, element_widt
     return output;
 }
 
-// input: RocList,
+pub fn listSetInPlace(
+    bytes: ?[*]u8,
+    index: usize,
+    element: Opaque,
+    element_width: usize,
+    dec: Dec,
+) callconv(.C) ?[*]u8 {
+    // INVARIANT: bounds checking happens on the roc side
+    //
+    // at the time of writing, the function is implemented roughly as
+    // `if inBounds then LowLevelListGet input index item else input`
+    // so we don't do a bounds check here. Hence, the list is also non-empty,
+    // because inserting into an empty list is always out of bounds
+
+    return listSetInPlaceHelp(bytes, index, element, element_width, dec);
+}
+
 pub fn listSet(
     bytes: ?[*]u8,
     length: usize,
@@ -1034,23 +1056,32 @@ pub fn listSet(
     const ptr: [*]usize = @ptrCast([*]usize, @alignCast(8, bytes));
 
     if ((ptr - 1)[0] == utils.REFCOUNT_ONE) {
-
-        // the element we will replace
-        var element_at_index = (bytes orelse undefined) + (index * element_width);
-
-        // decrement its refcount
-        dec(element_at_index);
-
-        // copy in the new element
-        @memcpy(element_at_index, element orelse undefined, element_width);
-
-        return bytes;
+        return listSetInPlaceHelp(bytes, index, element, element_width, dec);
     } else {
-        return listSetClone(bytes, length, alignment, index, element, element_width, dec);
+        return listSetImmutable(bytes, length, alignment, index, element, element_width, dec);
     }
 }
 
-inline fn listSetClone(
+inline fn listSetInPlaceHelp(
+    bytes: ?[*]u8,
+    index: usize,
+    element: Opaque,
+    element_width: usize,
+    dec: Dec,
+) ?[*]u8 {
+    // the element we will replace
+    var element_at_index = (bytes orelse undefined) + (index * element_width);
+
+    // decrement its refcount
+    dec(element_at_index);
+
+    // copy in the new element
+    @memcpy(element_at_index, element orelse undefined, element_width);
+
+    return bytes;
+}
+
+inline fn listSetImmutable(
     old_bytes: ?[*]u8,
     length: usize,
     alignment: u32,
@@ -1059,8 +1090,6 @@ inline fn listSetClone(
     element_width: usize,
     dec: Dec,
 ) ?[*]u8 {
-    @setCold(true);
-
     const data_bytes = length * element_width;
 
     var new_bytes = utils.allocateWithRefcount(data_bytes, alignment);
diff --git a/compiler/builtins/bitcode/src/main.zig b/compiler/builtins/bitcode/src/main.zig
index 46285ed163..71e0ff8df8 100644
--- a/compiler/builtins/bitcode/src/main.zig
+++ b/compiler/builtins/bitcode/src/main.zig
@@ -30,6 +30,8 @@ comptime {
     exportListFn(list.listConcat, "concat");
     exportListFn(list.listDrop, "drop");
     exportListFn(list.listSet, "set");
+    exportListFn(list.listSetInPlace, "set_in_place");
+    exportListFn(list.listSwap, "swap");
 }
 
 // Dict Module
diff --git a/compiler/builtins/bitcode/src/str.zig b/compiler/builtins/bitcode/src/str.zig
index 4423d83df8..a52f4dd33b 100644
--- a/compiler/builtins/bitcode/src/str.zig
+++ b/compiler/builtins/bitcode/src/str.zig
@@ -17,7 +17,7 @@ const InPlace = packed enum(u8) {
 
 const SMALL_STR_MAX_LENGTH = small_string_size - 1;
 const small_string_size = 2 * @sizeOf(usize);
-const blank_small_string: [16]u8 = init_blank_small_string(small_string_size);
+const blank_small_string: [@sizeOf(RocStr)]u8 = init_blank_small_string(small_string_size);
 
 fn init_blank_small_string(comptime n: usize) [n]u8 {
     var prime_list: [n]u8 = undefined;
@@ -85,12 +85,6 @@ pub const RocStr = extern struct {
         }
     }
 
-    pub fn toSlice(self: RocStr) []u8 {
-        const str_bytes_ptr: [*]u8 = self.str_bytes orelse unreachable;
-        const str_bytes: []u8 = str_bytes_ptr[0..self.str_len];
-        return str_bytes;
-    }
-
     // This takes ownership of the pointed-to bytes if they won't fit in a
     // small string, and returns a (pointer, len) tuple which points to them.
     pub fn withCapacity(length: usize) RocStr {
@@ -203,8 +197,8 @@ pub const RocStr = extern struct {
         return result;
     }
 
+    // NOTE: returns false for empty string!
     pub fn isSmallStr(self: RocStr) bool {
-        // NOTE: returns False for empty string!
         return @bitCast(isize, self.str_len) < 0;
     }
 
@@ -223,6 +217,82 @@ pub const RocStr = extern struct {
         return self.len() == 0;
     }
 
+    // If a string happens to be null-terminated already, then we can pass its
+    // bytes directly to functions (e.g. for opening files) that require
+    // null-terminated strings. Otherwise, we need to allocate and copy a new
+    // null-terminated string, which has a much higher performance cost!
+    fn isNullTerminated(self: RocStr) bool {
+        const len = self.len();
+        const longest_small_str = @sizeOf(RocStr) - 1;
+
+        // NOTE: We want to compare length here, *NOT* check for is_small_str!
+        // This is because we explicitly want the empty string to be handled in
+        // this branch, even though the empty string is not a small string.
+        //
+        // (The other branch dereferences the bytes pointer, which is not safe
+        // to do for the empty string.)
+        if (len <= longest_small_str) {
+            // If we're a small string, then usually the next byte after the
+            // end of the string will be zero. (Small strings set all their
+            // unused bytes to 0, so that comparison for equality can be fast.)
+            //
+            // However, empty strings are *not* null terminated, so if this is
+            // empty, it should return false.
+            //
+            // Also, if we are exactly a maximum-length small string,
+            // then the next byte is off the end of the struct;
+            // in that case, we are also not null-terminated!
+            return len != 0 and len != longest_small_str;
+        } else {
+            // This is a big string, and it's not empty, so we can safely
+            // dereference the pointer.
+            const ptr: [*]usize = @ptrCast([*]usize, @alignCast(8, self.str_bytes));
+            const capacity_or_refcount: isize = (ptr - 1)[0];
+
+            // If capacity_or_refcount is positive, then it's a capacity value.
+            //
+            // If we have excess capacity, then we can safely read the next
+            // byte after the end of the string. Maybe it happens to be zero!
+            if (capacity_or_refcount > @intCast(isize, len)) {
+                return self.str_bytes[len] == 0;
+            } else {
+                // This string was refcounted or immortal; we can't safely read
+                // the next byte, so assume the string is not null-terminated.
+                return false;
+            }
+        }
+    }
+
+    // Returns (@sizeOf(RocStr) - 1) for small strings and the empty string.
+    // Returns 0 for refcounted stirngs and immortal strings.
+    // Returns the stored capacity value for all other strings.
+    pub fn capacity(self: RocStr) usize {
+        const len = self.len();
+        const longest_small_str = @sizeOf(RocStr) - 1;
+
+        if (len <= longest_small_str) {
+            // Note that although empty strings technically have the full
+            // capacity of a small string available, they aren't marked as small
+            // strings, so if you want to make use of that capacity, you need
+            // to first change its flag to mark it as a small string!
+            return longest_small_str;
+        } else {
+            const ptr: [*]usize = @ptrCast([*]usize, @alignCast(8, self.str_bytes));
+            const capacity_or_refcount: isize = (ptr - 1)[0];
+
+            if (capacity_or_refcount > 0) {
+                // If capacity_or_refcount is positive, that means it's a
+                // capacity value.
+                return capacity_or_refcount;
+            } else {
+                // This is either a refcount or else this big string is stored
+                // in a readonly section; either way, it has no capacity,
+                // because we cannot mutate it in-place!
+                return 0;
+            }
+        }
+    }
+
     pub fn isUnique(self: RocStr) bool {
         // the empty list is unique (in the sense that copying it will not leak memory)
         if (self.isEmpty()) {
@@ -240,15 +310,13 @@ pub const RocStr = extern struct {
     }
 
     pub fn asSlice(self: RocStr) []u8 {
-        // Since this conditional would be prone to branch misprediction,
-        // make sure it will compile to a cmov.
         return self.asU8ptr()[0..self.len()];
     }
 
     pub fn asU8ptr(self: RocStr) [*]u8 {
         // Since this conditional would be prone to branch misprediction,
         // make sure it will compile to a cmov.
-        return if (self.isSmallStr() or self.isEmpty()) (&@bitCast([16]u8, self)) else (@ptrCast([*]u8, self.str_bytes));
+        return if (self.isSmallStr() or self.isEmpty()) (&@bitCast([@sizeOf(RocStr)]u8, self)) else (@ptrCast([*]u8, self.str_bytes));
     }
 
     // Given a pointer to some bytes, write the first (len) bytes of this
@@ -273,7 +341,7 @@ pub const RocStr = extern struct {
         const str2_ptr: [*]u8 = &str2;
         var roc_str2 = RocStr.init(str2_ptr, str2_len);
 
-        expect(roc_str1.eq(roc_str2));
+        try expect(roc_str1.eq(roc_str2));
 
         roc_str1.deinit();
         roc_str2.deinit();
@@ -295,7 +363,7 @@ pub const RocStr = extern struct {
             roc_str2.deinit();
         }
 
-        expect(!roc_str1.eq(roc_str2));
+        try expect(!roc_str1.eq(roc_str2));
     }
 
     test "RocStr.eq: not equal same length" {
@@ -314,7 +382,7 @@ pub const RocStr = extern struct {
             roc_str2.deinit();
         }
 
-        expect(!roc_str1.eq(roc_str2));
+        try expect(!roc_str1.eq(roc_str2));
     }
 };
 
@@ -449,8 +517,8 @@ test "strSplitInPlace: no delimiter" {
         delimiter.deinit();
     }
 
-    expectEqual(array.len, expected.len);
-    expect(array[0].eq(expected[0]));
+    try expectEqual(array.len, expected.len);
+    try expect(array[0].eq(expected[0]));
 }
 
 test "strSplitInPlace: empty end" {
@@ -490,10 +558,10 @@ test "strSplitInPlace: empty end" {
         delimiter.deinit();
     }
 
-    expectEqual(array.len, expected.len);
-    expect(array[0].eq(expected[0]));
-    expect(array[1].eq(expected[1]));
-    expect(array[2].eq(expected[2]));
+    try expectEqual(array.len, expected.len);
+    try expect(array[0].eq(expected[0]));
+    try expect(array[1].eq(expected[1]));
+    try expect(array[2].eq(expected[2]));
 }
 
 test "strSplitInPlace: delimiter on sides" {
@@ -532,10 +600,10 @@ test "strSplitInPlace: delimiter on sides" {
         delimiter.deinit();
     }
 
-    expectEqual(array.len, expected.len);
-    expect(array[0].eq(expected[0]));
-    expect(array[1].eq(expected[1]));
-    expect(array[2].eq(expected[2]));
+    try expectEqual(array.len, expected.len);
+    try expect(array[0].eq(expected[0]));
+    try expect(array[1].eq(expected[1]));
+    try expect(array[2].eq(expected[2]));
 }
 
 test "strSplitInPlace: three pieces" {
@@ -573,10 +641,10 @@ test "strSplitInPlace: three pieces" {
         delimiter.deinit();
     }
 
-    expectEqual(expected_array.len, array.len);
-    expect(array[0].eq(expected_array[0]));
-    expect(array[1].eq(expected_array[1]));
-    expect(array[2].eq(expected_array[2]));
+    try expectEqual(expected_array.len, array.len);
+    try expect(array[0].eq(expected_array[0]));
+    try expect(array[1].eq(expected_array[1]));
+    try expect(array[2].eq(expected_array[2]));
 }
 
 // This is used for `Str.split : Str, Str -> Array Str
@@ -639,7 +707,7 @@ test "countSegments: long delimiter" {
     }
 
     const segments_count = countSegments(str, delimiter);
-    expectEqual(segments_count, 1);
+    try expectEqual(segments_count, 1);
 }
 
 test "countSegments: delimiter at start" {
@@ -658,7 +726,7 @@ test "countSegments: delimiter at start" {
 
     const segments_count = countSegments(str, delimiter);
 
-    expectEqual(segments_count, 2);
+    try expectEqual(segments_count, 2);
 }
 
 test "countSegments: delimiter interspered" {
@@ -677,7 +745,7 @@ test "countSegments: delimiter interspered" {
 
     const segments_count = countSegments(str, delimiter);
 
-    expectEqual(segments_count, 3);
+    try expectEqual(segments_count, 3);
 }
 
 // Str.countGraphemeClusters
@@ -721,7 +789,7 @@ fn rocStrFromLiteral(bytes_arr: *const []u8) RocStr {}
 
 test "countGraphemeClusters: empty string" {
     const count = countGraphemeClusters(RocStr.empty());
-    expectEqual(count, 0);
+    try expectEqual(count, 0);
 }
 
 test "countGraphemeClusters: ascii characters" {
@@ -731,7 +799,7 @@ test "countGraphemeClusters: ascii characters" {
     defer str.deinit();
 
     const count = countGraphemeClusters(str);
-    expectEqual(count, 4);
+    try expectEqual(count, 4);
 }
 
 test "countGraphemeClusters: utf8 characters" {
@@ -741,7 +809,7 @@ test "countGraphemeClusters: utf8 characters" {
     defer str.deinit();
 
     const count = countGraphemeClusters(str);
-    expectEqual(count, 3);
+    try expectEqual(count, 3);
 }
 
 test "countGraphemeClusters: emojis" {
@@ -751,7 +819,7 @@ test "countGraphemeClusters: emojis" {
     defer str.deinit();
 
     const count = countGraphemeClusters(str);
-    expectEqual(count, 3);
+    try expectEqual(count, 3);
 }
 
 test "countGraphemeClusters: emojis and ut8 characters" {
@@ -761,7 +829,7 @@ test "countGraphemeClusters: emojis and ut8 characters" {
     defer str.deinit();
 
     const count = countGraphemeClusters(str);
-    expectEqual(count, 6);
+    try expectEqual(count, 6);
 }
 
 test "countGraphemeClusters: emojis, ut8, and ascii characters" {
@@ -771,7 +839,7 @@ test "countGraphemeClusters: emojis, ut8, and ascii characters" {
     defer str.deinit();
 
     const count = countGraphemeClusters(str);
-    expectEqual(count, 10);
+    try expectEqual(count, 10);
 }
 
 // Str.startsWith
@@ -821,27 +889,27 @@ pub fn startsWithCodePoint(string: RocStr, prefix: u32) callconv(.C) bool {
 test "startsWithCodePoint: ascii char" {
     const whole = RocStr.init("foobar", 6);
     const prefix = 'f';
-    expect(startsWithCodePoint(whole, prefix));
+    try expect(startsWithCodePoint(whole, prefix));
 }
 
 test "startsWithCodePoint: emoji" {
     const yes = RocStr.init("💖foobar", 10);
     const no = RocStr.init("foobar", 6);
     const prefix = '💖';
-    expect(startsWithCodePoint(yes, prefix));
-    expect(!startsWithCodePoint(no, prefix));
+    try expect(startsWithCodePoint(yes, prefix));
+    try expect(!startsWithCodePoint(no, prefix));
 }
 
 test "startsWith: foo starts with fo" {
     const foo = RocStr.init("foo", 3);
     const fo = RocStr.init("fo", 2);
-    expect(startsWith(foo, fo));
+    try expect(startsWith(foo, fo));
 }
 
 test "startsWith: 123456789123456789 starts with 123456789123456789" {
     const str = RocStr.init("123456789123456789", 18);
     defer str.deinit();
-    expect(startsWith(str, str));
+    try expect(startsWith(str, str));
 }
 
 test "startsWith: 12345678912345678910 starts with 123456789123456789" {
@@ -850,7 +918,7 @@ test "startsWith: 12345678912345678910 starts with 123456789123456789" {
     const prefix = RocStr.init("123456789123456789", 18);
     defer prefix.deinit();
 
-    expect(startsWith(str, prefix));
+    try expect(startsWith(str, prefix));
 }
 
 // Str.endsWith
@@ -882,13 +950,13 @@ test "endsWith: foo ends with oo" {
     defer foo.deinit();
     defer oo.deinit();
 
-    expect(endsWith(foo, oo));
+    try expect(endsWith(foo, oo));
 }
 
 test "endsWith: 123456789123456789 ends with 123456789123456789" {
     const str = RocStr.init("123456789123456789", 18);
     defer str.deinit();
-    expect(endsWith(str, str));
+    try expect(endsWith(str, str));
 }
 
 test "endsWith: 12345678912345678910 ends with 345678912345678910" {
@@ -897,7 +965,7 @@ test "endsWith: 12345678912345678910 ends with 345678912345678910" {
     defer str.deinit();
     defer suffix.deinit();
 
-    expect(endsWith(str, suffix));
+    try expect(endsWith(str, suffix));
 }
 
 test "endsWith: hello world ends with world" {
@@ -906,17 +974,18 @@ test "endsWith: hello world ends with world" {
     defer str.deinit();
     defer suffix.deinit();
 
-    expect(endsWith(str, suffix));
+    try expect(endsWith(str, suffix));
 }
 
 // Str.concat
-pub fn strConcatC(result_in_place: InPlace, arg1: RocStr, arg2: RocStr) callconv(.C) RocStr {
-    return @call(.{ .modifier = always_inline }, strConcat, .{ result_in_place, arg1, arg2 });
+pub fn strConcatC(arg1: RocStr, arg2: RocStr) callconv(.C) RocStr {
+    return @call(.{ .modifier = always_inline }, strConcat, .{ arg1, arg2 });
 }
 
-fn strConcat(result_in_place: InPlace, arg1: RocStr, arg2: RocStr) RocStr {
+fn strConcat(arg1: RocStr, arg2: RocStr) RocStr {
     if (arg1.isEmpty()) {
         // the second argument is borrowed, so we must increment its refcount before returning
+        const result_in_place = InPlace.Clone;
         return RocStr.clone(result_in_place, arg2);
     } else if (arg2.isEmpty()) {
         // the first argument is owned, so we can return it without cloning
@@ -974,11 +1043,11 @@ test "RocStr.concat: small concat small" {
         roc_str3.deinit();
     }
 
-    const result = strConcat(InPlace.Clone, roc_str1, roc_str2);
+    const result = strConcat(roc_str1, roc_str2);
 
     defer result.deinit();
 
-    expect(roc_str3.eq(result));
+    try expect(roc_str3.eq(result));
 }
 
 pub const RocListStr = extern struct {
@@ -1057,7 +1126,7 @@ test "RocStr.joinWith: result is big" {
 
     defer result.deinit();
 
-    expect(roc_result.eq(result));
+    try expect(roc_result.eq(result));
 }
 
 // Str.toBytes
@@ -1191,8 +1260,8 @@ fn validateUtf8BytesX(str: RocList) FromUtf8Result {
     return fromUtf8(str);
 }
 
-fn expectOk(result: FromUtf8Result) void {
-    expectEqual(result.is_ok, true);
+fn expectOk(result: FromUtf8Result) !void {
+    try expectEqual(result.is_ok, true);
 }
 
 fn sliceHelp(bytes: [*]const u8, length: usize) RocList {
@@ -1217,7 +1286,7 @@ test "validateUtf8Bytes: ascii" {
     const ptr: [*]const u8 = @ptrCast([*]const u8, raw);
     const list = sliceHelp(ptr, raw.len);
 
-    expectOk(validateUtf8BytesX(list));
+    try expectOk(validateUtf8BytesX(list));
 }
 
 test "validateUtf8Bytes: unicode œ" {
@@ -1225,7 +1294,7 @@ test "validateUtf8Bytes: unicode œ" {
     const ptr: [*]const u8 = @ptrCast([*]const u8, raw);
     const list = sliceHelp(ptr, raw.len);
 
-    expectOk(validateUtf8BytesX(list));
+    try expectOk(validateUtf8BytesX(list));
 }
 
 test "validateUtf8Bytes: unicode ∆" {
@@ -1233,7 +1302,7 @@ test "validateUtf8Bytes: unicode ∆" {
     const ptr: [*]const u8 = @ptrCast([*]const u8, raw);
     const list = sliceHelp(ptr, raw.len);
 
-    expectOk(validateUtf8BytesX(list));
+    try expectOk(validateUtf8BytesX(list));
 }
 
 test "validateUtf8Bytes: emoji" {
@@ -1241,7 +1310,7 @@ test "validateUtf8Bytes: emoji" {
     const ptr: [*]const u8 = @ptrCast([*]const u8, raw);
     const list = sliceHelp(ptr, raw.len);
 
-    expectOk(validateUtf8BytesX(list));
+    try expectOk(validateUtf8BytesX(list));
 }
 
 test "validateUtf8Bytes: unicode ∆ in middle of array" {
@@ -1249,15 +1318,15 @@ test "validateUtf8Bytes: unicode ∆ in middle of array" {
     const ptr: [*]const u8 = @ptrCast([*]const u8, raw);
     const list = sliceHelp(ptr, raw.len);
 
-    expectOk(validateUtf8BytesX(list));
+    try expectOk(validateUtf8BytesX(list));
 }
 
-fn expectErr(list: RocList, index: usize, err: Utf8DecodeError, problem: Utf8ByteProblem) void {
+fn expectErr(list: RocList, index: usize, err: Utf8DecodeError, problem: Utf8ByteProblem) !void {
     const str_ptr = @ptrCast([*]u8, list.bytes);
     const str_len = list.length;
 
-    expectError(err, numberOfNextCodepointBytes(str_ptr, str_len, index));
-    expectEqual(toErrUtf8ByteResponse(index, problem), validateUtf8Bytes(str_ptr, str_len));
+    try expectError(err, numberOfNextCodepointBytes(str_ptr, str_len, index));
+    try expectEqual(toErrUtf8ByteResponse(index, problem), validateUtf8Bytes(str_ptr, str_len));
 }
 
 test "validateUtf8Bytes: invalid start byte" {
@@ -1266,7 +1335,7 @@ test "validateUtf8Bytes: invalid start byte" {
     const ptr: [*]const u8 = @ptrCast([*]const u8, raw);
     const list = sliceHelp(ptr, raw.len);
 
-    expectErr(list, 2, error.Utf8InvalidStartByte, Utf8ByteProblem.InvalidStartByte);
+    try expectErr(list, 2, error.Utf8InvalidStartByte, Utf8ByteProblem.InvalidStartByte);
 }
 
 test "validateUtf8Bytes: unexpected eof for 2 byte sequence" {
@@ -1275,7 +1344,7 @@ test "validateUtf8Bytes: unexpected eof for 2 byte sequence" {
     const ptr: [*]const u8 = @ptrCast([*]const u8, raw);
     const list = sliceHelp(ptr, raw.len);
 
-    expectErr(list, 3, error.UnexpectedEof, Utf8ByteProblem.UnexpectedEndOfSequence);
+    try expectErr(list, 3, error.UnexpectedEof, Utf8ByteProblem.UnexpectedEndOfSequence);
 }
 
 test "validateUtf8Bytes: expected continuation for 2 byte sequence" {
@@ -1284,7 +1353,7 @@ test "validateUtf8Bytes: expected continuation for 2 byte sequence" {
     const ptr: [*]const u8 = @ptrCast([*]const u8, raw);
     const list = sliceHelp(ptr, raw.len);
 
-    expectErr(list, 3, error.Utf8ExpectedContinuation, Utf8ByteProblem.ExpectedContinuation);
+    try expectErr(list, 3, error.Utf8ExpectedContinuation, Utf8ByteProblem.ExpectedContinuation);
 }
 
 test "validateUtf8Bytes: unexpected eof for 3 byte sequence" {
@@ -1293,7 +1362,7 @@ test "validateUtf8Bytes: unexpected eof for 3 byte sequence" {
     const ptr: [*]const u8 = @ptrCast([*]const u8, raw);
     const list = sliceHelp(ptr, raw.len);
 
-    expectErr(list, 3, error.UnexpectedEof, Utf8ByteProblem.UnexpectedEndOfSequence);
+    try expectErr(list, 3, error.UnexpectedEof, Utf8ByteProblem.UnexpectedEndOfSequence);
 }
 
 test "validateUtf8Bytes: expected continuation for 3 byte sequence" {
@@ -1302,7 +1371,7 @@ test "validateUtf8Bytes: expected continuation for 3 byte sequence" {
     const ptr: [*]const u8 = @ptrCast([*]const u8, raw);
     const list = sliceHelp(ptr, raw.len);
 
-    expectErr(list, 3, error.Utf8ExpectedContinuation, Utf8ByteProblem.ExpectedContinuation);
+    try expectErr(list, 3, error.Utf8ExpectedContinuation, Utf8ByteProblem.ExpectedContinuation);
 }
 
 test "validateUtf8Bytes: unexpected eof for 4 byte sequence" {
@@ -1311,7 +1380,7 @@ test "validateUtf8Bytes: unexpected eof for 4 byte sequence" {
     const ptr: [*]const u8 = @ptrCast([*]const u8, raw);
     const list = sliceHelp(ptr, raw.len);
 
-    expectErr(list, 3, error.UnexpectedEof, Utf8ByteProblem.UnexpectedEndOfSequence);
+    try expectErr(list, 3, error.UnexpectedEof, Utf8ByteProblem.UnexpectedEndOfSequence);
 }
 
 test "validateUtf8Bytes: expected continuation for 4 byte sequence" {
@@ -1320,7 +1389,7 @@ test "validateUtf8Bytes: expected continuation for 4 byte sequence" {
     const ptr: [*]const u8 = @ptrCast([*]const u8, raw);
     const list = sliceHelp(ptr, raw.len);
 
-    expectErr(list, 3, error.Utf8ExpectedContinuation, Utf8ByteProblem.ExpectedContinuation);
+    try expectErr(list, 3, error.Utf8ExpectedContinuation, Utf8ByteProblem.ExpectedContinuation);
 }
 
 test "validateUtf8Bytes: overlong" {
@@ -1329,7 +1398,7 @@ test "validateUtf8Bytes: overlong" {
     const ptr: [*]const u8 = @ptrCast([*]const u8, raw);
     const list = sliceHelp(ptr, raw.len);
 
-    expectErr(list, 3, error.Utf8OverlongEncoding, Utf8ByteProblem.OverlongEncoding);
+    try expectErr(list, 3, error.Utf8OverlongEncoding, Utf8ByteProblem.OverlongEncoding);
 }
 
 test "validateUtf8Bytes: codepoint out too large" {
@@ -1338,7 +1407,7 @@ test "validateUtf8Bytes: codepoint out too large" {
     const ptr: [*]const u8 = @ptrCast([*]const u8, raw);
     const list = sliceHelp(ptr, raw.len);
 
-    expectErr(list, 3, error.Utf8CodepointTooLarge, Utf8ByteProblem.CodepointTooLarge);
+    try expectErr(list, 3, error.Utf8CodepointTooLarge, Utf8ByteProblem.CodepointTooLarge);
 }
 
 test "validateUtf8Bytes: surrogate halves" {
@@ -1347,5 +1416,5 @@ test "validateUtf8Bytes: surrogate halves" {
     const ptr: [*]const u8 = @ptrCast([*]const u8, raw);
     const list = sliceHelp(ptr, raw.len);
 
-    expectErr(list, 3, error.Utf8EncodesSurrogateHalf, Utf8ByteProblem.EncodesSurrogateHalf);
+    try expectErr(list, 3, error.Utf8EncodesSurrogateHalf, Utf8ByteProblem.EncodesSurrogateHalf);
 }
diff --git a/compiler/builtins/bitcode/src/utils.zig b/compiler/builtins/bitcode/src/utils.zig
index ee08aeeacf..b679c20153 100644
--- a/compiler/builtins/bitcode/src/utils.zig
+++ b/compiler/builtins/bitcode/src/utils.zig
@@ -20,18 +20,18 @@ comptime {
     }
 }
 
-fn testing_roc_alloc(size: usize, alignment: u32) callconv(.C) ?*c_void {
+fn testing_roc_alloc(size: usize, _: u32) callconv(.C) ?*c_void {
     return @ptrCast(?*c_void, std.testing.allocator.alloc(u8, size) catch unreachable);
 }
 
-fn testing_roc_realloc(c_ptr: *c_void, new_size: usize, old_size: usize, alignment: u32) callconv(.C) ?*c_void {
+fn testing_roc_realloc(c_ptr: *c_void, new_size: usize, old_size: usize, _: u32) callconv(.C) ?*c_void {
     const ptr = @ptrCast([*]u8, @alignCast(16, c_ptr));
     const slice = ptr[0..old_size];
 
     return @ptrCast(?*c_void, std.testing.allocator.realloc(slice, new_size) catch unreachable);
 }
 
-fn testing_roc_dealloc(c_ptr: *c_void, alignment: u32) callconv(.C) void {
+fn testing_roc_dealloc(c_ptr: *c_void, _: u32) callconv(.C) void {
     const ptr = @ptrCast([*]u8, @alignCast(16, c_ptr));
 
     std.testing.allocator.destroy(ptr);
@@ -53,8 +53,8 @@ pub const Inc = fn (?[*]u8) callconv(.C) void;
 pub const IncN = fn (?[*]u8, u64) callconv(.C) void;
 pub const Dec = fn (?[*]u8) callconv(.C) void;
 
-const REFCOUNT_MAX_ISIZE: comptime isize = 0;
-pub const REFCOUNT_ONE_ISIZE: comptime isize = std.math.minInt(isize);
+const REFCOUNT_MAX_ISIZE: isize = 0;
+pub const REFCOUNT_ONE_ISIZE: isize = std.math.minInt(isize);
 pub const REFCOUNT_ONE: usize = @bitCast(usize, REFCOUNT_ONE_ISIZE);
 
 pub const IntWidth = enum(u8) {
@@ -71,44 +71,6 @@ pub const IntWidth = enum(u8) {
     Usize,
 };
 
-pub fn intWidth(width: IntWidth) anytype {
-    switch (width) {
-        IntWidth.U8 => {
-            return u8;
-        },
-        IntWidth.U16 => {
-            return u16;
-        },
-        IntWidth.U32 => {
-            return u32;
-        },
-        IntWidth.U64 => {
-            return u64;
-        },
-        IntWidth.U128 => {
-            return u128;
-        },
-        IntWidth.I8 => {
-            return i8;
-        },
-        IntWidth.I16 => {
-            return i16;
-        },
-        IntWidth.I32 => {
-            return i32;
-        },
-        IntWidth.I64 => {
-            return i64;
-        },
-        IntWidth.I128 => {
-            return i128;
-        },
-        IntWidth.Usize => {
-            return usize;
-        },
-    }
-}
-
 pub fn decref(
     bytes_or_null: ?[*]u8,
     data_bytes: usize,
@@ -148,7 +110,7 @@ pub fn allocateWithRefcount(
     data_bytes: usize,
     alignment: u32,
 ) [*]u8 {
-    comptime const result_in_place = false;
+    const result_in_place = false;
 
     switch (alignment) {
         16 => {
diff --git a/compiler/builtins/build.rs b/compiler/builtins/build.rs
index c157c1cef7..05f5aa284c 100644
--- a/compiler/builtins/build.rs
+++ b/compiler/builtins/build.rs
@@ -9,6 +9,23 @@ use std::str;
 
 fn main() {
     let out_dir = env::var_os("OUT_DIR").unwrap();
+    let dest_obj_path = Path::new(&out_dir).join("builtins.o");
+    let dest_obj = dest_obj_path.to_str().expect("Invalid dest object path");
+
+    println!("cargo:rerun-if-changed=build.rs");
+    println!("cargo:rustc-env=BUILTINS_O={}", dest_obj);
+
+    // When we build on Netlify, zig is not installed (but also not used,
+    // since all we're doing is generating docs), so we can skip the steps
+    // that require having zig installed.
+    if env::var_os("NO_ZIG_INSTALLED").is_some() {
+        // We still need to do the other things before this point, because
+        // setting the env vars is needed for other parts of the build.
+        return;
+    }
+
+    let big_sur_path = "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib";
+    let use_build_script = Path::new(big_sur_path).exists();
 
     // "." is relative to where "build.rs" is
     let build_script_dir_path = fs::canonicalize(Path::new(".")).unwrap();
@@ -16,35 +33,31 @@ fn main() {
 
     let src_obj_path = bitcode_path.join("builtins.o");
     let src_obj = src_obj_path.to_str().expect("Invalid src object path");
-    println!("Compiling zig object to: {}", src_obj);
 
-    run_command(&bitcode_path, "zig", &["build", "object", "-Drelease=true"]);
+    let dest_ir_path = bitcode_path.join("builtins.ll");
+    let dest_ir = dest_ir_path.to_str().expect("Invalid dest ir path");
+
+    if use_build_script {
+        println!("Compiling zig object & ir to: {} and {}", src_obj, dest_ir);
+        run_command_with_no_args(&bitcode_path, "./build.sh");
+    } else {
+        println!("Compiling zig object to: {}", src_obj);
+        run_command(&bitcode_path, "zig", &["build", "object", "-Drelease=true"]);
+
+        println!("Compiling ir to: {}", dest_ir);
+        run_command(&bitcode_path, "zig", &["build", "ir", "-Drelease=true"]);
+    }
 
-    let dest_obj_path = Path::new(&out_dir).join("builtins.o");
-    let dest_obj = dest_obj_path.to_str().expect("Invalid dest object path");
     println!("Moving zig object to: {}", dest_obj);
 
     run_command(&bitcode_path, "mv", &[src_obj, dest_obj]);
 
-    let dest_ir_path = bitcode_path.join("builtins.ll");
-    let dest_ir = dest_ir_path.to_str().expect("Invalid dest ir path");
-    println!("Compiling ir to: {}", dest_ir);
-
-    run_command(&bitcode_path, "zig", &["build", "ir", "-Drelease=true"]);
-
-    let dest_bc_path = Path::new(&out_dir).join("builtins.bc");
+    let dest_bc_path = bitcode_path.join("builtins.bc");
     let dest_bc = dest_bc_path.to_str().expect("Invalid dest bc path");
     println!("Compiling bitcode to: {}", dest_bc);
 
-    run_command(
-        build_script_dir_path,
-        "llvm-as-10",
-        &[dest_ir, "-o", dest_bc],
-    );
+    run_command(build_script_dir_path, "llvm-as", &[dest_ir, "-o", dest_bc]);
 
-    println!("cargo:rerun-if-changed=build.rs");
-    println!("cargo:rustc-env=BUILTINS_BC={}", dest_bc);
-    println!("cargo:rustc-env=BUILTINS_O={}", dest_obj);
     get_zig_files(bitcode_path.as_path(), &|path| {
         let path: &Path = path;
         println!(
@@ -79,6 +92,25 @@ where
     }
 }
 
+fn run_command_with_no_args<P: AsRef<Path>>(path: P, command_str: &str) {
+    let output_result = Command::new(OsStr::new(&command_str))
+        .current_dir(path)
+        .output();
+    match output_result {
+        Ok(output) => match output.status.success() {
+            true => (),
+            false => {
+                let error_str = match str::from_utf8(&output.stderr) {
+                    Ok(stderr) => stderr.to_string(),
+                    Err(_) => format!("Failed to run \"{}\"", command_str),
+                };
+                panic!("{} failed: {}", command_str, error_str);
+            }
+        },
+        Err(reason) => panic!("{} failed: {}", command_str, reason),
+    }
+}
+
 fn get_zig_files(dir: &Path, cb: &dyn Fn(&Path)) -> io::Result<()> {
     if dir.is_dir() {
         for entry in fs::read_dir(dir)? {
diff --git a/compiler/builtins/docs/Bool.roc b/compiler/builtins/docs/Bool.roc
index 601a3d491f..077909e84d 100644
--- a/compiler/builtins/docs/Bool.roc
+++ b/compiler/builtins/docs/Bool.roc
@@ -55,36 +55,36 @@ and : Bool, Bool -> Bool
 ##
 ## In some languages, `&&` and `||` are special-cased in the compiler to skip
 ## evaluating the expression after the operator under certain circumstances.
-## # In Roc, this is not the case. See the performance notes for #Bool.and for details.
+## # In Roc, this is not the case. See the performance notes for [Bool.and] for details.
 or : Bool, Bool -> Bool
 
 ## Exclusive or
 xor : Bool, Bool -> Bool
 
+# TODO: removed `'` from signature because parser does not support it yet
+# Original signature: `isEq : 'val, 'val -> Bool`
 ## Returns `True` if the two values are *structurally equal*, and `False` otherwise.
 ##
 ## `a == b` is shorthand for `Bool.isEq a b`
 ##
 ## Structural equality works as follows:
 ##
-## 1. #Int and #Float values are equal if their numbers are equal.
-## 2. Records are equal if all their fields are equal.
-## 3. Global tags are equal if they are the same tag, and also their contents (if any) are equal.
-## 4. Private tags are equal if they are the same tag, in the same module, and also their contents (if any) are equal.
-## 5. Collections (#String, #List, #Map, #Set, and #Bytes) are equal if they are the same length, and also all their corresponding elements are equal.
+## 1. Global tags are equal if they are the same tag, and also their contents (if any) are equal.
+## 2. Private tags are equal if they are the same tag, in the same module, and also their contents (if any) are equal.
+## 3. Records are equal if all their fields are equal.
+## 4. Collections ([Str], [List], [Dict], and [Set]) are equal if they are the same length, and also all their corresponding elements are equal.
+## 5. [Num] values are equal if their numbers are equal, with one exception: if both arguments to `isEq` are *NaN*, then `isEq` returns `False`. See [Num.isNaN] for more about *NaN*.
 ##
 ## Note that `isEq` takes `'val` instead of `val`, which means `isEq` does not
 ## accept arguments whose types contain functions.
-# TODO: removed `'` from signature because parser does not support it yet
-# Original signature: `isEq : 'val, 'val -> Bool`
 isEq : val, val -> Bool
 
-## Calls #eq on the given values, then calls #not on the result.
+# TODO: removed `'` from signature because parser does not support it yet
+# Original signature: `isNotEq : 'val, 'val -> Bool`
+## Calls [isEq] on the given values, then calls [not] on the result.
 ##
 ## `a != b` is shorthand for `Bool.isNotEq a b`
 ##
 ## Note that `isNotEq` takes `'val` instead of `val`, which means `isNotEq` does not
 ## accept arguments whose types contain functions.
-# TODO: removed `'` from signature because parser does not support it yet
-# Original signature: `isNotEq : 'val, 'val -> Bool`
 isNotEq : val, val -> Bool
diff --git a/compiler/builtins/docs/Defaults.roc b/compiler/builtins/docs/Defaults.roc
deleted file mode 100644
index 046ba7dc3f..0000000000
--- a/compiler/builtins/docs/Defaults.roc
+++ /dev/null
@@ -1,7 +0,0 @@
-interface Defaults
-    exposes []
-    imports [
-        Dict.{ Dict },
-        Set.{ Set },
-        Num.{ Num, Int, Float }
-    ]
diff --git a/compiler/builtins/docs/Dict.roc b/compiler/builtins/docs/Dict.roc
index 9486764194..3aad9e4eee 100644
--- a/compiler/builtins/docs/Dict.roc
+++ b/compiler/builtins/docs/Dict.roc
@@ -19,3 +19,15 @@ map :
     Dict beforeKey beforeValue,
     ({ key: beforeKey, value: beforeValue } -> { key: afterKey, value: afterValue })
     -> Dict afterKey afterValue
+
+# DESIGN NOTES: The reason for panicking when given NaN is that:
+# * If we allowed NaN in, Dict.insert would no longer be idempotent.
+# * If we allowed NaN but overrode its semantics to make it feel like "NaN == NaN" we'd need isNaN checks in all hashing operations as well as all equality checks (during collision detection), not just insert. This would be much worse for performance than panicking on insert, which only requires one extra conditional on insert.
+# * It's obviously invalid; the whole point of NaN is that an error occurred. Giving a runtime error notifies you when this problem happens. Giving it only on insert is the best for performance, because it means you aren't paying for isNaN checks on lookups as well.
+
+# TODO: removed `'` from signature because parser does not support it yet
+# Original signature: insert : Dict 'key val, 'key, val -> Dict 'key val
+## Make sure never to insert a key of *NaN* into a [Dict]! Because *NaN* is
+## defined to be unequal to *NaN*, inserting a *NaN* key results in an entry
+## that can never be retrieved or removed from the [Dict].
+insert : Dict key val, key, val -> Dict key val
diff --git a/compiler/builtins/docs/List.roc b/compiler/builtins/docs/List.roc
index 02866d5d37..2f3248da4b 100644
--- a/compiler/builtins/docs/List.roc
+++ b/compiler/builtins/docs/List.roc
@@ -62,7 +62,7 @@ interface List2
 ## the same type. If you want to put a mix of #Int and #Str values into a list, try this:
 ##
 ## ```
-## mixedList : List [ IntElem Int, StrElem Str ]*
+## mixedList : List [ IntElem I64, StrElem Str ]*
 ## mixedList = [ IntElem 1, IntElem 2, StrElem "a", StrElem "b" ]
 ## ```
 ##
@@ -180,7 +180,7 @@ interface List2
 ## we can free it immediately because there are no other refcounts. However,
 ## in the case of `lists`, we have to iterate through the list and decrement
 ## the refcounts of each of its contained lists - because they, too, have
-## refcounts! Importantly, beacuse the first element had its refcount incremented
+## refcounts! Importantly, because the first element had its refcount incremented
 ## because the function returned `first`, that element will actually end up
 ## *not* getting freed at the end - but all the others will be.
 ##
@@ -232,9 +232,28 @@ reverse : List elem -> List elem
 
 ## Sorts a list using a function which specifies how two elements are ordered.
 ##
-##
+## When sorting by numeric values, it's more efficient to use [sortAsc] or
+## [sortDesc] instead.
 sort : List elem, (elem, elem -> [ Lt, Eq, Gt ]) -> List elem
 
+## Sorts a list in ascending order (lowest to highest), using a function which
+## specifies a way to represent each element as a number.
+##
+## This is more efficient than [sort] because it skips
+## calculating the `[ Lt, Eq, Gt ]` value and uses the number directly instead.
+##
+## To sort in descending order (highest to lowest), use [List.sortDesc] instead.
+sortAsc : List elem, (elem -> Num *) -> List elem
+
+## Sorts a list in descending order (highest to lowest), using a function which
+## specifies a way to represent each element as a number.
+##
+## This is more efficient than [sort] because it skips
+## calculating the `[ Lt, Eq, Gt ]` value and uses the number directly instead.
+##
+## To sort in ascending order (lowest to highest), use [List.sortAsc] instead.
+sortDesc : List elem, (elem -> Num *) -> List elem
+
 ## Convert each element in the list to something new, by calling a conversion
 ## function on each of them. Then return a new list of the converted values.
 ##
@@ -248,7 +267,7 @@ map : List before, (before -> after) -> List after
 
 ## This works like #List.map, except it also passes the index
 ## of the element to the conversion function.
-mapWithIndex : List before, (before, Int -> after) -> List after
+mapWithIndex : List before, (before, Nat -> after) -> List after
 
 ## This works like #List.map, except at any time you can return `Err` to
 ## cancel the entire operation immediately, and return that #Err.
@@ -279,7 +298,7 @@ update : List elem, Nat, (elem -> elem) -> List elem
 
 ## A more flexible version of #List.update, which returns an "updater" function
 ## that lets you delay performing the update until later.
-updater : List elem, Nat -> { elem, new : elem -> List elem }
+updater : List elem, Nat -> { elem, new : (elem -> List elem) }
 
 ## If all the elements in the list are #Ok, return a new list containing the
 ## contents of those #Ok tags. If any elements are #Err, return #Err.
@@ -629,7 +648,7 @@ walk : List elem, { start : state, step : (state, elem -> state) } -> state
 
 ## Note that in other languages, `walkBackwards` is sometimes called `reduceRight`,
 ## `fold`, `foldRight`, or `foldr`.
-walkBackwards : List elem, { start : state, step : (state, elem -> state ]) } -> state
+walkBackwards : List elem, { start : state, step : (state, elem -> state) } -> state
 
 ## Same as #List.walk, except you can stop walking early.
 ##
diff --git a/compiler/builtins/docs/Num.roc b/compiler/builtins/docs/Num.roc
index 813fe34e23..db8d41bd7f 100644
--- a/compiler/builtins/docs/Num.roc
+++ b/compiler/builtins/docs/Num.roc
@@ -9,13 +9,24 @@ interface Num2
 ## This is useful for functions that can work on either, for example #Num.add, whose type is:
 ##
 ## ```
-## add : Num range, Num range -> Num range
+## add : Num a, Num a -> Num a
 ## ```
 ##
-## The number 1.5 technically has the type `Num FloatingPoint`, so when you pass two of them to `Num.add`, the answer you get is `3.0 : Num FloatingPoint`.
+## The number 1.5 technically has the type `Num (Fraction *)`, so when you pass
+## two of them to [Num.add], the answer you get is `3.0 : Num (Fraction *)`.
+#
+## Similarly, the number 0x1 (that is, the integer 1 in hexadecimal notation)
+## technically has the type `Num (Integer *)`, so when you pass two of them to
+## [Num.add], the answer you get is `2 : Num (Integer *)`.
 ##
-## The type #Float is defined to be an alias for `Num FloatingPoint`, so `3.0 : Num FloatingPoint` is the same answer as `3.0 : Float`.  # # Similarly, the number 1 technically has the type `Num Integer`, so when you pass two of them to `Num.add`, the answer you get is `2 : Num Integer`.  # # The type #Int is defined to be an alias for `Num Integer`, so `2 : Num Integer` is the same answer as `2 : Int`.  #
-## In this way, the `Num` type makes it possible to have `1 + 1` return `2 : Int` and `1.5 + 1.5` return `3.0 : Float`.
+## The type [`Frac a`](#Frac) is defined to be an alias for `Num (Fraction a)`,
+## so `3.0 : Num (Fraction *)` is the same value as `3.0 : Frac *`.
+## Similarly, the type [`Int a`](#Int) is defined to be an alias for
+## `Num (Integer a)`, so `2 : Num (Integer *)` is the same value as
+## `2 : Int *`.
+##
+## In this way, the [Num] type makes it possible to have `1 + 0x1` return
+## `2 : Int *` and `1.5 + 1.5` return `3.0 : Frac`.
 ##
 ## ## Number Literals
 ##
@@ -29,29 +40,30 @@ interface Num2
 ## ends up having the type `Nat`.
 ##
 ## Sometimes number literals don't become more specific. For example,
-## the #Num.toStr function has the type `Num * -> Str`. This means that
+## the [Num.toStr] function has the type `Num * -> Str`. This means that
 ## when calling `Num.toStr (5 + 6)`, the expression `(5 + 6)`
 ## still has the type `Num *`. When this happens, `Num *` defaults to
-## being an #I32 - so this addition expression would overflow
+## being an [I64] - so this addition expression would overflow
 ## if either 5 or 6 were replaced with a number big enough to cause
-## addition overflow on an #I32.
+## addition overflow on an [I64] value.
 ##
-## If this default of #I32 is not big enough for your purposes,
-## you can add an `i64` to the end of the number literal, like so:
+## If this default of [I64] is not big enough for your purposes,
+## you can add an `i128` to the end of the number literal, like so:
 ##
-## >>> Num.toStr 5_000_000_000i64
+## >>> Num.toStr 5_000_000_000i128
 ##
-## This `i64` suffix specifies that you want this number literal to be
-## an #I64 instead of a `Num *`. All the other numeric types have
-## suffixes just like `i64`; here are some other examples:
+## This `i128` suffix specifies that you want this number literal to be
+## an [I128] instead of a `Num *`. All the other numeric types have
+## suffixes just like `i128`; here are some other examples:
 ##
-## * `215u8` is a `215` value of type #U8
-## * `76.4f32` is a `76.4` value of type #F32
-## * `12345ulen` is a `12345` value of type #Nat
+## * `215u8` is a `215` value of type [U8]
+## * `76.4f32` is a `76.4` value of type [F32]
+## * `123.45dec` is a `123.45` value of type [Dec]
+## * `12345nat` is a `12345` value of type [Nat]
 ##
 ## In practice, these are rarely needed. It's most common to write
 ## number literals without any suffix.
-Num range : [ @Num range ]
+Num a : [ @Num a ]
 
 ## A decimal number.
 ##
@@ -110,6 +122,30 @@ Dec : Frac [ @Decimal128 ]
 ## been done in a base-2 floating point calculation, which causes noticeable
 ## precision loss in this case.
 ##
+## The floating-point numbers ([F32] and [F64]) also have three values which
+## are not ordinary [finite numbers](https://en.wikipedia.org/wiki/Finite_number).
+## They are:
+## * ∞ ([infinity](https://en.wikipedia.org/wiki/Infinity))
+## * -∞ (negative infinity)
+## * *NaN* ([not a number](https://en.wikipedia.org/wiki/NaN))
+##
+## These values are different from ordinary numbers in that they only occur
+## when a floating-point calculation encounters an error. For example:
+## * Dividing a positive [F64] by `0.0` returns ∞.
+## * Dividing a negative [F64] by `0.0` returns -∞.
+## * Dividing a [F64] of `0.0` by `0.0` returns [*NaN*](Num.isNaN).
+##
+## These rules come from the [IEEE-754](https://en.wikipedia.org/wiki/IEEE_754)
+## floating point standard. Because almost all modern processors are built to
+## this standard, deviating from these rules has a significant performance
+## cost! Since the most common reason to choose [F64] or [F32] over [Dec] is
+## access to hardware-accelerated performance, Roc follows these rules exactly.
+##
+## There's no literal syntax for these error values, but you can check to see if
+## you ended up with one of them by using [isNaN], [isFinite], and [isInfinite].
+## Whenever a function in this module could return one of these values, that
+## possibility is noted in the function's documentation.
+##
 ## ## Performance Notes
 ##
 ## On typical modern CPUs, performance is similar between [Dec], [F64], and [F32]
@@ -128,7 +164,7 @@ Dec : Frac [ @Decimal128 ]
 ## an even bigger performance difference. [F32] and [F64] can do these in a
 ## single instruction, whereas [Dec] needs entire custom procedures - which use
 ## loops and conditionals. If you need to do performance-critical trigonometry
-## or square roots, either [F32] or [F64] is probably a better choice than the
+## or square roots, either [F64] or [F32] is probably a better choice than the
 ## usual default choice of [Dec], despite the precision problems they bring.
 Frac a : Num [ @Fraction a ]
 
@@ -194,6 +230,17 @@ I64 : Int [ @Signed64 ]
 U64 : Int [ @Unsigned64 ]
 I128 : Int [ @Signed128 ]
 U128 : Int [ @Unsigned128 ]
+
+## A [natural number](https://en.wikipedia.org/wiki/Natural_number) represented
+## as a 64-bit unsigned integer on 64-bit systems, a 32-bit unsigned integer
+## on 32-bit systems, and so on.
+##
+## This system-specific size makes it useful for certain data structure
+## functions like [List.len], because the number of elements many data strucures
+## can hold is also system-specific. For example, the maximum number of elements
+## a [List] can hold on a 64-bit system fits in a 64-bit unsigned integer, and
+## on a 32-bit system it fits in 32-bit unsigned integer. This makes [Nat] a
+## good fit for [List.len] regardless of system.
 Nat : Int [ @Natural ]
 
 ## A 64-bit signed integer. All number literals without decimal points are compatible with #Int values.
@@ -272,148 +319,6 @@ Nat : Int [ @Natural ]
 ## If you need to do math outside these bounds, consider using a larger numeric size.
 Int size : Num [ @Int size ]
 
-## A 64-bit floating-point number. All number literals with decimal points are #Float values.
-##
-## >>> 0.1
-##
-## >>> 1.0
-##
-## >>> 0.0
-##
-## If you like, you can put underscores in your #Float literals.
-## They have no effect on the number's value, but can make things easier to read.
-##
-## >>> 1_000_000.000_000_001
-##
-## Roc supports two types of floating-point numbers:
-##
-## - *Decimal* floating-point numbers
-## - *Binary* floating-point numbers
-##
-## Decimal floats are precise for decimal calculations. For example:
-##
-## >>> 0.1 + 0.2
-##
-## Operations on binary floats tend to run *much* faster than operations on
-## decimal floats, because almost all processors have dedicated instructions
-## for binary floats and not for decimal floats.
-## However, binary floats are less precise for decimal calculations.
-##
-## For example, here is the same `0.1 + 0.2` calculation again, this time putting
-## `f64` after the numbers to specify that they should be #F64 binary floats
-## instead of the default of decimal floats.
-##
-## >>> 0.1f64 + 0.2f64
-##
-## If decimal precision is unimportant, binary floats give better performance.
-## If decimal precision is important - for example, when representing money -
-## decimal floats tend to be worth the performance cost.
-##
-## Usually, Roc's compiler can infer a more specific type than #Float for
-## a particular float value, based on how it is used with other numbers. For example:
-##
-## >>> coordinates : { x : F32, y : F32 }
-## >>> coordinates = { x: 1, y: 2.5 }
-## >>>
-## >>> coordinates.x + 1
-##
-## On the last line, the compiler infers that the `1` in `+ 1` is an #F32
-## beacuse it's being added to `coordinates.x`, which was defined to be an #F32
-## on the first line.
-##
-## Sometimes the compiler has no information about which specific type to pick.
-## For example:
-##
-## >>> 0.1 + 0.2 == 0.3
-##
-## When this happens, the compiler defaults to choosing #D64 decimal floats.
-## If you want something else, you can write (for example) `0.1f32 + 0.2 == 0.3`
-## to compare them as #F32 values instead.
-##
-## Both decimal and binary #Float values conform to the [IEEE-754](https://en.wikipedia.org/wiki/IEEE_754#Interchange_formats)
-## specification for floating point numbers. Conforming to this specification
-## means Roc's binary floats have nearly universal hardware support, and its
-## decimal floats have [some hardware support](http://speleotrove.com/decimal/)
-## among the rare processors which support decimal float instructions at all.
-##
-## This specification covers several float formats. Here are the ones Roc supports:
-##
-## - #F32 (32-bit binary float)
-## - #F64 (64-bit binary float)
-## - #D32 (32-bit decimal float)
-## - #D64 (64-bit decimal float) # TODO show a table like we do with ints, with the min/max ranges
-##
-## Like #Int, it's possible for #Float operations to overflow. Like with ints,
-## you'll typically get a crash when this happens.
-##
-## * In a development build, you'll get an assertion failure.
-## * In an optimized build, you'll get [`Infinity` or `-Infinity`](https://en.wikipedia.org/wiki/IEEE_754-1985#Positive_and_negative_infinity).
-##
-## Although some languages treat have first-class representations for
-## `-Infinity`, `Infinity`, and the special `NaN` ("not a number")
-## floating-point values described in the IEEE-754, Roc does not.
-## Instead, Roc treats all of these as errors. If any Float operation
-## in a development build encounters one of these values, it will
-## result in an assertion failure.
-##
-## Stll, it's possible that these values may accidentally arise in
-## release builds. If this happens, they will behave according to the usual
-## IEEE-754 rules: any operation involving `NaN` will output `NaN`,
-## any operation involving `Infinity` or `-Infinity` will output either
-## `Infinity`, `-Infinity`, or `NaN`, and `NaN` is defined to be not
-## equal to itself - meaning `(x == x)` returns `False` if `x` is `NaN`.
-##
-## These are very error-prone values, so if you see an assertion fail in
-## developent because of one of them, take it seriously - and try to fix
-## the code so that it can't come up in a release!
-##
-## ## Loud versus Quiet errors
-##
-## Besides precision problems, another reason floats are error-prone
-## is that they have quiet error handling built in. For example, in
-## a 64-bit floating point number, there are certain patterns of those
-## 64 bits which do not represent valid floats; instead, they represent
-## invalid results of previous operations.
-##
-## Whenever any arithmetic operation is performed on an invalid float,
-## the result is also invalid. This is called *error propagation*, and
-## it is notoriously error-prone. In Roc, using equality operations like
-## `==` and `!=` on an invalid float causes a crash. (See #Float.verify
-## to check the validity of your float.)
-##
-## Beause invalid floats are so error-prone, Roc discourages using them.
-## Instead, by default it treats them the same way as overflow: by
-## crashing whenever any #Float function would otherwise return one.
-## You can also use functions like #Float.tryAdd to get an `Ok` or an error
-## back so you can gracefully recover from invalid values.
-##
-## Quiet errors can be useful sometimes. For example, you might want to
-## do three floating point calculations in a row, and then gracefully handle
-## the situation where any one of the three was invalid. In that situation,
-## quiet errors can be more efficient than using three `try` functions, because
-## it can have one condition at the end instead of three along the way.
-##
-## Another potential use for quiet errors is for risky performance optimizations.
-## When you are absolutely certain there is no chance of overflow or other
-## errors, using a *quiet* operation may save an entry in the instruction cache
-## by removing a branch that would always have been predicted correctly.
-## Always [measure the performance change](https://youtu.be/r-TLSBdHe1A)
-## if you do this! The compiler can optimize away those branches behind the scenes,
-## so you may find that using the quiet version expliitly
-## makes the code riskier to future change, without actually affecting performance.
-##
-## ## Performance Notes
-##
-## Currently, loud errors are implemented using an extra conditional. Although
-## this conditional will always be correctly branh-predicted unless an error
-## occurs, there is a small effect on the instruction cache, which means
-## quiet errors are very slightly more efficient.
-##
-## Long-term, it's possible that the Roc compiler may be able to implement
-## loud errors using *signalling errors* in some situations, which could
-## eliminate the performance difference between loud and quiet errors in
-## the situation where no error occurs.
-
 ## Convert
 
 ## Return a negative number when given a positive one, and vice versa.
@@ -426,16 +331,16 @@ Int size : Num [ @Int size ]
 ##
 ## >>> Num.neg 0.0
 ##
-## This is safe to use with any #Float, but it can cause overflow when used with certain #Int values.
+## This is safe to use with any #Frac, but it can cause overflow when used with certain #Int values.
 ##
 ## For example, calling #Num.neg on the lowest value of a signed integer (such as #Int.lowestI64 or #Int.lowestI32) will cause overflow.
 ## This is because, for any given size of signed integer (32-bit, 64-bit, etc.) its negated lowest value turns out to be 1 higher than
 ## the highest value it can represent. (For this reason, calling #Num.abs on the lowest signed value will also cause overflow.)
 ##
-## Additionally, calling #Num.neg on any unsigned integer (such as any #U64 or #U32 value) other than 0 will cause overflow.
+## Additionally, calling #Num.neg on any unsigned integer (such as any #U64 or #U32 value) other than zero will cause overflow.
 ##
-## (It will never crash when given a #Float, however, because of how floating point numbers represent positive and negative numbers.)
-neg : Num range -> Num range
+## (It will never crash when given a #Frac, however, because of how floating point numbers represent positive and negative numbers.)
+neg : Num a -> Num a
 
 ## Return the absolute value of the number.
 ##
@@ -451,14 +356,14 @@ neg : Num range -> Num range
 ##
 ## >>> Num.abs 0.0
 ##
-## This is safe to use with any #Float, but it can cause overflow when used with certain #Int values.
+## This is safe to use with any #Frac, but it can cause overflow when used with certain #Int values.
 ##
 ## For example, calling #Num.abs on the lowest value of a signed integer (such as #Int.lowestI64 or #Int.lowestI32) will cause overflow.
 ## This is because, for any given size of signed integer (32-bit, 64-bit, etc.) its negated lowest value turns out to be 1 higher than
 ## the highest value it can represent. (For this reason, calling #Num.neg on the lowest signed value will also cause overflow.)
 ##
 ## Calling this on an unsigned integer (like #U32 or #U64) never does anything.
-abs : Num range -> Num range
+abs : Num a -> Num a
 
 ## Check
 
@@ -485,7 +390,7 @@ isOdd : Num * -> Bool
 
 ## Add two numbers of the same type.
 ##
-## (To add an #Int and a #Float, first convert one so that they both have the same type. There are functions in the [`Float`](/Float) module that can convert both #Int to #Float and the other way around.)
+## (To add an #Int and a #Frac, first convert one so that they both have the same type. There are functions in the [`Frac`](/Frac) module that can convert both #Int to #Frac and the other way around.)
 ##
 ## `a + b` is shorthand for `Num.add a b`.
 ##
@@ -495,13 +400,24 @@ isOdd : Num * -> Bool
 ##
 ## `Num.add` can be convenient in pipelines.
 ##
-## >>> Float.pi
+## >>> Frac.pi
 ## >>>     |> Num.add 1.0
-add : Num range, Num range -> Num range
+##
+## If the answer to this operation can't fit in the return value (e.g. an
+## [I8] answer that's higher than 127 or lower than -128), the result is an
+## *overflow*. For [F64] and [F32], overflow results in an answer of either
+## ∞ or -∞. For all other number types, overflow results in a panic.
+add : Num a, Num a -> Num a
+
+## Add two numbers and check for overflow.
+##
+## This is the same as [Num.add] except if the operation overflows, instead of
+## panicking or returning ∞ or -∞, it will return `Err Overflow`.
+addCheckOverflow : Num a, Num a -> Result (Num a) [ Overflow ]*
 
 ## Subtract two numbers of the same type.
 ##
-## (To subtract an #Int and a #Float, first convert one so that they both have the same type. There are functions in the [`Float`](/Float) module that can convert both #Int to #Float and the other way around.)
+## (To subtract an #Int and a #Frac, first convert one so that they both have the same type. There are functions in the [`Frac`](/Frac) module that can convert both #Int to #Frac and the other way around.)
 ##
 ## `a - b` is shorthand for `Num.sub a b`.
 ##
@@ -511,13 +427,24 @@ add : Num range, Num range -> Num range
 ##
 ## `Num.sub` can be convenient in pipelines.
 ##
-## >>> Float.pi
+## >>> Frac.pi
 ## >>>     |> Num.sub 2.0
-sub : Num range, Num range -> Num range
+##
+## If the answer to this operation can't fit in the return value (e.g. an
+## [I8] answer that's higher than 127 or lower than -128), the result is an
+## *overflow*. For [F64] and [F32], overflow results in an answer of either
+## ∞ or -∞. For all other number types, overflow results in a panic.
+sub : Num a, Num a -> Num a
+
+## Subtract two numbers and check for overflow.
+##
+## This is the same as [Num.sub] except if the operation overflows, instead of
+## panicking or returning ∞ or -∞, it will return `Err Overflow`.
+subCheckOverflow : Num a, Num a -> Result (Num a) [ Overflow ]*
 
 ## Multiply two numbers of the same type.
 ##
-## (To multiply an #Int and a #Float, first convert one so that they both have the same type. There are functions in the [`Float`](/Float) module that can convert both #Int to #Float and the other way around.)
+## (To multiply an #Int and a #Frac, first convert one so that they both have the same type. There are functions in the [`Frac`](/Frac) module that can convert both #Int to #Frac and the other way around.)
 ##
 ## `a * b` is shorthand for `Num.mul a b`.
 ##
@@ -527,36 +454,115 @@ sub : Num range, Num range -> Num range
 ##
 ## `Num.mul` can be convenient in pipelines.
 ##
-## >>> Float.pi
+## >>> Frac.pi
 ## >>>     |> Num.mul 2.0
-mul : Num range, Num range -> Num range
+##
+## If the answer to this operation can't fit in the return value (e.g. an
+## [I8] answer that's higher than 127 or lower than -128), the result is an
+## *overflow*. For [F64] and [F32], overflow results in an answer of either
+## ∞ or -∞. For all other number types, overflow results in a panic.
+mul : Num a, Num a -> Num a
+
+## Multiply two numbers and check for overflow.
+##
+## This is the same as [Num.mul] except if the operation overflows, instead of
+## panicking or returning ∞ or -∞, it will return `Err Overflow`.
+mulCheckOverflow : Num a, Num a -> Result (Num a) [ Overflow ]*
 
 ## Convert
 
-## Convert a number to a string, formatted as the traditional base 10 (decimal).
+## Convert a number to a [Str].
+##
+## This is the same as calling `Num.format {}` - so for more details on
+## exact formatting, see [Num.format].
 ##
 ## >>> Num.toStr 42
 ##
-## Only #Float values will include a decimal point, and they will always include one.
+## Only #Frac values will include a decimal point, and they will always include one.
 ##
 ## >>> Num.toStr 4.2
 ##
 ## >>> Num.toStr 4.0
 ##
-## For other bases see #toHexStr, #toOctalStr, and #toBinaryStr.
+## When this function is given a non-[finite](Num.isFinite)
+## [F64] or [F32] value, the returned string will be `"NaN"`, `"∞"`, or `"-∞"`.
+##
+## To get strings in hexadecimal, octal, or binary format, use [Num.format].
 toStr : Num * -> Str
 
+## Convert a number into a [Str], formatted with the given options.
+##
+## Default options:
+## * `base: Decimal`
+## * `notation: Standard`
+## * `decimalMark: HideForIntegers "."`
+## * `decimalDigits: { min: 0, max: All }`
+## * `minIntDigits: 1`
+## * `wholeSep: { mark: ",", places: 3 }`
+##
+## ## Options
+##
+##
+## ### decimalMark
+##
+## * `AlwaysShow` always shows the decimal mark, no matter what.
+## * `HideForIntegers` hides the decimal mark if all the numbers after the decimal mark are 0.
+##
+## The [Str] included in either of these represents the mark itself.
+##
+## ### `decimalDigits
+##
+## With 0 decimal digits, the decimal mark will still be rendered if
+## `decimalMark` is set to `AlwaysShow`.
+##
+## If `max` is less than `min`, then first the number will be truncated to `max`
+## digits, and then zeroes will be added afterwards until it reaches `min` digits.
+##
+## >>> Num.format 1.23 { decPlaces: 0, decPointVis: AlwaysShow }
+##
+## ### minIntDigits
+##
+## If the integer portion of number is fewer than this many digits, zeroes will
+## be added in front of it until there are at least `minWholeDigits` digits.
+##
+## If this is set to zero, then numbers less than 1 will begin with `"."`
+## rather than `"0."`.
+##
+## ### wholeSep
+##
+## Examples:
+##
+## In some countries (e.g. USA and UK), a comma is used to separate thousands:
+## >>> Num.format 1_000_000 { base: Decimal, wholeSep: { mark: ",", places: 3 } }
+##
+## Sometimes when rendering bits, it's nice to group them into groups of 4:
+## >>> Num.format 1_000_000 { base: Binary, wholeSep: { mark: " ", places: 4 } }
+##
+## It's also common to render hexadecimal in groups of 2:
+## >>> Num.format 1_000_000 { base: Hexadecimal, wholeSep: { mark: " ", places: 2 } }
+format :
+    Num *,
+    {
+        base ? [ Decimal, Hexadecimal, Octal, Binary ],
+        notation ? [ Standard, Scientific ],
+        decimalMark ? [ AlwaysShow Str, HideForIntegers ],
+        decimalDigits ? { min : U16, max : [ All, Trunc U16, Round U16, Floor U16, Ceil U16 ] },
+        minWholeDigits ? U16,
+        wholeSep ? { mark : Str, places : U64 }
+    }
+    -> Str
+
 ## Round off the given float to the nearest integer.
-round : Float * -> Int *
-ceil : Float * -> Int *
-floor : Float * -> Int *
-trunc : Float * -> Int *
+round : Frac * -> Int *
+ceil : Frac * -> Int *
+floor : Frac * -> Int *
+trunc : Frac * -> Int *
 
 ## Convert an #Int to a #Nat. If the given number doesn't fit in #Nat, it will be truncated.
 ## Since #Nat has a different maximum number depending on the system you're building
 ## for, this may give a different answer on different systems.
 ##
-## For example, on a 32-bit sytem, #Num.maxNat will return the same answer as
+## For example, on a 32-bit system, #Num.maxNat will return the same answer as
 ## #Num.maxU32. This means that calling `Num.toNat 9_000_000_000` on a 32-bit
 ## system will return #Num.maxU32 instead of 9 billion, because 9 billion is
 ## higher than #Num.maxU32 and will not fit in a #Nat on a 32-bit system.
@@ -565,13 +571,13 @@ trunc : Float * -> Int *
 ## the #Nat value of 9_000_000_000. This is because on a 64-bit system, #Nat can
 ## hold up to #Num.maxU64, and 9_000_000_000 is lower than #Num.maxU64.
 ##
-## To convert a #Float to a #Nat, first call either #Num.round, #Num.ceil, or #Num.floor
+## To convert a #Frac to a #Nat, first call either #Num.round, #Num.ceil, or #Num.floor
 ## on it, then call this on the resulting #Int.
 toNat : Int * -> Nat
 
 ## Convert an #Int to an #I8. If the given number doesn't fit in #I8, it will be truncated.
 ##
-## To convert a #Float to an #I8, first call either #Num.round, #Num.ceil, or #Num.floor
+## To convert a #Frac to an #I8, first call either #Num.round, #Num.ceil, or #Num.floor
 ## on it, then call this on the resulting #Int.
 toI8 : Int * -> I8
 toI16 : Int * -> I16
@@ -595,13 +601,9 @@ toF32 : Num * -> F32
 ## there will be a loss of precision.
 toF64 : Num * -> F64
 
-## Convert a #Num to a #D32. If the given number can't be precisely represented in a #D32,
+## Convert a #Num to a #Dec. If the given number can't be precisely represented in a #Dec,
 ## there will be a loss of precision.
-toD32 : Num * -> D32
-
-## Convert a #Num to a #D64. If the given number can't be precisely represented in a #D64,
-## there will be a loss of precision.
-toD64 : Num * -> D64
+toDec : Num * -> Dec
 
 ## Divide two integers and #Num.round  the resulut.
 ##
@@ -624,16 +626,16 @@ toD64 : Num * -> D64
 ## >>> Num.divRound 8 -3
 ##
 ## This is the same as the #// operator.
-divRound : Int, Int -> Int
+divRound : Int a, Int a -> Int a
 
 ## Perform flooring modulo on two integers.
 ##
 ## Modulo is the same as remainder when working with positive numbers,
 ## but if either number is negative, then modulo works differently.
 ##
-## Additionally, flooring modulo uses #Float.floor on the result.
+## Additionally, flooring modulo uses #Frac.floor on the result.
 ##
-## (Use #Float.mod for non-flooring modulo.)
+## (Use #Frac.mod for non-flooring modulo.)
 ##
 ## Return `Err DivByZero` if the second integer is zero, because division by zero is undefined in mathematics.
 ##
@@ -646,40 +648,16 @@ divRound : Int, Int -> Int
 ## >>> -8 %% -3
 ##
 ## >>> Int.modFloor -8 -3
-#modFloor : Int, Int -> Result DivByZero Int
+#modFloor : Int a, Int a -> Result (Int a) [ DivByZero ]*
 
 
 ## Bitwise
 
-xor : Int, Int -> Int
+xor : Int a, Int a -> Int a
 
-and : Int, Int -> Int
+and : Int a, Int a -> Int a
 
-not : Int -> Int
-
-## Sort ascending - that is, with the lowest first, and the highest last.
-##
-##     List.sort Num.asc [ 3, 6, 0 ]
-##
-asc : Num a, Num a -> [ Eq, Lt, Gt ]
-
-## Sort descending - that is, with the highest first, and the lowest last.
-##
-##     List.sort Num.desc [ 3, 6, 0 ]
-##
-desc : Num a, Num a -> [ Eq, Lt, Gt ]
-
-## TODO should we offer hash32 etc even if someday it has to do a hash64 and truncate?
-##
-## This function can crash under these circumstances:
-##
-## * It receives a function, or any type that contains a function (for example a record, tag, or #List containing a function)
-## * It receives an erroneous #Float (`NaN`, `Infinity`, or `-Infinity` - these values can only originate from hosts)
-##
-## CAUTION: This function may give different answers in future releases of Roc,
-## so be aware that if you rely on the exact answer this gives today, your
-## code may break in a future Roc release.
-hash64 : a -> U64
+not : Int a -> Int a
 
 ## Limits
 
@@ -738,88 +716,81 @@ maxU32 : U32
 ## and zero is the lowest unsigned number. Unsigned numbers cannot be negative.
 minU32 : U32
 
-## The highest supported #Float value you can have, which is approximately 1.8 × 10^308.
+## The highest supported #F64 value you can have, which is approximately 1.8 × 10^308.
 ##
 ## If you go higher than this, your running Roc code will crash - so be careful not to!
-maxF64 : Float *
+maxF64 : F64
 
-## The lowest supported #Float value you can have, which is approximately -1.8 × 10^308.
+## The lowest supported #F64 value you can have, which is approximately -1.8 × 10^308.
 ##
 ## If you go lower than this, your running Roc code will crash - so be careful not to!
-minF64 : Float *
+minF64 : F64
 
-## The highest integer that can be represented as a #Float without # losing precision.
-## It is equal to 2^53, which is approximately 9 × 10^15.
+## The highest supported #F32 value you can have, which is approximately 1.8 × 10^308.
 ##
-## Some integers higher than this can be represented, but they may lose precision. For example:
-##
-## >>> Float.highestInt
-##
-## >>> Float.highestInt + 100 # Increasing may lose precision
-##
-## >>> Float.highestInt - 100 # Decreasing is fine - but watch out for lowestLosslessInt!
-maxPreciseInt : Float *
+## If you go higher than this, your running Roc code will crash - so be careful not to!
+maxF32 : F32
 
-## The lowest integer that can be represented as a #Float without losing precision.
-## It is equal to -2^53, which is approximately -9 × 10^15.
+## The lowest supported #F32 value you can have, which is approximately -1.8 × 10^308.
 ##
-## Some integers lower than this can be represented, but they may lose precision. For example:
+## If you go lower than this, your running Roc code will crash - so be careful not to!
+minF32 : F32
+
+## The highest supported #F64 value you can have, which is approximately 1.8 × 10^308.
 ##
-## >>> Float.lowestIntVal
+## If you go higher than this, your running Roc code will crash - so be careful not to!
+maxDec : Dec
+
+## The lowest supported #F64 value you can have, which is approximately -1.8 × 10^308.
 ##
-## >>> Float.lowestIntVal - 100 # Decreasing may lose precision
-##
-## >>> Float.lowestIntVal + 100 # Increasing is fine - but watch out for highestInt!
-maxPreciseInt : Float *
+## If you go lower than this, your running Roc code will crash - so be careful not to!
+maxDec : Dec
 
 ## Constants
 
 ## An approximation of e, specifically 2.718281828459045.
-e : Float *
+e : Frac *
 
 ## An approximation of pi, specifically 3.141592653589793.
-pi : Float *
-## Constants
-
-## An approximation of e, specifically 2.718281828459045.
-e : Float *
-
-## An approximation of pi, specifically 3.141592653589793.
-pi : Float *
-
-#ceiling : Float -> Int
-
-#floor : Float -> Int
+pi : Frac *
 
 ## Trigonometry
 
-#cos : Float -> Float
+cos : Frac a -> Frac a
 
-#acos : Float -> Float
+acos : Frac a -> Frac a
 
-#sin : Float -> Float
+sin : Frac a -> Frac a
 
-#asin : Float -> Float
+asin : Frac a -> Frac a
 
-#tan : Float -> Float
+tan : Frac a -> Frac a
 
-#atan : Float -> Float
+atan : Frac a -> Frac a
 
 ## Other Calculations (arithmetic?)
 
-## Divide two #Float numbers.
+## Divide one [Frac] by another.
 ##
 ## `a / b` is shorthand for `Num.div a b`.
 ##
-## Division by zero is undefined in mathematics. As such, you should make
-## sure never to pass zero as the denomaintor to this function!
+## [Division by zero is undefined in mathematics](https://en.wikipedia.org/wiki/Division_by_zero).
+## As such, you should make sure never to pass zero as the denomaintor to this function!
+## Calling [div] on a [Dec] denominator of zero will cause a panic.
 ##
-## If zero does get passed as the denominator...
+## Calling [div] on [F32] and [F64] values follows these rules:
+## * Dividing a positive [F64] or [F32] by zero returns ∞.
+## * Dividing a negative [F64] or [F32] by zero returns -∞.
+## * Dividing a zero [F64] or [F32] by zero returns [*NaN*](Num.isNaN).
 ##
-## * In a development build, you'll get an assertion failure.
-## * In a release build, the function will return `Infinity`, `-Infinity`, or `NaN` depending on the arguments.
+## > These rules come from the [IEEE-754](https://en.wikipedia.org/wiki/IEEE_754)
+## > floating point standard. Because almost all modern processors are built to
+## > this standard, deviating from these rules has a significant performance
+## > cost! Since the most common reason to choose [F64] or [F32] over [Dec] is
+## > access to hardware-accelerated performance, Roc follows these rules exactly.
 ##
-## To divide an #Int and a #Float, first convert the #Int to a #Float using one of the functions in this module.
+## To divide an [Int] and a [Frac], first convert the [Int] to a [Frac] using
+## one of the functions in this module like [toDec].
 ##
 ## >>> 5.0 / 7.0
 ##
@@ -827,45 +798,47 @@ pi : Float *
 ##
 ## `Num.div` can be convenient in pipelines.
 ##
-## >>> Float.pi
+## >>> Num.pi
 ## >>>     |> Num.div 2.0
-#div : Float, Float -> Result Float DivByZero
-div = \numerator, denominator ->
-    when numerator is
-        0.0 -> 0.0 # TODO return Result!
-        _ -> denominator
+div : Frac a, Frac a -> Frac a
 
-## Perform modulo on two #Float numbers.
+## Perform modulo on two [Frac]s.
 ##
 ## Modulo is the same as remainder when working with positive numbers,
 ## but if either number is negative, then modulo works differently.
 ##
-## Return `Err DivByZero` if the second number is zero, because division by zero is undefined in mathematics.
+## `a % b` is shorthand for `Num.mod a b`.
 ##
-## `a % b` is shorthand for `Float.mod a b`.
+## [Division by zero is undefined in mathematics](https://en.wikipedia.org/wiki/Division_by_zero),
+## and as such, so is modulo by zero. Because of this, you should make sure never
+## to pass zero for the second argument to this function!
+##
+## Passing [mod] a [Dec] value of zero for its second argument will cause a panic.
+## Passing [mod] a [F32] and [F64] value for its second argument will cause it
+## to return [*NaN*](Num.isNaN).
 ##
 ## >>> 5.0 % 7.0
 ##
-## >>> Float.mod 5 7
+## >>> Num.mod 5 7
 ##
-## `Float.mod` can be convenient in pipelines.
+## `Num.mod` can be convenient in pipelines.
 ##
-## >>> Float.pi
-## >>>     |> Float.mod 2.0
-mod : Float a, Float a -> Result (Float a) [ DivByZero ]*
+## >>> Num.pi
+## >>>     |> Num.mod 2.0
+mod : Frac a, Frac a -> Frac a
 
-## Raises a #Float to the power of another #Float.
+## Raises a #Frac to the power of another #Frac.
 ##
 ## `
 ## For an #Int alternative to this function, see #Num.raise.
-pow : Float a, Float a -> Float a
+pow : Frac a, Frac a -> Frac a
 
 ## Raises an integer to the power of another, by multiplying the integer by
 ## itself the given number of times.
 ##
 ## This process is known as [exponentiation by squaring](https://en.wikipedia.org/wiki/Exponentiation_by_squaring).
 ##
-## For a #Float alternative to this function, which supports negative exponents,
+## For a #Frac alternative to this function, which supports negative exponents,
 ## see #Num.exp.
 ##
 ## >>> Num.exp 5 0
@@ -882,33 +855,187 @@ pow : Float a, Float a -> Float a
 ## overflow
 expBySquaring : Int a, U8 -> Int a
 
-## Return the reciprocal of a #Float - that is, divides `1.0` by the given number.
+## Returns an approximation of the absolute value of a [Frac]'s square root.
 ##
-## Crashes if given `0.0`, because division by zero is undefined in mathematics.
+## The square root of a negative number is an irrational number, and [Frac] only
+## supports rational numbers. As such, you should make sure never to pass this
+## function a negative number! Calling [sqrt] on a negative [Dec] will cause a panic.
 ##
-## For a version that does not crash, use #tryRecip
-recip : Float a -> Result (Float a) [ DivByZero ]*
+## Calling [sqrt] on [F32] and [F64] values follows these rules:
+## * Passing a negative [F64] or [F32] returns [*NaN*](Num.isNaN).
+## * Passing [*NaN*](Num.isNaN) or -∞ also returns [*NaN*](Num.isNaN).
+## * Passing ∞ returns ∞.
+##
+## > These rules come from the [IEEE-754](https://en.wikipedia.org/wiki/IEEE_754)
+## > floating point standard. Because almost all modern processors are built to
+## > this standard, deviating from these rules has a significant performance
+## > cost! Since the most common reason to choose [F64] or [F32] over [Dec] is
+## > access to hardware-accelerated performance, Roc follows these rules exactly.
+##
+## >>> Frac.sqrt 4.0
+##
+## >>> Frac.sqrt 1.5
+##
+## >>> Frac.sqrt 0.0
+##
+## >>> Frac.sqrt -4.0f64
+##
+## >>> Frac.sqrt -4.0dec
+sqrt : Frac a -> Frac a
 
-## NOTE: Need to come up a suffix alternative to the "try" prefix.
-## This should be like (for example) recipTry so that it's more discoverable
-## in documentation and editor autocomplete when you type "recip"
-tryRecip : Float a -> Result (Float a) [ DivByZero ]*
+## Bit shifts
 
-## Return an approximation of the absolute value of the square root of the #Float.
+## [Logical bit shift](https://en.wikipedia.org/wiki/Bitwise_operation#Logical_shift) left.
 ##
-## Return #InvalidSqrt if given a negative number or an invalid #Float. The square root of a negative number is an irrational number, and #Float only supports rational numbers.
+## `a << b` is shorthand for `Num.shl a b`.
+shl : Int a, Int a -> Int a
+
+## [Arithmetic bit shift](https://en.wikipedia.org/wiki/Bitwise_operation#Arithmetic_shift) left.
 ##
-## >>> Float.sqrt 4.0
+## This is called `shlWrap` because any bits shifted
+## off the beginning of the number will be wrapped around to
+## the end. (In contrast, [shl] replaces discarded bits with zeroes.)
+shlWrap : Int a, Int a -> Int a
+
+## [Logical bit shift](https://en.wikipedia.org/wiki/Bitwise_operation#Logical_shift) right.
 ##
-## >>> Float.sqrt 1.5
+## `a >> b` is shorthand for `Num.shr a b`.
+shr : Int a, Int a -> Int a
+
+## [Arithmetic bit shift](https://en.wikipedia.org/wiki/Bitwise_operation#Arithmetic_shift) right.
 ##
-## >>> Float.sqrt 0.0
-##
-## >>> Float.sqrt -4.0
-sqrt : Float a -> [Ok (Float a), InvalidSqrt]*
+## This is called `shlWrap` because any bits shifted
+## off the end of the number will be wrapped around to
+## the beginning. (In contrast, [shr] replaces discarded bits with zeroes.)
+shrWrap : Int a, Int a -> Int a
 
 
 ## [Endianness](https://en.wikipedia.org/wiki/Endianness)
 Endi : [ Big, Little ]
 
+## The [Endi] argument does not matter for [U8] and [I8], since they have
+## only one byte.
 toBytes : Num *, Endi -> List U8
+
+## when Num.parseBytes bytes Big is
+##     Ok { val: f64, rest } -> ...
+##     Err (ExpectedNum (Float Binary64)) -> ...
+parseBytes : List U8, Endi -> Result { val : Num a, rest : List U8 } [ ExpectedNum a ]*
+
+## when Num.fromBytes bytes Big is
+##     Ok f64 -> ...
+##     Err (ExpectedNum (Float Binary64)) -> ...
+fromBytes : List U8, Endi -> Result (Num a) [ ExpectedNum a ]*
+
+## Comparison
+
+## Returns `True` if the first number is less than the second.
+##
+## `a < b` is shorthand for `Num.isLt a b`.
+##
+## If either argument is [*NaN*](Num.isNaN), returns `False` no matter what. (*NaN*
+## is [defined to be unordered](https://en.wikipedia.org/wiki/NaN#Comparison_with_NaN).)
+##
+## >>> 5
+## >>>     |> Num.isLt 6
+isLt : Num a, Num a -> Bool
+
+## Returns `True` if the first number is less than or equal to the second.
+##
+## `a <= b` is shorthand for `Num.isLte a b`.
+##
+## If either argument is [*NaN*](Num.isNaN), returns `False` no matter what. (*NaN*
+## is [defined to be unordered](https://en.wikipedia.org/wiki/NaN#Comparison_with_NaN).)
+isLte : Num a, Num a -> Bool
+
+## Returns `True` if the first number is greater than the second.
+##
+## `a > b` is shorthand for `Num.isGt a b`.
+##
+## If either argument is [*NaN*](Num.isNaN), returns `False` no matter what. (*NaN*
+## is [defined to be unordered](https://en.wikipedia.org/wiki/NaN#Comparison_with_NaN).)
+##
+## >>> 6
+## >>>     |> Num.isGt 5
+isGt : Num a, Num a -> Bool
+
+## Returns `True` if the first number is greater than or equal to the second.
+##
+## `a >= b` is shorthand for `Num.isGte a b`.
+##
+## If either argument is [*NaN*](Num.isNaN), returns `False` no matter what. (*NaN*
+## is [defined to be unordered](https://en.wikipedia.org/wiki/NaN#Comparison_with_NaN).)
+isGte : Num a, Num a -> Bool
+
+## Returns the higher of two numbers.
+##
+## If either argument is [*NaN*](Num.isNaN), returns `False` no matter what. (*NaN*
+## is [defined to be unordered](https://en.wikipedia.org/wiki/NaN#Comparison_with_NaN).)
+higher : Num a, Num a -> Num a
+
+## Returns the lower of two numbers.
+##
+## If either argument is [*NaN*](Num.isNaN), returns `False` no matter what. (*NaN*
+## is [defined to be unordered](https://en.wikipedia.org/wiki/NaN#Comparison_with_NaN).)
+lower : Num a, Num a -> Num a
+
+# Branchless implementation that works for all numeric types:
+#
+# let is_lt = arg1 < arg2;
+# let is_eq = arg1 == arg2;
+# return (is_lt as i8 - is_eq as i8) + 1;
+#
+# 1, 1 -> (0 - 1) + 1 == 0 # Eq
+# 5, 1 -> (0 - 0) + 1 == 1 # Gt
+# 1, 5 -> (1 - 0) + 1 == 2 # Lt
+
+## Returns `Lt` if the first number is less than the second, `Gt` if
+## the first is greater than the second, and `Eq` if they're equal.
+##
+## Although this can be passed to [List.sort], you'll get better performance
+## by using [List.sortAsc] or [List.sortDesc] instead.
+compare : Num a, Num a -> [ Lt, Eq, Gt ]
+
+## Special Floating-Point Values
+
+## When given a [F64] or [F32] value, returns `False` if that value is
+## [*NaN*](Num.isNaN), ∞ or -∞, and `True` otherwise.
+##
+## Always returns `True` when given a [Dec].
+##
+## This is the opposite of [isInfinite], except when given [*NaN*](Num.isNaN). Both
+## [isFinite] and [isInfinite] return `False` for [*NaN*](Num.isNaN).
+isFinite : Frac * -> Bool
+
+## When given a [F64] or [F32] value, returns `True` if that value is either
+## ∞ or -∞, and `False` otherwise.
+##
+## Always returns `False` when given a [Dec].
+##
+## This is the opposite of [isFinite], except when given [*NaN*](Num.isNaN). Both
+## [isFinite] and [isInfinite] return `False` for [*NaN*](Num.isNaN).
+isInfinite : Frac * -> Bool
+
+## When given a [F64] or [F32] value, returns `True` if that value is
+## *NaN* ([not a number](https://en.wikipedia.org/wiki/NaN)), and `False` otherwise.
+##
+## Always returns `False` when given a [Dec].
+##
+## >>> Num.isNaN 12.3
+##
+## >>> Num.isNaN (Num.sqrt -2)
+##
+## *NaN* is unusual from other numberic values in that:
+## * *NaN* is not equal to any other number, even itself. [Bool.isEq] always returns `False` if either argument is *NaN*.
+## * *NaN* has no ordering, so [isLt], [isLte], [isGt], and [isGte] always return `False` if either argument is *NaN*.
+##
+## These rules come from the [IEEE-754](https://en.wikipedia.org/wiki/IEEE_754)
+## floating point standard. Because almost all modern processors are built to
+## this standard, deviating from these rules has a significant performance
+## cost! Since the most common reason to choose [F64] or [F32] over [Dec] is
+## access to hardware-accelerated performance, Roc follows these rules exactly.
+##
+## Note that you should never put a *NaN* into a [Set], or use it as the key in
+## a [Dict]. The result is entries that can never be removed from those
+## collections! See the documentation for [Set.add] and [Dict.insert] for details.
+isNaN : Frac * -> Bool
diff --git a/compiler/builtins/docs/Set.roc b/compiler/builtins/docs/Set.roc
index bc5211b407..24c29dac0e 100644
--- a/compiler/builtins/docs/Set.roc
+++ b/compiler/builtins/docs/Set.roc
@@ -18,6 +18,9 @@ len : Set * -> Nat
 
 # TODO: removed `'` from signature because parser does not support it yet
 # Original signature: `add : Set 'elem, 'elem -> Set 'elem`
+## Make sure never to add a *NaN* to a [Set]! Because *NaN* is defined to be
+## unequal to *NaN*, adding a *NaN* results in an entry that can never be
+## retrieved or removed from the [Set].
 add : Set elem, elem -> Set elem
 
 ## Drops the given element from the set.
diff --git a/compiler/builtins/docs/Str.roc b/compiler/builtins/docs/Str.roc
index 4789db17b2..3a5ed0c88e 100644
--- a/compiler/builtins/docs/Str.roc
+++ b/compiler/builtins/docs/Str.roc
@@ -194,7 +194,7 @@ startsWith : Str, Str -> Bool
 ## if you want to check whether a string begins with something that's representable
 ## in a single code point, you can use (for example) `Str.startsWithCodePoint '鹏'`
 ## instead of `Str.startsWithCodePoint "鹏"`. ('鹏' evaluates to the [U32]
-## value `40527`.) This will not work for graphemes which take up mulitple code
+## value `40527`.) This will not work for graphemes which take up multiple code
 ## points, however; `Str.startsWithCodePoint '👩‍👩‍👦‍👦'` would be a compiler error
 ## because 👩‍👩‍👦‍👦 takes up multiple code points and cannot be represented as a
 ## single [U32]. You'd need to use `Str.startsWithCodePoint "🕊"` instead.
@@ -427,18 +427,68 @@ chomp : Str, Str -> Result Str [ Expected [ ExactStr Str ]* Str ]*
 ## equal to the given [U32], return whatever comes after that code point.
 chompCodePoint : Str, U32 -> Result Str [ Expected [ ExactCodePoint U32 ]* Str ]*
 
-## If the string begins with digits which can represent a valid #U8, return
-## that number along with the rest of the string after the digits.
-parseU8 : Str -> Result { val : U8, rest : Str } [ Expected [ NumU8 ]* Str ]*
-parseI8 : Str -> Result { val : I8, rest : Str } [ Expected [ NumI8 ]* Str ]*
-parseU16 : Str -> Result { val : U16, rest : Str } [ Expected [ NumU16 ]* Str ]*
-parseI16 : Str -> Result { val : I16, rest : Str } [ Expected [ NumI16 ]* Str ]*
-parseU32 : Str -> Result { val : U32, rest : Str } [ Expected [ NumU32 ]* Str ]*
-parseI32 : Str -> Result { val : I32, rest : Str } [ Expected [ NumI32 ]* Str ]*
-parseU64 : Str -> Result { val : U64, rest : Str } [ Expected [ NumU64 ]* Str ]*
-parseI64 : Str -> Result { val : I64, rest : Str } [ Expected [ NumI64 ]* Str ]*
-parseU128 : Str -> Result { val : U128, rest : Str } [ Expected [ NumU128 ]* Str ]*
-parseI128 : Str -> Result { val : I128, rest : Str } [ Expected [ NumI128 ]* Str ]*
+## If the string represents a valid #U8 number, return that number.
+##
+## For more advanced options, see [parseU8].
+toU8 : Str -> Result U8 [ InvalidU8 ]*
+toI8 : Str -> Result I8 [ InvalidI8 ]*
+toU16 : Str -> Result U16 [ InvalidU16 ]*
+toI16 : Str -> Result I16 [ InvalidI16 ]*
+toU32 : Str -> Result U32 [ InvalidU32 ]*
+toI32 : Str -> Result I32 [ InvalidI32 ]*
+toU64 : Str -> Result U64 [ InvalidU64 ]*
+toI64 : Str -> Result I64 [ InvalidI64 ]*
+toU128 : Str -> Result U128 [ InvalidU128 ]*
+toI128 : Str -> Result I128 [ InvalidI128 ]*
+toF64 : Str -> Result U128 [ InvalidF64 ]*
+toF32 : Str -> Result I128 [ InvalidF32 ]*
+toDec : Str -> Result Dec [ InvalidDec ]*
 
-parseF64 : Str -> Result { val : U128, rest : Str } [ Expected [ NumF64 ]* Str ]*
-parseF32 : Str -> Result { val : I128, rest : Str } [ Expected [ NumF32 ]* Str ]*
+## If the string represents a valid number, return that number.
+##
+## The exact number type to look for will be inferred from usage. Here's an
+## example where the `Err` branch matches `Integer Signed64`, which causes this to
+## parse an [I64] because [I64] is defined as `I64 : Num [ Integer [ Signed64 ] ]`.
+##
+## >>> when Str.toNum "12345" is
+## >>>     Ok i64 -> "The I64 was: \(i64)"
+## >>>     Err (ExpectedNum (Integer Signed64)) -> "Not a valid I64!"
+##
+## If the string is exactly `"NaN"`, `"∞"`, or `"-∞"`, they will be accepted
+## only when converting to [F64] or [F32] numbers, and will be translated accordingly.
+##
+## This never accepts numbers with underscores or commas in them. For more
+## advanced options, see [parseNum].
+toNum : Str -> Result (Num a) [ ExpectedNum a ]*
+
+## If the string begins with an [Int] or a [finite](Num.isFinite) [Frac], return
+## that number along with the rest of the string after it.
+##
+## The exact number type to look for will be inferred from usage. Here's an
+## example where the `Err` branch matches `Float Binary64`, which causes this to
+## parse an [F64] because [F64] is defined as `F64 : Num [ Fraction [ Float64 ] ]`.
+##
+## >>> when Str.parseNum input {} is
+## >>>     Ok { val: f64, rest } -> "The F64 was: \(f64)"
+## >>>     Err (ExpectedNum (Fraction Float64)) -> "Not a valid F64!"
+##
+## If the string begins with `"NaN"`, `"∞"`, and `"-∞"` (which do not represent
+## [finite](Num.isFinite) numbers), they will be accepted only when parsing
+## [F64] or [F32] numbers, and translated accordingly.
+parseNum : Str, NumParseConfig -> Result { val : Num a, rest : Str } [ ExpectedNum a ]*
+
+## Notes:
+## * You can allow a decimal mark for integers; they'll only parse if the numbers after it are all 0.
+## * For `wholeSep`, `Required` has a payload for how many digits (e.g. "required every 3 digits")
+## * For `wholeSep`, `Allowed` allows the separator to appear anywhere.
+NumParseConfig :
+    {
+        base ? [ Decimal, Hexadecimal, Octal, Binary ],
+        notation ? [ Standard, Scientific, Any ],
+        decimalMark ? [ Allowed Str, Required Str, Disallowed ],
+        decimalDigits ? [ Any, AtLeast U16, Exactly U16 ],
+        wholeDigits ? [ Any, AtLeast U16, Exactly U16 ],
+        leadingZeroes ? [ Allowed, Disallowed ],
+        trailingZeroes ? [ Allowed, Disallowed ],
+        wholeSep ? { mark : Str, policy : [ Allowed, Required U64 ] }
+    }
diff --git a/compiler/builtins/src/bitcode.rs b/compiler/builtins/src/bitcode.rs
index 732244d4ae..4ac7d0f0f3 100644
--- a/compiler/builtins/src/bitcode.rs
+++ b/compiler/builtins/src/bitcode.rs
@@ -1,28 +1,8 @@
-use std::fs::File;
-use std::io::prelude::Read;
-use std::vec::Vec;
-
-const BC_PATH: &str = env!(
-    "BUILTINS_BC",
-    "Env var BUILTINS_BC not found. Is there a problem with the build script?"
-);
-
 pub const OBJ_PATH: &str = env!(
     "BUILTINS_O",
     "Env var BUILTINS_O not found. Is there a problem with the build script?"
 );
 
-pub fn get_bytes() -> Vec<u8> {
-    // In the build script for the builtins module, we compile the builtins bitcode and set
-    // BUILTINS_BC to the path to the compiled output.
-    let mut builtins_bitcode = File::open(BC_PATH).expect("Unable to find builtins bitcode source");
-    let mut buffer = Vec::new();
-    builtins_bitcode
-        .read_to_end(&mut buffer)
-        .expect("Unable to read builtins bitcode");
-    buffer
-}
-
 pub const NUM_ASIN: &str = "roc_builtins.num.asin";
 pub const NUM_ACOS: &str = "roc_builtins.num.acos";
 pub const NUM_ATAN: &str = "roc_builtins.num.atan";
@@ -77,6 +57,7 @@ pub const LIST_CONTAINS: &str = "roc_builtins.list.contains";
 pub const LIST_REPEAT: &str = "roc_builtins.list.repeat";
 pub const LIST_APPEND: &str = "roc_builtins.list.append";
 pub const LIST_DROP: &str = "roc_builtins.list.drop";
+pub const LIST_SWAP: &str = "roc_builtins.list.swap";
 pub const LIST_SINGLE: &str = "roc_builtins.list.single";
 pub const LIST_JOIN: &str = "roc_builtins.list.join";
 pub const LIST_RANGE: &str = "roc_builtins.list.range";
@@ -84,3 +65,4 @@ pub const LIST_REVERSE: &str = "roc_builtins.list.reverse";
 pub const LIST_SORT_WITH: &str = "roc_builtins.list.sort_with";
 pub const LIST_CONCAT: &str = "roc_builtins.list.concat";
 pub const LIST_SET: &str = "roc_builtins.list.set";
+pub const LIST_SET_IN_PLACE: &str = "roc_builtins.list.set_in_place";
diff --git a/compiler/builtins/src/std.rs b/compiler/builtins/src/std.rs
index f6c4b07b19..158f0523b8 100644
--- a/compiler/builtins/src/std.rs
+++ b/compiler/builtins/src/std.rs
@@ -292,7 +292,7 @@ pub fn types() -> MutMap<Symbol, (SolvedType, Region)> {
     // minInt : Int range
     add_type!(Symbol::NUM_MIN_INT, int_type(flex(TVAR1)));
 
-    // div : Int, Int -> Result Int [ DivByZero ]*
+    // divInt : Int a, Int a -> Result (Int a) [ DivByZero ]*
     let div_by_zero = SolvedType::TagUnion(
         vec![(TagName::Global("DivByZero".into()), vec![])],
         Box::new(SolvedType::Wildcard),
@@ -856,6 +856,13 @@ pub fn types() -> MutMap<Symbol, (SolvedType, Region)> {
         Box::new(list_type(flex(TVAR1))),
     );
 
+    // swap : List elem, Nat, Nat -> List elem
+    add_top_level_function_type!(
+        Symbol::LIST_SWAP,
+        vec![list_type(flex(TVAR1)), nat_type(), nat_type()],
+        Box::new(list_type(flex(TVAR1))),
+    );
+
     // prepend : List elem, elem -> List elem
     add_top_level_function_type!(
         Symbol::LIST_PREPEND,
diff --git a/compiler/can/src/builtins.rs b/compiler/can/src/builtins.rs
index 7283316ba9..b6d13be1da 100644
--- a/compiler/can/src/builtins.rs
+++ b/compiler/can/src/builtins.rs
@@ -30,7 +30,7 @@ macro_rules! macro_magic {
 /// Some builtins cannot be constructed in code gen alone, and need to be defined
 /// as separate Roc defs. For example, List.get has this type:
 ///
-/// List.get : List elem, Int -> Result elem [ OutOfBounds ]*
+/// List.get : List elem, Nat -> Result elem [ OutOfBounds ]*
 ///
 /// Because this returns an open tag union for its Err type, it's not possible
 /// for code gen to return a hardcoded value for OutOfBounds. For example,
@@ -85,6 +85,7 @@ pub fn builtin_defs_map(symbol: Symbol, var_store: &mut VarStore) -> Option<Def>
         LIST_MAP2 => list_map2,
         LIST_MAP3 => list_map3,
         LIST_DROP => list_drop,
+        LIST_SWAP => list_swap,
         LIST_MAP_WITH_INDEX => list_map_with_index,
         LIST_KEEP_IF => list_keep_if,
         LIST_KEEP_OKS => list_keep_oks,
@@ -449,7 +450,7 @@ fn num_add(symbol: Symbol, var_store: &mut VarStore) -> Def {
     num_binop(symbol, var_store, LowLevel::NumAdd)
 }
 
-/// Num.addWrap : Int, Int -> Int
+/// Num.addWrap : Int a, Int a -> Int a
 fn num_add_wrap(symbol: Symbol, var_store: &mut VarStore) -> Def {
     num_binop(symbol, var_store, LowLevel::NumAddWrap)
 }
@@ -548,7 +549,7 @@ fn num_sub(symbol: Symbol, var_store: &mut VarStore) -> Def {
     num_binop(symbol, var_store, LowLevel::NumSub)
 }
 
-/// Num.subWrap : Int, Int -> Int
+/// Num.subWrap : Int a, Int a -> Int a
 fn num_sub_wrap(symbol: Symbol, var_store: &mut VarStore) -> Def {
     num_binop(symbol, var_store, LowLevel::NumSubWrap)
 }
@@ -647,7 +648,7 @@ fn num_mul(symbol: Symbol, var_store: &mut VarStore) -> Def {
     num_binop(symbol, var_store, LowLevel::NumMul)
 }
 
-/// Num.mulWrap : Int, Int -> Int
+/// Num.mulWrap : Int a, Int a -> Int a
 fn num_mul_wrap(symbol: Symbol, var_store: &mut VarStore) -> Def {
     num_binop(symbol, var_store, LowLevel::NumMulWrap)
 }
@@ -1151,7 +1152,7 @@ fn num_ceiling(symbol: Symbol, var_store: &mut VarStore) -> Def {
     )
 }
 
-/// Num.powInt : Int, Int -> Int
+/// Num.powInt : Int a, Int a -> Int a
 fn num_pow_int(symbol: Symbol, var_store: &mut VarStore) -> Def {
     let int_var = var_store.fresh();
 
@@ -1250,17 +1251,17 @@ fn num_asin(symbol: Symbol, var_store: &mut VarStore) -> Def {
     )
 }
 
-/// Num.bitwiseAnd : Int, Int -> Int
+/// Num.bitwiseAnd : Int a, Int a -> Int a
 fn num_bitwise_and(symbol: Symbol, var_store: &mut VarStore) -> Def {
     num_binop(symbol, var_store, LowLevel::NumBitwiseAnd)
 }
 
-/// Num.bitwiseXor : Int, Int -> Int
+/// Num.bitwiseXor : Int a, Int a -> Int a
 fn num_bitwise_xor(symbol: Symbol, var_store: &mut VarStore) -> Def {
     num_binop(symbol, var_store, LowLevel::NumBitwiseXor)
 }
 
-/// Num.bitwiseOr: Int, Int -> Int
+/// Num.bitwiseOr: Int a, Int a -> Int a
 fn num_bitwise_or(symbol: Symbol, var_store: &mut VarStore) -> Def {
     num_binop(symbol, var_store, LowLevel::NumBitwiseOr)
 }
@@ -1666,7 +1667,7 @@ fn list_concat(symbol: Symbol, var_store: &mut VarStore) -> Def {
     )
 }
 
-/// List.repeat : elem, Int -> List elem
+/// List.repeat : elem, Nat -> List elem
 fn list_repeat(symbol: Symbol, var_store: &mut VarStore) -> Def {
     let elem_var = var_store.fresh();
     let len_var = var_store.fresh();
@@ -1808,7 +1809,7 @@ fn list_get(symbol: Symbol, var_store: &mut VarStore) -> Def {
     )
 }
 
-/// List.set : List elem, Int, elem -> List elem
+/// List.set : List elem, Nat, elem -> List elem
 ///
 /// List.set :
 ///     Attr (w | u | v) (List (Attr u a)),
@@ -1882,6 +1883,36 @@ fn list_set(symbol: Symbol, var_store: &mut VarStore) -> Def {
         list_ret_var,
     )
 }
+
+/// List.swap : List elem, Nat, Nat -> List elem
+fn list_swap(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    let list_var = var_store.fresh();
+    let index1_var = var_store.fresh();
+    let index2_var = var_store.fresh();
+
+    let body = RunLowLevel {
+        op: LowLevel::ListSwap,
+        args: vec![
+            (list_var, Var(Symbol::ARG_1)),
+            (index1_var, Var(Symbol::ARG_2)),
+            (index2_var, Var(Symbol::ARG_3)),
+        ],
+        ret_var: list_var,
+    };
+
+    defn(
+        symbol,
+        vec![
+            (list_var, Symbol::ARG_1),
+            (index1_var, Symbol::ARG_2),
+            (index2_var, Symbol::ARG_3),
+        ],
+        var_store,
+        body,
+        list_var,
+    )
+}
+
 /// List.drop : List elem, Nat -> List elem
 fn list_drop(symbol: Symbol, var_store: &mut VarStore) -> Def {
     let list_var = var_store.fresh();
@@ -2267,7 +2298,7 @@ fn dict_get(symbol: Symbol, var_store: &mut VarStore) -> Def {
 
     let make_err = tag(
         "Err",
-        vec![tag("OutOfBounds", Vec::new(), var_store)],
+        vec![tag("KeyNotFound", Vec::new(), var_store)],
         var_store,
     );
 
@@ -2500,7 +2531,7 @@ fn set_walk(symbol: Symbol, var_store: &mut VarStore) -> Def {
     )
 }
 
-/// Num.rem : Int, Int -> Result Int [ DivByZero ]*
+/// Num.rem : Int a, Int a -> Result (Int a) [ DivByZero ]*
 fn num_rem(symbol: Symbol, var_store: &mut VarStore) -> Def {
     let num_var = var_store.fresh();
     let unbound_zero_var = var_store.fresh();
@@ -2559,7 +2590,7 @@ fn num_rem(symbol: Symbol, var_store: &mut VarStore) -> Def {
     )
 }
 
-/// Num.isMultipleOf : Int, Int -> Bool
+/// Num.isMultipleOf : Int a, Int a -> Bool
 fn num_is_multiple_of(symbol: Symbol, var_store: &mut VarStore) -> Def {
     lowlevel_2(symbol, LowLevel::NumIsMultipleOf, var_store)
 }
@@ -2665,7 +2696,7 @@ fn num_div_float(symbol: Symbol, var_store: &mut VarStore) -> Def {
     )
 }
 
-/// Num.div : Int, Int -> Result Int [ DivByZero ]*
+/// Num.div : Int a , Int a -> Result (Int a) [ DivByZero ]*
 fn num_div_int(symbol: Symbol, var_store: &mut VarStore) -> Def {
     let bool_var = var_store.fresh();
     let num_var = var_store.fresh();
diff --git a/compiler/can/src/def.rs b/compiler/can/src/def.rs
index 6f464cda1a..b4a4652924 100644
--- a/compiler/can/src/def.rs
+++ b/compiler/can/src/def.rs
@@ -561,14 +561,14 @@ pub fn sort_can_defs(
                     )));
 
                     declarations.push(Declaration::InvalidCycle(entries));
-
-                    // other groups may depend on the symbols defined here, so
-                    // also push this cycle onto the groups
-                    groups.push(cycle);
-                } else {
-                    // slightly inefficient, because we know this becomes exactly one DeclareRec already
-                    groups.push(cycle);
                 }
+
+                // if it's an invalid cycle, other groups may depend on the
+                // symbols defined here, so also push this cycle onto the groups
+                //
+                // if it's not an invalid cycle, this is slightly inefficient,
+                // because we know this becomes exactly one DeclareRec already
+                groups.push(cycle);
             }
 
             // now we have a collection of groups whose dependencies are not cyclic.
diff --git a/compiler/can/src/expr.rs b/compiler/can/src/expr.rs
index 9fd811c3e6..4116a51128 100644
--- a/compiler/can/src/expr.rs
+++ b/compiler/can/src/expr.rs
@@ -60,7 +60,6 @@ pub enum Expr {
     Float(Variable, Variable, f64),
     Str(InlinableString),
     List {
-        list_var: Variable, // required for uniqueness of the list
         elem_var: Variable,
         loc_elems: Vec<Located<Expr>>,
     },
@@ -304,7 +303,6 @@ pub fn canonicalize_expr<'a>(
             if loc_elems.is_empty() {
                 (
                     List {
-                        list_var: var_store.fresh(),
                         elem_var: var_store.fresh(),
                         loc_elems: Vec::new(),
                     },
@@ -331,7 +329,6 @@ pub fn canonicalize_expr<'a>(
 
                 (
                     List {
-                        list_var: var_store.fresh(),
                         elem_var: var_store.fresh(),
                         loc_elems: can_elems,
                     },
@@ -603,7 +600,7 @@ pub fn canonicalize_expr<'a>(
 
             // A "when" with no branches is a runtime error, but it will mess things up
             // if code gen mistakenly thinks this is a tail call just because its condition
-            // happend to be one. (The condition gave us our initial output value.)
+            // happened to be one. (The condition gave us our initial output value.)
             if branches.is_empty() {
                 output.tail_call = None;
             }
@@ -1234,7 +1231,6 @@ pub fn inline_calls(var_store: &mut VarStore, scope: &mut Scope, expr: Expr) ->
         | other @ ForeignCall { .. } => other,
 
         List {
-            list_var,
             elem_var,
             loc_elems,
         } => {
@@ -1250,7 +1246,6 @@ pub fn inline_calls(var_store: &mut VarStore, scope: &mut Scope, expr: Expr) ->
             }
 
             List {
-                list_var,
                 elem_var,
                 loc_elems: new_elems,
             }
diff --git a/compiler/can/src/operator.rs b/compiler/can/src/operator.rs
index e34340da64..4b6fbf14f2 100644
--- a/compiler/can/src/operator.rs
+++ b/compiler/can/src/operator.rs
@@ -345,7 +345,7 @@ pub fn desugar_expr<'a>(arena: &'a Bump, loc_expr: &'a Located<Expr<'a>>) -> &'a
             )
         }
         If(if_thens, final_else_branch) => {
-            // If does not get desugared into `when` so we can give more targetted error messages during type checking.
+            // If does not get desugared into `when` so we can give more targeted error messages during type checking.
             let desugared_final_else = &*arena.alloc(desugar_expr(arena, &final_else_branch));
 
             let mut desugared_if_thens = Vec::with_capacity_in(if_thens.len(), arena);
diff --git a/compiler/constrain/src/builtins.rs b/compiler/constrain/src/builtins.rs
index 807d542e73..4704c67539 100644
--- a/compiler/constrain/src/builtins.rs
+++ b/compiler/constrain/src/builtins.rs
@@ -13,7 +13,7 @@ use roc_types::types::Type::{self, *};
 #[inline(always)]
 pub fn int_literal(
     num_var: Variable,
-    percision_var: Variable,
+    precision_var: Variable,
     expected: Expected<Type>,
     region: Region,
 ) -> Constraint {
@@ -25,7 +25,7 @@ pub fn int_literal(
         And(vec![
             Eq(
                 num_type.clone(),
-                ForReason(reason, num_int(Type::Variable(percision_var)), region),
+                ForReason(reason, num_int(Type::Variable(precision_var)), region),
                 Category::Int,
                 region,
             ),
diff --git a/compiler/constrain/src/expr.rs b/compiler/constrain/src/expr.rs
index c4c5b5ed8e..05dff22dd7 100644
--- a/compiler/constrain/src/expr.rs
+++ b/compiler/constrain/src/expr.rs
@@ -96,7 +96,7 @@ pub fn constrain_expr(
     expected: Expected<Type>,
 ) -> Constraint {
     match expr {
-        Int(var, percision, _) => int_literal(*var, *percision, expected, region),
+        Int(var, precision, _) => int_literal(*var, *precision, expected, region),
         Num(var, _) => exists(
             vec![*var],
             Eq(
@@ -106,7 +106,7 @@ pub fn constrain_expr(
                 region,
             ),
         ),
-        Float(var, percision, _) => float_literal(*var, *percision, expected, region),
+        Float(var, precision, _) => float_literal(*var, *precision, expected, region),
         EmptyRecord => constrain_empty_record(region, expected),
         Expr::Record { record_var, fields } => {
             if fields.is_empty() {
@@ -220,7 +220,6 @@ pub fn constrain_expr(
         List {
             elem_var,
             loc_elems,
-            list_var: _unused,
         } => {
             if loc_elems.is_empty() {
                 exists(
diff --git a/compiler/constrain/src/module.rs b/compiler/constrain/src/module.rs
index ee580ad0a4..a6462eb570 100644
--- a/compiler/constrain/src/module.rs
+++ b/compiler/constrain/src/module.rs
@@ -146,7 +146,7 @@ pub fn pre_constrain_imports(
     // Translate referenced symbols into constraints. We do this on the main
     // thread because we need exclusive access to the exposed_types map, in order
     // to get the necessary constraint info for any aliases we imported. We also
-    // resolve builtin types now, so we can use a refernce to stdlib instead of
+    // resolve builtin types now, so we can use a reference to stdlib instead of
     // having to either clone it or recreate it from scratch on the other thread.
     for &symbol in references.iter() {
         let module_id = symbol.module_id();
diff --git a/compiler/fmt/tests/test_fmt.rs b/compiler/fmt/tests/test_fmt.rs
index ebcf367152..edf60c1696 100644
--- a/compiler/fmt/tests/test_fmt.rs
+++ b/compiler/fmt/tests/test_fmt.rs
@@ -116,7 +116,7 @@ mod test_fmt {
     }
 
     #[test]
-    fn force_space_at_begining_of_comment() {
+    fn force_space_at_beginning_of_comment() {
         expr_formats_to(
             indoc!(
                 r#"
diff --git a/compiler/gen_dev/Cargo.toml b/compiler/gen_dev/Cargo.toml
index b960c7468e..8fb6646e26 100644
--- a/compiler/gen_dev/Cargo.toml
+++ b/compiler/gen_dev/Cargo.toml
@@ -1,5 +1,6 @@
 [package]
 name = "roc_gen_dev"
+description = "The development backend for the Roc compiler"
 version = "0.1.0"
 authors = ["The Roc Contributors"]
 license = "UPL-1.0"
diff --git a/compiler/gen_dev/README.md b/compiler/gen_dev/README.md
index 6cbe429895..a9b544b93c 100644
--- a/compiler/gen_dev/README.md
+++ b/compiler/gen_dev/README.md
@@ -5,7 +5,7 @@ It goes from Roc's [mono ir](https://github.com/rtfeldman/roc/blob/trunk/compile
 
 ## General Process
 
-The backend is essentially defined as two recursive match statment over the mono ir.
+The backend is essentially defined as two recursive match statement over the mono ir.
 The first pass is used to do simple linear scan lifetime analysis.
 In the future it may be expanded to add a few other quick optimizations.
 The second pass is the actual meat of the backend that generates the byte buffer of output binary.
@@ -62,7 +62,7 @@ Here are example implementations for [arm](https://github.com/rtfeldman/roc/blob
 Adding a new builtin to the dev backend can be pretty simple.
 Here is [an example](https://github.com/rtfeldman/roc/pull/893/files) of adding `Num.Sub`.
 
-This is the general procede I follow with some helpful links:
+This is the general procedure I follow with some helpful links:
 
 1. Find a feature that is just n+1.
    For example, since we already have integers, adding a builtin that functions on them should be n+1.
diff --git a/compiler/gen_dev/src/generic64/mod.rs b/compiler/gen_dev/src/generic64/mod.rs
index af67498706..bf394204da 100644
--- a/compiler/gen_dev/src/generic64/mod.rs
+++ b/compiler/gen_dev/src/generic64/mod.rs
@@ -205,7 +205,7 @@ pub struct Backend64Bit<
     float_used_callee_saved_regs: MutSet<FloatReg>,
 
     stack_size: u32,
-    // The ammount of stack space needed to pass args for function calling.
+    // The amount of stack space needed to pass args for function calling.
     fn_call_stack_size: u32,
 }
 
@@ -409,7 +409,7 @@ impl<
                 Ok(())
             }
             x => Err(format!(
-                "recieving return type, {:?}, is not yet implemented",
+                "receiving return type, {:?}, is not yet implemented",
                 x
             )),
         }
diff --git a/compiler/gen_dev/src/generic64/x86_64.rs b/compiler/gen_dev/src/generic64/x86_64.rs
index d0b8053056..67ae107b5a 100644
--- a/compiler/gen_dev/src/generic64/x86_64.rs
+++ b/compiler/gen_dev/src/generic64/x86_64.rs
@@ -236,7 +236,7 @@ impl CallConv<X86_64GeneralReg, X86_64FloatReg> for X86_64SystemV {
             Layout::Builtin(Builtin::Float64) => {}
             x => {
                 return Err(format!(
-                    "recieving return type, {:?}, is not yet implemented",
+                    "receiving return type, {:?}, is not yet implemented",
                     x
                 ));
             }
@@ -530,7 +530,7 @@ impl CallConv<X86_64GeneralReg, X86_64FloatReg> for X86_64WindowsFastcall {
             Layout::Builtin(Builtin::Float64) => {}
             x => {
                 return Err(format!(
-                    "recieving return type, {:?}, is not yet implemented",
+                    "receiving return type, {:?}, is not yet implemented",
                     x
                 ));
             }
@@ -765,12 +765,11 @@ impl Assembler<X86_64GeneralReg, X86_64FloatReg> for X86_64Assembler {
         src1: X86_64GeneralReg,
         imm32: i32,
     ) {
-        if dst == src1 {
-            add_reg64_imm32(buf, dst, imm32);
-        } else {
+        if dst != src1 {
             mov_reg64_reg64(buf, dst, src1);
-            add_reg64_imm32(buf, dst, imm32);
         }
+
+        add_reg64_imm32(buf, dst, imm32);
     }
     #[inline(always)]
     fn add_reg64_reg64_reg64(
@@ -821,12 +820,11 @@ impl Assembler<X86_64GeneralReg, X86_64FloatReg> for X86_64Assembler {
         src1: X86_64GeneralReg,
         src2: X86_64GeneralReg,
     ) {
-        if dst == src1 {
-            imul_reg64_reg64(buf, dst, src2);
-        } else {
+        if dst != src1 {
             mov_reg64_reg64(buf, dst, src1);
-            imul_reg64_reg64(buf, dst, src2);
         }
+
+        imul_reg64_reg64(buf, dst, src2);
     }
 
     #[inline(always)]
@@ -926,12 +924,11 @@ impl Assembler<X86_64GeneralReg, X86_64FloatReg> for X86_64Assembler {
         src1: X86_64GeneralReg,
         imm32: i32,
     ) {
-        if dst == src1 {
-            sub_reg64_imm32(buf, dst, imm32);
-        } else {
+        if dst != src1 {
             mov_reg64_reg64(buf, dst, src1);
-            sub_reg64_imm32(buf, dst, imm32);
         }
+
+        sub_reg64_imm32(buf, dst, imm32);
     }
     #[inline(always)]
     fn sub_reg64_reg64_reg64(
@@ -940,12 +937,11 @@ impl Assembler<X86_64GeneralReg, X86_64FloatReg> for X86_64Assembler {
         src1: X86_64GeneralReg,
         src2: X86_64GeneralReg,
     ) {
-        if dst == src1 {
-            sub_reg64_reg64(buf, dst, src2);
-        } else {
+        if dst != src1 {
             mov_reg64_reg64(buf, dst, src1);
-            sub_reg64_reg64(buf, dst, src2);
         }
+
+        sub_reg64_reg64(buf, dst, src2);
     }
 
     #[inline(always)]
diff --git a/compiler/gen_dev/src/lib.rs b/compiler/gen_dev/src/lib.rs
index d6544bfbc1..af414118f0 100644
--- a/compiler/gen_dev/src/lib.rs
+++ b/compiler/gen_dev/src/lib.rs
@@ -106,6 +106,7 @@ where
                 call,
                 pass,
                 fail: _,
+                exception_id: _,
             } => {
                 // for now, treat invoke as a normal call
                 self.build_expr(symbol, &Expr::Call(call.clone()), layout)?;
@@ -144,7 +145,7 @@ where
     ) -> Result<(), String>;
 
     /// build_expr builds the expressions for the specified symbol.
-    /// The builder must keep track of the symbol because it may be refered to later.
+    /// The builder must keep track of the symbol because it may be referred to later.
     fn build_expr(
         &mut self,
         sym: &Symbol,
@@ -229,7 +230,7 @@ where
     }
 
     /// build_run_low_level builds the low level opertation and outputs to the specified symbol.
-    /// The builder must keep track of the symbol because it may be refered to later.
+    /// The builder must keep track of the symbol because it may be referred to later.
     fn build_run_low_level(
         &mut self,
         sym: &Symbol,
@@ -443,7 +444,13 @@ where
                             self.set_last_seen(*sym, stmt);
                         }
                     }
-                    Expr::AccessAtIndex { structure, .. } => {
+                    Expr::StructAtIndex { structure, .. } => {
+                        self.set_last_seen(*structure, stmt);
+                    }
+                    Expr::GetTagId { structure, .. } => {
+                        self.set_last_seen(*structure, stmt);
+                    }
+                    Expr::UnionAtIndex { structure, .. } => {
                         self.set_last_seen(*structure, stmt);
                     }
                     Expr::Array { elems, .. } => {
@@ -486,6 +493,7 @@ where
                 call,
                 pass,
                 fail: _,
+                exception_id: _,
             } => {
                 // for now, treat invoke as a normal call
                 self.set_last_seen(*symbol, stmt);
@@ -508,7 +516,7 @@ where
             Stmt::Ret(sym) => {
                 self.set_last_seen(*sym, stmt);
             }
-            Stmt::Rethrow => {}
+            Stmt::Resume(_exception_id) => {}
             Stmt::Refcounting(modify, following) => {
                 let sym = modify.get_symbol();
 
@@ -517,7 +525,7 @@ where
             }
             Stmt::Join {
                 parameters,
-                continuation,
+                body: continuation,
                 remainder,
                 ..
             } => {
diff --git a/compiler/gen_dev/src/object_builder.rs b/compiler/gen_dev/src/object_builder.rs
index 03bbc854de..e3b09d7b52 100644
--- a/compiler/gen_dev/src/object_builder.rs
+++ b/compiler/gen_dev/src/object_builder.rs
@@ -9,8 +9,7 @@ use object::{
 };
 use roc_collections::all::MutMap;
 use roc_module::symbol;
-use roc_mono::ir::Proc;
-use roc_mono::layout::Layout;
+use roc_mono::ir::{Proc, ProcLayout};
 use target_lexicon::{Architecture as TargetArch, BinaryFormat as TargetBF, Triple};
 
 // This is used by some code below which is currently commented out.
@@ -22,7 +21,7 @@ use target_lexicon::{Architecture as TargetArch, BinaryFormat as TargetBF, Tripl
 pub fn build_module<'a>(
     env: &'a Env,
     target: &Triple,
-    procedures: MutMap<(symbol::Symbol, Layout<'a>), Proc<'a>>,
+    procedures: MutMap<(symbol::Symbol, ProcLayout<'a>), Proc<'a>>,
 ) -> Result<Object, String> {
     match target {
         Triple {
@@ -145,7 +144,7 @@ fn generate_wrapper<'a, B: Backend<'a>>(
 
 fn build_object<'a, B: Backend<'a>>(
     env: &'a Env,
-    procedures: MutMap<(symbol::Symbol, Layout<'a>), Proc<'a>>,
+    procedures: MutMap<(symbol::Symbol, ProcLayout<'a>), Proc<'a>>,
     mut backend: B,
     mut output: Object,
 ) -> Result<Object, String> {
@@ -187,7 +186,7 @@ fn build_object<'a, B: Backend<'a>>(
     let mut procs = Vec::with_capacity_in(procedures.len(), env.arena);
     for ((sym, layout), proc) in procedures {
         let fn_name = layout_ids
-            .get(sym, &layout)
+            .get_toplevel(sym, &layout)
             .to_symbol_string(sym, &env.interns);
 
         let section_id = output.add_section(
diff --git a/compiler/gen_dev/tests/helpers/eval.rs b/compiler/gen_dev/tests/helpers/eval.rs
index 9800444959..f3403bb9d4 100644
--- a/compiler/gen_dev/tests/helpers/eval.rs
+++ b/compiler/gen_dev/tests/helpers/eval.rs
@@ -69,8 +69,8 @@ pub fn helper<'a>(
 
     let mut procedures = MutMap::default();
 
-    for ((symbol, top_level), proc) in top_procedures {
-        procedures.insert((symbol, arena.alloc(top_level).full()), proc);
+    for (key, proc) in top_procedures {
+        procedures.insert(key, proc);
     }
 
     /*
@@ -87,17 +87,12 @@ pub fn helper<'a>(
     println!("=================================\n");
     */
     debug_assert_eq!(exposed_to_host.len(), 1);
-    let main_fn_symbol = exposed_to_host.keys().copied().next().unwrap();
-
-    let main_fn_layout = procedures
-        .keys()
-        .find(|(s, _)| *s == main_fn_symbol)
-        .map(|t| t.1)
-        .unwrap();
+    let main_fn_symbol = loaded.entry_point.symbol;
+    let main_fn_layout = loaded.entry_point.layout;
 
     let mut layout_ids = roc_mono::layout::LayoutIds::default();
     let main_fn_name = layout_ids
-        .get(main_fn_symbol, &main_fn_layout)
+        .get_toplevel(main_fn_symbol, &main_fn_layout)
         .to_symbol_string(main_fn_symbol, &interns);
 
     let mut lines = Vec::new();
diff --git a/compiler/gen_llvm/Cargo.toml b/compiler/gen_llvm/Cargo.toml
new file mode 100644
index 0000000000..7fb3060814
--- /dev/null
+++ b/compiler/gen_llvm/Cargo.toml
@@ -0,0 +1,41 @@
+[package]
+name = "roc_gen_llvm"
+description = "The LLVM backend for the Roc compiler"
+version = "0.1.0"
+authors = ["The Roc Contributors"]
+license = "UPL-1.0"
+edition = "2018"
+
+[dependencies]
+roc_collections = { path = "../collections" }
+roc_region = { path = "../region" }
+roc_module = { path = "../module" }
+roc_problem = { path = "../problem" }
+roc_types = { path = "../types" }
+roc_builtins = { path = "../builtins" }
+roc_unify = { path = "../unify" }
+roc_solve = { path = "../solve" }
+roc_mono = { path = "../mono" }
+morphic_lib = { path = "../../vendor/morphic_lib" }
+im    = "14" # im and im-rc should always have the same version!
+im-rc = "14" # im and im-rc should always have the same version!
+bumpalo = { version = "3.6.1", features = ["collections"] }
+inlinable_string = "0.1"
+inkwell = { path = "../../vendor/inkwell" }
+target-lexicon = "0.10"
+
+[dev-dependencies]
+roc_can = { path = "../can" }
+roc_parse = { path = "../parse" }
+roc_load = { path = "../load" }
+roc_reporting = { path = "../reporting" }
+roc_build = { path = "../build" }
+roc_std = { path = "../../roc_std" }
+pretty_assertions = "0.5.1"
+maplit = "1.0.1"
+indoc = "0.3.3"
+quickcheck = "0.8"
+quickcheck_macros = "0.8"
+tokio = { version = "0.2", features = ["blocking", "fs", "sync", "rt-threaded"] }
+bumpalo = { version = "3.6.1", features = ["collections"] }
+libc = "0.2"
diff --git a/compiler/gen/src/lib.rs b/compiler/gen_llvm/src/lib.rs
similarity index 87%
rename from compiler/gen/src/lib.rs
rename to compiler/gen_llvm/src/lib.rs
index 092cf0b8a8..bef97f894c 100644
--- a/compiler/gen/src/lib.rs
+++ b/compiler/gen_llvm/src/lib.rs
@@ -2,7 +2,7 @@
 // See github.com/rtfeldman/roc/issues/800 for discussion of the large_enum_variant check.
 #![allow(clippy::large_enum_variant)]
 // we actually want to compare against the literal float bits
-#![allow(clippy::clippy::float_cmp)]
+#![allow(clippy::float_cmp)]
 
 pub mod llvm;
 
diff --git a/compiler/gen/src/llvm/bitcode.rs b/compiler/gen_llvm/src/llvm/bitcode.rs
similarity index 76%
rename from compiler/gen/src/llvm/bitcode.rs
rename to compiler/gen_llvm/src/llvm/bitcode.rs
index ab54010fa2..b92df1f79b 100644
--- a/compiler/gen/src/llvm/bitcode.rs
+++ b/compiler/gen_llvm/src/llvm/bitcode.rs
@@ -1,16 +1,16 @@
 /// Helpers for interacting with the zig that generates bitcode
 use crate::debug_info_init;
-use crate::llvm::build::{set_name, Env, C_CALL_CONV, FAST_CALL_CONV};
+use crate::llvm::build::{struct_from_fields, Env, C_CALL_CONV, FAST_CALL_CONV, TAG_DATA_INDEX};
 use crate::llvm::convert::basic_type_from_layout;
 use crate::llvm::refcounting::{
     decrement_refcount_layout, increment_n_refcount_layout, increment_refcount_layout,
 };
 use inkwell::attributes::{Attribute, AttributeLoc};
 use inkwell::types::{BasicType, BasicTypeEnum};
-use inkwell::values::{BasicValueEnum, CallSiteValue, FunctionValue, InstructionValue};
+use inkwell::values::{BasicValue, BasicValueEnum, CallSiteValue, FunctionValue, InstructionValue};
 use inkwell::AddressSpace;
 use roc_module::symbol::Symbol;
-use roc_mono::layout::{Layout, LayoutIds};
+use roc_mono::layout::{Layout, LayoutIds, UnionLayout};
 
 pub fn call_bitcode_fn<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
@@ -66,6 +66,126 @@ const ARGUMENT_SYMBOLS: [Symbol; 8] = [
     Symbol::ARG_8,
 ];
 
+pub fn build_has_tag_id<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    function: FunctionValue<'ctx>,
+    union_layout: UnionLayout<'a>,
+) -> FunctionValue<'ctx> {
+    let fn_name: &str = &format!("{}_has_tag_id", function.get_name().to_string_lossy());
+
+    // currently the code assumes we're dealing with a non-recursive layout
+    debug_assert!(matches!(union_layout, UnionLayout::NonRecursive(_)));
+
+    match env.module.get_function(fn_name) {
+        Some(function_value) => function_value,
+        None => build_has_tag_id_help(env, union_layout, &fn_name),
+    }
+}
+
+fn build_has_tag_id_help<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    union_layout: UnionLayout<'a>,
+    fn_name: &str,
+) -> FunctionValue<'ctx> {
+    let i8_ptr_type = env.context.i8_type().ptr_type(AddressSpace::Generic);
+    let argument_types: &[BasicTypeEnum] = &[env.context.i16_type().into(), i8_ptr_type.into()];
+
+    let block = env.builder.get_insert_block().expect("to be in a function");
+    let di_location = env.builder.get_current_debug_location().unwrap();
+
+    let output_type = crate::llvm::convert::zig_has_tag_id_type(env);
+
+    let function_value = crate::llvm::refcounting::build_header_help(
+        env,
+        &fn_name,
+        output_type.into(),
+        &argument_types,
+    );
+
+    // called from zig, must use C calling convention
+    function_value.set_call_conventions(C_CALL_CONV);
+
+    let kind_id = Attribute::get_named_enum_kind_id("alwaysinline");
+    debug_assert!(kind_id > 0);
+    let attr = env.context.create_enum_attribute(kind_id, 1);
+    function_value.add_attribute(AttributeLoc::Function, attr);
+
+    let entry = env.context.append_basic_block(function_value, "entry");
+    env.builder.position_at_end(entry);
+
+    debug_info_init!(env, function_value);
+
+    let it = function_value.get_param_iter();
+
+    let arguments =
+        bumpalo::collections::Vec::from_iter_in(it.take(argument_types.len()), env.arena);
+
+    for (argument, name) in arguments.iter().zip(ARGUMENT_SYMBOLS.iter()) {
+        argument.set_name(name.ident_string(&env.interns));
+    }
+
+    match arguments.as_slice() {
+        [tag_id, tag_value_ptr] => {
+            let tag_type = basic_type_from_layout(env, &Layout::Union(union_layout));
+
+            let argument_cast = env
+                .builder
+                .build_bitcast(
+                    *tag_value_ptr,
+                    tag_type.ptr_type(AddressSpace::Generic),
+                    "load_opaque",
+                )
+                .into_pointer_value();
+
+            let tag_value = env.builder.build_load(argument_cast, "get_value");
+
+            let actual_tag_id = {
+                let tag_id_i64 =
+                    crate::llvm::build::get_tag_id(env, function_value, &union_layout, tag_value);
+
+                env.builder
+                    .build_int_cast(tag_id_i64, env.context.i16_type(), "to_i16")
+            };
+
+            let answer = env.builder.build_int_compare(
+                inkwell::IntPredicate::EQ,
+                tag_id.into_int_value(),
+                actual_tag_id,
+                "compare",
+            );
+
+            let tag_data_ptr = {
+                let data_index = env
+                    .context
+                    .i64_type()
+                    .const_int(TAG_DATA_INDEX as u64, false);
+
+                let ptr = unsafe {
+                    env.builder.build_gep(
+                        tag_value_ptr.into_pointer_value(),
+                        &[data_index],
+                        "get_data_ptr",
+                    )
+                };
+                env.builder.build_bitcast(ptr, i8_ptr_type, "to_opaque")
+            };
+
+            let field_vals = [(0, answer.into()), (1, tag_data_ptr)];
+
+            let output = struct_from_fields(env, output_type, field_vals.iter().copied());
+
+            env.builder.build_return(Some(&output));
+
+            env.builder.position_at_end(block);
+            env.builder
+                .set_current_debug_location(env.context, di_location);
+
+            function_value
+        }
+        _ => unreachable!(),
+    }
+}
+
 pub fn build_transform_caller<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     function: FunctionValue<'ctx>,
@@ -110,6 +230,9 @@ fn build_transform_caller_help<'a, 'ctx, 'env>(
         &(bumpalo::vec![ in env.arena; BasicTypeEnum::PointerType(arg_type); argument_layouts.len() + 2 ]),
     );
 
+    // called from zig, must use C calling convention
+    function_value.set_call_conventions(C_CALL_CONV);
+
     let kind_id = Attribute::get_named_enum_kind_id("alwaysinline");
     debug_assert!(kind_id > 0);
     let attr = env.context.create_enum_attribute(kind_id, 1);
@@ -122,13 +245,13 @@ fn build_transform_caller_help<'a, 'ctx, 'env>(
 
     let mut it = function_value.get_param_iter();
     let closure_ptr = it.next().unwrap().into_pointer_value();
-    set_name(closure_ptr.into(), Symbol::ARG_1.ident_string(&env.interns));
+    closure_ptr.set_name(Symbol::ARG_1.ident_string(&env.interns));
 
     let arguments =
         bumpalo::collections::Vec::from_iter_in(it.take(argument_layouts.len()), env.arena);
 
     for (argument, name) in arguments.iter().zip(ARGUMENT_SYMBOLS[1..].iter()) {
-        set_name(*argument, name.ident_string(&env.interns));
+        argument.set_name(name.ident_string(&env.interns));
     }
 
     let mut arguments_cast =
@@ -148,9 +271,6 @@ fn build_transform_caller_help<'a, 'ctx, 'env>(
     }
 
     match closure_data_layout {
-        Layout::FunctionPointer(_, _) => {
-            // do nothing
-        }
         Layout::Closure(_, lambda_set, _) => {
             if let Layout::Struct(&[]) = lambda_set.runtime_representation() {
                 // do nothing
@@ -303,6 +423,9 @@ fn build_rc_wrapper<'a, 'ctx, 'env>(
                 ),
             };
 
+            // called from zig, must use C calling convention
+            function_value.set_call_conventions(C_CALL_CONV);
+
             let kind_id = Attribute::get_named_enum_kind_id("alwaysinline");
             debug_assert!(kind_id > 0);
             let attr = env.context.create_enum_attribute(kind_id, 1);
@@ -316,7 +439,7 @@ fn build_rc_wrapper<'a, 'ctx, 'env>(
             let mut it = function_value.get_param_iter();
             let value_ptr = it.next().unwrap().into_pointer_value();
 
-            set_name(value_ptr.into(), Symbol::ARG_1.ident_string(&env.interns));
+            value_ptr.set_name(Symbol::ARG_1.ident_string(&env.interns));
 
             let value_type = basic_type_from_layout(env, layout).ptr_type(AddressSpace::Generic);
 
@@ -334,7 +457,7 @@ fn build_rc_wrapper<'a, 'ctx, 'env>(
                 }
                 Mode::IncN => {
                     let n = it.next().unwrap().into_int_value();
-                    set_name(n.into(), Symbol::ARG_2.ident_string(&env.interns));
+                    n.set_name(Symbol::ARG_2.ident_string(&env.interns));
 
                     increment_n_refcount_layout(env, function_value, layout_ids, n, value, layout);
                 }
@@ -381,6 +504,9 @@ pub fn build_eq_wrapper<'a, 'ctx, 'env>(
                 &[arg_type.into(), arg_type.into()],
             );
 
+            // called from zig, must use C calling convention
+            function_value.set_call_conventions(C_CALL_CONV);
+
             let kind_id = Attribute::get_named_enum_kind_id("alwaysinline");
             debug_assert!(kind_id > 0);
             let attr = env.context.create_enum_attribute(kind_id, 1);
@@ -395,8 +521,8 @@ pub fn build_eq_wrapper<'a, 'ctx, 'env>(
             let value_ptr1 = it.next().unwrap().into_pointer_value();
             let value_ptr2 = it.next().unwrap().into_pointer_value();
 
-            set_name(value_ptr1.into(), Symbol::ARG_1.ident_string(&env.interns));
-            set_name(value_ptr2.into(), Symbol::ARG_2.ident_string(&env.interns));
+            value_ptr1.set_name(Symbol::ARG_1.ident_string(&env.interns));
+            value_ptr2.set_name(Symbol::ARG_2.ident_string(&env.interns));
 
             let value_type = basic_type_from_layout(env, layout).ptr_type(AddressSpace::Generic);
 
@@ -455,6 +581,9 @@ pub fn build_compare_wrapper<'a, 'ctx, 'env>(
                 &[arg_type.into(), arg_type.into(), arg_type.into()],
             );
 
+            // called from zig, must use C calling convention
+            function_value.set_call_conventions(C_CALL_CONV);
+
             // we expose this function to zig; must use c calling convention
             function_value.set_call_conventions(C_CALL_CONV);
 
@@ -473,9 +602,9 @@ pub fn build_compare_wrapper<'a, 'ctx, 'env>(
             let value_ptr1 = it.next().unwrap().into_pointer_value();
             let value_ptr2 = it.next().unwrap().into_pointer_value();
 
-            set_name(closure_ptr.into(), Symbol::ARG_1.ident_string(&env.interns));
-            set_name(value_ptr1.into(), Symbol::ARG_2.ident_string(&env.interns));
-            set_name(value_ptr2.into(), Symbol::ARG_3.ident_string(&env.interns));
+            closure_ptr.set_name(Symbol::ARG_1.ident_string(&env.interns));
+            value_ptr1.set_name(Symbol::ARG_2.ident_string(&env.interns));
+            value_ptr2.set_name(Symbol::ARG_3.ident_string(&env.interns));
 
             let value_type = basic_type_from_layout(env, layout);
             let value_ptr_type = value_type.ptr_type(AddressSpace::Generic);
@@ -496,7 +625,6 @@ pub fn build_compare_wrapper<'a, 'ctx, 'env>(
             let default = [value1, value2];
 
             let arguments_cast = match closure_data_layout {
-                Layout::FunctionPointer(_, _) => &default,
                 Layout::Closure(_, lambda_set, _) => {
                     if let Layout::Struct(&[]) = lambda_set.runtime_representation() {
                         &default
diff --git a/compiler/gen/src/llvm/build.rs b/compiler/gen_llvm/src/llvm/build.rs
similarity index 80%
rename from compiler/gen/src/llvm/build.rs
rename to compiler/gen_llvm/src/llvm/build.rs
index c1a0006844..201acf20d6 100644
--- a/compiler/gen/src/llvm/build.rs
+++ b/compiler/gen_llvm/src/llvm/build.rs
@@ -1,3 +1,5 @@
+use std::path::Path;
+
 use crate::llvm::bitcode::call_bitcode_fn;
 use crate::llvm::build_dict::{
     dict_contains, dict_difference, dict_empty, dict_get, dict_insert, dict_intersection,
@@ -8,7 +10,7 @@ use crate::llvm::build_list::{
     allocate_list, empty_list, empty_polymorphic_list, list_append, list_concat, list_contains,
     list_drop, list_get_unsafe, list_join, list_keep_errs, list_keep_if, list_keep_oks, list_len,
     list_map, list_map2, list_map3, list_map_with_index, list_prepend, list_range, list_repeat,
-    list_reverse, list_set, list_single, list_sort_with,
+    list_reverse, list_set, list_single, list_sort_with, list_swap,
 };
 use crate::llvm::build_str::{
     empty_str, str_concat, str_count_graphemes, str_ends_with, str_from_float, str_from_int,
@@ -17,15 +19,13 @@ use crate::llvm::build_str::{
 };
 use crate::llvm::compare::{generic_eq, generic_neq};
 use crate::llvm::convert::{
-    basic_type_from_builtin, basic_type_from_layout, block_of_memory, block_of_memory_slices,
-    get_fn_type, get_ptr_type, ptr_int,
+    basic_type_from_builtin, basic_type_from_layout, block_of_memory_slices, ptr_int,
 };
 use crate::llvm::refcounting::{
     decrement_refcount_layout, increment_refcount_layout, PointerToRefcount,
 };
 use bumpalo::collections::Vec;
 use bumpalo::Bump;
-use either::Either;
 use inkwell::basic_block::BasicBlock;
 use inkwell::builder::Builder;
 use inkwell::context::Context;
@@ -35,22 +35,25 @@ use inkwell::debug_info::{
 use inkwell::memory_buffer::MemoryBuffer;
 use inkwell::module::{Linkage, Module};
 use inkwell::passes::{PassManager, PassManagerBuilder};
-use inkwell::types::{BasicTypeEnum, FunctionType, IntType, StructType};
+use inkwell::types::{BasicType, BasicTypeEnum, FunctionType, IntType, StructType};
 use inkwell::values::BasicValueEnum::{self, *};
 use inkwell::values::{
-    BasicValue, CallSiteValue, FloatValue, FunctionValue, InstructionOpcode, InstructionValue,
-    IntValue, PointerValue, StructValue,
+    BasicValue, CallSiteValue, CallableValue, FloatValue, FunctionValue, InstructionOpcode,
+    InstructionValue, IntValue, PointerValue, StructValue,
 };
 use inkwell::OptimizationLevel;
 use inkwell::{AddressSpace, IntPredicate};
+use morphic_lib::{
+    CalleeSpecVar, FuncName, FuncSpec, FuncSpecSolutions, ModSolutions, UpdateMode, UpdateModeVar,
+};
 use roc_builtins::bitcode;
-use roc_collections::all::{ImMap, MutSet};
-use roc_module::ident::TagName;
+use roc_collections::all::{ImMap, MutMap, MutSet};
 use roc_module::low_level::LowLevel;
 use roc_module::symbol::{Interns, ModuleId, Symbol};
-use roc_mono::ir::{BranchInfo, CallType, JoinPointId, ModifyRc, TopLevelFunctionLayout, Wrapped};
-use roc_mono::layout::{Builtin, InPlace, LambdaSet, Layout, LayoutIds, UnionLayout};
-use target_lexicon::CallingConvention;
+use roc_mono::ir::{
+    BranchInfo, CallType, EntryPoint, ExceptionId, JoinPointId, ModifyRc, OptLevel, ProcLayout,
+};
+use roc_mono::layout::{Builtin, LambdaSet, Layout, LayoutIds, UnionLayout};
 
 /// This is for Inkwell's FunctionValue::verify - we want to know the verification
 /// output in debug builds, but we don't want it to print to stdout in release builds!
@@ -84,21 +87,6 @@ macro_rules! debug_info_init {
     }};
 }
 
-#[derive(Debug, Clone, Copy)]
-pub enum OptLevel {
-    Normal,
-    Optimize,
-}
-
-impl From<OptLevel> for OptimizationLevel {
-    fn from(level: OptLevel) -> Self {
-        match level {
-            OptLevel::Normal => OptimizationLevel::None,
-            OptLevel::Optimize => OptimizationLevel::Aggressive,
-        }
-    }
-}
-
 /// Iterate over all functions in an llvm module
 pub struct FunctionIterator<'ctx> {
     next: Option<FunctionValue<'ctx>>,
@@ -130,7 +118,7 @@ impl<'ctx> Iterator for FunctionIterator<'ctx> {
 #[derive(Default, Debug, Clone, PartialEq)]
 pub struct Scope<'a, 'ctx> {
     symbols: ImMap<Symbol, (Layout<'a>, BasicValueEnum<'ctx>)>,
-    pub top_level_thunks: ImMap<Symbol, (Layout<'a>, FunctionValue<'ctx>)>,
+    pub top_level_thunks: ImMap<Symbol, (ProcLayout<'a>, FunctionValue<'ctx>)>,
     join_points: ImMap<JoinPointId, (BasicBlock<'ctx>, &'a [PointerValue<'ctx>])>,
 }
 
@@ -144,11 +132,11 @@ impl<'a, 'ctx> Scope<'a, 'ctx> {
     pub fn insert_top_level_thunk(
         &mut self,
         symbol: Symbol,
-        layout: &'a TopLevelFunctionLayout<'a>,
+        layout: &'a ProcLayout<'a>,
         function_value: FunctionValue<'ctx>,
     ) {
         self.top_level_thunks
-            .insert(symbol, (layout.full(), function_value));
+            .insert(symbol, (*layout, function_value));
     }
     fn remove(&mut self, symbol: &Symbol) {
         self.symbols.remove(symbol);
@@ -313,6 +301,8 @@ impl<'a, 'ctx, 'env> Env<'a, 'ctx, 'env> {
             /* dwo_id */ 0,
             /* split_debug_inling */ false,
             /* debug_info_for_profiling */ false,
+            "",
+            "",
         )
     }
 
@@ -353,7 +343,9 @@ impl<'a, 'ctx, 'env> Env<'a, 'ctx, 'env> {
 }
 
 pub fn module_from_builtins<'ctx>(ctx: &'ctx Context, module_name: &str) -> Module<'ctx> {
-    let bitcode_bytes = bitcode::get_bytes();
+    // In the build script for the builtins module,
+    // we compile the builtins into LLVM bitcode
+    let bitcode_bytes: &[u8] = include_bytes!("../../../builtins/bitcode/builtins.bc");
 
     let memory_buffer = MemoryBuffer::create_from_memory_range(&bitcode_bytes, module_name);
 
@@ -592,22 +584,37 @@ pub fn construct_optimization_passes<'a>(
     (mpm, fpm)
 }
 
-pub fn promote_to_main_function<'a, 'ctx, 'env>(
+fn promote_to_main_function<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
-    layout_ids: &mut LayoutIds<'a>,
+    mod_solutions: &'a ModSolutions,
     symbol: Symbol,
-    layout: TopLevelFunctionLayout<'a>,
+    top_level: ProcLayout<'a>,
 ) -> (&'static str, FunctionValue<'ctx>) {
-    let fn_name = layout_ids
-        .get(symbol, &(env.arena.alloc(layout).full()))
-        .to_symbol_string(symbol, &env.interns);
+    let it = top_level.arguments.iter().copied();
+    let bytes = roc_mono::alias_analysis::func_name_bytes_help(symbol, it, top_level.result);
+    let func_name = FuncName(&bytes);
+    let func_solutions = mod_solutions.func_solutions(func_name).unwrap();
 
-    let roc_main_fn = env.module.get_function(&fn_name).unwrap();
+    let mut it = func_solutions.specs();
+    let func_spec = it.next().unwrap();
+    debug_assert!(
+        it.next().is_none(),
+        "we expect only one specialization of this symbol"
+    );
+
+    // NOTE fake layout; it is only used for debug prints
+    let roc_main_fn =
+        function_value_by_func_spec(env, *func_spec, symbol, &[], &Layout::Struct(&[]));
 
     let main_fn_name = "$Test.main";
 
     // Add main to the module.
-    let main_fn = expose_function_to_host_help(env, roc_main_fn, main_fn_name);
+    let main_fn = expose_function_to_host_help(
+        env,
+        &inlinable_string::InlinableString::from(main_fn_name),
+        roc_main_fn,
+        main_fn_name,
+    );
 
     (main_fn_name, main_fn)
 }
@@ -624,6 +631,7 @@ pub fn int_with_precision<'a, 'ctx, 'env>(
         Builtin::Int32 => env.context.i32_type().const_int(value as u64, false),
         Builtin::Int16 => env.context.i16_type().const_int(value as u64, false),
         Builtin::Int8 => env.context.i8_type().const_int(value as u64, false),
+        Builtin::Int1 => env.context.bool_type().const_int(value as u64, false),
         _ => panic!("Invalid layout for int literal = {:?}", precision),
     }
 }
@@ -781,6 +789,7 @@ pub fn build_exp_literal<'a, 'ctx, 'env>(
 pub fn build_exp_call<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     layout_ids: &mut LayoutIds<'a>,
+    func_spec_solutions: &FuncSpecSolutions,
     scope: &mut Scope<'a, 'ctx>,
     parent: FunctionValue<'ctx>,
     layout: &Layout<'a>,
@@ -793,7 +802,11 @@ pub fn build_exp_call<'a, 'ctx, 'env>(
 
     match call_type {
         CallType::ByName {
-            name, full_layout, ..
+            name,
+            specialization_id,
+            arg_layouts,
+            ret_layout,
+            ..
         } => {
             let mut arg_tuples: Vec<BasicValueEnum> =
                 Vec::with_capacity_in(arguments.len(), env.arena);
@@ -802,34 +815,62 @@ pub fn build_exp_call<'a, 'ctx, 'env>(
                 arg_tuples.push(load_symbol(scope, symbol));
             }
 
-            call_with_args(
+            let bytes = specialization_id.to_bytes();
+            let callee_var = CalleeSpecVar(&bytes);
+            let func_spec = func_spec_solutions.callee_spec(callee_var).unwrap();
+
+            roc_call_with_args(
                 env,
-                layout_ids,
-                &full_layout,
+                arg_layouts,
+                ret_layout,
                 *name,
-                parent,
+                func_spec,
                 arg_tuples.into_bump_slice(),
             )
         }
 
-        CallType::LowLevel { op, update_mode: _ } => {
-            run_low_level(env, layout_ids, scope, parent, layout, *op, arguments)
+        CallType::LowLevel { op, update_mode } => {
+            let bytes = update_mode.to_bytes();
+            let update_var = UpdateModeVar(&bytes);
+            let update_mode = func_spec_solutions.update_mode(update_var).ok();
+
+            run_low_level(
+                env,
+                layout_ids,
+                scope,
+                parent,
+                layout,
+                *op,
+                arguments,
+                update_mode,
+            )
         }
 
         CallType::HigherOrderLowLevel {
             op,
-            closure_layout,
             function_owns_closure_data,
-        } => run_higher_order_low_level(
-            env,
-            layout_ids,
-            scope,
-            layout,
-            *op,
-            *closure_layout,
-            *function_owns_closure_data,
-            arguments,
-        ),
+            specialization_id,
+            arg_layouts,
+            ret_layout,
+            ..
+        } => {
+            let bytes = specialization_id.to_bytes();
+            let callee_var = CalleeSpecVar(&bytes);
+            let func_spec = func_spec_solutions.callee_spec(callee_var).unwrap();
+
+            run_higher_order_low_level(
+                env,
+                layout_ids,
+                scope,
+                layout,
+                *op,
+                func_spec,
+                arg_layouts,
+                ret_layout,
+                *function_owns_closure_data,
+                arguments,
+            )
+        }
 
         CallType::Foreign {
             foreign_symbol,
@@ -840,6 +881,7 @@ pub fn build_exp_call<'a, 'ctx, 'env>(
             build_foreign_symbol(
                 env,
                 layout_ids,
+                func_spec_solutions,
                 scope,
                 parent,
                 foreign_symbol,
@@ -851,25 +893,58 @@ pub fn build_exp_call<'a, 'ctx, 'env>(
     }
 }
 
+pub const TAG_ID_INDEX: u32 = 1;
+pub const TAG_DATA_INDEX: u32 = 0;
+
+pub fn struct_from_fields<'a, 'ctx, 'env, I>(
+    env: &Env<'a, 'ctx, 'env>,
+    struct_type: StructType<'ctx>,
+    values: I,
+) -> StructValue<'ctx>
+where
+    I: Iterator<Item = (usize, BasicValueEnum<'ctx>)>,
+{
+    let mut struct_value = struct_type.const_zero().into();
+
+    // Insert field exprs into struct_val
+    for (index, field_val) in values {
+        let index: u32 = index as u32;
+
+        struct_value = env
+            .builder
+            .build_insert_value(struct_value, field_val, index, "insert_record_field")
+            .unwrap();
+    }
+
+    struct_value.into_struct_value()
+}
+
 pub fn build_exp_expr<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     layout_ids: &mut LayoutIds<'a>,
+    func_spec_solutions: &FuncSpecSolutions,
     scope: &mut Scope<'a, 'ctx>,
     parent: FunctionValue<'ctx>,
     layout: &Layout<'a>,
     expr: &roc_mono::ir::Expr<'a>,
 ) -> BasicValueEnum<'ctx> {
-    use inkwell::types::BasicType;
     use roc_mono::ir::Expr::*;
 
     match expr {
         Literal(literal) => build_exp_literal(env, layout, literal),
 
-        Call(call) => build_exp_call(env, layout_ids, scope, parent, layout, call),
+        Call(call) => build_exp_call(
+            env,
+            layout_ids,
+            func_spec_solutions,
+            scope,
+            parent,
+            layout,
+            call,
+        ),
 
         Struct(sorted_fields) => {
             let ctx = env.context;
-            let builder = env.builder;
 
             // Determine types
             let num_fields = sorted_fields.len();
@@ -889,526 +964,24 @@ pub fn build_exp_expr<'a, 'ctx, 'env>(
 
             // Create the struct_type
             let struct_type = ctx.struct_type(field_types.into_bump_slice(), false);
-            let mut struct_val = struct_type.const_zero().into();
 
             // Insert field exprs into struct_val
-            for (index, field_val) in field_vals.into_iter().enumerate() {
-                struct_val = builder
-                    .build_insert_value(struct_val, field_val, index as u32, "insert_record_field")
-                    .unwrap();
-            }
-
-            BasicValueEnum::StructValue(struct_val.into_struct_value())
-        }
-
-        Tag {
-            union_size,
-            arguments,
-            tag_layout,
-            ..
-        } if *union_size == 1
-            && matches!(tag_layout, Layout::Union(UnionLayout::NonRecursive(_))) =>
-        {
-            let it = arguments.iter();
-
-            let ctx = env.context;
-            let builder = env.builder;
-
-            // Determine types
-            let num_fields = arguments.len() + 1;
-            let mut field_types = Vec::with_capacity_in(num_fields, env.arena);
-            let mut field_vals = Vec::with_capacity_in(num_fields, env.arena);
-
-            for field_symbol in it {
-                let (val, field_layout) = load_symbol_and_layout(scope, field_symbol);
-                if !field_layout.is_dropped_because_empty() {
-                    let field_type = basic_type_from_layout(env, &field_layout);
-
-                    field_types.push(field_type);
-                    field_vals.push(val);
-                }
-            }
-
-            // If the struct has only one field that isn't zero-sized,
-            // unwrap it. This is what the layout expects us to do.
-            if field_vals.len() == 1 {
-                field_vals.pop().unwrap()
-            } else {
-                // Create the struct_type
-                let struct_type = ctx.struct_type(field_types.into_bump_slice(), false);
-                let mut struct_val = struct_type.const_zero().into();
-
-                // Insert field exprs into struct_val
-                for (index, field_val) in field_vals.into_iter().enumerate() {
-                    struct_val = builder
-                        .build_insert_value(
-                            struct_val,
-                            field_val,
-                            index as u32,
-                            "insert_single_tag_field",
-                        )
-                        .unwrap();
-                }
-
-                BasicValueEnum::StructValue(struct_val.into_struct_value())
-            }
+            struct_from_fields(env, struct_type, field_vals.into_iter().enumerate()).into()
         }
 
         Tag {
             arguments,
-            tag_layout: Layout::Union(UnionLayout::NonRecursive(fields)),
+            tag_layout: union_layout,
             union_size,
             tag_id,
             ..
-        } => {
-            let tag_layout = Layout::Union(UnionLayout::NonRecursive(fields));
-
-            debug_assert!(*union_size > 1);
-
-            let ctx = env.context;
-            let builder = env.builder;
-
-            // Determine types
-            let num_fields = arguments.len() + 1;
-            let mut field_types = Vec::with_capacity_in(num_fields, env.arena);
-            let mut field_vals = Vec::with_capacity_in(num_fields, env.arena);
-
-            let tag_field_layouts = &fields[*tag_id as usize];
-
-            for (field_symbol, tag_field_layout) in arguments.iter().zip(tag_field_layouts.iter()) {
-                let (val, _val_layout) = load_symbol_and_layout(scope, field_symbol);
-
-                // Zero-sized fields have no runtime representation.
-                // The layout of the struct expects them to be dropped!
-                if !tag_field_layout.is_dropped_because_empty() {
-                    let field_type = basic_type_from_layout(env, tag_field_layout);
-
-                    field_types.push(field_type);
-
-                    if let Layout::RecursivePointer = tag_field_layout {
-                        panic!(
-                            r"non-recursive tag unions cannot directly contain a recursive pointer"
-                        );
-                    } else {
-                        // this check fails for recursive tag unions, but can be helpful while debugging
-                        // debug_assert_eq!(tag_field_layout, val_layout);
-
-                        field_vals.push(val);
-                    }
-                }
-            }
-
-            // Create the struct_type
-            let struct_type = ctx.struct_type(field_types.into_bump_slice(), false);
-            let mut struct_val = struct_type.const_zero().into();
-
-            // Insert field exprs into struct_val
-            for (index, field_val) in field_vals.into_iter().enumerate() {
-                struct_val = builder
-                    .build_insert_value(
-                        struct_val,
-                        field_val,
-                        index as u32,
-                        "insert_multi_tag_field",
-                    )
-                    .unwrap();
-            }
-
-            // How we create tag values
-            //
-            // The memory layout of tags can be different. e.g. in
-            //
-            // [ Ok Int, Err Str ]
-            //
-            // the `Ok` tag stores a 64-bit integer, the `Err` tag stores a struct.
-            // All tags of a union must have the same length, for easy addressing (e.g. array lookups).
-            // So we need to ask for the maximum of all tag's sizes, even if most tags won't use
-            // all that memory, and certainly won't use it in the same way (the tags have fields of
-            // different types/sizes)
-            //
-            // In llvm, we must be explicit about the type of value we're creating: we can't just
-            // make a unspecified block of memory. So what we do is create a byte array of the
-            // desired size. Then when we know which tag we have (which is here, in this function),
-            // we need to cast that down to the array of bytes that llvm expects
-            //
-            // There is the bitcast instruction, but it doesn't work for arrays. So we need to jump
-            // through some hoops using store and load to get this to work: the array is put into a
-            // one-element struct, which can be cast to the desired type.
-            //
-            // This tricks comes from
-            // https://github.com/raviqqe/ssf/blob/bc32aae68940d5bddf5984128e85af75ca4f4686/ssf-llvm/src/expression_compiler.rs#L116
-
-            let internal_type = basic_type_from_layout(env, &tag_layout);
-
-            cast_tag_to_block_of_memory(builder, struct_val.into_struct_value(), internal_type)
-        }
-        Tag {
-            arguments,
-            tag_layout: Layout::Union(UnionLayout::Recursive(fields)),
-            union_size,
-            tag_id,
-            ..
-        } => {
-            let tag_layout = Layout::Union(UnionLayout::NonRecursive(fields));
-
-            debug_assert!(*union_size > 1);
-
-            let ctx = env.context;
-            let builder = env.builder;
-
-            // Determine types
-            let num_fields = arguments.len() + 1;
-            let mut field_types = Vec::with_capacity_in(num_fields, env.arena);
-            let mut field_vals = Vec::with_capacity_in(num_fields, env.arena);
-
-            let tag_field_layouts = &fields[*tag_id as usize];
-
-            for (field_symbol, tag_field_layout) in arguments.iter().zip(tag_field_layouts.iter()) {
-                let (val, _val_layout) = load_symbol_and_layout(scope, field_symbol);
-
-                // Zero-sized fields have no runtime representation.
-                // The layout of the struct expects them to be dropped!
-                if !tag_field_layout.is_dropped_because_empty() {
-                    let field_type = basic_type_from_layout(env, tag_field_layout);
-
-                    field_types.push(field_type);
-
-                    if let Layout::RecursivePointer = tag_field_layout {
-                        debug_assert!(val.is_pointer_value());
-
-                        // we store recursive pointers as `i64*`
-                        let ptr = env.builder.build_bitcast(
-                            val,
-                            ctx.i64_type().ptr_type(AddressSpace::Generic),
-                            "cast_recursive_pointer",
-                        );
-
-                        field_vals.push(ptr);
-                    } else {
-                        // this check fails for recursive tag unions, but can be helpful while debugging
-                        // debug_assert_eq!(tag_field_layout, val_layout);
-
-                        field_vals.push(val);
-                    }
-                }
-            }
-
-            // Create the struct_type
-            let data_ptr = reserve_with_refcount(env, &tag_layout);
-            let struct_type = ctx.struct_type(field_types.into_bump_slice(), false);
-            let struct_ptr = env
-                .builder
-                .build_bitcast(
-                    data_ptr,
-                    struct_type.ptr_type(AddressSpace::Generic),
-                    "block_of_memory_to_tag",
-                )
-                .into_pointer_value();
-
-            // Insert field exprs into struct_val
-            for (index, field_val) in field_vals.into_iter().enumerate() {
-                let field_ptr = builder
-                    .build_struct_gep(struct_ptr, index as u32, "struct_gep")
-                    .unwrap();
-
-                builder.build_store(field_ptr, field_val);
-            }
-
-            data_ptr.into()
-        }
-
-        Tag {
-            arguments,
-            tag_layout: Layout::Union(UnionLayout::NonNullableUnwrapped(fields)),
-            union_size,
-            tag_id,
-            ..
-        } => {
-            debug_assert_eq!(*union_size, 1);
-            debug_assert_eq!(*tag_id, 0);
-            debug_assert_eq!(arguments.len(), fields.len());
-
-            let struct_layout =
-                Layout::Union(UnionLayout::NonRecursive(env.arena.alloc([*fields])));
-
-            let ctx = env.context;
-            let builder = env.builder;
-
-            // Determine types
-            let num_fields = arguments.len() + 1;
-            let mut field_types = Vec::with_capacity_in(num_fields, env.arena);
-            let mut field_vals = Vec::with_capacity_in(num_fields, env.arena);
-
-            for (field_symbol, tag_field_layout) in arguments.iter().zip(fields.iter()) {
-                let val = load_symbol(scope, field_symbol);
-
-                // Zero-sized fields have no runtime representation.
-                // The layout of the struct expects them to be dropped!
-                if !tag_field_layout.is_dropped_because_empty() {
-                    let field_type = basic_type_from_layout(env, tag_field_layout);
-
-                    field_types.push(field_type);
-
-                    if let Layout::RecursivePointer = tag_field_layout {
-                        debug_assert!(val.is_pointer_value());
-
-                        // we store recursive pointers as `i64*`
-                        let ptr = env.builder.build_bitcast(
-                            val,
-                            ctx.i64_type().ptr_type(AddressSpace::Generic),
-                            "cast_recursive_pointer",
-                        );
-
-                        field_vals.push(ptr);
-                    } else {
-                        // this check fails for recursive tag unions, but can be helpful while debugging
-
-                        field_vals.push(val);
-                    }
-                }
-            }
-
-            // Create the struct_type
-            let data_ptr = reserve_with_refcount(env, &struct_layout);
-            let struct_type = ctx.struct_type(field_types.into_bump_slice(), false);
-            let struct_ptr = env
-                .builder
-                .build_bitcast(
-                    data_ptr,
-                    struct_type.ptr_type(AddressSpace::Generic),
-                    "block_of_memory_to_tag",
-                )
-                .into_pointer_value();
-
-            // Insert field exprs into struct_val
-            for (index, field_val) in field_vals.into_iter().enumerate() {
-                let field_ptr = builder
-                    .build_struct_gep(struct_ptr, index as u32, "struct_gep")
-                    .unwrap();
-
-                builder.build_store(field_ptr, field_val);
-            }
-
-            data_ptr.into()
-        }
-
-        Tag {
-            arguments,
-            tag_layout:
-                Layout::Union(UnionLayout::NullableWrapped {
-                    nullable_id,
-                    other_tags: fields,
-                }),
-            union_size,
-            tag_id,
-            ..
-        } => {
-            let tag_layout = Layout::Union(UnionLayout::NonRecursive(fields));
-            let tag_struct_type = basic_type_from_layout(env, &tag_layout);
-            if *tag_id == *nullable_id as u8 {
-                let output_type = tag_struct_type.ptr_type(AddressSpace::Generic);
-
-                return output_type.const_null().into();
-            }
-
-            debug_assert!(*union_size > 1);
-
-            let ctx = env.context;
-            let builder = env.builder;
-
-            // Determine types
-            let num_fields = arguments.len() + 1;
-            let mut field_types = Vec::with_capacity_in(num_fields, env.arena);
-            let mut field_vals = Vec::with_capacity_in(num_fields, env.arena);
-
-            let tag_field_layouts = {
-                use std::cmp::Ordering::*;
-                match tag_id.cmp(&(*nullable_id as u8)) {
-                    Equal => &[] as &[_],
-                    Less => &fields[*tag_id as usize],
-                    Greater => &fields[*tag_id as usize - 1],
-                }
-            };
-
-            for (field_symbol, tag_field_layout) in arguments.iter().zip(tag_field_layouts.iter()) {
-                let val = load_symbol(scope, field_symbol);
-
-                // Zero-sized fields have no runtime representation.
-                // The layout of the struct expects them to be dropped!
-                if !tag_field_layout.is_dropped_because_empty() {
-                    let field_type = basic_type_from_layout(env, tag_field_layout);
-
-                    field_types.push(field_type);
-
-                    if let Layout::RecursivePointer = tag_field_layout {
-                        debug_assert!(val.is_pointer_value());
-
-                        // we store recursive pointers as `i64*`
-                        let ptr = env.builder.build_bitcast(
-                            val,
-                            ctx.i64_type().ptr_type(AddressSpace::Generic),
-                            "cast_recursive_pointer",
-                        );
-
-                        field_vals.push(ptr);
-                    } else {
-                        // this check fails for recursive tag unions, but can be helpful while debugging
-                        // debug_assert_eq!(tag_field_layout, val_layout);
-
-                        field_vals.push(val);
-                    }
-                }
-            }
-
-            // Create the struct_type
-            let data_ptr = reserve_with_refcount(env, &tag_layout);
-            let struct_type = ctx.struct_type(field_types.into_bump_slice(), false);
-            let struct_ptr = env
-                .builder
-                .build_bitcast(
-                    data_ptr,
-                    struct_type.ptr_type(AddressSpace::Generic),
-                    "block_of_memory_to_tag",
-                )
-                .into_pointer_value();
-
-            // Insert field exprs into struct_val
-            for (index, field_val) in field_vals.into_iter().enumerate() {
-                let field_ptr = builder
-                    .build_struct_gep(struct_ptr, index as u32, "struct_gep")
-                    .unwrap();
-
-                builder.build_store(field_ptr, field_val);
-            }
-
-            data_ptr.into()
-        }
-
-        Tag {
-            arguments,
-            tag_layout:
-                Layout::Union(UnionLayout::NullableUnwrapped {
-                    nullable_id,
-                    other_fields,
-                    ..
-                }),
-            union_size,
-            tag_id,
-            tag_name,
-            ..
-        } => {
-            let other_fields = &other_fields[1..];
-
-            let tag_struct_type =
-                block_of_memory_slices(env.context, &[other_fields], env.ptr_bytes);
-
-            if *tag_id == *nullable_id as u8 {
-                let output_type = tag_struct_type.ptr_type(AddressSpace::Generic);
-
-                return output_type.const_null().into();
-            }
-
-            // this tag id is not the nullable one. For the type to be recursive, the other
-            // constructor must have at least one argument!
-            debug_assert!(!arguments.is_empty());
-
-            debug_assert!(*union_size == 2);
-
-            let ctx = env.context;
-            let builder = env.builder;
-
-            // Determine types
-            let num_fields = arguments.len() + 1;
-            let mut field_types = Vec::with_capacity_in(num_fields, env.arena);
-            let mut field_vals = Vec::with_capacity_in(num_fields, env.arena);
-
-            debug_assert!(!matches!(tag_name, TagName::Closure(_)));
-
-            let tag_field_layouts = other_fields;
-            let arguments = &arguments[1..];
-
-            debug_assert_eq!(arguments.len(), tag_field_layouts.len());
-
-            for (field_symbol, tag_field_layout) in arguments.iter().zip(tag_field_layouts.iter()) {
-                let val = load_symbol(scope, field_symbol);
-
-                // Zero-sized fields have no runtime representation.
-                // The layout of the struct expects them to be dropped!
-                if !tag_field_layout.is_dropped_because_empty() {
-                    let field_type = basic_type_from_layout(env, tag_field_layout);
-
-                    field_types.push(field_type);
-
-                    if let Layout::RecursivePointer = tag_field_layout {
-                        debug_assert!(val.is_pointer_value());
-
-                        // we store recursive pointers as `i64*`
-                        let ptr = env.builder.build_bitcast(
-                            val,
-                            ctx.i64_type().ptr_type(AddressSpace::Generic),
-                            "cast_recursive_pointer",
-                        );
-
-                        field_vals.push(ptr);
-                    } else {
-                        // this check fails for recursive tag unions, but can be helpful while debugging
-                        // debug_assert_eq!(tag_field_layout, val_layout);
-
-                        field_vals.push(val);
-                    }
-                }
-            }
-
-            // Create the struct_type
-            let data_ptr = reserve_with_refcount(
-                env,
-                &Layout::Union(UnionLayout::NonRecursive(&[other_fields])),
-            );
-
-            let struct_type = ctx.struct_type(field_types.into_bump_slice(), false);
-            let struct_ptr = env
-                .builder
-                .build_bitcast(
-                    data_ptr,
-                    struct_type.ptr_type(AddressSpace::Generic),
-                    "block_of_memory_to_tag",
-                )
-                .into_pointer_value();
-
-            // Insert field exprs into struct_val
-            for (index, field_val) in field_vals.into_iter().enumerate() {
-                let field_ptr = builder
-                    .build_struct_gep(struct_ptr, index as u32, "struct_gep")
-                    .unwrap();
-
-                builder.build_store(field_ptr, field_val);
-            }
-
-            data_ptr.into()
-        }
-
-        Tag { .. } => unreachable!("tags should have a Union or RecursiveUnion layout"),
+        } => build_tag(env, scope, union_layout, *union_size, *tag_id, arguments),
 
         Reset(_) => todo!(),
         Reuse { .. } => todo!(),
 
-        AccessAtIndex {
-            index,
-            structure,
-            wrapped: Wrapped::SingleElementRecord,
-            field_layouts,
-            ..
-        } => {
-            debug_assert_eq!(field_layouts.len(), 1);
-            debug_assert_eq!(*index, 0);
-            load_symbol(scope, structure)
-        }
-
-        AccessAtIndex {
-            index,
-            structure,
-            wrapped: Wrapped::RecordOrSingleTagUnion,
-            ..
+        StructAtIndex {
+            index, structure, ..
         } => {
             // extract field from a record
             match load_symbol_and_layout(scope, structure) {
@@ -1458,162 +1031,642 @@ pub fn build_exp_expr<'a, 'ctx, 'env>(
 
                     env.builder.build_load(ptr, "load_rosetree_like")
                 }
-                (other, layout) => unreachable!(
-                    "can only index into struct layout\nValue: {:?}\nLayout: {:?}\nIndex: {:?}",
-                    other, layout, index
-                ),
+                (other, layout) => {
+                    // potential cause: indexing into an unwrapped 1-element record/tag?
+                    unreachable!(
+                        "can only index into struct layout\nValue: {:?}\nLayout: {:?}\nIndex: {:?}",
+                        other, layout, index
+                    )
+                }
             }
         }
 
-        AccessAtIndex {
-            index,
-            structure,
-            field_layouts,
-            ..
-        } => {
-            use BasicValueEnum::*;
+        EmptyArray => empty_polymorphic_list(env),
+        Array { elem_layout, elems } => list_literal(env, scope, elem_layout, elems),
+        RuntimeErrorFunction(_) => todo!(),
 
+        UnionAtIndex {
+            tag_id,
+            structure,
+            index,
+            union_layout,
+        } => {
             let builder = env.builder;
 
-            // Determine types, assumes the descriminant is in the field layouts
-            let num_fields = field_layouts.len();
-            let mut field_types = Vec::with_capacity_in(num_fields, env.arena);
-
-            for field_layout in field_layouts.iter() {
-                let field_type = basic_type_from_layout(env, &field_layout);
-                field_types.push(field_type);
-            }
-
             // cast the argument bytes into the desired shape for this tag
-            let (argument, structure_layout) = load_symbol_and_layout(scope, structure);
+            let (argument, _structure_layout) = load_symbol_and_layout(scope, structure);
 
-            match argument {
-                StructValue(value) => {
+            match union_layout {
+                UnionLayout::NonRecursive(tag_layouts) => {
+                    debug_assert!(argument.is_struct_value());
+                    let field_layouts = tag_layouts[*tag_id as usize];
                     let struct_layout = Layout::Struct(field_layouts);
-                    let struct_type = env
-                        .context
-                        .struct_type(field_types.into_bump_slice(), false);
 
-                    let struct_value = access_index_struct_value(builder, value, struct_type);
+                    let struct_type = basic_type_from_layout(env, &struct_layout);
+
+                    let struct_value = access_index_struct_value(
+                        builder,
+                        argument.into_struct_value(),
+                        struct_type.into_struct_type(),
+                    );
 
                     let result = builder
                         .build_extract_value(struct_value, *index as u32, "")
                         .expect("desired field did not decode");
 
-                    if let Some(Layout::RecursivePointer) = field_layouts.get(*index as usize) {
-                        let desired_type =
-                            block_of_memory(env.context, &struct_layout, env.ptr_bytes);
-
-                        // the value is a pointer to the actual value; load that value!
-                        let ptr = env.builder.build_bitcast(
-                            result,
-                            desired_type.ptr_type(AddressSpace::Generic),
-                            "cast_struct_value_pointer",
-                        );
-
-                        builder.build_load(ptr.into_pointer_value(), "load_recursive_field")
-                    } else {
-                        result
-                    }
+                    result
                 }
-                PointerValue(value) => match structure_layout {
-                    Layout::Union(UnionLayout::NullableWrapped { nullable_id, .. })
-                        if *index == 0 =>
-                    {
-                        let ptr = value;
-                        let is_null = env.builder.build_is_null(ptr, "is_null");
+                UnionLayout::Recursive(tag_layouts) => {
+                    debug_assert!(argument.is_pointer_value());
 
-                        let ctx = env.context;
-                        let then_block = ctx.append_basic_block(parent, "then");
-                        let else_block = ctx.append_basic_block(parent, "else");
-                        let cont_block = ctx.append_basic_block(parent, "cont");
+                    let field_layouts = tag_layouts[*tag_id as usize];
 
-                        let result = builder.build_alloca(ctx.i64_type(), "result");
+                    let tag_id_type =
+                        basic_type_from_layout(env, &union_layout.tag_id_layout()).into_int_type();
 
-                        env.builder
-                            .build_conditional_branch(is_null, then_block, else_block);
+                    lookup_at_index_ptr2(
+                        env,
+                        union_layout,
+                        tag_id_type,
+                        field_layouts,
+                        *index as usize,
+                        argument.into_pointer_value(),
+                    )
+                }
+                UnionLayout::NonNullableUnwrapped(field_layouts) => {
+                    let struct_layout = Layout::Struct(&field_layouts);
 
-                        {
-                            env.builder.position_at_end(then_block);
-                            let tag_id = ctx.i64_type().const_int(*nullable_id as u64, false);
-                            env.builder.build_store(result, tag_id);
-                            env.builder.build_unconditional_branch(cont_block);
-                        }
+                    let struct_type = basic_type_from_layout(env, &struct_layout);
 
-                        {
-                            env.builder.position_at_end(else_block);
-                            let tag_id = extract_tag_discriminant_ptr(env, ptr);
-                            env.builder.build_store(result, tag_id);
-                            env.builder.build_unconditional_branch(cont_block);
-                        }
+                    lookup_at_index_ptr(
+                        env,
+                        union_layout,
+                        field_layouts,
+                        *index as usize,
+                        argument.into_pointer_value(),
+                        struct_type.into_struct_type(),
+                    )
+                }
+                UnionLayout::NullableWrapped {
+                    nullable_id,
+                    other_tags,
+                } => {
+                    debug_assert!(argument.is_pointer_value());
+                    debug_assert_ne!(*tag_id as i64, *nullable_id);
 
-                        env.builder.position_at_end(cont_block);
+                    let tag_index = if (*tag_id as i64) < *nullable_id {
+                        *tag_id
+                    } else {
+                        tag_id - 1
+                    };
 
-                        env.builder.build_load(result, "load_result")
-                    }
-                    Layout::Union(UnionLayout::NullableUnwrapped { nullable_id, .. }) => {
-                        if *index == 0 {
-                            let is_null = env.builder.build_is_null(value, "is_null");
+                    let field_layouts = other_tags[tag_index as usize];
 
-                            let ctx = env.context;
+                    let tag_id_type =
+                        basic_type_from_layout(env, &union_layout.tag_id_layout()).into_int_type();
 
-                            let then_value = ctx.i64_type().const_int(*nullable_id as u64, false);
-                            let else_value = ctx.i64_type().const_int(!*nullable_id as u64, false);
+                    lookup_at_index_ptr2(
+                        env,
+                        union_layout,
+                        tag_id_type,
+                        field_layouts,
+                        *index as usize,
+                        argument.into_pointer_value(),
+                    )
+                }
+                UnionLayout::NullableUnwrapped {
+                    nullable_id,
+                    other_fields,
+                } => {
+                    debug_assert!(argument.is_pointer_value());
+                    debug_assert_ne!(*tag_id != 0, *nullable_id);
 
-                            env.builder.build_select(
-                                is_null,
-                                then_value,
-                                else_value,
-                                "select_tag_id",
-                            )
-                        } else {
-                            let struct_type = env
-                                .context
-                                .struct_type(&field_types.into_bump_slice()[1..], false);
+                    let field_layouts = other_fields;
+                    let struct_layout = Layout::Struct(field_layouts);
 
-                            lookup_at_index_ptr(
-                                env,
-                                &field_layouts[1..],
-                                *index as usize - 1,
-                                value,
-                                struct_type,
-                                structure_layout,
-                            )
-                        }
-                    }
-                    _ => {
-                        let struct_type = env
-                            .context
-                            .struct_type(field_types.into_bump_slice(), false);
+                    let struct_type = basic_type_from_layout(env, &struct_layout);
 
-                        lookup_at_index_ptr(
-                            env,
-                            field_layouts,
-                            *index as usize,
-                            value,
-                            struct_type,
-                            structure_layout,
-                        )
-                    }
-                },
-                _ => panic!("cannot look up index in {:?}", argument),
+                    lookup_at_index_ptr(
+                        env,
+                        union_layout,
+                        field_layouts,
+                        // the tag id is not stored
+                        *index as usize,
+                        argument.into_pointer_value(),
+                        struct_type.into_struct_type(),
+                    )
+                }
             }
         }
-        EmptyArray => empty_polymorphic_list(env),
-        Array { elem_layout, elems } => {
-            list_literal(env, layout.in_place(), scope, elem_layout, elems)
+
+        GetTagId {
+            structure,
+            union_layout,
+        } => {
+            // cast the argument bytes into the desired shape for this tag
+            let (argument, _structure_layout) = load_symbol_and_layout(scope, structure);
+
+            get_tag_id(env, parent, &union_layout, argument).into()
+        }
+    }
+}
+
+pub fn build_tag<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    scope: &Scope<'a, 'ctx>,
+    union_layout: &UnionLayout<'a>,
+    union_size: u8,
+    tag_id: u8,
+    arguments: &[Symbol],
+) -> BasicValueEnum<'ctx> {
+    let tag_id_layout = union_layout.tag_id_layout();
+
+    match union_layout {
+        UnionLayout::NonRecursive(tags) => {
+            debug_assert!(union_size > 1);
+
+            let ctx = env.context;
+            let builder = env.builder;
+
+            // Determine types
+            let num_fields = arguments.len() + 1;
+            let mut field_types = Vec::with_capacity_in(num_fields, env.arena);
+            let mut field_vals = Vec::with_capacity_in(num_fields, env.arena);
+
+            let tag_field_layouts = &tags[tag_id as usize];
+
+            for (field_symbol, tag_field_layout) in arguments.iter().zip(tag_field_layouts.iter()) {
+                let (val, _val_layout) = load_symbol_and_layout(scope, field_symbol);
+
+                // Zero-sized fields have no runtime representation.
+                // The layout of the struct expects them to be dropped!
+                if !tag_field_layout.is_dropped_because_empty() {
+                    let field_type = basic_type_from_layout(env, tag_field_layout);
+
+                    field_types.push(field_type);
+
+                    if let Layout::RecursivePointer = tag_field_layout {
+                        panic!(
+                            r"non-recursive tag unions cannot directly contain a recursive pointer"
+                        );
+                    } else {
+                        // this check fails for recursive tag unions, but can be helpful while debugging
+                        // debug_assert_eq!(tag_field_layout, val_layout);
+
+                        field_vals.push(val);
+                    }
+                }
+            }
+
+            // Create the struct_type
+            let struct_type = ctx.struct_type(field_types.into_bump_slice(), false);
+
+            // Insert field exprs into struct_val
+            let struct_val =
+                struct_from_fields(env, struct_type, field_vals.into_iter().enumerate());
+
+            // How we create tag values
+            //
+            // The memory layout of tags can be different. e.g. in
+            //
+            // [ Ok Int, Err Str ]
+            //
+            // the `Ok` tag stores a 64-bit integer, the `Err` tag stores a struct.
+            // All tags of a union must have the same length, for easy addressing (e.g. array lookups).
+            // So we need to ask for the maximum of all tag's sizes, even if most tags won't use
+            // all that memory, and certainly won't use it in the same way (the tags have fields of
+            // different types/sizes)
+            //
+            // In llvm, we must be explicit about the type of value we're creating: we can't just
+            // make a unspecified block of memory. So what we do is create a byte array of the
+            // desired size. Then when we know which tag we have (which is here, in this function),
+            // we need to cast that down to the array of bytes that llvm expects
+            //
+            // There is the bitcast instruction, but it doesn't work for arrays. So we need to jump
+            // through some hoops using store and load to get this to work: the array is put into a
+            // one-element struct, which can be cast to the desired type.
+            //
+            // This tricks comes from
+            // https://github.com/raviqqe/ssf/blob/bc32aae68940d5bddf5984128e85af75ca4f4686/ssf-llvm/src/expression_compiler.rs#L116
+
+            let internal_type = block_of_memory_slices(env.context, tags, env.ptr_bytes);
+
+            let data = cast_tag_to_block_of_memory(builder, struct_val, internal_type);
+            let tag_id_type = basic_type_from_layout(env, &tag_id_layout).into_int_type();
+            let wrapper_type = env
+                .context
+                .struct_type(&[data.get_type(), tag_id_type.into()], false);
+
+            let tag_id_intval = tag_id_type.const_int(tag_id as u64, false);
+
+            let field_vals = [
+                (TAG_DATA_INDEX as usize, data),
+                (TAG_ID_INDEX as usize, tag_id_intval.into()),
+            ];
+
+            struct_from_fields(env, wrapper_type, field_vals.iter().copied()).into()
+        }
+        UnionLayout::Recursive(tags) => {
+            debug_assert!(union_size > 1);
+
+            let ctx = env.context;
+            let builder = env.builder;
+
+            // Determine types
+            let num_fields = arguments.len() + 1;
+            let mut field_types = Vec::with_capacity_in(num_fields, env.arena);
+            let mut field_vals = Vec::with_capacity_in(num_fields, env.arena);
+
+            let tag_field_layouts = &tags[tag_id as usize];
+
+            for (field_symbol, tag_field_layout) in arguments.iter().zip(tag_field_layouts.iter()) {
+                let (val, _val_layout) = load_symbol_and_layout(scope, field_symbol);
+
+                let field_type = basic_type_from_layout(env, tag_field_layout);
+
+                field_types.push(field_type);
+
+                if let Layout::RecursivePointer = tag_field_layout {
+                    debug_assert!(val.is_pointer_value());
+
+                    // we store recursive pointers as `i64*`
+                    let ptr = env.builder.build_bitcast(
+                        val,
+                        ctx.i64_type().ptr_type(AddressSpace::Generic),
+                        "cast_recursive_pointer",
+                    );
+
+                    field_vals.push(ptr);
+                } else {
+                    // this check fails for recursive tag unions, but can be helpful while debugging
+                    // debug_assert_eq!(tag_field_layout, val_layout);
+
+                    field_vals.push(val);
+                }
+            }
+
+            // Create the struct_type
+            let raw_data_ptr =
+                reserve_with_refcount_union_as_block_of_memory(env, *union_layout, tags);
+
+            let tag_id_ptr = builder
+                .build_struct_gep(raw_data_ptr, TAG_ID_INDEX, "tag_id_index")
+                .unwrap();
+
+            let tag_id_type = basic_type_from_layout(env, &tag_id_layout).into_int_type();
+
+            env.builder
+                .build_store(tag_id_ptr, tag_id_type.const_int(tag_id as u64, false));
+
+            let opaque_struct_ptr = builder
+                .build_struct_gep(raw_data_ptr, TAG_DATA_INDEX, "tag_data_index")
+                .unwrap();
+
+            let struct_type = env.context.struct_type(&field_types, false);
+            let struct_ptr = env
+                .builder
+                .build_bitcast(
+                    opaque_struct_ptr,
+                    struct_type.ptr_type(AddressSpace::Generic),
+                    "struct_ptr",
+                )
+                .into_pointer_value();
+
+            // Insert field exprs into struct_val
+            for (index, field_val) in field_vals.into_iter().enumerate() {
+                let field_ptr = builder
+                    .build_struct_gep(struct_ptr, index as u32, "field_struct_gep")
+                    .unwrap();
+
+                builder.build_store(field_ptr, field_val);
+            }
+
+            raw_data_ptr.into()
+        }
+        UnionLayout::NonNullableUnwrapped(fields) => {
+            debug_assert_eq!(union_size, 1);
+            debug_assert_eq!(tag_id, 0);
+            debug_assert_eq!(arguments.len(), fields.len());
+
+            let ctx = env.context;
+            let builder = env.builder;
+
+            // Determine types
+            let num_fields = arguments.len() + 1;
+            let mut field_types = Vec::with_capacity_in(num_fields, env.arena);
+            let mut field_vals = Vec::with_capacity_in(num_fields, env.arena);
+
+            for (field_symbol, tag_field_layout) in arguments.iter().zip(fields.iter()) {
+                let val = load_symbol(scope, field_symbol);
+
+                let field_type = basic_type_from_layout(env, tag_field_layout);
+
+                field_types.push(field_type);
+
+                if let Layout::RecursivePointer = tag_field_layout {
+                    debug_assert!(val.is_pointer_value());
+
+                    // we store recursive pointers as `i64*`
+                    let ptr = env.builder.build_bitcast(
+                        val,
+                        ctx.i64_type().ptr_type(AddressSpace::Generic),
+                        "cast_recursive_pointer",
+                    );
+
+                    field_vals.push(ptr);
+                } else {
+                    // this check fails for recursive tag unions, but can be helpful while debugging
+
+                    field_vals.push(val);
+                }
+            }
+
+            // Create the struct_type
+            let data_ptr =
+                reserve_with_refcount_union_as_block_of_memory(env, *union_layout, &[fields]);
+
+            let struct_type = ctx.struct_type(field_types.into_bump_slice(), false);
+            let struct_ptr = env
+                .builder
+                .build_bitcast(
+                    data_ptr,
+                    struct_type.ptr_type(AddressSpace::Generic),
+                    "block_of_memory_to_tag",
+                )
+                .into_pointer_value();
+
+            // Insert field exprs into struct_val
+            for (index, field_val) in field_vals.into_iter().enumerate() {
+                let field_ptr = builder
+                    .build_struct_gep(struct_ptr, index as u32, "struct_gep")
+                    .unwrap();
+
+                builder.build_store(field_ptr, field_val);
+            }
+
+            data_ptr.into()
+        }
+        UnionLayout::NullableWrapped {
+            nullable_id,
+            other_tags: tags,
+        } => {
+            if tag_id == *nullable_id as u8 {
+                let layout = Layout::Union(*union_layout);
+
+                return basic_type_from_layout(env, &layout)
+                    .into_pointer_type()
+                    .const_null()
+                    .into();
+            }
+
+            debug_assert!(union_size > 1);
+
+            let ctx = env.context;
+            let builder = env.builder;
+
+            // Determine types
+            let num_fields = arguments.len() + 1;
+            let mut field_types = Vec::with_capacity_in(num_fields, env.arena);
+            let mut field_vals = Vec::with_capacity_in(num_fields, env.arena);
+
+            let tag_field_layouts = {
+                use std::cmp::Ordering::*;
+                match tag_id.cmp(&(*nullable_id as u8)) {
+                    Equal => unreachable!("early return above"),
+                    Less => &tags[tag_id as usize],
+                    Greater => &tags[tag_id as usize - 1],
+                }
+            };
+
+            for (field_symbol, tag_field_layout) in arguments.iter().zip(tag_field_layouts.iter()) {
+                let val = load_symbol(scope, field_symbol);
+
+                // Zero-sized fields have no runtime representation.
+                // The layout of the struct expects them to be dropped!
+                if !tag_field_layout.is_dropped_because_empty() {
+                    let field_type = basic_type_from_layout(env, tag_field_layout);
+
+                    field_types.push(field_type);
+
+                    if let Layout::RecursivePointer = tag_field_layout {
+                        debug_assert!(val.is_pointer_value());
+
+                        // we store recursive pointers as `i64*`
+                        let ptr = env.builder.build_bitcast(
+                            val,
+                            ctx.i64_type().ptr_type(AddressSpace::Generic),
+                            "cast_recursive_pointer",
+                        );
+
+                        field_vals.push(ptr);
+                    } else {
+                        // this check fails for recursive tag unions, but can be helpful while debugging
+                        // debug_assert_eq!(tag_field_layout, val_layout);
+
+                        field_vals.push(val);
+                    }
+                }
+            }
+
+            // Create the struct_type
+            let raw_data_ptr =
+                reserve_with_refcount_union_as_block_of_memory(env, *union_layout, tags);
+
+            let tag_id_ptr = builder
+                .build_struct_gep(raw_data_ptr, TAG_ID_INDEX, "tag_id_index")
+                .unwrap();
+
+            let tag_id_type = basic_type_from_layout(env, &tag_id_layout).into_int_type();
+
+            env.builder
+                .build_store(tag_id_ptr, tag_id_type.const_int(tag_id as u64, false));
+
+            let opaque_struct_ptr = builder
+                .build_struct_gep(raw_data_ptr, TAG_DATA_INDEX, "tag_data_index")
+                .unwrap();
+
+            let struct_type = env.context.struct_type(&field_types, false);
+            let struct_ptr = env
+                .builder
+                .build_bitcast(
+                    opaque_struct_ptr,
+                    struct_type.ptr_type(AddressSpace::Generic),
+                    "struct_ptr",
+                )
+                .into_pointer_value();
+
+            // Insert field exprs into struct_val
+            for (index, field_val) in field_vals.into_iter().enumerate() {
+                let field_ptr = builder
+                    .build_struct_gep(struct_ptr, index as u32, "field_struct_gep")
+                    .unwrap();
+
+                builder.build_store(field_ptr, field_val);
+            }
+
+            raw_data_ptr.into()
+        }
+        UnionLayout::NullableUnwrapped {
+            nullable_id,
+            other_fields,
+        } => {
+            let tag_struct_type =
+                block_of_memory_slices(env.context, &[other_fields], env.ptr_bytes);
+
+            if tag_id == *nullable_id as u8 {
+                let output_type = tag_struct_type.ptr_type(AddressSpace::Generic);
+
+                return output_type.const_null().into();
+            }
+
+            // this tag id is not the nullable one. For the type to be recursive, the other
+            // constructor must have at least one argument!
+            debug_assert!(!arguments.is_empty());
+
+            debug_assert!(union_size == 2);
+
+            let ctx = env.context;
+            let builder = env.builder;
+
+            // Determine types
+            let num_fields = arguments.len() + 1;
+            let mut field_types = Vec::with_capacity_in(num_fields, env.arena);
+            let mut field_vals = Vec::with_capacity_in(num_fields, env.arena);
+
+            debug_assert_eq!(arguments.len(), other_fields.len());
+
+            for (field_symbol, tag_field_layout) in arguments.iter().zip(other_fields.iter()) {
+                let val = load_symbol(scope, field_symbol);
+
+                // Zero-sized fields have no runtime representation.
+                // The layout of the struct expects them to be dropped!
+                if !tag_field_layout.is_dropped_because_empty() {
+                    let field_type = basic_type_from_layout(env, tag_field_layout);
+
+                    field_types.push(field_type);
+
+                    if let Layout::RecursivePointer = tag_field_layout {
+                        debug_assert!(val.is_pointer_value());
+
+                        // we store recursive pointers as `i64*`
+                        let ptr = env.builder.build_bitcast(
+                            val,
+                            ctx.i64_type().ptr_type(AddressSpace::Generic),
+                            "cast_recursive_pointer",
+                        );
+
+                        field_vals.push(ptr);
+                    } else {
+                        // this check fails for recursive tag unions, but can be helpful while debugging
+                        // debug_assert_eq!(tag_field_layout, val_layout);
+
+                        field_vals.push(val);
+                    }
+                }
+            }
+
+            // Create the struct_type
+            let data_ptr =
+                reserve_with_refcount_union_as_block_of_memory(env, *union_layout, &[other_fields]);
+
+            let struct_type = ctx.struct_type(field_types.into_bump_slice(), false);
+            let struct_ptr = env
+                .builder
+                .build_bitcast(
+                    data_ptr,
+                    struct_type.ptr_type(AddressSpace::Generic),
+                    "block_of_memory_to_tag",
+                )
+                .into_pointer_value();
+
+            // Insert field exprs into struct_val
+            for (index, field_val) in field_vals.into_iter().enumerate() {
+                let field_ptr = builder
+                    .build_struct_gep(struct_ptr, index as u32, "struct_gep")
+                    .unwrap();
+
+                builder.build_store(field_ptr, field_val);
+            }
+
+            data_ptr.into()
+        }
+    }
+}
+
+pub fn get_tag_id<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    parent: FunctionValue<'ctx>,
+    union_layout: &UnionLayout<'a>,
+    argument: BasicValueEnum<'ctx>,
+) -> IntValue<'ctx> {
+    let builder = env.builder;
+
+    let tag_id_layout = union_layout.tag_id_layout();
+    let tag_id_int_type = basic_type_from_layout(env, &tag_id_layout).into_int_type();
+
+    match union_layout {
+        UnionLayout::NonRecursive(_) => {
+            let tag = argument.into_struct_value();
+
+            get_tag_id_non_recursive(env, tag)
+        }
+        UnionLayout::Recursive(_) => get_tag_id_wrapped(env, argument.into_pointer_value()),
+        UnionLayout::NonNullableUnwrapped(_) => tag_id_int_type.const_zero(),
+        UnionLayout::NullableWrapped { nullable_id, .. } => {
+            let argument_ptr = argument.into_pointer_value();
+            let is_null = env.builder.build_is_null(argument_ptr, "is_null");
+
+            let ctx = env.context;
+            let then_block = ctx.append_basic_block(parent, "then");
+            let else_block = ctx.append_basic_block(parent, "else");
+            let cont_block = ctx.append_basic_block(parent, "cont");
+
+            let result = builder.build_alloca(tag_id_int_type, "result");
+
+            env.builder
+                .build_conditional_branch(is_null, then_block, else_block);
+
+            {
+                env.builder.position_at_end(then_block);
+                let tag_id = tag_id_int_type.const_int(*nullable_id as u64, false);
+                env.builder.build_store(result, tag_id);
+                env.builder.build_unconditional_branch(cont_block);
+            }
+
+            {
+                env.builder.position_at_end(else_block);
+                let tag_id = get_tag_id_wrapped(env, argument_ptr);
+                env.builder.build_store(result, tag_id);
+                env.builder.build_unconditional_branch(cont_block);
+            }
+
+            env.builder.position_at_end(cont_block);
+
+            env.builder
+                .build_load(result, "load_result")
+                .into_int_value()
+        }
+        UnionLayout::NullableUnwrapped { nullable_id, .. } => {
+            let argument_ptr = argument.into_pointer_value();
+            let is_null = env.builder.build_is_null(argument_ptr, "is_null");
+
+            let then_value = tag_id_int_type.const_int(*nullable_id as u64, false);
+            let else_value = tag_id_int_type.const_int(!*nullable_id as u64, false);
+
+            env.builder
+                .build_select(is_null, then_value, else_value, "select_tag_id")
+                .into_int_value()
         }
-        RuntimeErrorFunction(_) => todo!(),
     }
 }
 
 fn lookup_at_index_ptr<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
+    union_layout: &UnionLayout<'a>,
     field_layouts: &[Layout<'_>],
     index: usize,
     value: PointerValue<'ctx>,
     struct_type: StructType<'ctx>,
-    structure_layout: &Layout<'_>,
 ) -> BasicValueEnum<'ctx> {
     let builder = env.builder;
 
@@ -1635,10 +1688,66 @@ fn lookup_at_index_ptr<'a, 'ctx, 'env>(
     if let Some(Layout::RecursivePointer) = field_layouts.get(index as usize) {
         // a recursive field is stored as a `i64*`, to use it we must cast it to
         // a pointer to the block of memory representation
+        let actual_type = basic_type_from_layout(env, &Layout::Union(*union_layout));
+        debug_assert!(actual_type.is_pointer_type());
+
         builder.build_bitcast(
             result,
-            basic_type_from_layout(env, structure_layout),
-            "cast_rec_pointer_lookup_at_index_ptr",
+            actual_type,
+            "cast_rec_pointer_lookup_at_index_ptr_old",
+        )
+    } else {
+        result
+    }
+}
+
+fn lookup_at_index_ptr2<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    union_layout: &UnionLayout<'a>,
+    tag_id_type: IntType<'ctx>,
+    field_layouts: &[Layout<'_>],
+    index: usize,
+    value: PointerValue<'ctx>,
+) -> BasicValueEnum<'ctx> {
+    let builder = env.builder;
+
+    let struct_layout = Layout::Struct(field_layouts);
+    let struct_type = basic_type_from_layout(env, &struct_layout);
+
+    let wrapper_type = env
+        .context
+        .struct_type(&[struct_type, tag_id_type.into()], false);
+
+    let ptr = env
+        .builder
+        .build_bitcast(
+            value,
+            wrapper_type.ptr_type(AddressSpace::Generic),
+            "cast_lookup_at_index_ptr",
+        )
+        .into_pointer_value();
+
+    let data_ptr = builder
+        .build_struct_gep(ptr, TAG_DATA_INDEX, "at_index_struct_gep")
+        .unwrap();
+
+    let elem_ptr = builder
+        .build_struct_gep(data_ptr, index as u32, "at_index_struct_gep")
+        .unwrap();
+
+    let result = builder.build_load(elem_ptr, "load_at_index_ptr");
+
+    if let Some(Layout::RecursivePointer) = field_layouts.get(index as usize) {
+        // a recursive field is stored as a `i64*`, to use it we must cast it to
+        // a pointer to the block of memory representation
+
+        let actual_type = basic_type_from_layout(env, &Layout::Union(*union_layout));
+        debug_assert!(actual_type.is_pointer_type());
+
+        builder.build_bitcast(
+            result,
+            actual_type,
+            "cast_rec_pointer_lookup_at_index_ptr_new",
         )
     } else {
         result
@@ -1648,17 +1757,67 @@ fn lookup_at_index_ptr<'a, 'ctx, 'env>(
 pub fn reserve_with_refcount<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     layout: &Layout<'a>,
+) -> PointerValue<'ctx> {
+    let stack_size = layout.stack_size(env.ptr_bytes);
+    let alignment_bytes = layout.alignment_bytes(env.ptr_bytes);
+
+    let basic_type = basic_type_from_layout(env, layout);
+
+    reserve_with_refcount_help(env, basic_type, stack_size, alignment_bytes)
+}
+
+fn reserve_with_refcount_union_as_block_of_memory<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    union_layout: UnionLayout<'a>,
+    fields: &[&[Layout<'a>]],
+) -> PointerValue<'ctx> {
+    let block_type = block_of_memory_slices(env.context, fields, env.ptr_bytes);
+
+    let basic_type = if union_layout.stores_tag_id() {
+        let tag_id_type = basic_type_from_layout(env, &union_layout.tag_id_layout());
+
+        env.context
+            .struct_type(&[block_type, tag_id_type], false)
+            .into()
+    } else {
+        block_type
+    };
+
+    let mut stack_size = fields
+        .iter()
+        .map(|tag| tag.iter().map(|l| l.stack_size(env.ptr_bytes)).sum())
+        .max()
+        .unwrap_or_default();
+
+    if union_layout.stores_tag_id() {
+        stack_size += union_layout.tag_id_layout().stack_size(env.ptr_bytes);
+    }
+
+    let alignment_bytes = fields
+        .iter()
+        .map(|tag| tag.iter().map(|l| l.alignment_bytes(env.ptr_bytes)))
+        .flatten()
+        .max()
+        .unwrap_or(0);
+
+    reserve_with_refcount_help(env, basic_type, stack_size, alignment_bytes)
+}
+
+fn reserve_with_refcount_help<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    basic_type: impl BasicType<'ctx>,
+    stack_size: u32,
+    alignment_bytes: u32,
 ) -> PointerValue<'ctx> {
     let ctx = env.context;
 
     let len_type = env.ptr_int();
 
-    let value_bytes = layout.stack_size(env.ptr_bytes);
-    let value_bytes_intvalue = len_type.const_int(value_bytes as u64, false);
+    let value_bytes_intvalue = len_type.const_int(stack_size as u64, false);
 
     let rc1 = crate::llvm::refcounting::refcount_1(ctx, env.ptr_bytes);
 
-    allocate_with_refcount_help(env, layout, value_bytes_intvalue, rc1)
+    allocate_with_refcount_help(env, basic_type, alignment_bytes, value_bytes_intvalue, rc1)
 }
 
 pub fn allocate_with_refcount<'a, 'ctx, 'env>(
@@ -1676,17 +1835,17 @@ pub fn allocate_with_refcount<'a, 'ctx, 'env>(
 
 pub fn allocate_with_refcount_help<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
-    layout: &Layout<'a>,
+    value_type: impl BasicType<'ctx>,
+    alignment_bytes: u32,
     number_of_data_bytes: IntValue<'ctx>,
     initial_refcount: IntValue<'ctx>,
 ) -> PointerValue<'ctx> {
     let builder = env.builder;
     let ctx = env.context;
 
-    let value_type = basic_type_from_layout(env, layout);
     let len_type = env.ptr_int();
 
-    let extra_bytes = layout.alignment_bytes(env.ptr_bytes).max(env.ptr_bytes);
+    let extra_bytes = alignment_bytes.max(env.ptr_bytes);
 
     let ptr = {
         // number of bytes we will allocated
@@ -1696,7 +1855,7 @@ pub fn allocate_with_refcount_help<'a, 'ctx, 'env>(
             "add_extra_bytes",
         );
 
-        env.call_alloc(number_of_bytes, layout.alignment_bytes(env.ptr_bytes))
+        env.call_alloc(number_of_bytes, alignment_bytes)
     };
 
     // We must return a pointer to the first element:
@@ -1718,7 +1877,7 @@ pub fn allocate_with_refcount_help<'a, 'ctx, 'env>(
 
         let index_intvalue = int_type.const_int(index, false);
 
-        let ptr_type = get_ptr_type(&value_type, AddressSpace::Generic);
+        let ptr_type = value_type.ptr_type(AddressSpace::Generic);
 
         unsafe {
             builder
@@ -1756,7 +1915,6 @@ pub fn allocate_with_refcount_help<'a, 'ctx, 'env>(
 
 fn list_literal<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
-    inplace: InPlace,
     scope: &Scope<'a, 'ctx>,
     elem_layout: &Layout<'a>,
     elems: &&[Symbol],
@@ -1770,7 +1928,7 @@ fn list_literal<'a, 'ctx, 'env>(
         let len_type = env.ptr_int();
         let len = len_type.const_int(len_u64, false);
 
-        allocate_list(env, inplace, elem_layout, len)
+        allocate_list(env, elem_layout, len)
     };
 
     // Copy the elements from the list literal into the array
@@ -1816,15 +1974,17 @@ fn list_literal<'a, 'ctx, 'env>(
 fn invoke_roc_function<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     layout_ids: &mut LayoutIds<'a>,
+    func_spec_solutions: &FuncSpecSolutions,
     scope: &mut Scope<'a, 'ctx>,
     parent: FunctionValue<'ctx>,
     symbol: Symbol,
     layout: Layout<'a>,
-    function_value: Either<FunctionValue<'ctx>, PointerValue<'ctx>>,
+    function_value: FunctionValue<'ctx>,
     arguments: &[Symbol],
     closure_argument: Option<BasicValueEnum<'ctx>>,
     pass: &'a roc_mono::ir::Stmt<'a>,
     fail: &'a roc_mono::ir::Stmt<'a>,
+    exception_id: ExceptionId,
 ) -> BasicValueEnum<'ctx> {
     let context = env.context;
 
@@ -1847,14 +2007,7 @@ fn invoke_roc_function<'a, 'ctx, 'env>(
             "tmp",
         );
 
-        match function_value {
-            Either::Left(function) => {
-                call.set_call_convention(function.get_call_conventions());
-            }
-            Either::Right(_) => {
-                call.set_call_convention(FAST_CALL_CONV);
-            }
-        }
+        call.set_call_convention(function_value.get_call_conventions());
 
         call.try_as_basic_value()
             .left()
@@ -1866,7 +2019,7 @@ fn invoke_roc_function<'a, 'ctx, 'env>(
 
         scope.insert(symbol, (layout, call_result));
 
-        build_exp_stmt(env, layout_ids, scope, parent, pass);
+        build_exp_stmt(env, layout_ids, func_spec_solutions, scope, parent, pass);
 
         scope.remove(&symbol);
     }
@@ -1881,16 +2034,22 @@ fn invoke_roc_function<'a, 'ctx, 'env>(
             context.struct_type(&[exception_ptr, selector_value], false)
         };
 
-        env.builder
-            .build_catch_all_landing_pad(
-                &landing_pad_type,
-                &BasicValueEnum::IntValue(context.i8_type().const_zero()),
-                context.i8_type().ptr_type(AddressSpace::Generic),
-                "invoke_landing_pad",
-            )
-            .into_struct_value();
+        let personality_function = get_gxx_personality_v0(env);
 
-        build_exp_stmt(env, layout_ids, scope, parent, fail);
+        let exception_object = env.builder.build_landing_pad(
+            landing_pad_type,
+            personality_function,
+            &[],
+            true,
+            "invoke_landing_pad",
+        );
+
+        scope.insert(
+            exception_id.into_inner(),
+            (Layout::Struct(&[]), exception_object),
+        );
+
+        build_exp_stmt(env, layout_ids, func_spec_solutions, scope, parent, fail);
     }
 
     call_result
@@ -1925,6 +2084,7 @@ fn decrement_with_size_check<'a, 'ctx, 'env>(
 pub fn build_exp_stmt<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     layout_ids: &mut LayoutIds<'a>,
+    func_spec_solutions: &FuncSpecSolutions,
     scope: &mut Scope<'a, 'ctx>,
     parent: FunctionValue<'ctx>,
     stmt: &roc_mono::ir::Stmt<'a>,
@@ -1949,7 +2109,15 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
             for (symbol, expr, layout) in queue {
                 debug_assert!(layout != &Layout::RecursivePointer);
 
-                let val = build_exp_expr(env, layout_ids, scope, parent, layout, &expr);
+                let val = build_exp_expr(
+                    env,
+                    layout_ids,
+                    func_spec_solutions,
+                    scope,
+                    parent,
+                    layout,
+                    &expr,
+                );
 
                 // Make a new scope which includes the binding we just encountered.
                 // This should be done *after* compiling the bound expr, since any
@@ -1962,7 +2130,7 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
                 stack.push(*symbol);
             }
 
-            let result = build_exp_stmt(env, layout_ids, scope, parent, cont);
+            let result = build_exp_stmt(env, layout_ids, func_spec_solutions, scope, parent, cont);
 
             for symbol in stack {
                 scope.remove(&symbol);
@@ -1987,12 +2155,13 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
             call,
             layout,
             pass,
-            fail: roc_mono::ir::Stmt::Rethrow,
+            fail: roc_mono::ir::Stmt::Resume(_),
+            exception_id: _,
         } => {
             // when the fail case is just Rethrow, there is no cleanup work to do
             // so we can just treat this invoke as a normal call
             let stmt = roc_mono::ir::Stmt::Let(*symbol, Expr::Call(call.clone()), *layout, pass);
-            build_exp_stmt(env, layout_ids, scope, parent, &stmt)
+            build_exp_stmt(env, layout_ids, func_spec_solutions, scope, parent, &stmt)
         }
 
         Invoke {
@@ -2001,26 +2170,36 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
             layout,
             pass,
             fail,
+            exception_id,
         } => match call.call_type {
             CallType::ByName {
                 name,
-                ref full_layout,
+                arg_layouts,
+                ref ret_layout,
+                specialization_id,
                 ..
             } => {
-                let function_value = function_value_by_name(env, layout_ids, *full_layout, name);
+                let bytes = specialization_id.to_bytes();
+                let callee_var = CalleeSpecVar(&bytes);
+                let func_spec = func_spec_solutions.callee_spec(callee_var).unwrap();
+
+                let function_value =
+                    function_value_by_func_spec(env, func_spec, name, arg_layouts, ret_layout);
 
                 invoke_roc_function(
                     env,
                     layout_ids,
+                    func_spec_solutions,
                     scope,
                     parent,
                     *symbol,
                     *layout,
-                    function_value.into(),
+                    function_value,
                     call.arguments,
                     None,
                     pass,
                     fail,
+                    *exception_id,
                 )
             }
 
@@ -2030,6 +2209,7 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
             } => build_foreign_symbol(
                 env,
                 layout_ids,
+                func_spec_solutions,
                 scope,
                 parent,
                 foreign_symbol,
@@ -2039,6 +2219,7 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
                     symbol: *symbol,
                     pass,
                     fail,
+                    exception_id: *exception_id,
                 },
             ),
 
@@ -2051,11 +2232,9 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
             }
         },
 
-        Rethrow => {
-            cxa_rethrow_exception(env);
-
-            // used in exception handling
-            env.builder.build_unreachable();
+        Resume(exception_id) => {
+            let exception_object = scope.get(&exception_id.into_inner()).unwrap().1;
+            env.builder.build_resume(exception_object);
 
             env.context.i64_type().const_zero().into()
         }
@@ -2077,13 +2256,20 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
                 ret_type,
             };
 
-            build_switch_ir(env, layout_ids, scope, parent, switch_args)
+            build_switch_ir(
+                env,
+                layout_ids,
+                func_spec_solutions,
+                scope,
+                parent,
+                switch_args,
+            )
         }
         Join {
             id,
             parameters,
             remainder,
-            continuation,
+            body: continuation,
         } => {
             let builder = env.builder;
             let context = env.context;
@@ -2108,7 +2294,14 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
             scope.join_points.insert(*id, (cont_block, joinpoint_args));
 
             // construct the blocks that may jump to this join point
-            build_exp_stmt(env, layout_ids, scope, parent, remainder);
+            build_exp_stmt(
+                env,
+                layout_ids,
+                func_spec_solutions,
+                scope,
+                parent,
+                remainder,
+            );
 
             let phi_block = builder.get_insert_block().unwrap();
 
@@ -2121,7 +2314,14 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
             }
 
             // put the continuation in
-            let result = build_exp_stmt(env, layout_ids, scope, parent, continuation);
+            let result = build_exp_stmt(
+                env,
+                layout_ids,
+                func_spec_solutions,
+                scope,
+                parent,
+                continuation,
+            );
 
             // remove this join point again
             scope.join_points.remove(&id);
@@ -2165,7 +2365,7 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
                         );
                     }
 
-                    build_exp_stmt(env, layout_ids, scope, parent, cont)
+                    build_exp_stmt(env, layout_ids, func_spec_solutions, scope, parent, cont)
                 }
                 Dec(symbol) => {
                     let (value, layout) = load_symbol_and_layout(scope, symbol);
@@ -2174,13 +2374,13 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
                         decrement_refcount_layout(env, parent, layout_ids, value, layout);
                     }
 
-                    build_exp_stmt(env, layout_ids, scope, parent, cont)
+                    build_exp_stmt(env, layout_ids, func_spec_solutions, scope, parent, cont)
                 }
                 DecRef(symbol) => {
                     let (value, layout) = load_symbol_and_layout(scope, symbol);
 
                     match layout {
-                        Layout::Builtin(Builtin::List(_, _)) => {
+                        Layout::Builtin(Builtin::List(_)) => {
                             debug_assert!(value.is_struct_value());
 
                             // because of how we insert DECREF for lists, we can't guarantee that
@@ -2226,7 +2426,7 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
                         }
                     }
 
-                    build_exp_stmt(env, layout_ids, scope, parent, cont)
+                    build_exp_stmt(env, layout_ids, func_spec_solutions, scope, parent, cont)
                 }
             }
         }
@@ -2327,8 +2527,6 @@ pub fn complex_bitcast<'ctx>(
     to_type: BasicTypeEnum<'ctx>,
     name: &str,
 ) -> BasicValueEnum<'ctx> {
-    use inkwell::types::BasicType;
-
     // builder.build_bitcast(from_value, to_type, "cast_basic_basic")
     // because this does not allow some (valid) bitcasts
 
@@ -2359,39 +2557,29 @@ pub fn complex_bitcast<'ctx>(
     }
 }
 
-fn extract_tag_discriminant_struct<'a, 'ctx, 'env>(
-    env: &Env<'a, 'ctx, 'env>,
-    from_value: StructValue<'ctx>,
-) -> IntValue<'ctx> {
-    let struct_type = env
-        .context
-        .struct_type(&[env.context.i64_type().into()], false);
-
-    let struct_value = complex_bitcast_struct_struct(
-        env.builder,
-        from_value,
-        struct_type,
-        "extract_tag_discriminant_struct",
-    );
-
-    env.builder
-        .build_extract_value(struct_value, 0, "")
-        .expect("desired field did not decode")
-        .into_int_value()
-}
-
-fn extract_tag_discriminant_ptr<'a, 'ctx, 'env>(
+/// get the tag id out of a pointer to a wrapped (i.e. stores the tag id at runtime) layout
+fn get_tag_id_wrapped<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     from_value: PointerValue<'ctx>,
 ) -> IntValue<'ctx> {
-    let tag_id_ptr_type = env.context.i64_type().ptr_type(AddressSpace::Generic);
-
-    let ptr = env
+    let tag_id_ptr = env
         .builder
-        .build_bitcast(from_value, tag_id_ptr_type, "extract_tag_discriminant_ptr")
-        .into_pointer_value();
+        .build_struct_gep(from_value, TAG_ID_INDEX, "tag_id_ptr")
+        .unwrap();
 
-    env.builder.build_load(ptr, "load_tag_id").into_int_value()
+    env.builder
+        .build_load(tag_id_ptr, "load_tag_id")
+        .into_int_value()
+}
+
+pub fn get_tag_id_non_recursive<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    tag: StructValue<'ctx>,
+) -> IntValue<'ctx> {
+    env.builder
+        .build_extract_value(tag, TAG_ID_INDEX, "get_tag_id")
+        .unwrap()
+        .into_int_value()
 }
 
 struct SwitchArgsIr<'a, 'ctx> {
@@ -2418,6 +2606,7 @@ fn const_i128<'a, 'ctx, 'env>(env: &Env<'a, 'ctx, 'env>, value: i128) -> IntValu
 fn build_switch_ir<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     layout_ids: &mut LayoutIds<'a>,
+    func_spec_solutions: &FuncSpecSolutions,
     scope: &Scope<'a, 'ctx>,
     parent: FunctionValue<'ctx>,
     switch_args: SwitchArgsIr<'a, 'ctx>,
@@ -2470,57 +2659,9 @@ fn build_switch_ir<'a, 'ctx, 'env>(
                 .into_int_value()
         }
         Layout::Union(variant) => {
-            use UnionLayout::*;
+            cond_layout = Layout::Builtin(Builtin::Int64);
 
-            match variant {
-                NonRecursive(_) => {
-                    // we match on the discriminant, not the whole Tag
-                    cond_layout = Layout::Builtin(Builtin::Int64);
-                    let full_cond = cond_value.into_struct_value();
-
-                    extract_tag_discriminant_struct(env, full_cond)
-                }
-                Recursive(_) => {
-                    // we match on the discriminant, not the whole Tag
-                    cond_layout = Layout::Builtin(Builtin::Int64);
-
-                    debug_assert!(cond_value.is_pointer_value());
-                    extract_tag_discriminant_ptr(env, cond_value.into_pointer_value())
-                }
-                NonNullableUnwrapped(_) => unreachable!("there is no tag to switch on"),
-                NullableWrapped { nullable_id, .. } => {
-                    // we match on the discriminant, not the whole Tag
-                    cond_layout = Layout::Builtin(Builtin::Int64);
-                    let full_cond_ptr = cond_value.into_pointer_value();
-
-                    let comparison: IntValue =
-                        env.builder.build_is_null(full_cond_ptr, "is_null_cond");
-
-                    let when_null = || {
-                        env.context
-                            .i64_type()
-                            .const_int(nullable_id as u64, false)
-                            .into()
-                    };
-                    let when_not_null = || extract_tag_discriminant_ptr(env, full_cond_ptr).into();
-
-                    crate::llvm::build_list::build_basic_phi2(
-                        env,
-                        parent,
-                        comparison,
-                        when_null,
-                        when_not_null,
-                        BasicTypeEnum::IntType(env.context.i64_type()),
-                    )
-                    .into_int_value()
-                }
-                NullableUnwrapped { .. } => {
-                    // there are only two options, so we do a `tag_id == 0` check and branch on that
-                    unreachable!(
-                        "we never switch on the tag id directly for NullableUnwrapped unions"
-                    )
-                }
-            }
+            get_tag_id(env, parent, &variant, cond_value)
         }
         Layout::Builtin(_) => cond_value.into_int_value(),
         other => todo!("Build switch value from layout: {:?}", other),
@@ -2540,7 +2681,14 @@ fn build_switch_ir<'a, 'ctx, 'env>(
                 {
                     builder.position_at_end(then_block);
 
-                    let branch_val = build_exp_stmt(env, layout_ids, scope, parent, true_branch);
+                    let branch_val = build_exp_stmt(
+                        env,
+                        layout_ids,
+                        func_spec_solutions,
+                        scope,
+                        parent,
+                        true_branch,
+                    );
 
                     if then_block.get_terminator().is_none() {
                         builder.build_unconditional_branch(cont_block);
@@ -2551,7 +2699,14 @@ fn build_switch_ir<'a, 'ctx, 'env>(
                 {
                     builder.position_at_end(else_block);
 
-                    let branch_val = build_exp_stmt(env, layout_ids, scope, parent, false_branch);
+                    let branch_val = build_exp_stmt(
+                        env,
+                        layout_ids,
+                        func_spec_solutions,
+                        scope,
+                        parent,
+                        false_branch,
+                    );
 
                     if else_block.get_terminator().is_none() {
                         builder.build_unconditional_branch(cont_block);
@@ -2579,18 +2734,14 @@ fn build_switch_ir<'a, 'ctx, 'env>(
             //   ]
             //
             // they either need to all be i8, or i64
-            let int_val = match cond_layout {
-                Layout::Builtin(Builtin::Usize) => {
-                    ptr_int(env.context, env.ptr_bytes).const_int(*int as u64, false)
-                }
-                Layout::Builtin(Builtin::Int64) => context.i64_type().const_int(*int as u64, false),
-                Layout::Builtin(Builtin::Int128) => const_i128(env, *int as i128),
-                Layout::Builtin(Builtin::Int32) => context.i32_type().const_int(*int as u64, false),
-                Layout::Builtin(Builtin::Int16) => context.i16_type().const_int(*int as u64, false),
-                Layout::Builtin(Builtin::Int8) => context.i8_type().const_int(*int as u64, false),
-                Layout::Builtin(Builtin::Int1) => context.bool_type().const_int(*int as u64, false),
-                _ => panic!("Can't cast to cond_layout = {:?}", cond_layout),
+            let condition_int_type = cond.get_type();
+
+            let int_val = if condition_int_type == context.i128_type() {
+                const_i128(env, *int as i128)
+            } else {
+                condition_int_type.const_int(*int as u64, false)
             };
+
             let block = context.append_basic_block(parent, format!("branch{}", int).as_str());
 
             cases.push((int_val, block));
@@ -2601,7 +2752,14 @@ fn build_switch_ir<'a, 'ctx, 'env>(
         for ((_, _, branch_expr), (_, block)) in branches.iter().zip(cases) {
             builder.position_at_end(block);
 
-            let branch_val = build_exp_stmt(env, layout_ids, scope, parent, branch_expr);
+            let branch_val = build_exp_stmt(
+                env,
+                layout_ids,
+                func_spec_solutions,
+                scope,
+                parent,
+                branch_expr,
+            );
 
             if block.get_terminator().is_none() {
                 builder.build_unconditional_branch(cont_block);
@@ -2612,7 +2770,14 @@ fn build_switch_ir<'a, 'ctx, 'env>(
         // The block for the conditional's default branch.
         builder.position_at_end(default_block);
 
-        let default_val = build_exp_stmt(env, layout_ids, scope, parent, default_branch);
+        let default_val = build_exp_stmt(
+            env,
+            layout_ids,
+            func_spec_solutions,
+            scope,
+            parent,
+            default_branch,
+        );
 
         if default_block.get_terminator().is_none() {
             builder.build_unconditional_branch(cont_block);
@@ -2640,18 +2805,6 @@ fn build_switch_ir<'a, 'ctx, 'env>(
     }
 }
 
-/// TODO could this be added to Inkwell itself as a method on BasicValueEnum?
-pub fn set_name(bv_enum: BasicValueEnum<'_>, name: &str) {
-    match bv_enum {
-        ArrayValue(val) => val.set_name(name),
-        IntValue(val) => val.set_name(name),
-        FloatValue(val) => val.set_name(name),
-        PointerValue(val) => val.set_name(name),
-        StructValue(val) => val.set_name(name),
-        VectorValue(val) => val.set_name(name),
-    }
-}
-
 /// Creates a new stack allocation instruction in the entry block of the function.
 pub fn create_entry_block_alloca<'a, 'ctx>(
     env: &Env<'a, 'ctx, '_>,
@@ -2672,21 +2825,22 @@ pub fn create_entry_block_alloca<'a, 'ctx>(
 
 fn expose_function_to_host<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
+    symbol: Symbol,
     roc_function: FunctionValue<'ctx>,
 ) {
-    let c_function_name: String =
-        format!("roc_{}_exposed", roc_function.get_name().to_str().unwrap());
+    // Assumption: there is only one specialization of a host-exposed function
+    let ident_string = symbol.ident_string(&env.interns);
+    let c_function_name: String = format!("roc__{}_1_exposed", ident_string);
 
-    expose_function_to_host_help(env, roc_function, &c_function_name);
+    expose_function_to_host_help(env, ident_string, roc_function, &c_function_name);
 }
 
 fn expose_function_to_host_help<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
+    ident_string: &inlinable_string::InlinableString,
     roc_function: FunctionValue<'ctx>,
     c_function_name: &str,
 ) -> FunctionValue<'ctx> {
-    use inkwell::types::BasicType;
-
     let roc_wrapper_function = make_exception_catcher(env, roc_function);
 
     let roc_function_type = roc_wrapper_function.get_type();
@@ -2744,8 +2898,7 @@ fn expose_function_to_host_help<'a, 'ctx, 'env>(
 
     // STEP 3: build a {} -> u64 function that gives the size of the return type
     let size_function_type = env.context.i64_type().fn_type(&[], false);
-    let size_function_name: String =
-        format!("roc_{}_size", roc_function.get_name().to_str().unwrap());
+    let size_function_name: String = format!("roc__{}_size", ident_string);
 
     let size_function = add_func(
         env.module,
@@ -2774,18 +2927,17 @@ fn invoke_and_catch<'a, 'ctx, 'env, F, T>(
     env: &Env<'a, 'ctx, 'env>,
     parent: FunctionValue<'ctx>,
     function: F,
+    calling_convention: u32,
     arguments: &[BasicValueEnum<'ctx>],
     return_type: T,
 ) -> BasicValueEnum<'ctx>
 where
-    F: Into<either::Either<FunctionValue<'ctx>, PointerValue<'ctx>>>,
     T: inkwell::types::BasicType<'ctx>,
+    F: Into<CallableValue<'ctx>>,
 {
     let context = env.context;
     let builder = env.builder;
 
-    let u8_ptr = env.context.i8_type().ptr_type(AddressSpace::Generic);
-
     let call_result_type = context.struct_type(
         &[context.i64_type().into(), return_type.as_basic_type_enum()],
         false,
@@ -2806,7 +2958,7 @@ where
             catch_block,
             "call_roc_function",
         );
-        call.set_call_convention(FAST_CALL_CONV);
+        call.set_call_convention(calling_convention);
         call.try_as_basic_value().left().unwrap()
     };
 
@@ -2814,71 +2966,7 @@ where
     {
         builder.position_at_end(catch_block);
 
-        let landing_pad_type = {
-            let exception_ptr = context.i8_type().ptr_type(AddressSpace::Generic).into();
-            let selector_value = context.i32_type().into();
-
-            context.struct_type(&[exception_ptr, selector_value], false)
-        };
-
-        let info = builder
-            .build_catch_all_landing_pad(
-                &landing_pad_type,
-                &BasicValueEnum::IntValue(context.i8_type().const_zero()),
-                context.i8_type().ptr_type(AddressSpace::Generic),
-                "main_landing_pad",
-            )
-            .into_struct_value();
-
-        let exception_ptr = builder
-            .build_extract_value(info, 0, "exception_ptr")
-            .unwrap();
-
-        let thrown = cxa_begin_catch(env, exception_ptr);
-
-        let error_msg = {
-            let exception_type = u8_ptr;
-            let ptr = builder.build_bitcast(
-                thrown,
-                exception_type.ptr_type(AddressSpace::Generic),
-                "cast",
-            );
-
-            builder.build_load(ptr.into_pointer_value(), "error_msg")
-        };
-
-        let return_type = context.struct_type(&[context.i64_type().into(), u8_ptr.into()], false);
-
-        let return_value = {
-            let v1 = return_type.const_zero();
-
-            // flag is non-zero, indicating failure
-            let flag = context.i64_type().const_int(1, false);
-
-            let v2 = builder
-                .build_insert_value(v1, flag, 0, "set_error")
-                .unwrap();
-
-            let v3 = builder
-                .build_insert_value(v2, error_msg, 1, "set_exception")
-                .unwrap();
-
-            v3
-        };
-
-        // bitcast result alloca so we can store our concrete type { flag, error_msg } in there
-        let result_alloca_bitcast = builder
-            .build_bitcast(
-                result_alloca,
-                return_type.ptr_type(AddressSpace::Generic),
-                "result_alloca_bitcast",
-            )
-            .into_pointer_value();
-
-        // store our return value
-        builder.build_store(result_alloca_bitcast, return_value);
-
-        cxa_end_catch(env);
+        build_catch_all_landing_pad(env, result_alloca);
 
         builder.build_unconditional_branch(cont_block);
     }
@@ -2911,14 +2999,89 @@ where
 
     builder.position_at_end(cont_block);
 
-    let result = builder.build_load(result_alloca, "result");
+    builder.build_load(result_alloca, "result")
+}
 
-    // MUST set the personality at the very end;
-    // doing it earlier can cause the personality to be ignored
-    let personality_func = get_gxx_personality_v0(env);
-    parent.set_personality_function(personality_func);
+fn build_catch_all_landing_pad<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    result_alloca: PointerValue<'ctx>,
+) {
+    let context = env.context;
+    let builder = env.builder;
 
-    result
+    let u8_ptr = env.context.i8_type().ptr_type(AddressSpace::Generic);
+
+    let landing_pad_type = {
+        let exception_ptr = context.i8_type().ptr_type(AddressSpace::Generic).into();
+        let selector_value = context.i32_type().into();
+
+        context.struct_type(&[exception_ptr, selector_value], false)
+    };
+
+    // null pointer functions as a catch-all catch clause
+    let null = u8_ptr.const_zero();
+
+    let personality_function = get_gxx_personality_v0(env);
+
+    let info = builder
+        .build_landing_pad(
+            landing_pad_type,
+            personality_function,
+            &[null.into()],
+            false,
+            "main_landing_pad",
+        )
+        .into_struct_value();
+
+    let exception_ptr = builder
+        .build_extract_value(info, 0, "exception_ptr")
+        .unwrap();
+
+    let thrown = cxa_begin_catch(env, exception_ptr);
+
+    let error_msg = {
+        let exception_type = u8_ptr;
+        let ptr = builder.build_bitcast(
+            thrown,
+            exception_type.ptr_type(AddressSpace::Generic),
+            "cast",
+        );
+
+        builder.build_load(ptr.into_pointer_value(), "error_msg")
+    };
+
+    let return_type = context.struct_type(&[context.i64_type().into(), u8_ptr.into()], false);
+
+    let return_value = {
+        let v1 = return_type.const_zero();
+
+        // flag is non-zero, indicating failure
+        let flag = context.i64_type().const_int(1, false);
+
+        let v2 = builder
+            .build_insert_value(v1, flag, 0, "set_error")
+            .unwrap();
+
+        let v3 = builder
+            .build_insert_value(v2, error_msg, 1, "set_exception")
+            .unwrap();
+
+        v3
+    };
+
+    // bitcast result alloca so we can store our concrete type { flag, error_msg } in there
+    let result_alloca_bitcast = builder
+        .build_bitcast(
+            result_alloca,
+            return_type.ptr_type(AddressSpace::Generic),
+            "result_alloca_bitcast",
+        )
+        .into_pointer_value();
+
+    // store our return value
+    builder.build_store(result_alloca_bitcast, return_value);
+
+    cxa_end_catch(env);
 }
 
 fn make_exception_catcher<'a, 'ctx, 'env>(
@@ -2984,35 +3147,197 @@ fn make_exception_catching_wrapper<'a, 'ctx, 'env>(
         env,
         wrapper_function,
         roc_function,
+        roc_function.get_call_conventions(),
         &arguments,
         roc_function_type.get_return_type().unwrap(),
     );
 
     builder.build_return(Some(&result));
 
-    // MUST set the personality at the very end;
-    // doing it earlier can cause the personality to be ignored
-    let personality_func = get_gxx_personality_v0(env);
-    wrapper_function.set_personality_function(personality_func);
-
     wrapper_function
 }
 
-pub fn build_proc_header<'a, 'ctx, 'env>(
+pub fn build_proc_headers<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
-    layout_ids: &mut LayoutIds<'a>,
+    mod_solutions: &'a ModSolutions,
+    procedures: MutMap<(Symbol, ProcLayout<'a>), roc_mono::ir::Proc<'a>>,
+    scope: &mut Scope<'a, 'ctx>,
+    // alias_analysis_solutions: AliasAnalysisSolutions,
+) -> Vec<
+    'a,
+    (
+        roc_mono::ir::Proc<'a>,
+        &'a [(&'a FuncSpecSolutions, FunctionValue<'ctx>)],
+    ),
+> {
+    // Populate Procs further and get the low-level Expr from the canonical Expr
+    let mut headers = Vec::with_capacity_in(procedures.len(), env.arena);
+    for ((symbol, layout), proc) in procedures {
+        let name_bytes = roc_mono::alias_analysis::func_name_bytes(&proc);
+        let func_name = FuncName(&name_bytes);
+
+        let func_solutions = mod_solutions.func_solutions(func_name).unwrap();
+
+        let it = func_solutions.specs();
+        let mut function_values = Vec::with_capacity_in(it.size_hint().0, env.arena);
+        for specialization in it {
+            let fn_val = build_proc_header(env, *specialization, symbol, &proc);
+
+            if proc.args.is_empty() {
+                // this is a 0-argument thunk, i.e. a top-level constant definition
+                // it must be in-scope everywhere in the module!
+                scope.insert_top_level_thunk(symbol, env.arena.alloc(layout), fn_val);
+            }
+
+            let func_spec_solutions = func_solutions.spec(specialization).unwrap();
+
+            function_values.push((func_spec_solutions, fn_val));
+        }
+        headers.push((proc, function_values.into_bump_slice()));
+    }
+
+    headers
+}
+
+pub fn build_procedures<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    opt_level: OptLevel,
+    procedures: MutMap<(Symbol, ProcLayout<'a>), roc_mono::ir::Proc<'a>>,
+    entry_point: EntryPoint<'a>,
+    debug_output_file: Option<&Path>,
+) {
+    build_procedures_help(env, opt_level, procedures, entry_point, debug_output_file);
+}
+
+pub fn build_procedures_return_main<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    opt_level: OptLevel,
+    procedures: MutMap<(Symbol, ProcLayout<'a>), roc_mono::ir::Proc<'a>>,
+    entry_point: EntryPoint<'a>,
+) -> (&'static str, FunctionValue<'ctx>) {
+    let mod_solutions = build_procedures_help(env, opt_level, procedures, entry_point, None);
+
+    promote_to_main_function(env, mod_solutions, entry_point.symbol, entry_point.layout)
+}
+
+fn build_procedures_help<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    opt_level: OptLevel,
+    procedures: MutMap<(Symbol, ProcLayout<'a>), roc_mono::ir::Proc<'a>>,
+    entry_point: EntryPoint<'a>,
+    debug_output_file: Option<&Path>,
+) -> &'a ModSolutions {
+    let mut layout_ids = roc_mono::layout::LayoutIds::default();
+    let mut scope = Scope::default();
+
+    let it = procedures.iter().map(|x| x.1);
+
+    let solutions = match roc_mono::alias_analysis::spec_program(entry_point, it) {
+        Err(e) => panic!("Error in alias analysis: {}", e),
+        Ok(solutions) => solutions,
+    };
+
+    let solutions = env.arena.alloc(solutions);
+
+    let mod_solutions = solutions
+        .mod_solutions(roc_mono::alias_analysis::MOD_APP)
+        .unwrap();
+
+    // Add all the Proc headers to the module.
+    // We have to do this in a separate pass first,
+    // because their bodies may reference each other.
+    let headers = build_proc_headers(env, &mod_solutions, procedures, &mut scope);
+
+    let (_, function_pass) = construct_optimization_passes(env.module, opt_level);
+
+    for (proc, fn_vals) in headers {
+        for (func_spec_solutions, fn_val) in fn_vals {
+            let mut current_scope = scope.clone();
+
+            // only have top-level thunks for this proc's module in scope
+            // this retain is not needed for correctness, but will cause less confusion when debugging
+            let home = proc.name.module_id();
+            current_scope.retain_top_level_thunks_for_module(home);
+
+            build_proc(
+                &env,
+                mod_solutions,
+                &mut layout_ids,
+                func_spec_solutions,
+                scope.clone(),
+                &proc,
+                *fn_val,
+            );
+
+            // call finalize() before any code generation/verification
+            env.dibuilder.finalize();
+
+            if fn_val.verify(true) {
+                function_pass.run_on(&fn_val);
+            } else {
+                let mode = "NON-OPTIMIZED";
+
+                eprintln!(
+                    "\n\nFunction {:?} failed LLVM verification in {} build. Its content was:\n",
+                    fn_val.get_name().to_str().unwrap(),
+                    mode,
+                );
+
+                fn_val.print_to_stderr();
+
+                if let Some(app_ll_file) = debug_output_file {
+                    env.module.print_to_file(&app_ll_file).unwrap();
+
+                    panic!(
+                        r"😱 LLVM errors when defining function {:?}; I wrote the full LLVM IR to {:?}",
+                        fn_val.get_name().to_str().unwrap(),
+                        app_ll_file,
+                    );
+                } else {
+                    panic!(
+                    "The preceding code was from {:?}, which failed LLVM verification in {} build.",
+                     fn_val.get_name().to_str().unwrap(),
+                    mode,
+                    )
+                }
+            }
+        }
+    }
+
+    mod_solutions
+}
+
+fn func_spec_name<'a>(
+    arena: &'a Bump,
+    interns: &Interns,
+    symbol: Symbol,
+    func_spec: FuncSpec,
+) -> bumpalo::collections::String<'a> {
+    use std::fmt::Write;
+
+    let mut buf = bumpalo::collections::String::with_capacity_in(1, arena);
+
+    let ident_string = symbol.ident_string(interns);
+    let module_string = interns.module_ids.get_name(symbol.module_id()).unwrap();
+    write!(buf, "{}_{}_", module_string, ident_string).unwrap();
+
+    for byte in func_spec.0.iter() {
+        write!(buf, "{:x?}", byte).unwrap();
+    }
+
+    buf
+}
+
+fn build_proc_header<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    func_spec: FuncSpec,
     symbol: Symbol,
-    top_level: TopLevelFunctionLayout<'a>,
     proc: &roc_mono::ir::Proc<'a>,
 ) -> FunctionValue<'ctx> {
-    let layout = env.arena.alloc(top_level).full();
-
     let args = proc.args;
     let arena = env.arena;
 
-    let fn_name = layout_ids
-        .get(symbol, &layout)
-        .to_symbol_string(symbol, &env.interns);
+    let fn_name = func_spec_name(env.arena, &env.interns, symbol, func_spec);
 
     let ret_type = basic_type_from_layout(env, &proc.ret_layout);
     let mut arg_basic_types = Vec::with_capacity_in(args.len(), arena);
@@ -3023,7 +3348,7 @@ pub fn build_proc_header<'a, 'ctx, 'env>(
         arg_basic_types.push(arg_type);
     }
 
-    let fn_type = get_fn_type(&ret_type, &arg_basic_types);
+    let fn_type = ret_type.fn_type(&arg_basic_types, false);
 
     let fn_val = add_func(
         env.module,
@@ -3037,7 +3362,7 @@ pub fn build_proc_header<'a, 'ctx, 'env>(
     fn_val.set_subprogram(subprogram);
 
     if env.exposed_to_host.contains(&symbol) {
-        expose_function_to_host(env, fn_val);
+        expose_function_to_host(env, symbol, fn_val);
     }
 
     fn_val
@@ -3052,8 +3377,6 @@ pub fn build_closure_caller<'a, 'ctx, 'env>(
     lambda_set: LambdaSet<'a>,
     result: &Layout<'a>,
 ) {
-    use inkwell::types::BasicType;
-
     let context = &env.context;
     let builder = env.builder;
 
@@ -3061,7 +3384,7 @@ pub fn build_closure_caller<'a, 'ctx, 'env>(
 
     // e.g. `roc__main_1_Fx_caller`
     let function_name = format!(
-        "roc_{}_{}_caller",
+        "roc__{}_{}_caller",
         def_name,
         alias_symbol.ident_string(&env.interns)
     );
@@ -3119,8 +3442,14 @@ pub fn build_closure_caller<'a, 'ctx, 'env>(
         *param = builder.build_load(param.into_pointer_value(), "load_param");
     }
 
-    let call_result =
-        invoke_and_catch(env, function_value, evaluator, &[closure_data], result_type);
+    let call_result = invoke_and_catch(
+        env,
+        function_value,
+        evaluator,
+        evaluator.get_call_conventions(),
+        &[closure_data],
+        result_type,
+    );
 
     builder.build_store(output, call_result);
 
@@ -3136,138 +3465,22 @@ pub fn build_closure_caller<'a, 'ctx, 'env>(
     );
 
     // STEP 4: build a {} -> u64 function that gives the size of the closure
-    let layout = Layout::Closure(arguments, lambda_set, result);
-    build_host_exposed_alias_size(env, def_name, alias_symbol, &layout);
-}
-
-fn build_function_caller<'a, 'ctx, 'env>(
-    env: &'a Env<'a, 'ctx, 'env>,
-    def_name: &str,
-    alias_symbol: Symbol,
-    arguments: &[Layout<'a>],
-    result: &Layout<'a>,
-) {
-    use inkwell::types::BasicType;
-
-    let context = &env.context;
-    let builder = env.builder;
-
-    // STEP 1: build function header
-
-    // e.g. `roc__main_1_Fx_caller`
-    let function_name = format!(
-        "roc_{}_{}_caller",
-        def_name,
-        alias_symbol.ident_string(&env.interns)
-    );
-
-    let mut argument_types = Vec::with_capacity_in(arguments.len() + 3, env.arena);
-
-    for layout in arguments {
-        let arg_type = basic_type_from_layout(env, layout);
-        let arg_ptr_type = arg_type.ptr_type(AddressSpace::Generic);
-
-        argument_types.push(arg_ptr_type.into());
-    }
-
-    let function_pointer_type = {
-        let mut args = Vec::new_in(env.arena);
-        args.extend(arguments.iter().cloned());
-
-        // pretend the closure layout is empty
-        args.push(Layout::Struct(&[]));
-
-        let function_layout = Layout::FunctionPointer(&args, result);
-
-        // this is already a (function) pointer type
-        basic_type_from_layout(env, &function_layout)
-    };
-    argument_types.push(function_pointer_type);
-
-    let closure_argument_type = {
-        let basic_type = basic_type_from_layout(env, &Layout::Struct(&[]));
-
-        basic_type.ptr_type(AddressSpace::Generic)
-    };
-    argument_types.push(closure_argument_type.into());
-
-    let result_type = basic_type_from_layout(env, result);
-
-    let roc_call_result_type =
-        context.struct_type(&[context.i64_type().into(), result_type], false);
-
-    let output_type = { roc_call_result_type.ptr_type(AddressSpace::Generic) };
-    argument_types.push(output_type.into());
-
-    let function_type = context.void_type().fn_type(&argument_types, false);
-
-    let function_value = add_func(
-        env.module,
-        function_name.as_str(),
-        function_type,
-        Linkage::External,
-        C_CALL_CONV,
-    );
-
-    // STEP 2: build function body
-
-    let entry = context.append_basic_block(function_value, "entry");
-
-    builder.position_at_end(entry);
-
-    let mut parameters = function_value.get_params();
-    let output = parameters.pop().unwrap().into_pointer_value();
-    let _closure_data_ptr = parameters.pop().unwrap().into_pointer_value();
-    let function_ptr = parameters.pop().unwrap().into_pointer_value();
-
-    // let closure_data = context.struct_type(&[], false).const_zero().into();
-
-    let actual_function_type =
-        basic_type_from_layout(env, &Layout::FunctionPointer(arguments, result));
-
-    let function_ptr = builder
-        .build_bitcast(function_ptr, actual_function_type, "cast")
-        .into_pointer_value();
-
-    let mut parameters = parameters;
-
-    for param in parameters.iter_mut() {
-        debug_assert!(param.is_pointer_value());
-        *param = builder.build_load(param.into_pointer_value(), "load_param");
-    }
-
-    let call_result = invoke_and_catch(env, function_value, function_ptr, &parameters, result_type);
-
-    builder.build_store(output, call_result);
-
-    builder.build_return(None);
-
-    // STEP 3: build a {} -> u64 function that gives the size of the return type
-    build_host_exposed_alias_size_help(
-        env,
-        def_name,
-        alias_symbol,
-        Some("result"),
-        roc_call_result_type.into(),
-    );
-
-    // STEP 4: build a {} -> u64 function that gives the size of the function
-    let layout = Layout::FunctionPointer(arguments, result);
-    build_host_exposed_alias_size(env, def_name, alias_symbol, &layout);
+    let layout = lambda_set.runtime_representation();
+    build_host_exposed_alias_size(env, def_name, alias_symbol, layout);
 }
 
 fn build_host_exposed_alias_size<'a, 'ctx, 'env>(
     env: &'a Env<'a, 'ctx, 'env>,
     def_name: &str,
     alias_symbol: Symbol,
-    layout: &Layout<'a>,
+    layout: Layout<'a>,
 ) {
     build_host_exposed_alias_size_help(
         env,
         def_name,
         alias_symbol,
         None,
-        basic_type_from_layout(env, layout),
+        basic_type_from_layout(env, &layout),
     )
 }
 
@@ -3284,14 +3497,14 @@ fn build_host_exposed_alias_size_help<'a, 'ctx, 'env>(
     let size_function_type = env.context.i64_type().fn_type(&[], false);
     let size_function_name: String = if let Some(label) = opt_label {
         format!(
-            "roc_{}_{}_{}_size",
+            "roc__{}_{}_{}_size",
             def_name,
             alias_symbol.ident_string(&env.interns),
             label
         )
     } else {
         format!(
-            "roc_{}_{}_size",
+            "roc__{}_{}_size",
             def_name,
             alias_symbol.ident_string(&env.interns)
         )
@@ -3309,62 +3522,68 @@ fn build_host_exposed_alias_size_help<'a, 'ctx, 'env>(
 
     builder.position_at_end(entry);
 
-    use inkwell::types::BasicType;
     let size: BasicValueEnum = basic_type.size_of().unwrap().into();
     builder.build_return(Some(&size));
 }
 
 pub fn build_proc<'a, 'ctx, 'env>(
     env: &'a Env<'a, 'ctx, 'env>,
+    mod_solutions: &'a ModSolutions,
     layout_ids: &mut LayoutIds<'a>,
+    func_spec_solutions: &FuncSpecSolutions,
     mut scope: Scope<'a, 'ctx>,
-    proc: roc_mono::ir::Proc<'a>,
+    proc: &roc_mono::ir::Proc<'a>,
     fn_val: FunctionValue<'ctx>,
 ) {
     use roc_mono::ir::HostExposedLayouts;
+    use roc_mono::layout::RawFunctionLayout;
     let copy = proc.host_exposed_layouts.clone();
-    let fn_name = fn_val.get_name().to_string_lossy();
     match copy {
         HostExposedLayouts::NotHostExposed => {}
         HostExposedLayouts::HostExposed { rigids: _, aliases } => {
             for (name, (symbol, top_level, layout)) in aliases {
                 match layout {
-                    Layout::Closure(arguments, closure, result) => {
+                    RawFunctionLayout::Function(arguments, closure, result) => {
                         // define closure size and return value size, e.g.
                         //
                         // * roc__mainForHost_1_Update_size() -> i64
                         // * roc__mainForHost_1_Update_result_size() -> i64
 
-                        let evaluator_layout = env.arena.alloc(top_level).full();
-                        let evaluator_name = layout_ids
-                            .get(symbol, &evaluator_layout)
-                            .to_symbol_string(symbol, &env.interns);
-
-                        let evaluator = function_value_by_name_help(
-                            env,
-                            evaluator_layout,
+                        let it = top_level.arguments.iter().copied();
+                        let bytes = roc_mono::alias_analysis::func_name_bytes_help(
                             symbol,
-                            &evaluator_name,
+                            it,
+                            top_level.result,
                         );
+                        let func_name = FuncName(&bytes);
+                        let func_solutions = mod_solutions.func_solutions(func_name).unwrap();
+
+                        let mut it = func_solutions.specs();
+                        let func_spec = it.next().unwrap();
+                        debug_assert!(
+                            it.next().is_none(),
+                            "we expect only one specialization of this symbol"
+                        );
+
+                        let evaluator = function_value_by_func_spec(
+                            env,
+                            *func_spec,
+                            symbol,
+                            top_level.arguments,
+                            &top_level.result,
+                        );
+
+                        let ident_string = proc.name.ident_string(&env.interns);
+                        let fn_name: String = format!("{}_1", ident_string);
 
                         build_closure_caller(
                             env, &fn_name, evaluator, name, arguments, closure, result,
                         )
                     }
-                    Layout::FunctionPointer(arguments, result) => {
-                        // define function size (equal to pointer size) and return value size, e.g.
-                        //
-                        // * roc__mainForHost_1_Update_size() -> i64
-                        // * roc__mainForHost_1_Update_result_size() -> i64
-                        build_function_caller(env, &fn_name, name, arguments, result)
-                    }
 
-                    Layout::Builtin(_) => {}
-                    Layout::PhantomEmptyStruct => {}
-                    Layout::Struct(_) => {}
-                    Layout::Union(_) => {}
-                    Layout::RecursivePointer => {}
-                    Layout::Pointer(_) => {}
+                    RawFunctionLayout::ZeroArgumentThunk(_) => {
+                        // do nothing
+                    }
                 }
             }
         }
@@ -3383,11 +3602,18 @@ pub fn build_proc<'a, 'ctx, 'env>(
 
     // Add args to scope
     for (arg_val, (layout, arg_symbol)) in fn_val.get_param_iter().zip(args) {
-        set_name(arg_val, arg_symbol.ident_string(&env.interns));
+        arg_val.set_name(arg_symbol.ident_string(&env.interns));
         scope.insert(*arg_symbol, (*layout, arg_val));
     }
 
-    let body = build_exp_stmt(env, layout_ids, &mut scope, fn_val, &proc.body);
+    let body = build_exp_stmt(
+        env,
+        layout_ids,
+        func_spec_solutions,
+        &mut scope,
+        fn_val,
+        &proc.body,
+    );
 
     // only add a return if codegen did not already add one
     if let Some(block) = builder.get_insert_block() {
@@ -3407,23 +3633,23 @@ pub fn verify_fn(fn_val: FunctionValue<'_>) {
     }
 }
 
-fn function_value_by_name<'a, 'ctx, 'env>(
+fn function_value_by_func_spec<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
-    layout_ids: &mut LayoutIds<'a>,
-    layout: Layout<'a>,
+    func_spec: FuncSpec,
     symbol: Symbol,
+    arguments: &[Layout<'a>],
+    result: &Layout<'a>,
 ) -> FunctionValue<'ctx> {
-    let fn_name = layout_ids
-        .get(symbol, &layout)
-        .to_symbol_string(symbol, &env.interns);
+    let fn_name = func_spec_name(env.arena, &env.interns, symbol, func_spec);
     let fn_name = fn_name.as_str();
 
-    function_value_by_name_help(env, layout, symbol, fn_name)
+    function_value_by_name_help(env, arguments, result, symbol, fn_name)
 }
 
 fn function_value_by_name_help<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
-    layout: Layout<'a>,
+    arguments: &[Layout<'a>],
+    result: &Layout<'a>,
     symbol: Symbol,
     fn_name: &str,
 ) -> FunctionValue<'ctx> {
@@ -3431,7 +3657,8 @@ fn function_value_by_name_help<'a, 'ctx, 'env>(
         if symbol.is_builtin() {
             eprintln!(
                 "Unrecognized builtin function: {:?}\nLayout: {:?}\n",
-                fn_name, layout
+                fn_name,
+                (arguments, result)
             );
             eprintln!("Is the function defined? If so, maybe there is a problem with the layout");
 
@@ -3443,7 +3670,9 @@ fn function_value_by_name_help<'a, 'ctx, 'env>(
             // Unrecognized non-builtin function:
             eprintln!(
                 "Unrecognized non-builtin function: {:?}\n\nSymbol: {:?}\nLayout: {:?}\n",
-                fn_name, symbol, layout
+                fn_name,
+                symbol,
+                (arguments, result)
             );
             eprintln!("Is the function defined? If so, maybe there is a problem with the layout");
 
@@ -3457,15 +3686,16 @@ fn function_value_by_name_help<'a, 'ctx, 'env>(
 
 // #[allow(clippy::cognitive_complexity)]
 #[inline(always)]
-fn call_with_args<'a, 'ctx, 'env>(
+fn roc_call_with_args<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
-    layout_ids: &mut LayoutIds<'a>,
-    layout: &Layout<'a>,
+    argument_layouts: &[Layout<'a>],
+    result_layout: &Layout<'a>,
     symbol: Symbol,
-    _parent: FunctionValue<'ctx>,
+    func_spec: FuncSpec,
     args: &[BasicValueEnum<'ctx>],
 ) -> BasicValueEnum<'ctx> {
-    let fn_val = function_value_by_name(env, layout_ids, *layout, symbol);
+    let fn_val =
+        function_value_by_func_spec(env, func_spec, symbol, argument_layouts, result_layout);
 
     let call = env.builder.build_call(fn_val, args, "call");
 
@@ -3478,8 +3708,8 @@ fn call_with_args<'a, 'ctx, 'env>(
 
 /// Translates a target_lexicon::Triple to a LLVM calling convention u32
 /// as described in https://llvm.org/doxygen/namespacellvm_1_1CallingConv.html
-pub fn get_call_conventions(cc: CallingConvention) -> u32 {
-    use CallingConvention::*;
+pub fn get_call_conventions(cc: target_lexicon::CallingConvention) -> u32 {
+    use target_lexicon::CallingConvention::*;
 
     // For now, we're returning 0 for the C calling convention on all of these.
     // Not sure if we should be picking something more specific!
@@ -3491,9 +3721,9 @@ pub fn get_call_conventions(cc: CallingConvention) -> u32 {
 }
 
 /// Source: https://llvm.org/doxygen/namespacellvm_1_1CallingConv.html
-pub static C_CALL_CONV: u32 = 0;
-pub static FAST_CALL_CONV: u32 = 8;
-pub static COLD_CALL_CONV: u32 = 9;
+pub const C_CALL_CONV: u32 = 0;
+pub const FAST_CALL_CONV: u32 = 8;
+pub const COLD_CALL_CONV: u32 = 9;
 
 pub struct RocFunctionCall<'ctx> {
     pub caller: PointerValue<'ctx>,
@@ -3547,7 +3777,9 @@ fn run_higher_order_low_level<'a, 'ctx, 'env>(
     scope: &Scope<'a, 'ctx>,
     return_layout: &Layout<'a>,
     op: LowLevel,
-    function_layout: Layout<'a>,
+    func_spec: FuncSpec,
+    argument_layouts: &[Layout<'a>],
+    result_layout: &Layout<'a>,
     function_owns_closure_data: bool,
     args: &[Symbol],
 ) -> BasicValueEnum<'ctx> {
@@ -3560,18 +3792,13 @@ fn run_higher_order_low_level<'a, 'ctx, 'env>(
         ($index:expr) => {{
             let function_symbol = args[$index];
 
-            let fn_name = layout_ids
-                .get(function_symbol, &function_layout)
-                .to_symbol_string(function_symbol, &env.interns);
-
-            env.module
-                .get_function(fn_name.as_str())
-                .unwrap_or_else(|| {
-                    panic!(
-                        "Could not get pointer to unknown function {:?} {:?}",
-                        fn_name, function_layout
-                    )
-                })
+            function_value_by_func_spec(
+                env,
+                func_spec,
+                function_symbol,
+                argument_layouts,
+                return_layout,
+            )
         }};
     }
 
@@ -3587,7 +3814,7 @@ fn run_higher_order_low_level<'a, 'ctx, 'env>(
 
             match list_layout {
                 Layout::Builtin(Builtin::EmptyList) => default,
-                Layout::Builtin(Builtin::List(_, element_layout)) => {
+                Layout::Builtin(Builtin::List(element_layout)) => {
                     let argument_layouts = &[**element_layout, *default_layout];
 
                     let roc_function_call = roc_function_call(
@@ -3604,6 +3831,7 @@ fn run_higher_order_low_level<'a, 'ctx, 'env>(
                         env,
                         layout_ids,
                         roc_function_call,
+                        result_layout,
                         list,
                         element_layout,
                         default,
@@ -3629,8 +3857,8 @@ fn run_higher_order_low_level<'a, 'ctx, 'env>(
             match (list_layout, return_layout) {
                 (Layout::Builtin(Builtin::EmptyList), _) => empty_list(env),
                 (
-                    Layout::Builtin(Builtin::List(_, element_layout)),
-                    Layout::Builtin(Builtin::List(_, result_layout)),
+                    Layout::Builtin(Builtin::List(element_layout)),
+                    Layout::Builtin(Builtin::List(result_layout)),
                 ) => {
                     let argument_layouts = &[**element_layout];
 
@@ -3660,9 +3888,9 @@ fn run_higher_order_low_level<'a, 'ctx, 'env>(
 
             match (list1_layout, list2_layout, return_layout) {
                 (
-                    Layout::Builtin(Builtin::List(_, element1_layout)),
-                    Layout::Builtin(Builtin::List(_, element2_layout)),
-                    Layout::Builtin(Builtin::List(_, result_layout)),
+                    Layout::Builtin(Builtin::List(element1_layout)),
+                    Layout::Builtin(Builtin::List(element2_layout)),
+                    Layout::Builtin(Builtin::List(result_layout)),
                 ) => {
                     let argument_layouts = &[**element1_layout, **element2_layout];
 
@@ -3704,10 +3932,10 @@ fn run_higher_order_low_level<'a, 'ctx, 'env>(
 
             match (list1_layout, list2_layout, list3_layout, return_layout) {
                 (
-                    Layout::Builtin(Builtin::List(_, element1_layout)),
-                    Layout::Builtin(Builtin::List(_, element2_layout)),
-                    Layout::Builtin(Builtin::List(_, element3_layout)),
-                    Layout::Builtin(Builtin::List(_, result_layout)),
+                    Layout::Builtin(Builtin::List(element1_layout)),
+                    Layout::Builtin(Builtin::List(element2_layout)),
+                    Layout::Builtin(Builtin::List(element3_layout)),
+                    Layout::Builtin(Builtin::List(result_layout)),
                 ) => {
                     let argument_layouts =
                         &[**element1_layout, **element2_layout, **element3_layout];
@@ -3754,8 +3982,8 @@ fn run_higher_order_low_level<'a, 'ctx, 'env>(
             match (list_layout, return_layout) {
                 (Layout::Builtin(Builtin::EmptyList), _) => empty_list(env),
                 (
-                    Layout::Builtin(Builtin::List(_, element_layout)),
-                    Layout::Builtin(Builtin::List(_, result_layout)),
+                    Layout::Builtin(Builtin::List(element_layout)),
+                    Layout::Builtin(Builtin::List(result_layout)),
                 ) => {
                     let argument_layouts = &[Layout::Builtin(Builtin::Usize), **element_layout];
 
@@ -3786,7 +4014,7 @@ fn run_higher_order_low_level<'a, 'ctx, 'env>(
 
             match list_layout {
                 Layout::Builtin(Builtin::EmptyList) => empty_list(env),
-                Layout::Builtin(Builtin::List(_, element_layout)) => {
+                Layout::Builtin(Builtin::List(element_layout)) => {
                     let argument_layouts = &[**element_layout];
 
                     let roc_function_call = roc_function_call(
@@ -3818,8 +4046,8 @@ fn run_higher_order_low_level<'a, 'ctx, 'env>(
                 (_, Layout::Builtin(Builtin::EmptyList))
                 | (Layout::Builtin(Builtin::EmptyList), _) => empty_list(env),
                 (
-                    Layout::Builtin(Builtin::List(_, before_layout)),
-                    Layout::Builtin(Builtin::List(_, after_layout)),
+                    Layout::Builtin(Builtin::List(before_layout)),
+                    Layout::Builtin(Builtin::List(after_layout)),
                 ) => {
                     let argument_layouts = &[**before_layout];
 
@@ -3837,7 +4065,7 @@ fn run_higher_order_low_level<'a, 'ctx, 'env>(
                         env,
                         layout_ids,
                         roc_function_call,
-                        &function_layout,
+                        result_layout,
                         list,
                         before_layout,
                         after_layout,
@@ -3862,8 +4090,8 @@ fn run_higher_order_low_level<'a, 'ctx, 'env>(
                 (_, Layout::Builtin(Builtin::EmptyList))
                 | (Layout::Builtin(Builtin::EmptyList), _) => empty_list(env),
                 (
-                    Layout::Builtin(Builtin::List(_, before_layout)),
-                    Layout::Builtin(Builtin::List(_, after_layout)),
+                    Layout::Builtin(Builtin::List(before_layout)),
+                    Layout::Builtin(Builtin::List(after_layout)),
                 ) => {
                     let argument_layouts = &[**before_layout];
 
@@ -3881,7 +4109,7 @@ fn run_higher_order_low_level<'a, 'ctx, 'env>(
                         env,
                         layout_ids,
                         roc_function_call,
-                        &function_layout,
+                        result_layout,
                         list,
                         before_layout,
                         after_layout,
@@ -3913,7 +4141,7 @@ fn run_higher_order_low_level<'a, 'ctx, 'env>(
 
             match list_layout {
                 Layout::Builtin(Builtin::EmptyList) => empty_list(env),
-                Layout::Builtin(Builtin::List(_, element_layout)) => {
+                Layout::Builtin(Builtin::List(element_layout)) => {
                     use crate::llvm::bitcode::build_compare_wrapper;
 
                     let argument_layouts = &[**element_layout, **element_layout];
@@ -3972,7 +4200,6 @@ fn run_higher_order_low_level<'a, 'ctx, 'env>(
 
                     dict_walk(
                         env,
-                        layout_ids,
                         roc_function_call,
                         dict,
                         default,
@@ -3997,6 +4224,7 @@ fn run_low_level<'a, 'ctx, 'env>(
     layout: &Layout<'a>,
     op: LowLevel,
     args: &[Symbol],
+    update_mode: Option<UpdateMode>,
 ) -> BasicValueEnum<'ctx> {
     use LowLevel::*;
 
@@ -4007,13 +4235,13 @@ fn run_low_level<'a, 'ctx, 'env>(
             // Str.concat : Str, Str -> Str
             debug_assert_eq!(args.len(), 2);
 
-            str_concat(env, layout.in_place(), scope, args[0], args[1])
+            str_concat(env, scope, args[0], args[1])
         }
         StrJoinWith => {
             // Str.joinWith : List Str, Str -> Str
             debug_assert_eq!(args.len(), 2);
 
-            str_join_with(env, layout.in_place(), scope, args[0], args[1])
+            str_join_with(env, scope, args[0], args[1])
         }
         StrStartsWith => {
             // Str.startsWith : Str, Str -> Bool
@@ -4067,7 +4295,7 @@ fn run_low_level<'a, 'ctx, 'env>(
             // Str.split : Str, Str -> List Str
             debug_assert_eq!(args.len(), 2);
 
-            str_split(env, scope, layout.in_place(), args[0], args[1])
+            str_split(env, scope, args[0], args[1])
         }
         StrIsEmpty => {
             // Str.isEmpty : Str -> Str
@@ -4102,7 +4330,7 @@ fn run_low_level<'a, 'ctx, 'env>(
 
             let (arg, arg_layout) = load_symbol_and_layout(scope, &args[0]);
 
-            list_single(env, layout.in_place(), arg, arg_layout)
+            list_single(env, arg, arg_layout)
         }
         ListRepeat => {
             // List.repeat : Int, elem -> List elem
@@ -4119,7 +4347,7 @@ fn run_low_level<'a, 'ctx, 'env>(
 
             let (list, list_layout) = load_symbol_and_layout(scope, &args[0]);
 
-            list_reverse(env, layout.in_place(), list, list_layout)
+            list_reverse(env, list, list_layout)
         }
         ListConcat => {
             debug_assert_eq!(args.len(), 2);
@@ -4128,14 +4356,7 @@ fn run_low_level<'a, 'ctx, 'env>(
 
             let second_list = load_symbol(scope, &args[1]);
 
-            list_concat(
-                env,
-                layout.in_place(),
-                parent,
-                first_list,
-                second_list,
-                list_layout,
-            )
+            list_concat(env, parent, first_list, second_list, list_layout)
         }
         ListContains => {
             // List.contains : List elem, elem -> Bool
@@ -4168,7 +4389,29 @@ fn run_low_level<'a, 'ctx, 'env>(
             let original_wrapper = load_symbol(scope, &args[0]).into_struct_value();
             let (elem, elem_layout) = load_symbol_and_layout(scope, &args[1]);
 
-            list_append(env, layout.in_place(), original_wrapper, elem, elem_layout)
+            list_append(env, original_wrapper, elem, elem_layout)
+        }
+        ListSwap => {
+            // List.swap : List elem, Nat, Nat -> List elem
+            debug_assert_eq!(args.len(), 3);
+
+            let (list, list_layout) = load_symbol_and_layout(scope, &args[0]);
+            let original_wrapper = list.into_struct_value();
+
+            let index_1 = load_symbol(scope, &args[1]);
+            let index_2 = load_symbol(scope, &args[2]);
+
+            match list_layout {
+                Layout::Builtin(Builtin::EmptyList) => empty_list(env),
+                Layout::Builtin(Builtin::List(element_layout)) => list_swap(
+                    env,
+                    original_wrapper,
+                    index_1.into_int_value(),
+                    index_2.into_int_value(),
+                    element_layout,
+                ),
+                _ => unreachable!("Invalid layout {:?} in List.swap", list_layout),
+            }
         }
         ListDrop => {
             // List.drop : List elem, Nat -> List elem
@@ -4181,7 +4424,7 @@ fn run_low_level<'a, 'ctx, 'env>(
 
             match list_layout {
                 Layout::Builtin(Builtin::EmptyList) => empty_list(env),
-                Layout::Builtin(Builtin::List(_, element_layout)) => list_drop(
+                Layout::Builtin(Builtin::List(element_layout)) => list_drop(
                     env,
                     layout_ids,
                     original_wrapper,
@@ -4198,7 +4441,7 @@ fn run_low_level<'a, 'ctx, 'env>(
             let original_wrapper = load_symbol(scope, &args[0]).into_struct_value();
             let (elem, elem_layout) = load_symbol_and_layout(scope, &args[1]);
 
-            list_prepend(env, layout.in_place(), original_wrapper, elem, elem_layout)
+            list_prepend(env, original_wrapper, elem, elem_layout)
         }
         ListJoin => {
             // List.join : List (List elem) -> List elem
@@ -4206,7 +4449,7 @@ fn run_low_level<'a, 'ctx, 'env>(
 
             let (list, outer_list_layout) = load_symbol_and_layout(scope, &args[0]);
 
-            list_join(env, layout.in_place(), parent, list, outer_list_layout)
+            list_join(env, parent, list, outer_list_layout)
         }
         NumAbs | NumNeg | NumRound | NumSqrtUnchecked | NumLogUnchecked | NumSin | NumCos
         | NumCeiling | NumFloor | NumToFloat | NumIsFinite | NumAtan | NumAcos | NumAsin => {
@@ -4424,7 +4667,7 @@ fn run_low_level<'a, 'ctx, 'env>(
             BasicValueEnum::IntValue(bool_val)
         }
         ListGetUnsafe => {
-            // List.get : List elem, Int -> [ Ok elem, OutOfBounds ]*
+            // List.get : List elem, Nat -> [ Ok elem, OutOfBounds ]*
             debug_assert_eq!(args.len(), 2);
 
             let (wrapper_struct, list_layout) = load_symbol_and_layout(scope, &args[0]);
@@ -4440,27 +4683,6 @@ fn run_low_level<'a, 'ctx, 'env>(
                 wrapper_struct,
             )
         }
-        ListSetInPlace => {
-            let (list, list_layout) = load_symbol_and_layout(scope, &args[0]);
-            let (index, _) = load_symbol_and_layout(scope, &args[1]);
-            let (element, _) = load_symbol_and_layout(scope, &args[2]);
-
-            match list_layout {
-                Layout::Builtin(Builtin::EmptyList) => {
-                    // no elements, so nothing to remove
-                    empty_list(env)
-                }
-                Layout::Builtin(Builtin::List(_, element_layout)) => list_set(
-                    env,
-                    layout_ids,
-                    list,
-                    index.into_int_value(),
-                    element,
-                    element_layout,
-                ),
-                _ => unreachable!("invalid dict layout"),
-            }
-        }
         ListSet => {
             let (list, list_layout) = load_symbol_and_layout(scope, &args[0]);
             let (index, _) = load_symbol_and_layout(scope, &args[1]);
@@ -4471,13 +4693,14 @@ fn run_low_level<'a, 'ctx, 'env>(
                     // no elements, so nothing to remove
                     empty_list(env)
                 }
-                Layout::Builtin(Builtin::List(_, element_layout)) => list_set(
+                Layout::Builtin(Builtin::List(element_layout)) => list_set(
                     env,
                     layout_ids,
                     list,
                     index.into_int_value(),
                     element,
                     element_layout,
+                    update_mode.unwrap(),
                 ),
                 _ => unreachable!("invalid dict layout"),
             }
@@ -4648,7 +4871,7 @@ fn run_low_level<'a, 'ctx, 'env>(
 
             match list_layout {
                 Layout::Builtin(Builtin::EmptyList) => dict_empty(env),
-                Layout::Builtin(Builtin::List(_, key_layout)) => {
+                Layout::Builtin(Builtin::List(key_layout)) => {
                     set_from_list(env, layout_ids, list, key_layout)
                 }
                 _ => unreachable!("invalid dict layout"),
@@ -4693,6 +4916,7 @@ enum ForeignCallOrInvoke<'a> {
     Call,
     Invoke {
         symbol: Symbol,
+        exception_id: ExceptionId,
         pass: &'a roc_mono::ir::Stmt<'a>,
         fail: &'a roc_mono::ir::Stmt<'a>,
     },
@@ -4716,7 +4940,7 @@ fn build_foreign_symbol_return_result<'a, 'ctx, 'env>(
         arg_types.push(arg_type);
     }
 
-    let function_type = get_fn_type(&return_type, &arg_types);
+    let function_type = return_type.fn_type(&arg_types, false);
     let function = get_foreign_symbol(env, foreign.clone(), function_type);
 
     (function, arg_vals.into_bump_slice())
@@ -4753,6 +4977,7 @@ fn build_foreign_symbol_write_result_into_ptr<'a, 'ctx, 'env>(
 fn build_foreign_symbol<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     layout_ids: &mut LayoutIds<'a>,
+    func_spec_solutions: &FuncSpecSolutions,
     scope: &mut Scope<'a, 'ctx>,
     parent: FunctionValue<'ctx>,
     foreign: &roc_module::ident::ForeignSymbol,
@@ -4787,7 +5012,12 @@ fn build_foreign_symbol<'a, 'ctx, 'env>(
                 call.try_as_basic_value().left().unwrap()
             }
         }
-        ForeignCallOrInvoke::Invoke { symbol, pass, fail } => {
+        ForeignCallOrInvoke::Invoke {
+            symbol,
+            pass,
+            fail,
+            exception_id,
+        } => {
             let pass_block = env.context.append_basic_block(parent, "invoke_pass");
             let fail_block = env.context.append_basic_block(parent, "invoke_fail");
 
@@ -4811,7 +5041,7 @@ fn build_foreign_symbol<'a, 'ctx, 'env>(
 
                 scope.insert(symbol, (*ret_layout, call_result));
 
-                build_exp_stmt(env, layout_ids, scope, parent, pass);
+                build_exp_stmt(env, layout_ids, func_spec_solutions, scope, parent, pass);
 
                 scope.remove(&symbol);
             }
@@ -4828,16 +5058,22 @@ fn build_foreign_symbol<'a, 'ctx, 'env>(
                         .struct_type(&[exception_ptr, selector_value], false)
                 };
 
-                env.builder
-                    .build_catch_all_landing_pad(
-                        &landing_pad_type,
-                        &BasicValueEnum::IntValue(env.context.i8_type().const_zero()),
-                        env.context.i8_type().ptr_type(AddressSpace::Generic),
-                        "invoke_landing_pad",
-                    )
-                    .into_struct_value();
+                let personality_function = get_gxx_personality_v0(env);
 
-                build_exp_stmt(env, layout_ids, scope, parent, fail);
+                let exception_object = env.builder.build_landing_pad(
+                    landing_pad_type,
+                    personality_function,
+                    &[],
+                    true,
+                    "invoke_landing_pad",
+                );
+
+                scope.insert(
+                    exception_id.into_inner(),
+                    (Layout::Struct(&[]), exception_object),
+                );
+
+                build_exp_stmt(env, layout_ids, func_spec_solutions, scope, parent, fail);
             }
 
             call_result
@@ -5004,7 +5240,7 @@ fn build_int_binop<'a, 'ctx, 'env>(
             //        rem == 0
             //    }
             //
-            // NOTE we'd like the branches to be swapped for better branch prediciton,
+            // NOTE we'd like the branches to be swapped for better branch prediction,
             // but llvm normalizes to the above ordering in -O3
             let zero = rhs.get_type().const_zero();
             let neg_1 = rhs.get_type().const_int(-1i64 as u64, false);
@@ -5351,7 +5587,7 @@ fn build_int_unary_op<'a, 'ctx, 'env>(
             int_abs_raise_on_overflow(env, arg, arg_layout)
         }
         NumToFloat => {
-            // TODO: Handle differnt sized numbers
+            // TODO: Handle different sized numbers
             // This is an Int, so we need to convert it.
             bd.build_cast(
                 InstructionOpcode::SIToFP,
@@ -5475,7 +5711,7 @@ fn build_float_unary_op<'a, 'ctx, 'env>(
 
     let bd = env.builder;
 
-    // TODO: Handle differnt sized floats
+    // TODO: Handle different sized floats
     match op {
         NumNeg => bd.build_float_neg(arg, "negate_float").into(),
         NumAbs => env.call_intrinsic(LLVM_FABS_F64, &[arg.into()]),
@@ -5616,7 +5852,7 @@ fn throw_exception<'a, 'ctx, 'env>(env: &Env<'a, 'ctx, 'env>, message: &str) {
     let builder = env.builder;
 
     let info = {
-        // we represend both void and char pointers with `u8*`
+        // we represented both void and char pointers with `u8*`
         let u8_ptr = context.i8_type().ptr_type(AddressSpace::Generic);
 
         // allocate an exception (that can hold a pointer to a string)
@@ -5737,32 +5973,6 @@ fn cxa_throw_exception<'a, 'ctx, 'env>(env: &Env<'a, 'ctx, 'env>, info: BasicVal
     call.set_call_convention(C_CALL_CONV);
 }
 
-fn cxa_rethrow_exception(env: &Env<'_, '_, '_>) {
-    let name = "__cxa_rethrow";
-
-    let module = env.module;
-    let context = env.context;
-
-    let function = match module.get_function(&name) {
-        Some(gvalue) => gvalue,
-        None => {
-            let cxa_rethrow = add_func(
-                module,
-                name,
-                context.void_type().fn_type(&[], false),
-                Linkage::External,
-                C_CALL_CONV,
-            );
-
-            cxa_rethrow
-        }
-    };
-    let call = env.builder.build_call(function, &[], "rethrow");
-
-    call.set_call_convention(C_CALL_CONV);
-    // call.try_as_basic_value().left().unwrap()
-}
-
 fn get_foreign_symbol<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     foreign_symbol: roc_module::ident::ForeignSymbol,
@@ -5797,7 +6007,7 @@ fn get_gxx_personality_v0<'a, 'ctx, 'env>(env: &Env<'a, 'ctx, 'env>) -> Function
         None => {
             let personality_func = add_func(
                 module,
-                "__gxx_personality_v0",
+                name,
                 context.i64_type().fn_type(&[], false),
                 Linkage::External,
                 C_CALL_CONV,
@@ -5849,7 +6059,7 @@ fn cxa_begin_catch<'a, 'ctx, 'env>(
 
             let cxa_begin_catch = add_func(
                 module,
-                "__cxa_begin_catch",
+                name,
                 u8_ptr.fn_type(&[u8_ptr.into()], false),
                 Linkage::External,
                 C_CALL_CONV,
diff --git a/compiler/gen/src/llvm/build_dict.rs b/compiler/gen_llvm/src/llvm/build_dict.rs
similarity index 97%
rename from compiler/gen/src/llvm/build_dict.rs
rename to compiler/gen_llvm/src/llvm/build_dict.rs
index d95a557ad5..73f799466e 100644
--- a/compiler/gen/src/llvm/build_dict.rs
+++ b/compiler/gen_llvm/src/llvm/build_dict.rs
@@ -3,14 +3,14 @@ use crate::llvm::bitcode::{
     build_dec_wrapper, build_eq_wrapper, build_inc_wrapper, call_bitcode_fn, call_void_bitcode_fn,
 };
 use crate::llvm::build::{
-    complex_bitcast, load_symbol, load_symbol_and_layout, set_name, Env, RocFunctionCall, Scope,
+    complex_bitcast, load_symbol, load_symbol_and_layout, Env, RocFunctionCall, Scope,
 };
 use crate::llvm::build_list::{layout_width, pass_as_opaque};
-use crate::llvm::convert::{as_const_zero, basic_type_from_layout};
+use crate::llvm::convert::basic_type_from_layout;
 use crate::llvm::refcounting::Mode;
 use inkwell::attributes::{Attribute, AttributeLoc};
 use inkwell::types::BasicType;
-use inkwell::values::{BasicValueEnum, FunctionValue, StructValue};
+use inkwell::values::{BasicValue, BasicValueEnum, FunctionValue, StructValue};
 use inkwell::AddressSpace;
 use roc_builtins::bitcode;
 use roc_module::symbol::Symbol;
@@ -326,7 +326,7 @@ pub fn dict_get<'a, 'ctx, 'env>(
     let done_block = env.context.append_basic_block(parent, "done");
 
     let value_bt = basic_type_from_layout(env, value_layout);
-    let default = as_const_zero(&value_bt);
+    let default = value_bt.const_zero();
 
     env.builder
         .build_conditional_branch(flag, if_not_null, done_block);
@@ -635,7 +635,6 @@ fn dict_intersect_or_difference<'a, 'ctx, 'env>(
 #[allow(clippy::too_many_arguments)]
 pub fn dict_walk<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
-    layout_ids: &mut LayoutIds<'a>,
     roc_function_call: RocFunctionCall<'ctx>,
     dict: BasicValueEnum<'ctx>,
     accum: BasicValueEnum<'ctx>,
@@ -660,9 +659,6 @@ pub fn dict_walk<'a, 'ctx, 'env>(
 
     let output_ptr = builder.build_alloca(accum_bt, "output_ptr");
 
-    let inc_key_fn = build_inc_wrapper(env, layout_ids, key_layout);
-    let inc_value_fn = build_inc_wrapper(env, layout_ids, value_layout);
-
     call_void_bitcode_fn(
         env,
         &[
@@ -676,8 +672,6 @@ pub fn dict_walk<'a, 'ctx, 'env>(
             layout_width(env, key_layout),
             layout_width(env, value_layout),
             layout_width(env, accum_layout),
-            inc_key_fn.as_global_value().as_pointer_value().into(),
-            inc_value_fn.as_global_value().as_pointer_value().into(),
             env.builder.build_bitcast(output_ptr, u8_ptr, "to_opaque"),
         ],
         &bitcode::DICT_WALK,
@@ -836,8 +830,8 @@ fn build_hash_wrapper<'a, 'ctx, 'env>(
             let seed_arg = it.next().unwrap().into_int_value();
             let value_ptr = it.next().unwrap().into_pointer_value();
 
-            set_name(seed_arg.into(), Symbol::ARG_1.ident_string(&env.interns));
-            set_name(value_ptr.into(), Symbol::ARG_2.ident_string(&env.interns));
+            seed_arg.set_name(Symbol::ARG_1.ident_string(&env.interns));
+            value_ptr.set_name(Symbol::ARG_2.ident_string(&env.interns));
 
             let value_type = basic_type_from_layout(env, layout).ptr_type(AddressSpace::Generic);
 
diff --git a/compiler/gen/src/llvm/build_hash.rs b/compiler/gen_llvm/src/llvm/build_hash.rs
similarity index 83%
rename from compiler/gen/src/llvm/build_hash.rs
rename to compiler/gen_llvm/src/llvm/build_hash.rs
index 95db536a88..61a74dd232 100644
--- a/compiler/gen/src/llvm/build_hash.rs
+++ b/compiler/gen_llvm/src/llvm/build_hash.rs
@@ -1,11 +1,13 @@
 use crate::debug_info_init;
 use crate::llvm::bitcode::call_bitcode_fn;
 use crate::llvm::build::Env;
-use crate::llvm::build::{cast_block_of_memory_to_tag, complex_bitcast, set_name, FAST_CALL_CONV};
+use crate::llvm::build::{cast_block_of_memory_to_tag, get_tag_id, FAST_CALL_CONV, TAG_DATA_INDEX};
 use crate::llvm::build_str;
 use crate::llvm::convert::basic_type_from_layout;
 use bumpalo::collections::Vec;
-use inkwell::values::{BasicValueEnum, FunctionValue, IntValue, PointerValue, StructValue};
+use inkwell::values::{
+    BasicValue, BasicValueEnum, FunctionValue, IntValue, PointerValue, StructValue,
+};
 use roc_builtins::bitcode;
 use roc_module::symbol::Symbol;
 use roc_mono::layout::{Builtin, Layout, LayoutIds, UnionLayout};
@@ -56,11 +58,6 @@ fn build_hash_layout<'a, 'ctx, 'env>(
             val.into_struct_value(),
         ),
 
-        Layout::PhantomEmptyStruct => {
-            // just does nothing and returns the seed
-            seed
-        }
-
         Layout::Union(union_layout) => {
             build_hash_tag(env, layout_ids, layout, union_layout, seed, val)
         }
@@ -91,11 +88,7 @@ fn build_hash_layout<'a, 'ctx, 'env>(
             }
         },
 
-        Layout::Pointer(_) => {
-            unreachable!("unused")
-        }
-
-        Layout::FunctionPointer(_, _) | Layout::Closure(_, _, _) => {
+        Layout::Closure(_, _, _) => {
             unreachable!("the type system will guarantee these are never hashed")
         }
     }
@@ -157,7 +150,7 @@ fn hash_builtin<'a, 'ctx, 'env>(
         Builtin::Set(_) => {
             todo!("Implement Hash for Set")
         }
-        Builtin::List(_, element_layout) => build_hash_list(
+        Builtin::List(element_layout) => build_hash_list(
             env,
             layout_ids,
             layout,
@@ -241,8 +234,8 @@ fn build_hash_struct_help<'a, 'ctx, 'env>(
     let seed = it.next().unwrap().into_int_value();
     let value = it.next().unwrap().into_struct_value();
 
-    set_name(seed.into(), Symbol::ARG_1.ident_string(&env.interns));
-    set_name(value.into(), Symbol::ARG_2.ident_string(&env.interns));
+    seed.set_name(Symbol::ARG_1.ident_string(&env.interns));
+    value.set_name(Symbol::ARG_2.ident_string(&env.interns));
 
     let entry = ctx.append_basic_block(parent, "entry");
     env.builder.position_at_end(entry);
@@ -382,8 +375,8 @@ fn build_hash_tag_help<'a, 'ctx, 'env>(
     let seed = it.next().unwrap().into_int_value();
     let value = it.next().unwrap();
 
-    set_name(seed.into(), Symbol::ARG_1.ident_string(&env.interns));
-    set_name(value, Symbol::ARG_2.ident_string(&env.interns));
+    seed.set_name(Symbol::ARG_1.ident_string(&env.interns));
+    value.set_name(Symbol::ARG_2.ident_string(&env.interns));
 
     let entry = ctx.append_basic_block(parent, "entry");
     env.builder.position_at_end(entry);
@@ -408,13 +401,15 @@ fn hash_tag<'a, 'ctx, 'env>(
     let merge_block = env.context.append_basic_block(parent, "merge_block");
     env.builder.position_at_end(merge_block);
 
-    let merge_phi = env.builder.build_phi(env.context.i64_type(), "merge_hash");
+    let tag_id_layout = union_layout.tag_id_layout();
+    let tag_id_basic_type = basic_type_from_layout(env, &tag_id_layout);
+
+    let merge_phi = env.builder.build_phi(seed.get_type(), "merge_hash");
 
     env.builder.position_at_end(entry_block);
     match union_layout {
         NonRecursive(tags) => {
-            // SAFETY we know that non-recursive tags cannot be NULL
-            let tag_id = nonrec_tag_id(env, tag.into_struct_value());
+            let current_tag_id = get_tag_id(env, parent, union_layout, tag);
 
             let mut cases = Vec::with_capacity_in(tags.len(), env.arena);
 
@@ -422,7 +417,6 @@ fn hash_tag<'a, 'ctx, 'env>(
                 let block = env.context.append_basic_block(parent, "tag_id_modify");
                 env.builder.position_at_end(block);
 
-                // TODO drop tag id?
                 let struct_layout = Layout::Struct(field_layouts);
 
                 let wrapper_type = basic_type_from_layout(env, &struct_layout);
@@ -431,6 +425,24 @@ fn hash_tag<'a, 'ctx, 'env>(
                 let as_struct =
                     cast_block_of_memory_to_tag(env.builder, tag.into_struct_value(), wrapper_type);
 
+                // hash the tag id
+                let hash_bytes = store_and_use_as_u8_ptr(
+                    env,
+                    tag_id_basic_type
+                        .into_int_type()
+                        .const_int(tag_id as u64, false)
+                        .into(),
+                    &tag_id_layout,
+                );
+
+                let seed = hash_bitcode_fn(
+                    env,
+                    seed,
+                    hash_bytes,
+                    tag_id_layout.stack_size(env.ptr_bytes),
+                );
+
+                // hash the tag data
                 let answer = build_hash_struct(
                     env,
                     layout_ids,
@@ -444,7 +456,7 @@ fn hash_tag<'a, 'ctx, 'env>(
                 env.builder.build_unconditional_branch(merge_block);
 
                 cases.push((
-                    env.context.i64_type().const_int(tag_id as u64, false),
+                    current_tag_id.get_type().const_int(tag_id as u64, false),
                     block,
                 ));
             }
@@ -453,11 +465,10 @@ fn hash_tag<'a, 'ctx, 'env>(
 
             let default = cases.pop().unwrap().1;
 
-            env.builder.build_switch(tag_id, default, &cases);
+            env.builder.build_switch(current_tag_id, default, &cases);
         }
         Recursive(tags) => {
-            // SAFETY recursive tag unions are not NULL
-            let tag_id = unsafe { rec_tag_id_unsafe(env, tag.into_pointer_value()) };
+            let current_tag_id = get_tag_id(env, parent, union_layout, tag);
 
             let mut cases = Vec::with_capacity_in(tags.len(), env.arena);
 
@@ -465,6 +476,23 @@ fn hash_tag<'a, 'ctx, 'env>(
                 let block = env.context.append_basic_block(parent, "tag_id_modify");
                 env.builder.position_at_end(block);
 
+                // hash the tag id
+                let hash_bytes = store_and_use_as_u8_ptr(
+                    env,
+                    tag_id_basic_type
+                        .into_int_type()
+                        .const_int(tag_id as u64, false)
+                        .into(),
+                    &tag_id_layout,
+                );
+                let seed = hash_bitcode_fn(
+                    env,
+                    seed,
+                    hash_bytes,
+                    tag_id_layout.stack_size(env.ptr_bytes),
+                );
+
+                // hash the tag data
                 let answer = hash_ptr_to_struct(
                     env,
                     layout_ids,
@@ -478,7 +506,7 @@ fn hash_tag<'a, 'ctx, 'env>(
                 env.builder.build_unconditional_branch(merge_block);
 
                 cases.push((
-                    env.context.i64_type().const_int(tag_id as u64, false),
+                    current_tag_id.get_type().const_int(tag_id as u64, false),
                     block,
                 ));
             }
@@ -487,11 +515,10 @@ fn hash_tag<'a, 'ctx, 'env>(
 
             let default = cases.pop().unwrap().1;
 
-            env.builder.build_switch(tag_id, default, &cases);
+            env.builder.build_switch(current_tag_id, default, &cases);
         }
         NullableUnwrapped { other_fields, .. } => {
             let tag = tag.into_pointer_value();
-            let other_fields = &other_fields[1..];
 
             let is_null = env.builder.build_is_null(tag, "is_null");
 
@@ -520,7 +547,10 @@ fn hash_tag<'a, 'ctx, 'env>(
                 env.builder.build_unconditional_branch(merge_block);
             }
         }
-        NullableWrapped { other_tags, .. } => {
+        NullableWrapped {
+            other_tags,
+            nullable_id,
+        } => {
             let tag = tag.into_pointer_value();
 
             let is_null = env.builder.build_is_null(tag, "is_null");
@@ -541,31 +571,57 @@ fn hash_tag<'a, 'ctx, 'env>(
             }
 
             {
-                env.builder.position_at_end(hash_other_block);
-
-                // SAFETY recursive tag unions are not NULL
-                let tag_id = unsafe { rec_tag_id_unsafe(env, tag) };
-
                 let mut cases = Vec::with_capacity_in(other_tags.len(), env.arena);
 
-                for (tag_id, field_layouts) in other_tags.iter().enumerate() {
+                for (mut tag_id, field_layouts) in other_tags.iter().enumerate() {
+                    if tag_id >= *nullable_id as usize {
+                        tag_id += 1;
+                    }
+
                     let block = env.context.append_basic_block(parent, "tag_id_modify");
                     env.builder.position_at_end(block);
 
-                    let answer =
-                        hash_ptr_to_struct(env, layout_ids, union_layout, field_layouts, seed, tag);
+                    // hash the tag id
+                    let hash_bytes = store_and_use_as_u8_ptr(
+                        env,
+                        tag_id_basic_type
+                            .into_int_type()
+                            .const_int(tag_id as u64, false)
+                            .into(),
+                        &tag_id_layout,
+                    );
+                    let seed1 = hash_bitcode_fn(
+                        env,
+                        seed,
+                        hash_bytes,
+                        tag_id_layout.stack_size(env.ptr_bytes),
+                    );
+
+                    // hash tag data
+                    let answer = hash_ptr_to_struct(
+                        env,
+                        layout_ids,
+                        union_layout,
+                        field_layouts,
+                        seed1,
+                        tag,
+                    );
 
                     merge_phi.add_incoming(&[(&answer, block)]);
                     env.builder.build_unconditional_branch(merge_block);
 
                     cases.push((
-                        env.context.i64_type().const_int(tag_id as u64, false),
+                        tag_id_basic_type
+                            .into_int_type()
+                            .const_int(tag_id as u64, false),
                         block,
                     ));
                 }
 
                 env.builder.position_at_end(hash_other_block);
 
+                let tag_id = get_tag_id(env, parent, union_layout, tag.into());
+
                 let default = cases.pop().unwrap().1;
 
                 env.builder.build_switch(tag_id, default, &cases);
@@ -662,8 +718,8 @@ fn build_hash_list_help<'a, 'ctx, 'env>(
     let seed = it.next().unwrap().into_int_value();
     let value = it.next().unwrap().into_struct_value();
 
-    set_name(seed.into(), Symbol::ARG_1.ident_string(&env.interns));
-    set_name(value.into(), Symbol::ARG_2.ident_string(&env.interns));
+    seed.set_name(Symbol::ARG_1.ident_string(&env.interns));
+    value.set_name(Symbol::ARG_2.ident_string(&env.interns));
 
     let entry = ctx.append_basic_block(parent, "entry");
     env.builder.position_at_end(entry);
@@ -772,18 +828,27 @@ fn hash_ptr_to_struct<'a, 'ctx, 'env>(
 ) -> IntValue<'ctx> {
     use inkwell::types::BasicType;
 
-    let struct_layout = Layout::Struct(field_layouts);
-
-    let wrapper_type = basic_type_from_layout(env, &struct_layout);
-    debug_assert!(wrapper_type.is_struct_type());
+    let wrapper_type = basic_type_from_layout(env, &Layout::Union(*union_layout));
 
     // cast the opaque pointer to a pointer of the correct shape
+    let wrapper_ptr = env
+        .builder
+        .build_bitcast(tag, wrapper_type, "opaque_to_correct")
+        .into_pointer_value();
+
+    let struct_ptr = env
+        .builder
+        .build_struct_gep(wrapper_ptr, TAG_DATA_INDEX, "get_tag_data")
+        .unwrap();
+
+    let struct_layout = Layout::Struct(field_layouts);
+    let struct_type = basic_type_from_layout(env, &struct_layout);
     let struct_ptr = env
         .builder
         .build_bitcast(
-            tag,
-            wrapper_type.ptr_type(inkwell::AddressSpace::Generic),
-            "opaque_to_correct",
+            struct_ptr,
+            struct_type.ptr_type(inkwell::AddressSpace::Generic),
+            "cast_tag_data",
         )
         .into_pointer_value();
 
@@ -811,14 +876,13 @@ fn store_and_use_as_u8_ptr<'a, 'ctx, 'env>(
     let alloc = env.builder.build_alloca(basic_type, "store");
     env.builder.build_store(alloc, value);
 
+    let u8_ptr = env
+        .context
+        .i8_type()
+        .ptr_type(inkwell::AddressSpace::Generic);
+
     env.builder
-        .build_bitcast(
-            alloc,
-            env.context
-                .i8_type()
-                .ptr_type(inkwell::AddressSpace::Generic),
-            "as_u8_ptr",
-        )
+        .build_bitcast(alloc, u8_ptr, "as_u8_ptr")
         .into_pointer_value()
 }
 
@@ -837,34 +901,3 @@ fn hash_bitcode_fn<'a, 'ctx, 'env>(
     )
     .into_int_value()
 }
-
-fn nonrec_tag_id<'a, 'ctx, 'env>(
-    env: &Env<'a, 'ctx, 'env>,
-    tag: StructValue<'ctx>,
-) -> IntValue<'ctx> {
-    complex_bitcast(
-        env.builder,
-        tag.into(),
-        env.context.i64_type().into(),
-        "load_tag_id",
-    )
-    .into_int_value()
-}
-
-unsafe fn rec_tag_id_unsafe<'a, 'ctx, 'env>(
-    env: &Env<'a, 'ctx, 'env>,
-    tag: PointerValue<'ctx>,
-) -> IntValue<'ctx> {
-    let ptr = env
-        .builder
-        .build_bitcast(
-            tag,
-            env.context
-                .i64_type()
-                .ptr_type(inkwell::AddressSpace::Generic),
-            "cast_for_tag_id",
-        )
-        .into_pointer_value();
-
-    env.builder.build_load(ptr, "load_tag_id").into_int_value()
-}
diff --git a/compiler/gen/src/llvm/build_list.rs b/compiler/gen_llvm/src/llvm/build_list.rs
similarity index 88%
rename from compiler/gen/src/llvm/build_list.rs
rename to compiler/gen_llvm/src/llvm/build_list.rs
index f71e4a507a..7aa7e07ead 100644
--- a/compiler/gen/src/llvm/build_list.rs
+++ b/compiler/gen_llvm/src/llvm/build_list.rs
@@ -1,20 +1,21 @@
 #![allow(clippy::too_many_arguments)]
 use crate::llvm::bitcode::{
-    build_dec_wrapper, build_eq_wrapper, build_inc_n_wrapper, build_inc_wrapper,
-    build_transform_caller, call_bitcode_fn, call_void_bitcode_fn,
+    build_dec_wrapper, build_eq_wrapper, build_has_tag_id, build_inc_n_wrapper, build_inc_wrapper,
+    call_bitcode_fn, call_void_bitcode_fn,
 };
 use crate::llvm::build::{
     allocate_with_refcount_help, cast_basic_basic, complex_bitcast, Env, RocFunctionCall,
 };
-use crate::llvm::convert::{basic_type_from_layout, get_ptr_type};
+use crate::llvm::convert::basic_type_from_layout;
 use crate::llvm::refcounting::increment_refcount_layout;
 use inkwell::builder::Builder;
 use inkwell::context::Context;
-use inkwell::types::{BasicTypeEnum, PointerType};
+use inkwell::types::{BasicType, BasicTypeEnum, PointerType};
 use inkwell::values::{BasicValueEnum, FunctionValue, IntValue, PointerValue, StructValue};
 use inkwell::{AddressSpace, IntPredicate};
+use morphic_lib::UpdateMode;
 use roc_builtins::bitcode;
-use roc_mono::layout::{Builtin, InPlace, Layout, LayoutIds, MemoryMode};
+use roc_mono::layout::{Builtin, Layout, LayoutIds};
 
 fn list_returned_from_zig<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
@@ -83,7 +84,6 @@ pub fn pass_as_opaque<'a, 'ctx, 'env>(
 /// List.single : a -> List a
 pub fn list_single<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
-    _inplace: InPlace,
     element: BasicValueEnum<'ctx>,
     element_layout: &Layout<'a>,
 ) -> BasicValueEnum<'ctx> {
@@ -124,7 +124,6 @@ pub fn list_repeat<'a, 'ctx, 'env>(
 /// List.prepend : List elem, elem -> List elem
 pub fn list_prepend<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
-    inplace: InPlace,
     original_wrapper: StructValue<'ctx>,
     elem: BasicValueEnum<'ctx>,
     elem_layout: &Layout<'a>,
@@ -135,7 +134,7 @@ pub fn list_prepend<'a, 'ctx, 'env>(
     // Load the usize length from the wrapper.
     let len = list_len(builder, original_wrapper);
     let elem_type = basic_type_from_layout(env, elem_layout);
-    let ptr_type = get_ptr_type(&elem_type, AddressSpace::Generic);
+    let ptr_type = elem_type.ptr_type(AddressSpace::Generic);
     let list_ptr = load_list_ptr(builder, original_wrapper, ptr_type);
 
     // The output list length, which is the old list length + 1
@@ -146,7 +145,7 @@ pub fn list_prepend<'a, 'ctx, 'env>(
     );
 
     // Allocate space for the new array that we'll copy into.
-    let clone_ptr = allocate_list(env, inplace, elem_layout, new_list_len);
+    let clone_ptr = allocate_list(env, elem_layout, new_list_len);
 
     builder.build_store(clone_ptr, elem);
 
@@ -189,19 +188,18 @@ pub fn list_prepend<'a, 'ctx, 'env>(
 /// List.join : List (List elem) -> List elem
 pub fn list_join<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
-    _inplace: InPlace,
     _parent: FunctionValue<'ctx>,
     outer_list: BasicValueEnum<'ctx>,
     outer_list_layout: &Layout<'a>,
 ) -> BasicValueEnum<'ctx> {
     match outer_list_layout {
         Layout::Builtin(Builtin::EmptyList)
-        | Layout::Builtin(Builtin::List(_, Layout::Builtin(Builtin::EmptyList))) => {
+        | Layout::Builtin(Builtin::List(Layout::Builtin(Builtin::EmptyList))) => {
             // If the input list is empty, or if it is a list of empty lists
             // then simply return an empty list
             empty_list(env)
         }
-        Layout::Builtin(Builtin::List(_, Layout::Builtin(Builtin::List(_, element_layout)))) => {
+        Layout::Builtin(Builtin::List(Layout::Builtin(Builtin::List(element_layout)))) => {
             call_bitcode_fn_returns_list(
                 env,
                 &[
@@ -221,23 +219,16 @@ pub fn list_join<'a, 'ctx, 'env>(
 /// List.reverse : List elem -> List elem
 pub fn list_reverse<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
-    _output_inplace: InPlace,
     list: BasicValueEnum<'ctx>,
     list_layout: &Layout<'a>,
 ) -> BasicValueEnum<'ctx> {
-    let (_, element_layout) = match *list_layout {
-        Layout::Builtin(Builtin::EmptyList) => (
-            InPlace::InPlace,
+    let element_layout = match *list_layout {
+        Layout::Builtin(Builtin::EmptyList) => {
             // this pointer will never actually be dereferenced
-            Layout::Builtin(Builtin::Int64),
-        ),
-        Layout::Builtin(Builtin::List(memory_mode, elem_layout)) => (
-            match memory_mode {
-                MemoryMode::Unique => InPlace::InPlace,
-                MemoryMode::Refcounted => InPlace::Clone,
-            },
-            *elem_layout,
-        ),
+            Layout::Builtin(Builtin::Int64)
+        }
+
+        Layout::Builtin(Builtin::List(elem_layout)) => *elem_layout,
 
         _ => unreachable!("Invalid layout {:?} in List.reverse", list_layout),
     };
@@ -264,9 +255,9 @@ pub fn list_get_unsafe<'a, 'ctx, 'env>(
     let builder = env.builder;
 
     match list_layout {
-        Layout::Builtin(Builtin::List(_, elem_layout)) => {
+        Layout::Builtin(Builtin::List(elem_layout)) => {
             let elem_type = basic_type_from_layout(env, elem_layout);
-            let ptr_type = get_ptr_type(&elem_type, AddressSpace::Generic);
+            let ptr_type = elem_type.ptr_type(AddressSpace::Generic);
             // Load the pointer to the array data
             let array_data_ptr = load_list_ptr(builder, wrapper_struct, ptr_type);
 
@@ -293,7 +284,6 @@ pub fn list_get_unsafe<'a, 'ctx, 'env>(
 /// List.append : List elem, elem -> List elem
 pub fn list_append<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
-    _inplace: InPlace,
     original_wrapper: StructValue<'ctx>,
     element: BasicValueEnum<'ctx>,
     element_layout: &Layout<'a>,
@@ -310,7 +300,28 @@ pub fn list_append<'a, 'ctx, 'env>(
     )
 }
 
-/// List.drop : List elem, Nat -> List elem
+/// List.swap : List elem, Nat, Nat -> List elem
+pub fn list_swap<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    original_wrapper: StructValue<'ctx>,
+    index_1: IntValue<'ctx>,
+    index_2: IntValue<'ctx>,
+    element_layout: &Layout<'a>,
+) -> BasicValueEnum<'ctx> {
+    call_bitcode_fn_returns_list(
+        env,
+        &[
+            pass_list_as_i128(env, original_wrapper.into()),
+            env.alignment_intvalue(&element_layout),
+            layout_width(env, &element_layout),
+            index_1.into(),
+            index_2.into(),
+        ],
+        &bitcode::LIST_SWAP,
+    )
+}
+
+/// List.drop : List elem, Nat, Nat -> List elem
 pub fn list_drop<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     layout_ids: &mut LayoutIds<'a>,
@@ -340,6 +351,7 @@ pub fn list_set<'a, 'ctx, 'env>(
     index: IntValue<'ctx>,
     element: BasicValueEnum<'ctx>,
     element_layout: &'a Layout<'a>,
+    update_mode: UpdateMode,
 ) -> BasicValueEnum<'ctx> {
     let dec_element_fn = build_dec_wrapper(env, layout_ids, element_layout);
 
@@ -349,19 +361,32 @@ pub fn list_set<'a, 'ctx, 'env>(
         env.context.i8_type().ptr_type(AddressSpace::Generic),
     );
 
-    let new_bytes = call_bitcode_fn(
-        env,
-        &[
-            bytes.into(),
-            length.into(),
-            env.alignment_intvalue(&element_layout),
-            index.into(),
-            pass_element_as_opaque(env, element),
-            layout_width(env, element_layout),
-            dec_element_fn.as_global_value().as_pointer_value().into(),
-        ],
-        &bitcode::LIST_SET,
-    );
+    let new_bytes = match update_mode {
+        UpdateMode::InPlace => call_bitcode_fn(
+            env,
+            &[
+                bytes.into(),
+                index.into(),
+                pass_element_as_opaque(env, element),
+                layout_width(env, element_layout),
+                dec_element_fn.as_global_value().as_pointer_value().into(),
+            ],
+            bitcode::LIST_SET_IN_PLACE,
+        ),
+        UpdateMode::Immutable => call_bitcode_fn(
+            env,
+            &[
+                bytes.into(),
+                length.into(),
+                env.alignment_intvalue(&element_layout),
+                index.into(),
+                pass_element_as_opaque(env, element),
+                layout_width(env, element_layout),
+                dec_element_fn.as_global_value().as_pointer_value().into(),
+            ],
+            bitcode::LIST_SET,
+        ),
+    };
 
     store_list(env, new_bytes.into_pointer_value(), length)
 }
@@ -400,6 +425,7 @@ pub fn list_walk_generic<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     layout_ids: &mut LayoutIds<'a>,
     roc_function_call: RocFunctionCall<'ctx>,
+    function_call_return_layout: &Layout<'a>,
     list: BasicValueEnum<'ctx>,
     element_layout: &Layout<'a>,
     default: BasicValueEnum<'ctx>,
@@ -440,6 +466,18 @@ pub fn list_walk_generic<'a, 'ctx, 'env>(
             );
         }
         ListWalk::WalkUntil | ListWalk::WalkBackwardsUntil => {
+            let function = env
+                .builder
+                .get_insert_block()
+                .unwrap()
+                .get_parent()
+                .unwrap();
+
+            let has_tag_id = match function_call_return_layout {
+                Layout::Union(union_layout) => build_has_tag_id(env, function, *union_layout),
+                _ => unreachable!(),
+            };
+
             let dec_element_fn = build_dec_wrapper(env, layout_ids, element_layout);
             call_void_bitcode_fn(
                 env,
@@ -452,7 +490,9 @@ pub fn list_walk_generic<'a, 'ctx, 'env>(
                     pass_as_opaque(env, default_ptr),
                     env.alignment_intvalue(&element_layout),
                     layout_width(env, element_layout),
+                    layout_width(env, function_call_return_layout),
                     layout_width(env, default_layout),
+                    has_tag_id.as_global_value().as_pointer_value().into(),
                     dec_element_fn.as_global_value().as_pointer_value().into(),
                     pass_as_opaque(env, result_ptr),
                 ],
@@ -586,20 +626,26 @@ pub fn list_keep_oks<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     layout_ids: &mut LayoutIds<'a>,
     roc_function_call: RocFunctionCall<'ctx>,
-    function_layout: &Layout<'a>,
+    // Layout of the `Result after *`
+    result_layout: &Layout<'a>,
     list: BasicValueEnum<'ctx>,
     before_layout: &Layout<'a>,
     after_layout: &Layout<'a>,
 ) -> BasicValueEnum<'ctx> {
-    // Layout of the `Result after *`
-    let result_layout = match function_layout {
-        Layout::FunctionPointer(_, ret) => ret,
-        Layout::Closure(_, _, ret) => ret,
-        _ => unreachable!("not a callable layout"),
-    };
-
     let dec_result_fn = build_dec_wrapper(env, layout_ids, result_layout);
 
+    let function = env
+        .builder
+        .get_insert_block()
+        .unwrap()
+        .get_parent()
+        .unwrap();
+
+    let has_tag_id = match result_layout {
+        Layout::Union(union_layout) => build_has_tag_id(env, function, *union_layout),
+        _ => unreachable!(),
+    };
+
     call_bitcode_fn(
         env,
         &[
@@ -612,6 +658,7 @@ pub fn list_keep_oks<'a, 'ctx, 'env>(
             layout_width(env, before_layout),
             layout_width(env, result_layout),
             layout_width(env, after_layout),
+            has_tag_id.as_global_value().as_pointer_value().into(),
             dec_result_fn.as_global_value().as_pointer_value().into(),
         ],
         bitcode::LIST_KEEP_OKS,
@@ -623,20 +670,26 @@ pub fn list_keep_errs<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     layout_ids: &mut LayoutIds<'a>,
     roc_function_call: RocFunctionCall<'ctx>,
-    function_layout: &Layout<'a>,
+    // Layout of the `Result * err`
+    result_layout: &Layout<'a>,
     list: BasicValueEnum<'ctx>,
     before_layout: &Layout<'a>,
     after_layout: &Layout<'a>,
 ) -> BasicValueEnum<'ctx> {
-    // Layout of the `Result after *`
-    let result_layout = match function_layout {
-        Layout::FunctionPointer(_, ret) => ret,
-        Layout::Closure(_, _, ret) => ret,
-        _ => unreachable!("not a callable layout"),
-    };
-
     let dec_result_fn = build_dec_wrapper(env, layout_ids, result_layout);
 
+    let function = env
+        .builder
+        .get_insert_block()
+        .unwrap()
+        .get_parent()
+        .unwrap();
+
+    let has_tag_id = match result_layout {
+        Layout::Union(union_layout) => build_has_tag_id(env, function, *union_layout),
+        _ => unreachable!(),
+    };
+
     call_bitcode_fn(
         env,
         &[
@@ -649,60 +702,13 @@ pub fn list_keep_errs<'a, 'ctx, 'env>(
             layout_width(env, before_layout),
             layout_width(env, result_layout),
             layout_width(env, after_layout),
+            has_tag_id.as_global_value().as_pointer_value().into(),
             dec_result_fn.as_global_value().as_pointer_value().into(),
         ],
         bitcode::LIST_KEEP_ERRS,
     )
 }
 
-pub fn list_keep_result<'a, 'ctx, 'env>(
-    env: &Env<'a, 'ctx, 'env>,
-    layout_ids: &mut LayoutIds<'a>,
-    transform: FunctionValue<'ctx>,
-    transform_layout: Layout<'a>,
-    closure_data: BasicValueEnum<'ctx>,
-    closure_data_layout: Layout<'a>,
-    list: BasicValueEnum<'ctx>,
-    before_layout: &Layout<'a>,
-    after_layout: &Layout<'a>,
-    op: &str,
-) -> BasicValueEnum<'ctx> {
-    let builder = env.builder;
-
-    let result_layout = match transform_layout {
-        Layout::FunctionPointer(_, ret) => ret,
-        Layout::Closure(_, _, ret) => ret,
-        _ => unreachable!("not a callable layout"),
-    };
-
-    let closure_data_ptr = builder.build_alloca(closure_data.get_type(), "closure_data_ptr");
-    env.builder.build_store(closure_data_ptr, closure_data);
-
-    let stepper_caller =
-        build_transform_caller(env, transform, closure_data_layout, &[*before_layout])
-            .as_global_value()
-            .as_pointer_value();
-
-    let inc_closure = build_inc_wrapper(env, layout_ids, &transform_layout);
-    let dec_result_fn = build_dec_wrapper(env, layout_ids, result_layout);
-
-    call_bitcode_fn(
-        env,
-        &[
-            pass_list_as_i128(env, list),
-            pass_as_opaque(env, closure_data_ptr),
-            stepper_caller.into(),
-            env.alignment_intvalue(&before_layout),
-            layout_width(env, before_layout),
-            layout_width(env, after_layout),
-            layout_width(env, result_layout),
-            inc_closure.as_global_value().as_pointer_value().into(),
-            dec_result_fn.as_global_value().as_pointer_value().into(),
-        ],
-        op,
-    )
-}
-
 /// List.sortWith : List a, (a, a -> Ordering) -> List a
 pub fn list_sort_with<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
@@ -849,7 +855,6 @@ pub fn list_map3<'a, 'ctx, 'env>(
 /// List.concat : List elem, List elem -> List elem
 pub fn list_concat<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
-    _inplace: InPlace,
     _parent: FunctionValue<'ctx>,
     first_list: BasicValueEnum<'ctx>,
     second_list: BasicValueEnum<'ctx>,
@@ -861,7 +866,7 @@ pub fn list_concat<'a, 'ctx, 'env>(
             // then simply return an empty list
             empty_list(env)
         }
-        Layout::Builtin(Builtin::List(_, elem_layout)) => call_bitcode_fn_returns_list(
+        Layout::Builtin(Builtin::List(elem_layout)) => call_bitcode_fn_returns_list(
             env,
             &[
                 pass_list_as_i128(env, first_list),
@@ -1115,7 +1120,6 @@ pub fn load_list_ptr<'ctx>(
 
 pub fn allocate_list<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
-    inplace: InPlace,
     elem_layout: &Layout<'a>,
     number_of_elements: IntValue<'ctx>,
 ) -> PointerValue<'ctx> {
@@ -1128,16 +1132,13 @@ pub fn allocate_list<'a, 'ctx, 'env>(
     let number_of_data_bytes =
         builder.build_int_mul(bytes_per_element, number_of_elements, "data_length");
 
-    let rc1 = match inplace {
-        InPlace::InPlace => number_of_elements,
-        InPlace::Clone => {
-            // the refcount of a new list is initially 1
-            // we assume that the list is indeed used (dead variables are eliminated)
-            crate::llvm::refcounting::refcount_1(ctx, env.ptr_bytes)
-        }
-    };
+    // the refcount of a new list is initially 1
+    // we assume that the list is indeed used (dead variables are eliminated)
+    let rc1 = crate::llvm::refcounting::refcount_1(ctx, env.ptr_bytes);
 
-    allocate_with_refcount_help(env, elem_layout, number_of_data_bytes, rc1)
+    let basic_type = basic_type_from_layout(env, elem_layout);
+    let alignment_bytes = elem_layout.alignment_bytes(env.ptr_bytes);
+    allocate_with_refcount_help(env, basic_type, alignment_bytes, number_of_data_bytes, rc1)
 }
 
 pub fn store_list<'a, 'ctx, 'env>(
diff --git a/compiler/gen/src/llvm/build_str.rs b/compiler/gen_llvm/src/llvm/build_str.rs
similarity index 95%
rename from compiler/gen/src/llvm/build_str.rs
rename to compiler/gen_llvm/src/llvm/build_str.rs
index 4692f5eaa8..ee8576ceb8 100644
--- a/compiler/gen/src/llvm/build_str.rs
+++ b/compiler/gen_llvm/src/llvm/build_str.rs
@@ -6,7 +6,7 @@ use inkwell::values::{BasicValueEnum, FunctionValue, IntValue, PointerValue, Str
 use inkwell::AddressSpace;
 use roc_builtins::bitcode;
 use roc_module::symbol::Symbol;
-use roc_mono::layout::{Builtin, InPlace, Layout};
+use roc_mono::layout::{Builtin, Layout};
 
 use super::build::load_symbol;
 
@@ -16,7 +16,6 @@ pub static CHAR_LAYOUT: Layout = Layout::Builtin(Builtin::Int8);
 pub fn str_split<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     scope: &Scope<'a, 'ctx>,
-    inplace: InPlace,
     str_symbol: Symbol,
     delimiter_symbol: Symbol,
 ) -> BasicValueEnum<'ctx> {
@@ -33,7 +32,7 @@ pub fn str_split<'a, 'ctx, 'env>(
     .into_int_value();
 
     // a pointer to the elements
-    let ret_list_ptr = allocate_list(env, inplace, &Layout::Builtin(Builtin::Str), segment_count);
+    let ret_list_ptr = allocate_list(env, &Layout::Builtin(Builtin::Str), segment_count);
 
     // get the RocStr type defined by zig
     let roc_str_type = env.module.get_struct_type("str.RocStr").unwrap();
@@ -109,7 +108,6 @@ pub fn destructure<'ctx>(
 /// Str.concat : Str, Str -> Str
 pub fn str_concat<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
-    inplace: InPlace,
     scope: &Scope<'a, 'ctx>,
     str1_symbol: Symbol,
     str2_symbol: Symbol,
@@ -120,14 +118,7 @@ pub fn str_concat<'a, 'ctx, 'env>(
 
     call_bitcode_fn(
         env,
-        &[
-            env.context
-                .i8_type()
-                .const_int(inplace as u64, false)
-                .into(),
-            str1_i128.into(),
-            str2_i128.into(),
-        ],
+        &[str1_i128.into(), str2_i128.into()],
         &bitcode::STR_CONCAT,
     )
 }
@@ -135,7 +126,6 @@ pub fn str_concat<'a, 'ctx, 'env>(
 /// Str.join : List Str, Str -> Str
 pub fn str_join_with<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
-    _inplace: InPlace,
     scope: &Scope<'a, 'ctx>,
     list_symbol: Symbol,
     str_symbol: Symbol,
diff --git a/compiler/gen/src/llvm/compare.rs b/compiler/gen_llvm/src/llvm/compare.rs
similarity index 91%
rename from compiler/gen/src/llvm/compare.rs
rename to compiler/gen_llvm/src/llvm/compare.rs
index 0cf32263d0..2a9d1d3fa3 100644
--- a/compiler/gen/src/llvm/compare.rs
+++ b/compiler/gen_llvm/src/llvm/compare.rs
@@ -1,10 +1,13 @@
 use crate::llvm::build::Env;
-use crate::llvm::build::{cast_block_of_memory_to_tag, complex_bitcast, set_name, FAST_CALL_CONV};
+use crate::llvm::build::{cast_block_of_memory_to_tag, get_tag_id, FAST_CALL_CONV};
 use crate::llvm::build_list::{list_len, load_list_ptr};
 use crate::llvm::build_str::str_equal;
-use crate::llvm::convert::{basic_type_from_layout, get_ptr_type};
+use crate::llvm::convert::basic_type_from_layout;
 use bumpalo::collections::Vec;
-use inkwell::values::{BasicValueEnum, FunctionValue, IntValue, PointerValue, StructValue};
+use inkwell::types::BasicType;
+use inkwell::values::{
+    BasicValue, BasicValueEnum, FunctionValue, IntValue, PointerValue, StructValue,
+};
 use inkwell::{AddressSpace, FloatPredicate, IntPredicate};
 use roc_module::symbol::Symbol;
 use roc_mono::layout::{Builtin, Layout, LayoutIds, UnionLayout};
@@ -99,7 +102,7 @@ fn build_eq_builtin<'a, 'ctx, 'env>(
         Builtin::Float16 => float_cmp(FloatPredicate::OEQ, "eq_f16"),
 
         Builtin::Str => str_equal(env, lhs_val, rhs_val),
-        Builtin::List(_, elem) => build_list_eq(
+        Builtin::List(elem) => build_list_eq(
             env,
             layout_ids,
             &Layout::Builtin(*builtin),
@@ -159,11 +162,6 @@ fn build_eq<'a, 'ctx, 'env>(
             rhs_val,
         ),
 
-        Layout::PhantomEmptyStruct => {
-            // always equal to itself
-            env.context.bool_type().const_int(1, false).into()
-        }
-
         Layout::RecursivePointer => match when_recursive {
             WhenRecursive::Unreachable => {
                 unreachable!("recursion pointers should never be compared directly")
@@ -197,11 +195,7 @@ fn build_eq<'a, 'ctx, 'env>(
             }
         },
 
-        Layout::Pointer(_) => {
-            unreachable!("unused")
-        }
-
-        Layout::FunctionPointer(_, _) | Layout::Closure(_, _, _) => {
+        Layout::Closure(_, _, _) => {
             unreachable!("the type system will guarantee these are never compared")
         }
     }
@@ -258,7 +252,7 @@ fn build_neq_builtin<'a, 'ctx, 'env>(
 
             result.into()
         }
-        Builtin::List(_, elem) => {
+        Builtin::List(elem) => {
             let is_equal = build_list_eq(
                 env,
                 layout_ids,
@@ -338,20 +332,11 @@ fn build_neq<'a, 'ctx, 'env>(
             result.into()
         }
 
-        Layout::PhantomEmptyStruct => {
-            // always equal to itself
-            env.context.bool_type().const_int(1, false).into()
-        }
-
         Layout::RecursivePointer => {
             unreachable!("recursion pointers should never be compared directly")
         }
 
-        Layout::Pointer(_) => {
-            unreachable!("unused")
-        }
-
-        Layout::FunctionPointer(_, _) | Layout::Closure(_, _, _) => {
+        Layout::Closure(_, _, _) => {
             unreachable!("the type system will guarantee these are never compared")
         }
     }
@@ -446,8 +431,8 @@ fn build_list_eq_help<'a, 'ctx, 'env>(
     let list1 = it.next().unwrap().into_struct_value();
     let list2 = it.next().unwrap().into_struct_value();
 
-    set_name(list1.into(), Symbol::ARG_1.ident_string(&env.interns));
-    set_name(list2.into(), Symbol::ARG_2.ident_string(&env.interns));
+    list1.set_name(Symbol::ARG_1.ident_string(&env.interns));
+    list2.set_name(Symbol::ARG_2.ident_string(&env.interns));
 
     let entry = ctx.append_basic_block(parent, "entry");
     env.builder.position_at_end(entry);
@@ -475,7 +460,7 @@ fn build_list_eq_help<'a, 'ctx, 'env>(
 
         let builder = env.builder;
         let element_type = basic_type_from_layout(env, element_layout);
-        let ptr_type = get_ptr_type(&element_type, AddressSpace::Generic);
+        let ptr_type = element_type.ptr_type(AddressSpace::Generic);
         let ptr1 = load_list_ptr(env.builder, list1, ptr_type);
         let ptr2 = load_list_ptr(env.builder, list2, ptr_type);
 
@@ -654,8 +639,8 @@ fn build_struct_eq_help<'a, 'ctx, 'env>(
     let struct1 = it.next().unwrap().into_struct_value();
     let struct2 = it.next().unwrap().into_struct_value();
 
-    set_name(struct1.into(), Symbol::ARG_1.ident_string(&env.interns));
-    set_name(struct2.into(), Symbol::ARG_2.ident_string(&env.interns));
+    struct1.set_name(Symbol::ARG_1.ident_string(&env.interns));
+    struct2.set_name(Symbol::ARG_2.ident_string(&env.interns));
 
     let entry = ctx.append_basic_block(parent, "entry");
     let start = ctx.append_basic_block(parent, "start");
@@ -835,8 +820,8 @@ fn build_tag_eq_help<'a, 'ctx, 'env>(
     let tag1 = it.next().unwrap();
     let tag2 = it.next().unwrap();
 
-    set_name(tag1, Symbol::ARG_1.ident_string(&env.interns));
-    set_name(tag2, Symbol::ARG_2.ident_string(&env.interns));
+    tag1.set_name(Symbol::ARG_1.ident_string(&env.interns));
+    tag2.set_name(Symbol::ARG_2.ident_string(&env.interns));
 
     let entry = ctx.append_basic_block(parent, "entry");
 
@@ -861,9 +846,8 @@ fn build_tag_eq_help<'a, 'ctx, 'env>(
 
     match union_layout {
         NonRecursive(tags) => {
-            // SAFETY we know that non-recursive tags cannot be NULL
-            let id1 = nonrec_tag_id(env, tag1.into_struct_value());
-            let id2 = nonrec_tag_id(env, tag2.into_struct_value());
+            let id1 = get_tag_id(env, parent, union_layout, tag1);
+            let id2 = get_tag_id(env, parent, union_layout, tag2);
 
             let compare_tag_fields = ctx.append_basic_block(parent, "compare_tag_fields");
 
@@ -912,10 +896,7 @@ fn build_tag_eq_help<'a, 'ctx, 'env>(
 
                 env.builder.build_return(Some(&answer));
 
-                cases.push((
-                    env.context.i64_type().const_int(tag_id as u64, false),
-                    block,
-                ));
+                cases.push((id1.get_type().const_int(tag_id as u64, false), block));
             }
 
             env.builder.position_at_end(compare_tag_fields);
@@ -941,9 +922,8 @@ fn build_tag_eq_help<'a, 'ctx, 'env>(
 
             env.builder.position_at_end(compare_tag_ids);
 
-            // SAFETY we know that non-recursive tags cannot be NULL
-            let id1 = unsafe { rec_tag_id_unsafe(env, tag1.into_pointer_value()) };
-            let id2 = unsafe { rec_tag_id_unsafe(env, tag2.into_pointer_value()) };
+            let id1 = get_tag_id(env, parent, union_layout, tag1);
+            let id2 = get_tag_id(env, parent, union_layout, tag2);
 
             let compare_tag_fields = ctx.append_basic_block(parent, "compare_tag_fields");
 
@@ -975,10 +955,7 @@ fn build_tag_eq_help<'a, 'ctx, 'env>(
 
                 env.builder.build_return(Some(&answer));
 
-                cases.push((
-                    env.context.i64_type().const_int(tag_id as u64, false),
-                    block,
-                ));
+                cases.push((id1.get_type().const_int(tag_id as u64, false), block));
             }
 
             env.builder.position_at_end(compare_tag_fields);
@@ -988,9 +965,6 @@ fn build_tag_eq_help<'a, 'ctx, 'env>(
             env.builder.build_switch(id1, default, &cases);
         }
         NullableUnwrapped { other_fields, .. } => {
-            // drop the tag id; it is not stored
-            let other_fields = &other_fields[1..];
-
             let ptr_equal = env.builder.build_int_compare(
                 IntPredicate::EQ,
                 env.builder
@@ -1096,9 +1070,8 @@ fn build_tag_eq_help<'a, 'ctx, 'env>(
 
             env.builder.position_at_end(compare_other);
 
-            // SAFETY we know at this point that tag1/tag2 are not NULL
-            let id1 = unsafe { rec_tag_id_unsafe(env, tag1.into_pointer_value()) };
-            let id2 = unsafe { rec_tag_id_unsafe(env, tag2.into_pointer_value()) };
+            let id1 = get_tag_id(env, parent, union_layout, tag1);
+            let id2 = get_tag_id(env, parent, union_layout, tag2);
 
             let compare_tag_fields = ctx.append_basic_block(parent, "compare_tag_fields");
 
@@ -1131,10 +1104,7 @@ fn build_tag_eq_help<'a, 'ctx, 'env>(
 
                 env.builder.build_return(Some(&answer));
 
-                cases.push((
-                    env.context.i64_type().const_int(tag_id as u64, false),
-                    block,
-                ));
+                cases.push((id1.get_type().const_int(tag_id as u64, false), block));
             }
 
             env.builder.position_at_end(compare_tag_fields);
@@ -1182,8 +1152,6 @@ fn eq_ptr_to_struct<'a, 'ctx, 'env>(
     tag1: PointerValue<'ctx>,
     tag2: PointerValue<'ctx>,
 ) -> IntValue<'ctx> {
-    use inkwell::types::BasicType;
-
     let struct_layout = Layout::Struct(field_layouts);
 
     let wrapper_type = basic_type_from_layout(env, &struct_layout);
@@ -1228,32 +1196,3 @@ fn eq_ptr_to_struct<'a, 'ctx, 'env>(
     )
     .into_int_value()
 }
-
-fn nonrec_tag_id<'a, 'ctx, 'env>(
-    env: &Env<'a, 'ctx, 'env>,
-    tag: StructValue<'ctx>,
-) -> IntValue<'ctx> {
-    complex_bitcast(
-        env.builder,
-        tag.into(),
-        env.context.i64_type().into(),
-        "load_tag_id",
-    )
-    .into_int_value()
-}
-
-unsafe fn rec_tag_id_unsafe<'a, 'ctx, 'env>(
-    env: &Env<'a, 'ctx, 'env>,
-    tag: PointerValue<'ctx>,
-) -> IntValue<'ctx> {
-    let ptr = env
-        .builder
-        .build_bitcast(
-            tag,
-            env.context.i64_type().ptr_type(AddressSpace::Generic),
-            "cast_for_tag_id",
-        )
-        .into_pointer_value();
-
-    env.builder.build_load(ptr, "load_tag_id").into_int_value()
-}
diff --git a/compiler/gen/src/llvm/convert.rs b/compiler/gen_llvm/src/llvm/convert.rs
similarity index 60%
rename from compiler/gen/src/llvm/convert.rs
rename to compiler/gen_llvm/src/llvm/convert.rs
index 29e874c992..1cb299c0ae 100644
--- a/compiler/gen/src/llvm/convert.rs
+++ b/compiler/gen_llvm/src/llvm/convert.rs
@@ -1,88 +1,9 @@
 use bumpalo::collections::Vec;
 use inkwell::context::Context;
-use inkwell::types::BasicTypeEnum::{self, *};
-use inkwell::types::{ArrayType, BasicType, FunctionType, IntType, PointerType, StructType};
-use inkwell::values::BasicValueEnum;
+use inkwell::types::{BasicType, BasicTypeEnum, IntType, StructType};
 use inkwell::AddressSpace;
 use roc_mono::layout::{Builtin, Layout, UnionLayout};
 
-/// TODO could this be added to Inkwell itself as a method on BasicValueEnum?
-pub fn get_ptr_type<'ctx>(
-    bt_enum: &BasicTypeEnum<'ctx>,
-    address_space: AddressSpace,
-) -> PointerType<'ctx> {
-    match bt_enum {
-        ArrayType(typ) => typ.ptr_type(address_space),
-        IntType(typ) => typ.ptr_type(address_space),
-        FloatType(typ) => typ.ptr_type(address_space),
-        PointerType(typ) => typ.ptr_type(address_space),
-        StructType(typ) => typ.ptr_type(address_space),
-        VectorType(typ) => typ.ptr_type(address_space),
-    }
-}
-
-/// TODO could this be added to Inkwell itself as a method on BasicValueEnum?
-pub fn get_fn_type<'ctx>(
-    bt_enum: &BasicTypeEnum<'ctx>,
-    arg_types: &[BasicTypeEnum<'ctx>],
-) -> FunctionType<'ctx> {
-    match bt_enum {
-        ArrayType(typ) => typ.fn_type(arg_types, false),
-        IntType(typ) => typ.fn_type(arg_types, false),
-        FloatType(typ) => typ.fn_type(arg_types, false),
-        PointerType(typ) => typ.fn_type(arg_types, false),
-        StructType(typ) => typ.fn_type(arg_types, false),
-        VectorType(typ) => typ.fn_type(arg_types, false),
-    }
-}
-
-/// TODO could this be added to Inkwell itself as a method on BasicValueEnum?
-pub fn get_array_type<'ctx>(bt_enum: &BasicTypeEnum<'ctx>, size: u32) -> ArrayType<'ctx> {
-    match bt_enum {
-        ArrayType(typ) => typ.array_type(size),
-        IntType(typ) => typ.array_type(size),
-        FloatType(typ) => typ.array_type(size),
-        PointerType(typ) => typ.array_type(size),
-        StructType(typ) => typ.array_type(size),
-        VectorType(typ) => typ.array_type(size),
-    }
-}
-
-/// TODO could this be added to Inkwell itself as a method on BasicValueEnum?
-pub fn as_const_zero<'ctx>(bt_enum: &BasicTypeEnum<'ctx>) -> BasicValueEnum<'ctx> {
-    match bt_enum {
-        ArrayType(typ) => typ.const_zero().into(),
-        IntType(typ) => typ.const_zero().into(),
-        FloatType(typ) => typ.const_zero().into(),
-        PointerType(typ) => typ.const_zero().into(),
-        StructType(typ) => typ.const_zero().into(),
-        VectorType(typ) => typ.const_zero().into(),
-    }
-}
-
-fn basic_type_from_function_layout<'a, 'ctx, 'env>(
-    env: &crate::llvm::build::Env<'a, 'ctx, 'env>,
-    args: &[Layout<'_>],
-    closure_type: Option<BasicTypeEnum<'ctx>>,
-    ret_layout: &Layout<'_>,
-) -> BasicTypeEnum<'ctx> {
-    let ret_type = basic_type_from_layout(env, &ret_layout);
-    let mut arg_basic_types = Vec::with_capacity_in(args.len(), env.arena);
-
-    for arg_layout in args.iter() {
-        arg_basic_types.push(basic_type_from_layout(env, arg_layout));
-    }
-
-    if let Some(closure) = closure_type {
-        arg_basic_types.push(closure);
-    }
-
-    let fn_type = get_fn_type(&ret_type, arg_basic_types.into_bump_slice());
-    let ptr_type = fn_type.ptr_type(AddressSpace::Generic);
-
-    ptr_type.as_basic_type_enum()
-}
-
 fn basic_type_from_record<'a, 'ctx, 'env>(
     env: &crate::llvm::build::Env<'a, 'ctx, 'env>,
     fields: &[Layout<'_>],
@@ -105,38 +26,49 @@ pub fn basic_type_from_layout<'a, 'ctx, 'env>(
     use Layout::*;
 
     match layout {
-        FunctionPointer(args, ret_layout) => {
-            basic_type_from_function_layout(env, args, None, ret_layout)
-        }
         Closure(_args, closure_layout, _ret_layout) => {
             let closure_data_layout = closure_layout.runtime_representation();
             basic_type_from_layout(env, &closure_data_layout)
         }
-        Pointer(layout) => basic_type_from_layout(env, &layout)
-            .ptr_type(AddressSpace::Generic)
-            .into(),
-        PhantomEmptyStruct => env.context.struct_type(&[], false).into(),
         Struct(sorted_fields) => basic_type_from_record(env, sorted_fields),
         Union(variant) => {
             use UnionLayout::*;
+
+            let tag_id_type = basic_type_from_layout(env, &variant.tag_id_layout());
+
             match variant {
-                Recursive(tags)
-                | NullableWrapped {
+                NullableWrapped {
                     other_tags: tags, ..
                 } => {
-                    let block = block_of_memory_slices(env.context, tags, env.ptr_bytes);
-                    block.ptr_type(AddressSpace::Generic).into()
+                    let data = block_of_memory_slices(env.context, tags, env.ptr_bytes);
+
+                    env.context
+                        .struct_type(&[data, tag_id_type], false)
+                        .ptr_type(AddressSpace::Generic)
+                        .into()
                 }
                 NullableUnwrapped { other_fields, .. } => {
                     let block =
-                        block_of_memory_slices(env.context, &[&other_fields[1..]], env.ptr_bytes);
+                        block_of_memory_slices(env.context, &[&other_fields], env.ptr_bytes);
                     block.ptr_type(AddressSpace::Generic).into()
                 }
                 NonNullableUnwrapped(fields) => {
                     let block = block_of_memory_slices(env.context, &[fields], env.ptr_bytes);
                     block.ptr_type(AddressSpace::Generic).into()
                 }
-                NonRecursive(_) => block_of_memory(env.context, layout, env.ptr_bytes),
+                Recursive(tags) => {
+                    let data = block_of_memory_slices(env.context, tags, env.ptr_bytes);
+
+                    env.context
+                        .struct_type(&[data, tag_id_type], false)
+                        .ptr_type(AddressSpace::Generic)
+                        .into()
+                }
+                NonRecursive(tags) => {
+                    let data = block_of_memory_slices(env.context, tags, env.ptr_bytes);
+
+                    env.context.struct_type(&[data, tag_id_type], false).into()
+                }
             }
         }
         RecursivePointer => {
@@ -174,7 +106,7 @@ pub fn basic_type_from_builtin<'a, 'ctx, 'env>(
         Float16 => context.f16_type().as_basic_type_enum(),
         Dict(_, _) | EmptyDict => zig_dict_type(env).into(),
         Set(_) | EmptySet => zig_dict_type(env).into(),
-        List(_, _) | EmptyList => zig_list_type(env).into(),
+        List(_) | EmptyList => zig_list_type(env).into(),
         Str | EmptyStr => zig_str_type(env).into(),
     }
 }
@@ -197,13 +129,43 @@ pub fn block_of_memory_slices<'ctx>(
     block_of_memory_help(context, union_size)
 }
 
+pub fn union_data_is_struct<'a, 'ctx, 'env>(
+    env: &crate::llvm::build::Env<'a, 'ctx, 'env>,
+    layouts: &[Layout<'_>],
+) -> StructType<'ctx> {
+    let data_type = basic_type_from_record(env, layouts);
+    union_data_is_struct_type(env.context, data_type.into_struct_type())
+}
+
+pub fn union_data_is_struct_type<'ctx>(
+    context: &'ctx Context,
+    struct_type: StructType<'ctx>,
+) -> StructType<'ctx> {
+    let tag_id_type = context.i64_type();
+    context.struct_type(&[struct_type.into(), tag_id_type.into()], false)
+}
+
+pub fn union_data_block_of_memory<'ctx>(
+    context: &'ctx Context,
+    tag_id_int_type: IntType<'ctx>,
+    layouts: &[&[Layout<'_>]],
+    ptr_bytes: u32,
+) -> StructType<'ctx> {
+    let data_type = block_of_memory_slices(context, layouts, ptr_bytes);
+    context.struct_type(&[data_type, tag_id_int_type.into()], false)
+}
+
 pub fn block_of_memory<'ctx>(
     context: &'ctx Context,
     layout: &Layout<'_>,
     ptr_bytes: u32,
 ) -> BasicTypeEnum<'ctx> {
     // TODO make this dynamic
-    let union_size = layout.stack_size(ptr_bytes as u32);
+    let mut union_size = layout.stack_size(ptr_bytes as u32);
+
+    if let Layout::Union(UnionLayout::NonRecursive { .. }) = layout {
+        union_size -= ptr_bytes;
+    }
 
     block_of_memory_help(context, union_size)
 }
@@ -266,3 +228,9 @@ pub fn zig_str_type<'a, 'ctx, 'env>(
 ) -> StructType<'ctx> {
     env.module.get_struct_type("str.RocStr").unwrap()
 }
+
+pub fn zig_has_tag_id_type<'a, 'ctx, 'env>(
+    env: &crate::llvm::build::Env<'a, 'ctx, 'env>,
+) -> StructType<'ctx> {
+    env.module.get_struct_type("list.HasTagId").unwrap()
+}
diff --git a/compiler/gen/src/llvm/externs.rs b/compiler/gen_llvm/src/llvm/externs.rs
similarity index 96%
rename from compiler/gen/src/llvm/externs.rs
rename to compiler/gen_llvm/src/llvm/externs.rs
index 2bb2a01c6e..48bea9283b 100644
--- a/compiler/gen/src/llvm/externs.rs
+++ b/compiler/gen_llvm/src/llvm/externs.rs
@@ -1,8 +1,9 @@
-use crate::llvm::build::{add_func, set_name, C_CALL_CONV};
+use crate::llvm::build::{add_func, C_CALL_CONV};
 use crate::llvm::convert::ptr_int;
 use inkwell::builder::Builder;
 use inkwell::context::Context;
 use inkwell::module::{Linkage, Module};
+use inkwell::values::BasicValue;
 use inkwell::AddressSpace;
 
 /// Define functions for roc_alloc, roc_realloc, and roc_dealloc
@@ -69,8 +70,8 @@ pub fn add_default_roc_externs<'ctx>(
 
             debug_assert!(params.next().is_none());
 
-            set_name(ptr_arg, "ptr");
-            set_name(size_arg, "size");
+            ptr_arg.set_name("ptr");
+            size_arg.set_name("size");
 
             if cfg!(debug_assertions) {
                 crate::llvm::build::verify_fn(fn_val);
diff --git a/compiler/gen/src/llvm/mod.rs b/compiler/gen_llvm/src/llvm/mod.rs
similarity index 100%
rename from compiler/gen/src/llvm/mod.rs
rename to compiler/gen_llvm/src/llvm/mod.rs
diff --git a/compiler/gen/src/llvm/refcounting.rs b/compiler/gen_llvm/src/llvm/refcounting.rs
similarity index 88%
rename from compiler/gen/src/llvm/refcounting.rs
rename to compiler/gen_llvm/src/llvm/refcounting.rs
index cf0a254163..4dd072f622 100644
--- a/compiler/gen/src/llvm/refcounting.rs
+++ b/compiler/gen_llvm/src/llvm/refcounting.rs
@@ -1,21 +1,24 @@
 use crate::debug_info_init;
 use crate::llvm::build::{
-    add_func, cast_basic_basic, cast_block_of_memory_to_tag, set_name, Env, FAST_CALL_CONV,
-    LLVM_SADD_WITH_OVERFLOW_I64,
+    add_func, cast_basic_basic, cast_block_of_memory_to_tag, get_tag_id, get_tag_id_non_recursive,
+    Env, FAST_CALL_CONV, LLVM_SADD_WITH_OVERFLOW_I64, TAG_DATA_INDEX,
 };
 use crate::llvm::build_list::{incrementing_elem_loop, list_len, load_list};
 use crate::llvm::convert::{
-    basic_type_from_layout, block_of_memory, block_of_memory_slices, ptr_int,
+    basic_type_from_layout, block_of_memory_slices, ptr_int, union_data_block_of_memory,
 };
 use bumpalo::collections::Vec;
+use inkwell::basic_block::BasicBlock;
 use inkwell::context::Context;
 use inkwell::module::Linkage;
 use inkwell::types::{AnyTypeEnum, BasicType, BasicTypeEnum};
-use inkwell::values::{BasicValueEnum, FunctionValue, IntValue, PointerValue, StructValue};
+use inkwell::values::{
+    BasicValue, BasicValueEnum, FunctionValue, IntValue, PointerValue, StructValue,
+};
 use inkwell::{AddressSpace, IntPredicate};
 use roc_module::symbol::Interns;
 use roc_module::symbol::Symbol;
-use roc_mono::layout::{Builtin, Layout, LayoutIds, MemoryMode, UnionLayout};
+use roc_mono::layout::{Builtin, Layout, LayoutIds, UnionLayout};
 
 pub const REFCOUNT_MAX: usize = 0_usize;
 
@@ -92,6 +95,14 @@ impl<'ctx> PointerToRefcount<'ctx> {
         Self::from_ptr_to_data(env, data_ptr)
     }
 
+    pub fn is_1<'a, 'env>(&self, env: &Env<'a, 'ctx, 'env>) -> IntValue<'ctx> {
+        let current = self.get_refcount(env);
+        let one = refcount_1(env.context, env.ptr_bytes);
+
+        env.builder
+            .build_int_compare(IntPredicate::EQ, current, one, "is_one")
+    }
+
     pub fn get_refcount<'a, 'env>(&self, env: &Env<'a, 'ctx, 'env>) -> IntValue<'ctx> {
         env.builder
             .build_load(self.value, "get_refcount")
@@ -390,7 +401,7 @@ fn modify_refcount_struct_help<'a, 'ctx, 'env>(
     let arg_symbol = Symbol::ARG_1;
     let arg_val = fn_val.get_param_iter().next().unwrap();
 
-    set_name(arg_val, arg_symbol.ident_string(&env.interns));
+    arg_val.set_name(arg_symbol.ident_string(&env.interns));
 
     let parent = fn_val;
 
@@ -469,21 +480,17 @@ fn modify_refcount_builtin<'a, 'ctx, 'env>(
     use Builtin::*;
 
     match builtin {
-        List(memory_mode, element_layout) => {
-            if let MemoryMode::Refcounted = memory_mode {
-                let function = modify_refcount_list(
-                    env,
-                    layout_ids,
-                    mode,
-                    when_recursive,
-                    layout,
-                    element_layout,
-                );
+        List(element_layout) => {
+            let function = modify_refcount_list(
+                env,
+                layout_ids,
+                mode,
+                when_recursive,
+                layout,
+                element_layout,
+            );
 
-                Some(function)
-            } else {
-                None
-            }
+            Some(function)
         }
         Set(element_layout) => {
             let key_layout = &Layout::Struct(&[]);
@@ -646,6 +653,7 @@ fn modify_refcount_layout_build_function<'a, 'ctx, 'env>(
                         layout_ids,
                         mode,
                         &WhenRecursive::Loop(*variant),
+                        *variant,
                         tags,
                         true,
                     );
@@ -654,14 +662,13 @@ fn modify_refcount_layout_build_function<'a, 'ctx, 'env>(
                 }
 
                 NullableUnwrapped { other_fields, .. } => {
-                    let other_fields = &other_fields[1..];
-
                     let function = build_rec_union(
                         env,
                         layout_ids,
                         mode,
                         &WhenRecursive::Loop(*variant),
-                        &*env.arena.alloc([other_fields]),
+                        *variant,
+                        env.arena.alloc([*other_fields]),
                         true,
                     );
 
@@ -674,6 +681,7 @@ fn modify_refcount_layout_build_function<'a, 'ctx, 'env>(
                         layout_ids,
                         mode,
                         &WhenRecursive::Loop(*variant),
+                        *variant,
                         &*env.arena.alloc([*fields]),
                         true,
                     );
@@ -686,6 +694,7 @@ fn modify_refcount_layout_build_function<'a, 'ctx, 'env>(
                         layout_ids,
                         mode,
                         &WhenRecursive::Loop(*variant),
+                        *variant,
                         tags,
                         false,
                     );
@@ -724,11 +733,9 @@ fn modify_refcount_layout_build_function<'a, 'ctx, 'env>(
             Some(function)
         }
 
-        PhantomEmptyStruct => None,
-
         Layout::RecursivePointer => match when_recursive {
             WhenRecursive::Unreachable => {
-                unreachable!("recursion pointers should never be hashed directly")
+                unreachable!("recursion pointers cannot be in/decremented directly")
             }
             WhenRecursive::Loop(union_layout) => {
                 let layout = Layout::Union(*union_layout);
@@ -745,8 +752,6 @@ fn modify_refcount_layout_build_function<'a, 'ctx, 'env>(
                 Some(function)
             }
         },
-
-        FunctionPointer(_, _) | Pointer(_) => None,
     }
 }
 
@@ -827,7 +832,7 @@ fn modify_refcount_list_help<'a, 'ctx, 'env>(
     let arg_symbol = Symbol::ARG_1;
     let arg_val = fn_val.get_param_iter().next().unwrap();
 
-    set_name(arg_val, arg_symbol.ident_string(&env.interns));
+    arg_val.set_name(arg_symbol.ident_string(&env.interns));
 
     let parent = fn_val;
     let original_wrapper = arg_val.into_struct_value();
@@ -946,7 +951,7 @@ fn modify_refcount_str_help<'a, 'ctx, 'env>(
     let arg_symbol = Symbol::ARG_1;
     let arg_val = fn_val.get_param_iter().next().unwrap();
 
-    set_name(arg_val, arg_symbol.ident_string(&env.interns));
+    arg_val.set_name(arg_symbol.ident_string(&env.interns));
 
     let parent = fn_val;
 
@@ -1065,7 +1070,7 @@ fn modify_refcount_dict_help<'a, 'ctx, 'env>(
     let arg_symbol = Symbol::ARG_1;
     let arg_val = fn_val.get_param_iter().next().unwrap();
 
-    set_name(arg_val, arg_symbol.ident_string(&env.interns));
+    arg_val.set_name(arg_symbol.ident_string(&env.interns));
 
     let parent = fn_val;
 
@@ -1179,7 +1184,7 @@ pub enum Mode {
 }
 
 impl Mode {
-    fn to_call_mode<'ctx>(&self, function: FunctionValue<'ctx>) -> CallMode<'ctx> {
+    fn to_call_mode(self, function: FunctionValue<'_>) -> CallMode<'_> {
         match self {
             Mode::Inc => {
                 let amount = function.get_nth_param(1).unwrap().into_int_value();
@@ -1202,10 +1207,11 @@ fn build_rec_union<'a, 'ctx, 'env>(
     layout_ids: &mut LayoutIds<'a>,
     mode: Mode,
     when_recursive: &WhenRecursive<'a>,
-    fields: &'a [&'a [Layout<'a>]],
+    union_layout: UnionLayout<'a>,
+    tags: &'a [&'a [Layout<'a>]],
     is_nullable: bool,
 ) -> FunctionValue<'ctx> {
-    let layout = Layout::Union(UnionLayout::Recursive(fields));
+    let layout = Layout::Union(UnionLayout::Recursive(tags));
 
     let (_, fn_name) = function_name_from_mode(
         layout_ids,
@@ -1222,9 +1228,7 @@ fn build_rec_union<'a, 'ctx, 'env>(
             let block = env.builder.get_insert_block().expect("to be in a function");
             let di_location = env.builder.get_current_debug_location().unwrap();
 
-            let basic_type = block_of_memory_slices(env.context, fields, env.ptr_bytes)
-                .ptr_type(AddressSpace::Generic)
-                .into();
+            let basic_type = basic_type_from_layout(env, &Layout::Union(union_layout));
             let function_value = build_header(env, basic_type, mode, &fn_name);
 
             build_rec_union_help(
@@ -1232,7 +1236,8 @@ fn build_rec_union<'a, 'ctx, 'env>(
                 layout_ids,
                 mode,
                 when_recursive,
-                fields,
+                union_layout,
+                tags,
                 function_value,
                 is_nullable,
             );
@@ -1248,12 +1253,14 @@ fn build_rec_union<'a, 'ctx, 'env>(
     function
 }
 
+#[allow(clippy::too_many_arguments)]
 fn build_rec_union_help<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     layout_ids: &mut LayoutIds<'a>,
     mode: Mode,
     when_recursive: &WhenRecursive<'a>,
-    tags: &[&[Layout<'a>]],
+    union_layout: UnionLayout<'a>,
+    tags: &'a [&'a [roc_mono::layout::Layout<'a>]],
     fn_val: FunctionValue<'ctx>,
     is_nullable: bool,
 ) {
@@ -1262,8 +1269,6 @@ fn build_rec_union_help<'a, 'ctx, 'env>(
     let context = &env.context;
     let builder = env.builder;
 
-    let pick = |a, b| if let Mode::Inc = mode { a } else { b };
-
     // Add a basic block for the entry point
     let entry = context.append_basic_block(fn_val, "entry");
 
@@ -1276,15 +1281,101 @@ fn build_rec_union_help<'a, 'ctx, 'env>(
 
     let arg_val = fn_val.get_param_iter().next().unwrap();
 
-    set_name(arg_val, arg_symbol.ident_string(&env.interns));
+    arg_val.set_name(arg_symbol.ident_string(&env.interns));
 
     let parent = fn_val;
 
-    let layout = Layout::Union(UnionLayout::Recursive(tags));
-
     debug_assert!(arg_val.is_pointer_value());
     let value_ptr = arg_val.into_pointer_value();
 
+    // to increment/decrement the cons-cell itself
+    let refcount_ptr = PointerToRefcount::from_ptr_to_data(env, value_ptr);
+    let call_mode = mode_to_call_mode(fn_val, mode);
+
+    let should_recurse_block = env.context.append_basic_block(parent, "should_recurse");
+
+    let ctx = env.context;
+    if is_nullable {
+        let is_null = env.builder.build_is_null(value_ptr, "is_null");
+
+        let then_block = ctx.append_basic_block(parent, "then");
+
+        env.builder
+            .build_conditional_branch(is_null, then_block, should_recurse_block);
+
+        {
+            env.builder.position_at_end(then_block);
+            env.builder.build_return(None);
+        }
+    } else {
+        env.builder.build_unconditional_branch(should_recurse_block);
+    }
+
+    env.builder.position_at_end(should_recurse_block);
+
+    let layout = Layout::Union(union_layout);
+
+    match mode {
+        Mode::Inc => {
+            // inc is cheap; we never recurse
+            refcount_ptr.modify(call_mode, &layout, env);
+            env.builder.build_return(None);
+        }
+
+        Mode::Dec => {
+            let do_recurse_block = env.context.append_basic_block(parent, "do_recurse");
+            let no_recurse_block = env.context.append_basic_block(parent, "no_recurse");
+
+            builder.build_conditional_branch(
+                refcount_ptr.is_1(env),
+                do_recurse_block,
+                no_recurse_block,
+            );
+
+            {
+                env.builder.position_at_end(no_recurse_block);
+
+                refcount_ptr.modify(call_mode, &layout, env);
+                env.builder.build_return(None);
+            }
+
+            {
+                env.builder.position_at_end(do_recurse_block);
+
+                build_rec_union_recursive_decrement(
+                    env,
+                    layout_ids,
+                    when_recursive,
+                    parent,
+                    fn_val,
+                    union_layout,
+                    tags,
+                    value_ptr,
+                    refcount_ptr,
+                    do_recurse_block,
+                )
+            }
+        }
+    }
+}
+
+#[allow(clippy::too_many_arguments)]
+fn build_rec_union_recursive_decrement<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    layout_ids: &mut LayoutIds<'a>,
+    when_recursive: &WhenRecursive<'a>,
+    parent: FunctionValue<'ctx>,
+    decrement_fn: FunctionValue<'ctx>,
+    union_layout: UnionLayout<'a>,
+    tags: &[&[Layout<'a>]],
+    value_ptr: PointerValue<'ctx>,
+    refcount_ptr: PointerToRefcount<'ctx>,
+    match_block: BasicBlock<'ctx>,
+) {
+    let mode = Mode::Dec;
+    let call_mode = mode_to_call_mode(decrement_fn, mode);
+    let builder = env.builder;
+
     // branches that are not/don't contain anything refcounted
     // if there is only one branch, we don't need to switch
     let switch_needed: bool = (|| {
@@ -1300,31 +1391,12 @@ fn build_rec_union_help<'a, 'ctx, 'env>(
         false
     })();
 
-    // to increment/decrement the cons-cell itself
-    let refcount_ptr = PointerToRefcount::from_ptr_to_data(env, value_ptr);
-    let call_mode = mode_to_call_mode(fn_val, mode);
-
-    let ctx = env.context;
-    let cont_block = ctx.append_basic_block(parent, "cont");
-    if is_nullable {
-        let is_null = env.builder.build_is_null(value_ptr, "is_null");
-
-        let then_block = ctx.append_basic_block(parent, "then");
-
-        env.builder
-            .build_conditional_branch(is_null, then_block, cont_block);
-
-        {
-            env.builder.position_at_end(then_block);
-            env.builder.build_return(None);
-        }
-    } else {
-        env.builder.build_unconditional_branch(cont_block);
-    }
-
     // next, make a jump table for all possible values of the tag_id
     let mut cases = Vec::with_capacity_in(tags.len(), env.arena);
 
+    let tag_id_int_type =
+        basic_type_from_layout(env, &union_layout.tag_id_layout()).into_int_type();
+
     for (tag_id, field_layouts) in tags.iter().enumerate() {
         // if none of the fields are or contain anything refcounted, just move on
         if !field_layouts
@@ -1334,9 +1406,7 @@ fn build_rec_union_help<'a, 'ctx, 'env>(
             continue;
         }
 
-        let block = env
-            .context
-            .append_basic_block(parent, pick("tag_id_increment", "tag_id_decrement"));
+        let block = env.context.append_basic_block(parent, "tag_id_decrement");
 
         env.builder.position_at_end(block);
 
@@ -1372,7 +1442,23 @@ fn build_rec_union_help<'a, 'ctx, 'env>(
                 debug_assert!(ptr_as_i64_ptr.is_pointer_value());
 
                 // therefore we must cast it to our desired type
-                let union_type = block_of_memory_slices(env.context, tags, env.ptr_bytes);
+                let union_type = match union_layout {
+                    UnionLayout::NonRecursive(_) => unreachable!(),
+                    UnionLayout::Recursive(_) | UnionLayout::NullableWrapped { .. } => {
+                        union_data_block_of_memory(
+                            env.context,
+                            tag_id_int_type,
+                            tags,
+                            env.ptr_bytes,
+                        )
+                        .into()
+                    }
+                    UnionLayout::NonNullableUnwrapped { .. }
+                    | UnionLayout::NullableUnwrapped { .. } => {
+                        block_of_memory_slices(env.context, tags, env.ptr_bytes)
+                    }
+                };
+
                 let recursive_field_ptr = cast_basic_basic(
                     env.builder,
                     ptr_as_i64_ptr,
@@ -1386,10 +1472,9 @@ fn build_rec_union_help<'a, 'ctx, 'env>(
                     .build_struct_gep(struct_ptr, i as u32, "gep_recursive_pointer")
                     .unwrap();
 
-                let field = env.builder.build_load(
-                    elem_pointer,
-                    pick("increment_struct_field", "decrement_struct_field"),
-                );
+                let field = env
+                    .builder
+                    .build_load(elem_pointer, "decrement_struct_field");
 
                 deferred_nonrec.push((field, field_layout));
             }
@@ -1401,14 +1486,14 @@ fn build_rec_union_help<'a, 'ctx, 'env>(
         // lists. To achieve it, we must first load all fields that we want to inc/dec (done above)
         // and store them on the stack, then modify (and potentially free) the current cell, then
         // actually inc/dec the fields.
-        refcount_ptr.modify(call_mode, &layout, env);
+        refcount_ptr.modify(call_mode, &Layout::Union(union_layout), env);
 
         for (field, field_layout) in deferred_nonrec {
             modify_refcount_layout_help(
                 env,
                 parent,
                 layout_ids,
-                mode.to_call_mode(fn_val),
+                mode.to_call_mode(decrement_fn),
                 when_recursive,
                 field,
                 field_layout,
@@ -1417,22 +1502,19 @@ fn build_rec_union_help<'a, 'ctx, 'env>(
 
         for ptr in deferred_rec {
             // recursively decrement the field
-            let call = call_help(env, fn_val, mode.to_call_mode(fn_val), ptr);
+            let call = call_help(env, decrement_fn, mode.to_call_mode(decrement_fn), ptr);
             call.set_tail_call(true);
         }
 
         // this function returns void
         builder.build_return(None);
 
-        cases.push((
-            env.context.i64_type().const_int(tag_id as u64, false),
-            block,
-        ));
+        cases.push((tag_id_int_type.const_int(tag_id as u64, false), block));
     }
 
-    cases.reverse();
+    env.builder.position_at_end(match_block);
 
-    env.builder.position_at_end(cont_block);
+    cases.reverse();
 
     if cases.len() == 1 && !switch_needed {
         // there is only one tag in total; we don't need a switch
@@ -1443,11 +1525,9 @@ fn build_rec_union_help<'a, 'ctx, 'env>(
         env.builder.build_unconditional_branch(only_branch);
     } else {
         // read the tag_id
-        let current_tag_id = rec_union_read_tag(env, value_ptr);
+        let current_tag_id = get_tag_id(env, parent, &union_layout, value_ptr.into());
 
-        let merge_block = env
-            .context
-            .append_basic_block(parent, pick("increment_merge", "decrement_merge"));
+        let merge_block = env.context.append_basic_block(parent, "decrement_merge");
 
         // switch on it
         env.builder
@@ -1456,30 +1536,13 @@ fn build_rec_union_help<'a, 'ctx, 'env>(
         env.builder.position_at_end(merge_block);
 
         // increment/decrement the cons-cell itself
-        refcount_ptr.modify(call_mode, &layout, env);
+        refcount_ptr.modify(call_mode, &Layout::Union(union_layout), env);
 
         // this function returns void
         builder.build_return(None);
     }
 }
 
-fn rec_union_read_tag<'a, 'ctx, 'env>(
-    env: &Env<'a, 'ctx, 'env>,
-    value_ptr: PointerValue<'ctx>,
-) -> IntValue<'ctx> {
-    // Assumption: the tag is the first thing stored
-    // so cast the pointer to the data to a `i64*`
-    let tag_ptr_type = env.context.i64_type().ptr_type(AddressSpace::Generic);
-    let tag_ptr = env
-        .builder
-        .build_bitcast(value_ptr, tag_ptr_type, "cast_tag_ptr")
-        .into_pointer_value();
-
-    env.builder
-        .build_load(tag_ptr, "load_tag_id")
-        .into_int_value()
-}
-
 fn function_name_from_mode<'a>(
     layout_ids: &mut LayoutIds<'a>,
     interns: &Interns,
@@ -1524,7 +1587,7 @@ fn modify_refcount_union<'a, 'ctx, 'env>(
     let function = match env.module.get_function(fn_name.as_str()) {
         Some(function_value) => function_value,
         None => {
-            let basic_type = block_of_memory(env.context, &layout, env.ptr_bytes);
+            let basic_type = basic_type_from_layout(env, &layout);
             let function_value = build_header(env, basic_type, mode, &fn_name);
 
             modify_refcount_union_help(
@@ -1571,7 +1634,7 @@ fn modify_refcount_union_help<'a, 'ctx, 'env>(
     let arg_symbol = Symbol::ARG_1;
     let arg_val = fn_val.get_param_iter().next().unwrap();
 
-    set_name(arg_val, arg_symbol.ident_string(&env.interns));
+    arg_val.set_name(arg_symbol.ident_string(&env.interns));
 
     let parent = fn_val;
 
@@ -1580,19 +1643,7 @@ fn modify_refcount_union_help<'a, 'ctx, 'env>(
     let wrapper_struct = arg_val.into_struct_value();
 
     // read the tag_id
-    let tag_id = {
-        // the first element of the wrapping struct is an array of i64
-        let first_array = env
-            .builder
-            .build_extract_value(wrapper_struct, 0, "read_tag_id")
-            .unwrap()
-            .into_array_value();
-
-        env.builder
-            .build_extract_value(first_array, 0, "read_tag_id_2")
-            .unwrap()
-            .into_int_value()
-    };
+    let tag_id = get_tag_id_non_recursive(env, wrapper_struct);
 
     let tag_id_u8 = env
         .builder
@@ -1620,7 +1671,12 @@ fn modify_refcount_union_help<'a, 'ctx, 'env>(
         let wrapper_type = basic_type_from_layout(env, &Layout::Struct(field_layouts));
 
         debug_assert!(wrapper_type.is_struct_type());
-        let wrapper_struct = cast_block_of_memory_to_tag(env.builder, wrapper_struct, wrapper_type);
+        let data_bytes = env
+            .builder
+            .build_extract_value(wrapper_struct, TAG_DATA_INDEX, "read_tag_id")
+            .unwrap()
+            .into_struct_value();
+        let wrapper_struct = cast_block_of_memory_to_tag(env.builder, data_bytes, wrapper_type);
 
         for (i, field_layout) in field_layouts.iter().enumerate() {
             if let Layout::RecursivePointer = field_layout {
@@ -1689,7 +1745,7 @@ pub fn refcount_offset<'a, 'ctx, 'env>(env: &Env<'a, 'ctx, 'env>, layout: &Layou
     let value_bytes = layout.stack_size(env.ptr_bytes) as u64;
 
     match layout {
-        Layout::Builtin(Builtin::List(_, _)) => env.ptr_bytes as u64,
+        Layout::Builtin(Builtin::List(_)) => env.ptr_bytes as u64,
         Layout::Builtin(Builtin::Str) => env.ptr_bytes as u64,
         Layout::RecursivePointer | Layout::Union(_) => env.ptr_bytes as u64,
         _ => (env.ptr_bytes as u64).max(value_bytes),
diff --git a/compiler/gen/src/run_roc.rs b/compiler/gen_llvm/src/run_roc.rs
similarity index 95%
rename from compiler/gen/src/run_roc.rs
rename to compiler/gen_llvm/src/run_roc.rs
index 1e1c28b9b2..884ef41fcc 100644
--- a/compiler/gen/src/run_roc.rs
+++ b/compiler/gen_llvm/src/run_roc.rs
@@ -38,7 +38,7 @@ macro_rules! run_jit_function {
 
     ($lib: expr, $main_fn_name: expr, $ty:ty, $transform:expr, $errors:expr) => {{
         use inkwell::context::Context;
-        use roc_gen::run_roc::RocCallResult;
+        use roc_gen_llvm::run_roc::RocCallResult;
         use std::mem::MaybeUninit;
 
         unsafe {
@@ -77,7 +77,7 @@ macro_rules! run_jit_function_dynamic_type {
 
     ($lib: expr, $main_fn_name: expr, $bytes:expr, $transform:expr, $errors:expr) => {{
         use inkwell::context::Context;
-        use roc_gen::run_roc::RocCallResult;
+        use roc_gen_llvm::run_roc::RocCallResult;
 
         unsafe {
             let main: libloading::Symbol<unsafe extern "C" fn(*const u8)> = $lib
@@ -86,7 +86,7 @@ macro_rules! run_jit_function_dynamic_type {
                 .ok_or(format!("Unable to JIT compile `{}`", $main_fn_name))
                 .expect("errored");
 
-            let size = roc_gen::run_roc::ROC_CALL_RESULT_DISCRIMINANT_SIZE + $bytes;
+            let size = roc_gen_llvm::run_roc::ROC_CALL_RESULT_DISCRIMINANT_SIZE + $bytes;
             let layout = std::alloc::Layout::array::<u8>(size).unwrap();
             let result = std::alloc::alloc(layout);
             main(result);
diff --git a/compiler/load/Cargo.toml b/compiler/load/Cargo.toml
index 01d0a82f5b..6ed6e3cc63 100644
--- a/compiler/load/Cargo.toml
+++ b/compiler/load/Cargo.toml
@@ -19,6 +19,7 @@ roc_parse = { path = "../parse" }
 roc_solve = { path = "../solve" }
 roc_mono = { path = "../mono" }
 roc_reporting = { path = "../reporting" }
+morphic_lib = { path = "../../vendor/morphic_lib" }
 ven_pretty = { path = "../../vendor/pretty" }
 bumpalo = { version = "3.6.1", features = ["collections"] }
 inlinable_string = "0.1"
diff --git a/compiler/load/src/docs.rs b/compiler/load/src/docs.rs
index c971188257..1fa5db439d 100644
--- a/compiler/load/src/docs.rs
+++ b/compiler/load/src/docs.rs
@@ -1,10 +1,12 @@
-use crate::docs::DocEntry::DetatchedDoc;
-use crate::docs::TypeAnnotation::{Apply, BoundVariable, Record, TagUnion};
+use crate::docs::DocEntry::DetachedDoc;
+use crate::docs::TypeAnnotation::{
+    Apply, BoundVariable, Function, NoTypeAnn, ObscuredRecord, ObscuredTagUnion, Record, TagUnion,
+};
+use crate::file::LoadedModule;
 use inlinable_string::InlinableString;
 use roc_can::scope::Scope;
-use roc_collections::all::MutMap;
 use roc_module::ident::ModuleName;
-use roc_module::symbol::{IdentIds, Interns, ModuleId};
+use roc_module::symbol::IdentIds;
 use roc_parse::ast;
 use roc_parse::ast::CommentOrNewline;
 use roc_parse::ast::{AssignedField, Def};
@@ -17,7 +19,7 @@ pub struct Documentation {
     pub name: String,
     pub version: String,
     pub docs: String,
-    pub modules: Vec<(MutMap<ModuleId, ModuleDocumentation>, Interns)>,
+    pub modules: Vec<LoadedModule>,
 }
 
 #[derive(Debug)]
@@ -30,14 +32,14 @@ pub struct ModuleDocumentation {
 #[derive(Debug, Clone)]
 pub enum DocEntry {
     DocDef(DocDef),
-    DetatchedDoc(String),
+    DetachedDoc(String),
 }
 
 #[derive(Debug, Clone)]
 pub struct DocDef {
     pub name: String,
     pub type_vars: Vec<String>,
-    pub type_annotation: Option<TypeAnnotation>,
+    pub type_annotation: TypeAnnotation,
     pub docs: Option<String>,
 }
 
@@ -45,8 +47,14 @@ pub struct DocDef {
 pub enum TypeAnnotation {
     TagUnion {
         tags: Vec<Tag>,
-        extension: Option<Box<TypeAnnotation>>,
+        extension: Box<TypeAnnotation>,
     },
+    Function {
+        args: Vec<TypeAnnotation>,
+        output: Box<TypeAnnotation>,
+    },
+    ObscuredTagUnion,
+    ObscuredRecord,
     BoundVariable(String),
     Apply {
         name: String,
@@ -54,7 +62,10 @@ pub enum TypeAnnotation {
     },
     Record {
         fields: Vec<RecordField>,
+        extension: Box<TypeAnnotation>,
     },
+    Wildcard,
+    NoTypeAnn,
 }
 #[derive(Debug, Clone)]
 pub enum RecordField {
@@ -97,10 +108,10 @@ pub fn generate_module_docs<'a>(
     }
 }
 
-fn detatched_docs_from_comments_and_new_lines<'a>(
+fn detached_docs_from_comments_and_new_lines<'a>(
     comments_or_new_lines: &'a [roc_parse::ast::CommentOrNewline<'a>],
 ) -> Vec<String> {
-    let mut detatched_docs: Vec<String> = Vec::new();
+    let mut detached_docs: Vec<String> = Vec::new();
 
     let mut docs = String::new();
 
@@ -112,13 +123,16 @@ fn detatched_docs_from_comments_and_new_lines<'a>(
             }
 
             CommentOrNewline::LineComment(_) | CommentOrNewline::Newline => {
-                detatched_docs.push(docs.clone());
+                if !docs.is_empty() {
+                    detached_docs.push(docs.clone());
+                }
+
                 docs = String::new();
             }
         }
     }
 
-    detatched_docs
+    detached_docs
 }
 
 fn generate_entry_doc<'a>(
@@ -136,8 +150,8 @@ fn generate_entry_doc<'a>(
         Def::SpaceBefore(sub_def, comments_or_new_lines) => {
             // Comments before a definition are attached to the current defition
 
-            for detatched_doc in detatched_docs_from_comments_and_new_lines(comments_or_new_lines) {
-                acc.push(DetatchedDoc(detatched_doc));
+            for detached_doc in detached_docs_from_comments_and_new_lines(comments_or_new_lines) {
+                acc.push(DetachedDoc(detached_doc));
             }
 
             generate_entry_doc(ident_ids, acc, Some(comments_or_new_lines), sub_def)
@@ -161,7 +175,7 @@ fn generate_entry_doc<'a>(
                 {
                     let doc_def = DocDef {
                         name: identifier.to_string(),
-                        type_annotation: None,
+                        type_annotation: NoTypeAnn,
                         type_vars: Vec::new(),
                         docs: before_comments_or_new_lines.and_then(comments_or_new_lines_to_docs),
                     };
@@ -172,7 +186,11 @@ fn generate_entry_doc<'a>(
 
             _ => (acc, None),
         },
-        Def::AnnotatedBody { ann_pattern, .. } => match ann_pattern.value {
+        Def::AnnotatedBody {
+            ann_pattern,
+            ann_type,
+            ..
+        } => match ann_pattern.value {
             Pattern::Identifier(identifier) => {
                 // Check if the definition is exposed
                 if ident_ids
@@ -181,7 +199,7 @@ fn generate_entry_doc<'a>(
                 {
                     let doc_def = DocDef {
                         name: identifier.to_string(),
-                        type_annotation: None,
+                        type_annotation: type_to_docs(false, ann_type.value),
                         type_vars: Vec::new(),
                         docs: before_comments_or_new_lines.and_then(comments_or_new_lines_to_docs),
                     };
@@ -204,7 +222,7 @@ fn generate_entry_doc<'a>(
 
             let doc_def = DocDef {
                 name: name.value.to_string(),
-                type_annotation: type_to_docs(ann.value),
+                type_annotation: type_to_docs(false, ann.value),
                 type_vars,
                 docs: before_comments_or_new_lines.and_then(comments_or_new_lines_to_docs),
             };
@@ -221,7 +239,7 @@ fn generate_entry_doc<'a>(
     }
 }
 
-fn type_to_docs(type_annotation: ast::TypeAnnotation) -> Option<TypeAnnotation> {
+fn type_to_docs(in_func_type_ann: bool, type_annotation: ast::TypeAnnotation) -> TypeAnnotation {
     match type_annotation {
         ast::TypeAnnotation::TagUnion {
             tags,
@@ -233,7 +251,7 @@ fn type_to_docs(type_annotation: ast::TypeAnnotation) -> Option<TypeAnnotation>
             let mut any_tags_are_private = false;
 
             for tag in tags {
-                match tag_to_doc(tag.value) {
+                match tag_to_doc(in_func_type_ann, tag.value) {
                     None => {
                         any_tags_are_private = true;
                         break;
@@ -245,20 +263,24 @@ fn type_to_docs(type_annotation: ast::TypeAnnotation) -> Option<TypeAnnotation>
             }
 
             if any_tags_are_private {
-                None
+                if in_func_type_ann {
+                    ObscuredTagUnion
+                } else {
+                    NoTypeAnn
+                }
             } else {
                 let extension = match ext {
-                    None => None,
-                    Some(ext_type_ann) => type_to_docs(ext_type_ann.value).map(Box::new),
+                    None => NoTypeAnn,
+                    Some(ext_type_ann) => type_to_docs(in_func_type_ann, ext_type_ann.value),
                 };
 
-                Some(TagUnion {
+                TagUnion {
                     tags: tags_to_render,
-                    extension,
-                })
+                    extension: Box::new(extension),
+                }
             }
         }
-        ast::TypeAnnotation::BoundVariable(var_name) => Some(BoundVariable(var_name.to_string())),
+        ast::TypeAnnotation::BoundVariable(var_name) => BoundVariable(var_name.to_string()),
         ast::TypeAnnotation::Apply(module_name, type_name, type_ann_parts) => {
             let mut name = String::new();
 
@@ -272,16 +294,14 @@ fn type_to_docs(type_annotation: ast::TypeAnnotation) -> Option<TypeAnnotation>
             let mut parts: Vec<TypeAnnotation> = Vec::new();
 
             for type_ann_part in type_ann_parts {
-                if let Some(part) = type_to_docs(type_ann_part.value) {
-                    parts.push(part);
-                }
+                parts.push(type_to_docs(in_func_type_ann, type_ann_part.value));
             }
 
-            Some(Apply { name, parts })
+            Apply { name, parts }
         }
         ast::TypeAnnotation::Record {
             fields,
-            ext: _,
+            ext,
             final_comments: _,
         } => {
             let mut doc_fields = Vec::new();
@@ -289,7 +309,7 @@ fn type_to_docs(type_annotation: ast::TypeAnnotation) -> Option<TypeAnnotation>
             let mut any_fields_include_private_tags = false;
 
             for field in fields {
-                match record_field_to_doc(field.value) {
+                match record_field_to_doc(in_func_type_ann, field.value) {
                     None => {
                         any_fields_include_private_tags = true;
                         break;
@@ -300,37 +320,61 @@ fn type_to_docs(type_annotation: ast::TypeAnnotation) -> Option<TypeAnnotation>
                 }
             }
             if any_fields_include_private_tags {
-                None
+                if in_func_type_ann {
+                    ObscuredRecord
+                } else {
+                    NoTypeAnn
+                }
             } else {
-                Some(Record { fields: doc_fields })
+                let extension = match ext {
+                    None => NoTypeAnn,
+                    Some(ext_type_ann) => type_to_docs(in_func_type_ann, ext_type_ann.value),
+                };
+
+                Record {
+                    fields: doc_fields,
+                    extension: Box::new(extension),
+                }
             }
         }
-        ast::TypeAnnotation::SpaceBefore(&sub_type_ann, _) => type_to_docs(sub_type_ann),
-        ast::TypeAnnotation::SpaceAfter(&sub_type_ann, _) => type_to_docs(sub_type_ann),
-        _ => {
-            // TODO "Implement type to docs")
-
-            None
+        ast::TypeAnnotation::SpaceBefore(&sub_type_ann, _) => {
+            type_to_docs(in_func_type_ann, sub_type_ann)
         }
+        ast::TypeAnnotation::SpaceAfter(&sub_type_ann, _) => {
+            type_to_docs(in_func_type_ann, sub_type_ann)
+        }
+        ast::TypeAnnotation::Function(ast_arg_anns, output_ann) => {
+            let mut doc_arg_anns = Vec::new();
+
+            for ast_arg_ann in ast_arg_anns {
+                doc_arg_anns.push(type_to_docs(true, ast_arg_ann.value));
+            }
+
+            Function {
+                args: doc_arg_anns,
+                output: Box::new(type_to_docs(true, output_ann.value)),
+            }
+        }
+        ast::TypeAnnotation::Wildcard => TypeAnnotation::Wildcard,
+        _ => NoTypeAnn,
     }
 }
 
-fn record_field_to_doc(field: ast::AssignedField<'_, ast::TypeAnnotation>) -> Option<RecordField> {
+fn record_field_to_doc(
+    in_func_ann: bool,
+    field: ast::AssignedField<'_, ast::TypeAnnotation>,
+) -> Option<RecordField> {
     match field {
-        AssignedField::RequiredValue(name, _, type_ann) => {
-            type_to_docs(type_ann.value).map(|type_ann_docs| RecordField::RecordField {
-                name: name.value.to_string(),
-                type_annotation: type_ann_docs,
-            })
-        }
-        AssignedField::SpaceBefore(&sub_field, _) => record_field_to_doc(sub_field),
-        AssignedField::SpaceAfter(&sub_field, _) => record_field_to_doc(sub_field),
-        AssignedField::OptionalValue(name, _, type_ann) => {
-            type_to_docs(type_ann.value).map(|type_ann_docs| RecordField::OptionalField {
-                name: name.value.to_string(),
-                type_annotation: type_ann_docs,
-            })
-        }
+        AssignedField::RequiredValue(name, _, type_ann) => Some(RecordField::RecordField {
+            name: name.value.to_string(),
+            type_annotation: type_to_docs(in_func_ann, type_ann.value),
+        }),
+        AssignedField::SpaceBefore(&sub_field, _) => record_field_to_doc(in_func_ann, sub_field),
+        AssignedField::SpaceAfter(&sub_field, _) => record_field_to_doc(in_func_ann, sub_field),
+        AssignedField::OptionalValue(name, _, type_ann) => Some(RecordField::OptionalField {
+            name: name.value.to_string(),
+            type_annotation: type_to_docs(in_func_ann, type_ann.value),
+        }),
         AssignedField::LabelOnly(label) => Some(RecordField::LabelOnly {
             name: label.value.to_string(),
         }),
@@ -340,7 +384,7 @@ fn record_field_to_doc(field: ast::AssignedField<'_, ast::TypeAnnotation>) -> Op
 
 // The Option here represents if it is private. Private tags
 // evaluate to `None`.
-fn tag_to_doc(tag: ast::Tag) -> Option<Tag> {
+fn tag_to_doc(in_func_ann: bool, tag: ast::Tag) -> Option<Tag> {
     match tag {
         ast::Tag::Global { name, args } => Some(Tag {
             name: name.value.to_string(),
@@ -348,17 +392,15 @@ fn tag_to_doc(tag: ast::Tag) -> Option<Tag> {
                 let mut type_vars = Vec::new();
 
                 for arg in args {
-                    if let Some(type_var) = type_to_docs(arg.value) {
-                        type_vars.push(type_var);
-                    }
+                    type_vars.push(type_to_docs(in_func_ann, arg.value));
                 }
 
                 type_vars
             },
         }),
         ast::Tag::Private { .. } => None,
-        ast::Tag::SpaceBefore(&sub_tag, _) => tag_to_doc(sub_tag),
-        ast::Tag::SpaceAfter(&sub_tag, _) => tag_to_doc(sub_tag),
+        ast::Tag::SpaceBefore(&sub_tag, _) => tag_to_doc(in_func_ann, sub_tag),
+        ast::Tag::SpaceAfter(&sub_tag, _) => tag_to_doc(in_func_ann, sub_tag),
         ast::Tag::Malformed(_) => None,
     }
 }
diff --git a/compiler/load/src/file.rs b/compiler/load/src/file.rs
index 0f48ffc4be..24d07b1a47 100644
--- a/compiler/load/src/file.rs
+++ b/compiler/load/src/file.rs
@@ -19,8 +19,8 @@ use roc_module::symbol::{
     Symbol,
 };
 use roc_mono::ir::{
-    CapturedSymbols, ExternalSpecializations, PartialProc, PendingSpecialization, Proc, Procs,
-    TopLevelFunctionLayout,
+    CapturedSymbols, EntryPoint, ExternalSpecializations, PartialProc, PendingSpecialization, Proc,
+    ProcLayout, Procs,
 };
 use roc_mono::layout::{Layout, LayoutCache, LayoutProblem};
 use roc_parse::ast::{self, StrLiteral, TypeAnnotation};
@@ -621,6 +621,8 @@ pub struct LoadedModule {
     pub type_problems: MutMap<ModuleId, Vec<solve::TypeError>>,
     pub declarations_by_id: MutMap<ModuleId, Vec<Declaration>>,
     pub exposed_to_host: MutMap<Symbol, Variable>,
+    pub exposed_aliases: MutMap<Symbol, Alias>,
+    pub exposed_values: Vec<Symbol>,
     pub header_sources: MutMap<ModuleId, (PathBuf, Box<str>)>,
     pub sources: MutMap<ModuleId, (PathBuf, Box<str>)>,
     pub timings: MutMap<ModuleId, ModuleTiming>,
@@ -660,6 +662,8 @@ enum HeaderFor<'a> {
         config_shorthand: &'a str,
         /// the type scheme of the main function (required by the platform)
         platform_main_type: TypedIdent<'a>,
+        /// provided symbol to host (commonly `mainForHost`)
+        main_for_host: Symbol,
     },
     Interface,
 }
@@ -705,7 +709,8 @@ pub struct MonomorphizedModule<'a> {
     pub can_problems: MutMap<ModuleId, Vec<roc_problem::can::Problem>>,
     pub type_problems: MutMap<ModuleId, Vec<solve::TypeError>>,
     pub mono_problems: MutMap<ModuleId, Vec<roc_mono::ir::MonoProblem>>,
-    pub procedures: MutMap<(Symbol, TopLevelFunctionLayout<'a>), Proc<'a>>,
+    pub procedures: MutMap<(Symbol, ProcLayout<'a>), Proc<'a>>,
+    pub entry_point: EntryPoint<'a>,
     pub exposed_to_host: MutMap<Symbol, Variable>,
     pub header_sources: MutMap<ModuleId, (PathBuf, Box<str>)>,
     pub sources: MutMap<ModuleId, (PathBuf, Box<str>)>,
@@ -760,6 +765,8 @@ enum Msg<'a> {
     FinishedAllTypeChecking {
         solved_subs: Solved<Subs>,
         exposed_vars_by_symbol: MutMap<Symbol, Variable>,
+        exposed_aliases_by_symbol: MutMap<Symbol, Alias>,
+        exposed_values: Vec<Symbol>,
         documentation: MutMap<ModuleId, ModuleDocumentation>,
     },
     FoundSpecializations {
@@ -776,7 +783,7 @@ enum Msg<'a> {
         ident_ids: IdentIds,
         layout_cache: LayoutCache<'a>,
         external_specializations_requested: BumpMap<ModuleId, ExternalSpecializations<'a>>,
-        procedures: MutMap<(Symbol, TopLevelFunctionLayout<'a>), Proc<'a>>,
+        procedures: MutMap<(Symbol, ProcLayout<'a>), Proc<'a>>,
         problems: Vec<roc_mono::ir::MonoProblem>,
         module_timing: ModuleTiming,
         subs: Subs,
@@ -804,10 +811,16 @@ enum PlatformPath<'a> {
     RootIsPkgConfig,
 }
 
+#[derive(Debug)]
+struct PlatformData {
+    module_id: ModuleId,
+    provides: Symbol,
+}
+
 #[derive(Debug)]
 struct State<'a> {
     pub root_id: ModuleId,
-    pub platform_id: Option<ModuleId>,
+    pub platform_data: Option<PlatformData>,
     pub goal_phase: Phase,
     pub stdlib: &'a StdLib,
     pub exposed_types: SubsByModule,
@@ -818,7 +831,7 @@ struct State<'a> {
 
     pub module_cache: ModuleCache<'a>,
     pub dependencies: Dependencies<'a>,
-    pub procedures: MutMap<(Symbol, TopLevelFunctionLayout<'a>), Proc<'a>>,
+    pub procedures: MutMap<(Symbol, ProcLayout<'a>), Proc<'a>>,
     pub exposed_to_host: MutMap<Symbol, Variable>,
 
     /// This is the "final" list of IdentIds, after canonicalization and constraint gen
@@ -849,7 +862,7 @@ struct State<'a> {
     pub needs_specialization: MutSet<ModuleId>,
 
     pub all_pending_specializations:
-        MutMap<Symbol, MutMap<TopLevelFunctionLayout<'a>, PendingSpecialization<'a>>>,
+        MutMap<Symbol, MutMap<ProcLayout<'a>, PendingSpecialization<'a>>>,
 
     pub specializations_in_flight: u32,
 
@@ -1447,7 +1460,7 @@ where
 
             let mut state = State {
                 root_id,
-                platform_id: None,
+                platform_data: None,
                 goal_phase,
                 stdlib,
                 output_path: None,
@@ -1501,6 +1514,8 @@ where
                     Msg::FinishedAllTypeChecking {
                         solved_subs,
                         exposed_vars_by_symbol,
+                        exposed_aliases_by_symbol,
+                        exposed_values,
                         documentation,
                     } => {
                         // We're done! There should be no more messages pending.
@@ -1516,6 +1531,8 @@ where
                         return Ok(LoadResult::TypeChecked(finish(
                             state,
                             solved_subs,
+                            exposed_values,
+                            exposed_aliases_by_symbol,
                             exposed_vars_by_symbol,
                             documentation,
                         )));
@@ -1674,10 +1691,13 @@ fn update<'a>(
                     debug_assert!(matches!(state.platform_path, PlatformPath::NotSpecified));
                     state.platform_path = PlatformPath::Valid(to_platform.clone());
                 }
-                PkgConfig { .. } => {
-                    debug_assert_eq!(state.platform_id, None);
+                PkgConfig { main_for_host, .. } => {
+                    debug_assert!(matches!(state.platform_data, None));
 
-                    state.platform_id = Some(header.module_id);
+                    state.platform_data = Some(PlatformData {
+                        module_id: header.module_id,
+                        provides: main_for_host,
+                    });
 
                     if header.is_root_module {
                         debug_assert!(matches!(state.platform_path, PlatformPath::NotSpecified));
@@ -1896,9 +1916,9 @@ fn update<'a>(
 
             // if there is a platform, the Package-Config module provides host-exposed,
             // otherwise the App module exposes host-exposed
-            let is_host_exposed = match state.platform_id {
+            let is_host_exposed = match state.platform_data {
                 None => module_id == state.root_id,
-                Some(platform_id) => module_id == platform_id,
+                Some(ref platform_data) => module_id == platform_data.module_id,
             };
 
             if is_host_exposed {
@@ -1927,6 +1947,8 @@ fn update<'a>(
                     .send(Msg::FinishedAllTypeChecking {
                         solved_subs,
                         exposed_vars_by_symbol: solved_module.exposed_vars_by_symbol,
+                        exposed_values: solved_module.exposed_symbols,
+                        exposed_aliases_by_symbol: solved_module.aliases,
                         documentation,
                     })
                     .map_err(|_| LoadingProblem::MsgChannelDied)?;
@@ -2025,7 +2047,7 @@ fn update<'a>(
         }
         MadeSpecializations {
             module_id,
-            mut ident_ids,
+            ident_ids,
             subs,
             procedures,
             external_specializations_requested,
@@ -2061,22 +2083,16 @@ fn update<'a>(
                     println!("{}", result);
                 }
 
-                if false {
-                    let it = state.procedures.iter().map(|x| x.1);
-
-                    if let Err(e) = roc_mono::alias_analysis::spec_program(it) {
-                        println!("Error in alias analysis: {:?}", e)
-                    }
-                }
-
                 Proc::insert_refcount_operations(arena, &mut state.procedures);
 
-                Proc::optimize_refcount_operations(
-                    arena,
-                    module_id,
-                    &mut ident_ids,
-                    &mut state.procedures,
-                );
+                // This is not safe with the new non-recursive RC updates that we do for tag unions
+                //
+                //                Proc::optimize_refcount_operations(
+                //                    arena,
+                //                    module_id,
+                //                    &mut ident_ids,
+                //                    &mut state.procedures,
+                //                );
 
                 state.constrained_ident_ids.insert(module_id, ident_ids);
 
@@ -2160,6 +2176,7 @@ fn finish_specialization(
         module_cache,
         output_path,
         platform_path,
+        platform_data,
         ..
     } = state;
 
@@ -2207,6 +2224,34 @@ fn finish_specialization(
 
     let platform_path = path_to_platform.into();
 
+    let entry_point = {
+        let symbol = match platform_data {
+            None => {
+                debug_assert_eq!(exposed_to_host.len(), 1);
+                *exposed_to_host.iter().next().unwrap().0
+            }
+            Some(PlatformData { provides, .. }) => provides,
+        };
+
+        match procedures.keys().find(|(s, _)| *s == symbol) {
+            Some((_, layout)) => EntryPoint {
+                layout: *layout,
+                symbol,
+            },
+            None => {
+                // the entry point is not specialized. This can happen if the repl output
+                // is a function value
+                EntryPoint {
+                    layout: roc_mono::ir::ProcLayout {
+                        arguments: &[],
+                        result: Layout::Struct(&[]),
+                    },
+                    symbol,
+                }
+            }
+        }
+    };
+
     Ok(MonomorphizedModule {
         can_problems,
         mono_problems,
@@ -2218,6 +2263,7 @@ fn finish_specialization(
         subs,
         interns,
         procedures,
+        entry_point,
         sources,
         header_sources,
         timings: state.timings,
@@ -2227,6 +2273,8 @@ fn finish_specialization(
 fn finish(
     state: State,
     solved: Solved<Subs>,
+    exposed_values: Vec<Symbol>,
+    exposed_aliases_by_symbol: MutMap<Symbol, Alias>,
     exposed_vars_by_symbol: MutMap<Symbol, Variable>,
     documentation: MutMap<ModuleId, ModuleDocumentation>,
 ) -> LoadedModule {
@@ -2261,6 +2309,8 @@ fn finish(
         can_problems: state.module_cache.can_problems,
         type_problems: state.module_cache.type_problems,
         declarations_by_id: state.declarations_by_id,
+        exposed_aliases: exposed_aliases_by_symbol,
+        exposed_values,
         exposed_to_host: exposed_vars_by_symbol.into_iter().collect(),
         header_sources,
         sources,
@@ -2991,7 +3041,7 @@ fn send_header_two<'a>(
         HashMap::with_capacity_and_hasher(scope_size, default_hasher());
     let home: ModuleId;
 
-    let ident_ids = {
+    let mut ident_ids = {
         // Lock just long enough to perform the minimal operations necessary.
         let mut module_ids = (*module_ids).lock();
         let mut ident_ids_by_module = (*ident_ids_by_module).lock();
@@ -3114,9 +3164,17 @@ fn send_header_two<'a>(
     // to decrement its "pending" count.
     let module_name = ModuleNameEnum::PkgConfig;
 
+    let main_for_host = {
+        let ident_str: InlinableString = provides[0].value.as_str().into();
+        let ident_id = ident_ids.get_or_insert(&ident_str);
+
+        Symbol::new(home, ident_id)
+    };
+
     let extra = HeaderFor::PkgConfig {
         config_shorthand: shorthand,
         platform_main_type: requires[0].value.clone(),
+        main_for_host,
     };
 
     let mut package_qualified_imported_modules = MutSet::default();
@@ -3246,6 +3304,7 @@ fn run_solve<'a>(
 
     let solved_module = SolvedModule {
         exposed_vars_by_symbol,
+        exposed_symbols: exposed_symbols.into_iter().collect::<Vec<_>>(),
         solved_types,
         problems,
         aliases: solved_env.aliases,
@@ -3482,7 +3541,6 @@ fn fabricate_effects_module<'a>(
                     &mut var_store,
                     annotation,
                 );
-
                 exposed_symbols.insert(symbol);
 
                 declarations.push(Declaration::Declare(def));
@@ -3821,7 +3879,8 @@ fn make_specializations<'a>(
         ident_ids: &mut ident_ids,
         ptr_bytes,
         update_mode_counter: 0,
-        call_specialization_counter: 0,
+        // call_specialization_counter=0 is reserved
+        call_specialization_counter: 1,
     };
 
     // TODO: for now this final specialization pass is sequential,
@@ -3884,7 +3943,8 @@ fn build_pending_specializations<'a>(
         ident_ids: &mut ident_ids,
         ptr_bytes,
         update_mode_counter: 0,
-        call_specialization_counter: 0,
+        // call_specialization_counter=0 is reserved
+        call_specialization_counter: 1,
     };
 
     // Add modules' decls to Procs
@@ -4003,7 +4063,7 @@ fn add_def_to_module<'a>(
 
                         procs.insert_exposed(
                             symbol,
-                            TopLevelFunctionLayout::from_layout(mono_env.arena, layout),
+                            ProcLayout::from_layout(mono_env.arena, layout),
                             mono_env.arena,
                             mono_env.subs,
                             def.annotation,
@@ -4034,14 +4094,14 @@ fn add_def_to_module<'a>(
                     if is_exposed {
                         let annotation = def.expr_var;
 
-                        let layout = match layout_cache.from_var(
+                        let top_level = match layout_cache.from_var(
                             mono_env.arena,
                             annotation,
                             mono_env.subs,
                         ) {
                             Ok(l) => {
                                 // remember, this is a 0-argument thunk
-                                Layout::FunctionPointer(&[], mono_env.arena.alloc(l))
+                                ProcLayout::new(mono_env.arena, &[], l)
                             }
                             Err(LayoutProblem::Erroneous) => {
                                 let message = "top level function has erroneous type";
@@ -4062,7 +4122,7 @@ fn add_def_to_module<'a>(
 
                         procs.insert_exposed(
                             symbol,
-                            TopLevelFunctionLayout::from_layout(mono_env.arena, layout),
+                            top_level,
                             mono_env.arena,
                             mono_env.subs,
                             def.annotation,
diff --git a/compiler/module/src/low_level.rs b/compiler/module/src/low_level.rs
index 2ed35be875..98a1f48cf8 100644
--- a/compiler/module/src/low_level.rs
+++ b/compiler/module/src/low_level.rs
@@ -18,7 +18,6 @@ pub enum LowLevel {
     ListLen,
     ListGetUnsafe,
     ListSet,
-    ListSetInPlace,
     ListSingle,
     ListRepeat,
     ListReverse,
@@ -40,6 +39,7 @@ pub enum LowLevel {
     ListKeepErrs,
     ListSortWith,
     ListDrop,
+    ListSwap,
     DictSize,
     DictEmpty,
     DictInsert,
@@ -124,7 +124,6 @@ impl LowLevel {
             | ListLen
             | ListGetUnsafe
             | ListSet
-            | ListSetInPlace
             | ListDrop
             | ListSingle
             | ListRepeat
@@ -135,6 +134,7 @@ impl LowLevel {
             | ListPrepend
             | ListJoin
             | ListRange
+            | ListSwap
             | DictSize
             | DictEmpty
             | DictInsert
diff --git a/compiler/module/src/symbol.rs b/compiler/module/src/symbol.rs
index 90fc894970..d107bdffe4 100644
--- a/compiler/module/src/symbol.rs
+++ b/compiler/module/src/symbol.rs
@@ -278,10 +278,10 @@ impl ModuleId {
         // This is a no-op that should get DCE'd
     }
 
-    pub fn to_string<'a>(&self, interns: &'a Interns) -> &'a InlinableString {
+    pub fn to_string(self, interns: &Interns) -> &InlinableString {
         interns
             .module_ids
-            .get_name(*self)
+            .get_name(self)
             .unwrap_or_else(|| panic!("Could not find ModuleIds for {:?}", self))
     }
 }
@@ -396,17 +396,15 @@ impl<'a> PackageModuleIds<'a> {
     fn insert_debug_name(module_id: ModuleId, module_name: &PQModuleName) {
         let mut names = DEBUG_MODULE_ID_NAMES.lock().expect("Failed to acquire lock for Debug interning into DEBUG_MODULE_ID_NAMES, presumably because a thread panicked.");
 
-        if !names.contains_key(&module_id.0) {
-            match module_name {
-                PQModuleName::Unqualified(module) => {
-                    names.insert(module_id.0, module.to_string().into());
-                }
+        names
+            .entry(module_id.0)
+            .or_insert_with(|| match module_name {
+                PQModuleName::Unqualified(module) => module.to_string().into(),
                 PQModuleName::Qualified(package, module) => {
                     let name = format!("{}.{}", package, module).into();
-                    names.insert(module_id.0, name);
+                    name
                 }
-            }
-        }
+            });
     }
 
     #[cfg(not(debug_assertions))]
@@ -464,9 +462,9 @@ impl ModuleIds {
         let mut names = DEBUG_MODULE_ID_NAMES.lock().expect("Failed to acquire lock for Debug interning into DEBUG_MODULE_ID_NAMES, presumably because a thread panicked.");
 
         // TODO make sure modules are never added more than once!
-        if !names.contains_key(&module_id.0) {
-            names.insert(module_id.0, module_name.to_string().into());
-        }
+        names
+            .entry(module_id.0)
+            .or_insert_with(|| module_name.to_string().into());
     }
 
     #[cfg(not(debug_assertions))]
@@ -904,6 +902,7 @@ define_builtins! {
         14 STR_UT8_BYTE_PROBLEM: "Utf8ByteProblem" // the Utf8ByteProblem type alias
         15 STR_TO_BYTES: "toBytes"
         16 STR_STARTS_WITH_CODE_POINT: "startsWithCodePoint"
+        17 STR_ALIAS_ANALYSIS_STATIC: "#aliasAnalysisStatic" // string with the static lifetime
     }
     4 LIST: "List" => {
         0 LIST_LIST: "List" imported // the List.List type alias
@@ -939,6 +938,7 @@ define_builtins! {
         30 LIST_RANGE: "range"
         31 LIST_SORT_WITH: "sortWith"
         32 LIST_DROP: "drop"
+        33 LIST_SWAP: "swap"
     }
     5 RESULT: "Result" => {
         0 RESULT_RESULT: "Result" imported // the Result.Result type alias
diff --git a/compiler/mono/src/alias_analysis.rs b/compiler/mono/src/alias_analysis.rs
index ebca28d1a5..2b4354d26e 100644
--- a/compiler/mono/src/alias_analysis.rs
+++ b/compiler/mono/src/alias_analysis.rs
@@ -1,8 +1,8 @@
 use morphic_lib::TypeContext;
 use morphic_lib::{
-    BlockExpr, BlockId, CalleeSpecVar, EntryPointName, ExprContext, FuncDef, FuncDefBuilder,
-    FuncName, ModDefBuilder, ModName, ProgramBuilder, Result, TypeId, TypeName, UpdateModeVar,
-    ValueId,
+    BlockExpr, BlockId, CalleeSpecVar, ConstDefBuilder, ConstName, EntryPointName, ExprContext,
+    FuncDef, FuncDefBuilder, FuncName, ModDefBuilder, ModName, ProgramBuilder, Result, TypeId,
+    UpdateModeVar, ValueId,
 };
 use roc_collections::all::MutMap;
 use roc_module::low_level::LowLevel;
@@ -13,25 +13,144 @@ use crate::ir::{Call, CallType, Expr, Literal, ModifyRc, Proc, Stmt};
 use crate::layout::{Builtin, Layout, ListLayout, UnionLayout};
 
 // just using one module for now
-const MOD_LIST: ModName = ModName(b"UserApp");
-const MOD_APP: ModName = ModName(b"UserApp");
+pub const MOD_APP: ModName = ModName(b"UserApp");
 
-pub fn spec_program<'a, I>(procs: I) -> Result<morphic_lib::Solutions>
+pub const STATIC_STR_NAME: ConstName = ConstName(&Symbol::STR_ALIAS_ANALYSIS_STATIC.to_ne_bytes());
+
+const ENTRY_POINT_NAME: &[u8] = b"mainForHost";
+
+pub fn func_name_bytes(proc: &Proc) -> [u8; SIZE] {
+    func_name_bytes_help(proc.name, proc.args.iter().map(|x| x.0), proc.ret_layout)
+}
+
+const DEBUG: bool = false;
+const SIZE: usize = if DEBUG { 50 } else { 16 };
+
+pub fn func_name_bytes_help<'a, I>(
+    symbol: Symbol,
+    argument_layouts: I,
+    return_layout: Layout<'a>,
+) -> [u8; SIZE]
+where
+    I: Iterator<Item = Layout<'a>>,
+{
+    let mut name_bytes = [0u8; SIZE];
+
+    use std::collections::hash_map::DefaultHasher;
+    use std::hash::Hash;
+    use std::hash::Hasher;
+
+    let layout_hash = {
+        let mut hasher = DefaultHasher::new();
+
+        for layout in argument_layouts {
+            match layout {
+                Layout::Closure(_, lambda_set, _) => {
+                    lambda_set.runtime_representation().hash(&mut hasher);
+                }
+                _ => {
+                    layout.hash(&mut hasher);
+                }
+            }
+        }
+
+        match return_layout {
+            Layout::Closure(_, lambda_set, _) => {
+                lambda_set.runtime_representation().hash(&mut hasher);
+            }
+            _ => {
+                return_layout.hash(&mut hasher);
+            }
+        }
+
+        hasher.finish()
+    };
+
+    let sbytes = symbol.to_ne_bytes();
+    let lbytes = layout_hash.to_ne_bytes();
+
+    let it = sbytes
+        .iter()
+        .chain(lbytes.iter())
+        .zip(name_bytes.iter_mut());
+
+    for (source, target) in it {
+        *target = *source;
+    }
+
+    if DEBUG {
+        for (i, c) in (format!("{:?}", symbol)).chars().take(25).enumerate() {
+            name_bytes[25 + i] = c as u8;
+        }
+    }
+
+    name_bytes
+}
+
+fn bytes_as_ascii(bytes: &[u8]) -> String {
+    use std::fmt::Write;
+
+    let mut buf = String::new();
+
+    for byte in bytes {
+        write!(buf, "{:02X}", byte).unwrap();
+    }
+
+    buf
+}
+
+pub fn spec_program<'a, I>(
+    entry_point: crate::ir::EntryPoint<'a>,
+    procs: I,
+) -> Result<morphic_lib::Solutions>
 where
     I: Iterator<Item = &'a Proc<'a>>,
 {
-    let mut main_function = None;
     let main_module = {
         let mut m = ModDefBuilder::new();
 
+        // a const that models all static strings
+        let static_str_def = {
+            let mut cbuilder = ConstDefBuilder::new();
+            let block = cbuilder.add_block();
+            let cell = cbuilder.add_new_heap_cell(block)?;
+            let value_id = cbuilder.add_make_tuple(block, &[cell])?;
+            let root = BlockExpr(block, value_id);
+            let str_type_id = str_type(&mut cbuilder)?;
+
+            cbuilder.build(str_type_id, root)?
+        };
+        m.add_const(STATIC_STR_NAME, static_str_def)?;
+
+        // the entry point wrapper
+        let roc_main_bytes = func_name_bytes_help(
+            entry_point.symbol,
+            entry_point.layout.arguments.iter().copied(),
+            entry_point.layout.result,
+        );
+        let roc_main = FuncName(&roc_main_bytes);
+
+        let entry_point_function = build_entry_point(entry_point.layout, roc_main)?;
+        let entry_point_name = FuncName(ENTRY_POINT_NAME);
+        m.add_func(entry_point_name, entry_point_function)?;
+
+        // all other functions
         for proc in procs {
+            let bytes = func_name_bytes(proc);
+            let func_name = FuncName(&bytes);
+
+            if DEBUG {
+                eprintln!(
+                    "{:?}: {:?} with {:?} args",
+                    proc.name,
+                    bytes_as_ascii(&bytes),
+                    (proc.args, proc.ret_layout),
+                );
+            }
+
             let spec = proc_spec(proc)?;
 
-            m.add_func(FuncName(&proc.name.to_ne_bytes()), spec)?;
-
-            if format!("{:?}", proc.name).contains("mainForHost") {
-                main_function = Some(proc.name);
-            }
+            m.add_func(func_name, spec)?;
         }
 
         m.build()?
@@ -40,18 +159,42 @@ where
     let program = {
         let mut p = ProgramBuilder::new();
         p.add_mod(MOD_APP, main_module)?;
-        p.add_entry_point(
-            EntryPointName(b"mainForHost"),
-            MOD_APP,
-            FuncName(&main_function.unwrap().to_ne_bytes()),
-        )?;
+
+        let entry_point_name = FuncName(ENTRY_POINT_NAME);
+        p.add_entry_point(EntryPointName(ENTRY_POINT_NAME), MOD_APP, entry_point_name)?;
 
         p.build()?
     };
 
+    if DEBUG {
+        eprintln!("{}", program.to_source_string());
+    }
+
     morphic_lib::solve(program)
 }
 
+fn build_entry_point(layout: crate::ir::ProcLayout, func_name: FuncName) -> Result<FuncDef> {
+    let mut builder = FuncDefBuilder::new();
+    let block = builder.add_block();
+
+    // to the modelling language, the arguments appear out of thin air
+    let argument_type = build_tuple_type(&mut builder, layout.arguments)?;
+    let argument = builder.add_unknown_with(block, &[], argument_type)?;
+
+    let name_bytes = [0; 16];
+    let spec_var = CalleeSpecVar(&name_bytes);
+    let result = builder.add_call(block, spec_var, MOD_APP, func_name, argument)?;
+
+    // to the modelling language, the result disappears into the void
+    let unit_type = builder.add_tuple_type(&[])?;
+    let unit_value = builder.add_unknown_with(block, &[result], unit_type)?;
+
+    let root = BlockExpr(block, unit_value);
+    let spec = builder.build(unit_type, unit_type, root)?;
+
+    Ok(spec)
+}
+
 fn proc_spec(proc: &Proc) -> Result<FuncDef> {
     let mut builder = FuncDefBuilder::new();
     let mut env = Env::default();
@@ -72,13 +215,16 @@ fn proc_spec(proc: &Proc) -> Result<FuncDef> {
     let root = BlockExpr(block, value_id);
     let arg_type_id = layout_spec(&mut builder, &Layout::Struct(&argument_layouts))?;
     let ret_type_id = layout_spec(&mut builder, &proc.ret_layout)?;
-    builder.build(arg_type_id, ret_type_id, root)
+
+    let spec = builder.build(arg_type_id, ret_type_id, root)?;
+
+    Ok(spec)
 }
 
 #[derive(Default)]
 struct Env {
     symbols: MutMap<Symbol, ValueId>,
-    join_points: MutMap<crate::ir::JoinPointId, morphic_lib::JoinPointId>,
+    join_points: MutMap<crate::ir::JoinPointId, morphic_lib::ContinuationId>,
 }
 
 fn stmt_spec(
@@ -91,11 +237,25 @@ fn stmt_spec(
     use Stmt::*;
 
     match stmt {
-        Let(symbol, expr, layout, continuation) => {
-            let value_id = expr_spec(builder, env, block, layout, expr)?;
+        Let(symbol, expr, expr_layout, mut continuation) => {
+            let value_id = expr_spec(builder, env, block, expr_layout, expr)?;
             env.symbols.insert(*symbol, value_id);
+
+            let mut queue = vec![symbol];
+
+            while let Let(symbol, expr, expr_layout, c) = continuation {
+                let value_id = expr_spec(builder, env, block, expr_layout, expr)?;
+                env.symbols.insert(*symbol, value_id);
+
+                queue.push(symbol);
+                continuation = c;
+            }
+
             let result = stmt_spec(builder, env, block, layout, continuation)?;
-            env.symbols.remove(symbol);
+
+            for symbol in queue {
+                env.symbols.remove(symbol);
+            }
 
             Ok(result)
         }
@@ -105,6 +265,7 @@ fn stmt_spec(
             layout: call_layout,
             pass,
             fail,
+            exception_id: _,
         } => {
             // a call that might throw an exception
 
@@ -127,7 +288,7 @@ fn stmt_spec(
             cond_layout: _,
             branches,
             default_branch,
-            ret_layout,
+            ret_layout: _lies,
         } => {
             let mut cases = Vec::with_capacity(branches.len() + 1);
 
@@ -138,7 +299,7 @@ fn stmt_spec(
 
             for branch in it {
                 let block = builder.add_block();
-                let value_id = stmt_spec(builder, env, block, ret_layout, branch)?;
+                let value_id = stmt_spec(builder, env, block, layout, branch)?;
                 cases.push(BlockExpr(block, value_id));
             }
 
@@ -146,12 +307,27 @@ fn stmt_spec(
         }
         Ret(symbol) => Ok(env.symbols[symbol]),
         Refcounting(modify_rc, continuation) => match modify_rc {
-            ModifyRc::Inc(symbol, _) | ModifyRc::Dec(symbol) | ModifyRc::DecRef(symbol) => {
-                let result_type = builder.add_tuple_type(&[])?;
+            ModifyRc::Inc(symbol, _) => {
                 let argument = env.symbols[symbol];
 
-                // this is how RC is modelled; it recursively touches all heap cells
-                builder.add_unknown_with(block, &[argument], result_type)?;
+                // a recursive touch is never worse for optimizations than a normal touch
+                // and a bit more permissive in its type
+                builder.add_recursive_touch(block, argument)?;
+
+                stmt_spec(builder, env, block, layout, continuation)
+            }
+
+            ModifyRc::Dec(symbol) => {
+                let argument = env.symbols[symbol];
+
+                builder.add_recursive_touch(block, argument)?;
+
+                stmt_spec(builder, env, block, layout, continuation)
+            }
+            ModifyRc::DecRef(symbol) => {
+                let argument = env.symbols[symbol];
+
+                builder.add_recursive_touch(block, argument)?;
 
                 stmt_spec(builder, env, block, layout, continuation)
             }
@@ -159,7 +335,7 @@ fn stmt_spec(
         Join {
             id,
             parameters,
-            continuation,
+            body,
             remainder,
         } => {
             let mut type_ids = Vec::new();
@@ -173,31 +349,37 @@ fn stmt_spec(
             let jp_arg_type_id = builder.add_tuple_type(&type_ids)?;
 
             let (jpid, jp_argument) =
-                builder.declare_join_point(block, jp_arg_type_id, ret_type_id)?;
+                builder.declare_continuation(block, jp_arg_type_id, ret_type_id)?;
 
+            // NOTE join point arguments can shadow variables from the outer scope
+            // the ordering of steps here is important
+
+            // add this ID so both body and remainder can reference it
+            env.join_points.insert(*id, jpid);
+
+            // first, with the current variable bindings, process the remainder
+            let cont_block = builder.add_block();
+            let cont_value_id = stmt_spec(builder, env, cont_block, layout, remainder)?;
+
+            // only then introduce variables bound by the jump point, and process its body
             let join_body_sub_block = {
-                env.join_points.insert(*id, jpid);
                 let jp_body_block = builder.add_block();
 
                 // unpack the argument
                 for (i, p) in parameters.iter().enumerate() {
                     let value_id =
                         builder.add_get_tuple_field(jp_body_block, jp_argument, i as u32)?;
+
                     env.symbols.insert(p.symbol, value_id);
                 }
 
-                let jp_body_value_id = stmt_spec(builder, env, jp_body_block, layout, remainder)?;
+                let jp_body_value_id = stmt_spec(builder, env, jp_body_block, layout, body)?;
+
                 BlockExpr(jp_body_block, jp_body_value_id)
             };
 
-            // NOTE the symbols bound by the join point can shadow the argument symbols of the
-            // surrounding function, so we don't remove them from the env here
-
-            let cont_block = builder.add_block();
-            let cont_value_id = stmt_spec(builder, env, cont_block, layout, continuation)?;
-
             env.join_points.remove(id);
-            builder.define_join_point(jpid, join_body_sub_block)?;
+            builder.define_continuation(jpid, join_body_sub_block)?;
 
             builder.add_sub_block(block, BlockExpr(cont_block, cont_value_id))
         }
@@ -208,7 +390,7 @@ fn stmt_spec(
             let jpid = env.join_points[id];
             builder.add_jump(block, jpid, argument, ret_type_id)
         }
-        Rethrow | RuntimeError(_) => {
+        Resume(_) | RuntimeError(_) => {
             let type_id = layout_spec(builder, layout)?;
 
             builder.add_terminate(block, type_id)
@@ -260,17 +442,17 @@ fn call_spec(
     match &call.call_type {
         ByName {
             name: symbol,
-            full_layout: _,
-            ret_layout: _,
-            arg_layouts: _,
+            ret_layout,
+            arg_layouts,
             specialization_id,
         } => {
             let array = specialization_id.to_bytes();
             let spec_var = CalleeSpecVar(&array);
 
             let arg_value_id = build_tuple_value(builder, env, block, call.arguments)?;
-            let slice = &symbol.to_ne_bytes();
-            let name = FuncName(slice);
+            let it = arg_layouts.iter().copied();
+            let bytes = func_name_bytes_help(*symbol, it, *ret_layout);
+            let name = FuncName(&bytes);
             let module = MOD_APP;
             builder.add_call(block, spec_var, module, name, arg_value_id)
         }
@@ -297,7 +479,223 @@ fn call_spec(
             *update_mode,
             call.arguments,
         ),
-        HigherOrderLowLevel { .. } => todo!(),
+        HigherOrderLowLevel {
+            specialization_id,
+            closure_env_layout,
+            op,
+            arg_layouts,
+            ret_layout,
+            ..
+        } => {
+            let array = specialization_id.to_bytes();
+            let spec_var = CalleeSpecVar(&array);
+
+            let symbol = {
+                use roc_module::low_level::LowLevel::*;
+
+                match op {
+                    ListMap | ListMapWithIndex => call.arguments[1],
+                    ListMap2 => call.arguments[2],
+                    ListMap3 => call.arguments[3],
+                    ListWalk | ListWalkUntil | ListWalkBackwards | DictWalk => call.arguments[2],
+                    ListKeepIf | ListKeepOks | ListKeepErrs => call.arguments[1],
+                    ListSortWith => call.arguments[1],
+                    _ => unreachable!(),
+                }
+            };
+
+            let it = arg_layouts.iter().copied();
+            let bytes = func_name_bytes_help(symbol, it, *ret_layout);
+            let name = FuncName(&bytes);
+            let module = MOD_APP;
+
+            {
+                use roc_module::low_level::LowLevel::*;
+
+                match op {
+                    DictWalk => {
+                        let dict = env.symbols[&call.arguments[0]];
+                        let default = env.symbols[&call.arguments[1]];
+                        let closure_env = env.symbols[&call.arguments[3]];
+
+                        let bag = builder.add_get_tuple_field(block, dict, DICT_BAG_INDEX)?;
+                        let _cell = builder.add_get_tuple_field(block, dict, DICT_CELL_INDEX)?;
+
+                        let first = builder.add_bag_get(block, bag)?;
+
+                        let key = builder.add_get_tuple_field(block, first, 0)?;
+                        let val = builder.add_get_tuple_field(block, first, 1)?;
+
+                        let argument = if closure_env_layout.is_none() {
+                            builder.add_make_tuple(block, &[key, val, default])?
+                        } else {
+                            builder.add_make_tuple(block, &[key, val, default, closure_env])?
+                        };
+                        builder.add_call(block, spec_var, module, name, argument)?;
+                    }
+
+                    ListWalk | ListWalkBackwards | ListWalkUntil => {
+                        let list = env.symbols[&call.arguments[0]];
+                        let default = env.symbols[&call.arguments[1]];
+                        let closure_env = env.symbols[&call.arguments[3]];
+
+                        let bag = builder.add_get_tuple_field(block, list, LIST_BAG_INDEX)?;
+                        let _cell = builder.add_get_tuple_field(block, list, LIST_CELL_INDEX)?;
+
+                        let first = builder.add_bag_get(block, bag)?;
+
+                        let argument = if closure_env_layout.is_none() {
+                            builder.add_make_tuple(block, &[first, default])?
+                        } else {
+                            builder.add_make_tuple(block, &[first, default, closure_env])?
+                        };
+                        builder.add_call(block, spec_var, module, name, argument)?;
+                    }
+
+                    ListMapWithIndex => {
+                        let list = env.symbols[&call.arguments[0]];
+                        let closure_env = env.symbols[&call.arguments[2]];
+
+                        let bag = builder.add_get_tuple_field(block, list, LIST_BAG_INDEX)?;
+                        let _cell = builder.add_get_tuple_field(block, list, LIST_CELL_INDEX)?;
+
+                        let first = builder.add_bag_get(block, bag)?;
+                        let index = builder.add_make_tuple(block, &[])?;
+
+                        let argument = if closure_env_layout.is_none() {
+                            builder.add_make_tuple(block, &[first, index])?
+                        } else {
+                            builder.add_make_tuple(block, &[first, index, closure_env])?
+                        };
+                        builder.add_call(block, spec_var, module, name, argument)?;
+                    }
+
+                    ListMap => {
+                        let list1 = env.symbols[&call.arguments[0]];
+                        let closure_env = env.symbols[&call.arguments[2]];
+
+                        let bag1 = builder.add_get_tuple_field(block, list1, LIST_BAG_INDEX)?;
+                        let _cell1 = builder.add_get_tuple_field(block, list1, LIST_CELL_INDEX)?;
+
+                        let elem1 = builder.add_bag_get(block, bag1)?;
+
+                        let argument = if closure_env_layout.is_none() {
+                            builder.add_make_tuple(block, &[elem1])?
+                        } else {
+                            builder.add_make_tuple(block, &[elem1, closure_env])?
+                        };
+                        builder.add_call(block, spec_var, module, name, argument)?;
+                    }
+
+                    ListSortWith => {
+                        let list1 = env.symbols[&call.arguments[0]];
+                        let closure_env = env.symbols[&call.arguments[2]];
+
+                        let bag1 = builder.add_get_tuple_field(block, list1, LIST_BAG_INDEX)?;
+                        let _cell1 = builder.add_get_tuple_field(block, list1, LIST_CELL_INDEX)?;
+
+                        let elem1 = builder.add_bag_get(block, bag1)?;
+
+                        let argument = if closure_env_layout.is_none() {
+                            builder.add_make_tuple(block, &[elem1, elem1])?
+                        } else {
+                            builder.add_make_tuple(block, &[elem1, elem1, closure_env])?
+                        };
+                        builder.add_call(block, spec_var, module, name, argument)?;
+                    }
+
+                    ListMap2 => {
+                        let list1 = env.symbols[&call.arguments[0]];
+                        let list2 = env.symbols[&call.arguments[1]];
+                        let closure_env = env.symbols[&call.arguments[3]];
+
+                        let bag1 = builder.add_get_tuple_field(block, list1, LIST_BAG_INDEX)?;
+                        let _cell1 = builder.add_get_tuple_field(block, list1, LIST_CELL_INDEX)?;
+                        let elem1 = builder.add_bag_get(block, bag1)?;
+
+                        let bag2 = builder.add_get_tuple_field(block, list2, LIST_BAG_INDEX)?;
+                        let _cell2 = builder.add_get_tuple_field(block, list2, LIST_CELL_INDEX)?;
+                        let elem2 = builder.add_bag_get(block, bag2)?;
+
+                        let argument = if closure_env_layout.is_none() {
+                            builder.add_make_tuple(block, &[elem1, elem2])?
+                        } else {
+                            builder.add_make_tuple(block, &[elem1, elem2, closure_env])?
+                        };
+                        builder.add_call(block, spec_var, module, name, argument)?;
+                    }
+
+                    ListMap3 => {
+                        let list1 = env.symbols[&call.arguments[0]];
+                        let list2 = env.symbols[&call.arguments[1]];
+                        let list3 = env.symbols[&call.arguments[2]];
+                        let closure_env = env.symbols[&call.arguments[4]];
+
+                        let bag1 = builder.add_get_tuple_field(block, list1, LIST_BAG_INDEX)?;
+                        let _cell1 = builder.add_get_tuple_field(block, list1, LIST_CELL_INDEX)?;
+                        let elem1 = builder.add_bag_get(block, bag1)?;
+
+                        let bag2 = builder.add_get_tuple_field(block, list2, LIST_BAG_INDEX)?;
+                        let _cell2 = builder.add_get_tuple_field(block, list2, LIST_CELL_INDEX)?;
+                        let elem2 = builder.add_bag_get(block, bag2)?;
+
+                        let bag3 = builder.add_get_tuple_field(block, list3, LIST_BAG_INDEX)?;
+                        let _cell3 = builder.add_get_tuple_field(block, list3, LIST_CELL_INDEX)?;
+                        let elem3 = builder.add_bag_get(block, bag3)?;
+
+                        let argument = if closure_env_layout.is_none() {
+                            builder.add_make_tuple(block, &[elem1, elem2, elem3])?
+                        } else {
+                            builder.add_make_tuple(block, &[elem1, elem2, elem3, closure_env])?
+                        };
+                        builder.add_call(block, spec_var, module, name, argument)?;
+                    }
+
+                    ListKeepIf | ListKeepOks | ListKeepErrs => {
+                        let list = env.symbols[&call.arguments[0]];
+                        let closure_env = env.symbols[&call.arguments[2]];
+
+                        let bag = builder.add_get_tuple_field(block, list, LIST_BAG_INDEX)?;
+                        // let _cell = builder.add_get_tuple_field(block, list, LIST_CELL_INDEX)?;
+
+                        let first = builder.add_bag_get(block, bag)?;
+
+                        let argument = if closure_env_layout.is_none() {
+                            builder.add_make_tuple(block, &[first])?
+                        } else {
+                            builder.add_make_tuple(block, &[first, closure_env])?
+                        };
+                        let result = builder.add_call(block, spec_var, module, name, argument)?;
+                        let unit = builder.add_tuple_type(&[])?;
+                        builder.add_unknown_with(block, &[result], unit)?;
+                    }
+
+                    _ => {
+                        // fake a call to the function argument
+                        // to make sure the function is specialized
+
+                        // very invalid
+                        let arg_value_id = build_tuple_value(builder, env, block, &[])?;
+
+                        builder.add_call(block, spec_var, module, name, arg_value_id)?;
+                    }
+                }
+            }
+
+            // TODO overly pessimstic
+            // filter_map because one of the arguments is a function name, which
+            // is not defined in the env
+            let arguments: Vec<_> = call
+                .arguments
+                .iter()
+                .filter_map(|symbol| env.symbols.get(symbol))
+                .copied()
+                .collect();
+
+            let result_type = layout_spec(builder, layout)?;
+
+            builder.add_unknown_with(block, &arguments, result_type)
+        }
     }
 }
 
@@ -341,12 +739,17 @@ fn lowlevel_spec(
 
             builder.add_sub_block(block, sub_block)
         }
-        Eq | NotEq => new_bool(builder, block),
-        NumLte | NumLt | NumGt | NumGte => new_order(builder, block),
+        Eq | NotEq => {
+            // just dream up a unit value
+            builder.add_make_tuple(block, &[])
+        }
+        NumLte | NumLt | NumGt | NumGte => {
+            // just dream up a unit value
+            builder.add_make_tuple(block, &[])
+        }
         ListLen => {
-            let list = env.symbols[&arguments[0]];
-
-            builder.add_get_tuple_field(block, list, LIST_LEN_INDEX)
+            // just dream up a unit value
+            builder.add_make_tuple(block, &[])
         }
         ListGetUnsafe => {
             // NOTE the ListGet lowlevel op is only evaluated if the index is in-bounds
@@ -372,21 +775,18 @@ fn lowlevel_spec(
 
             Ok(list)
         }
-        other => todo!("lowlevel op not implemented: {:?}", other),
+        _other => {
+            // TODO overly pessimstic
+            let arguments: Vec<_> = arguments.iter().map(|symbol| env.symbols[symbol]).collect();
+
+            let result_type = layout_spec(builder, layout)?;
+
+            builder.add_unknown_with(block, &arguments, result_type)
+        }
     }
 }
 
 fn build_variant_types(
-    builder: &mut FuncDefBuilder,
-    layout: &Layout,
-) -> Option<Result<Vec<TypeId>>> {
-    match layout {
-        Layout::Union(union_layout) => Some(build_variant_types_help(builder, union_layout)),
-        _ => None,
-    }
-}
-
-fn build_variant_types_help(
     builder: &mut FuncDefBuilder,
     union_layout: &UnionLayout,
 ) -> Result<Vec<TypeId>> {
@@ -400,24 +800,29 @@ fn build_variant_types_help(
                 result.push(build_tuple_type(builder, tag)?);
             }
         }
-        Recursive(_) => todo!(),
-        NonNullableUnwrapped(_) => todo!(),
+        Recursive(_) => unreachable!(),
+        NonNullableUnwrapped(_) => unreachable!(),
         NullableWrapped {
             nullable_id: _,
             other_tags: _,
-        } => todo!(),
+        } => unreachable!(),
         NullableUnwrapped {
             nullable_id: _,
             other_fields: _,
-        } => todo!(),
+        } => unreachable!(),
     }
 
     Ok(result)
 }
 
+fn worst_case_type(context: &mut impl TypeContext) -> Result<TypeId> {
+    let cell = context.add_heap_cell_type();
+    context.add_bag_type(cell)
+}
+
 fn expr_spec(
     builder: &mut FuncDefBuilder,
-    env: &Env,
+    env: &mut Env,
     block: BlockId,
     layout: &Layout,
     expr: &Expr,
@@ -433,37 +838,48 @@ fn expr_spec(
             tag_id,
             union_size: _,
             arguments,
-        } => {
-            let value_id = build_tuple_value(builder, env, block, arguments)?;
-            let variant_types = build_variant_types(builder, tag_layout).unwrap()?;
-            builder.add_make_union(block, &variant_types, *tag_id as u32, value_id)
-        }
-        Struct(fields) => build_tuple_value(builder, env, block, fields),
-        AccessAtIndex {
-            index,
-            field_layouts: _,
-            structure,
-            wrapped,
-        } => {
-            use crate::ir::Wrapped;
-
-            let value_id = env.symbols[structure];
-
-            match wrapped {
-                Wrapped::EmptyRecord => {
-                    // this is a unit value
-                    builder.add_make_tuple(block, &[])
-                }
-                Wrapped::SingleElementRecord => {
-                    todo!("do we unwrap single-element records still?")
-                }
-                Wrapped::RecordOrSingleTagUnion => {
-                    builder.add_get_tuple_field(block, value_id, *index as u32)
-                }
-                Wrapped::MultiTagUnion => {
-                    builder.add_get_tuple_field(block, value_id, *index as u32)
-                }
+        } => match tag_layout {
+            UnionLayout::NonRecursive(_) => {
+                let value_id = build_tuple_value(builder, env, block, arguments)?;
+                let variant_types = build_variant_types(builder, tag_layout)?;
+                builder.add_make_union(block, &variant_types, *tag_id as u32, value_id)
             }
+            UnionLayout::Recursive(_)
+            | UnionLayout::NonNullableUnwrapped(_)
+            | UnionLayout::NullableWrapped { .. }
+            | UnionLayout::NullableUnwrapped { .. } => {
+                let result_type = worst_case_type(builder)?;
+                let value_id = build_tuple_value(builder, env, block, arguments)?;
+                builder.add_unknown_with(block, &[value_id], result_type)
+            }
+        },
+        Struct(fields) => build_tuple_value(builder, env, block, fields),
+        UnionAtIndex {
+            index,
+            tag_id,
+            structure,
+            union_layout,
+        } => match union_layout {
+            UnionLayout::NonRecursive(_) => {
+                let index = (*index) as u32;
+                let tag_value_id = env.symbols[structure];
+                let tuple_value_id =
+                    builder.add_unwrap_union(block, tag_value_id, *tag_id as u32)?;
+
+                builder.add_get_tuple_field(block, tuple_value_id, index)
+            }
+            _ => {
+                // for the moment recursive tag unions don't quite work
+                let value_id = env.symbols[structure];
+                let result_type = layout_spec(builder, layout)?;
+                builder.add_unknown_with(block, &[value_id], result_type)
+            }
+        },
+        StructAtIndex {
+            index, structure, ..
+        } => {
+            let value_id = env.symbols[structure];
+            builder.add_get_tuple_field(block, value_id, *index as u32)
         }
         Array { elem_layout, elems } => {
             let type_id = layout_spec(builder, elem_layout)?;
@@ -478,7 +894,9 @@ fn expr_spec(
                 bag = builder.add_bag_insert(block, bag, value_id)?;
             }
 
-            Ok(bag)
+            let cell = builder.add_new_heap_cell(block)?;
+
+            builder.add_make_tuple(block, &[cell, bag])
         }
 
         EmptyArray => {
@@ -504,6 +922,7 @@ fn expr_spec(
 
             builder.add_terminate(block, type_id)
         }
+        GetTagId { .. } => builder.add_make_tuple(block, &[]),
     }
 }
 
@@ -525,16 +944,25 @@ fn layout_spec(builder: &mut FuncDefBuilder, layout: &Layout) -> Result<TypeId>
 
     match layout {
         Builtin(builtin) => builtin_spec(builder, builtin),
-        PhantomEmptyStruct => todo!(),
         Struct(fields) => build_tuple_type(builder, fields),
-        Union(union_layout) => {
-            let variant_types = build_variant_types_help(builder, union_layout)?;
-            builder.add_union_type(&variant_types)
-        }
-        RecursivePointer => todo!(),
-        FunctionPointer(_, _) => todo!(),
-        Closure(_, _, _) => todo!(),
-        Pointer(_) => todo!(),
+        Union(union_layout) => match union_layout {
+            UnionLayout::NonRecursive(_) => {
+                let variant_types = build_variant_types(builder, union_layout)?;
+                builder.add_union_type(&variant_types)
+            }
+            UnionLayout::Recursive(_) => worst_case_type(builder),
+            UnionLayout::NonNullableUnwrapped(_) => worst_case_type(builder),
+            UnionLayout::NullableWrapped {
+                nullable_id: _,
+                other_tags: _,
+            } => worst_case_type(builder),
+            UnionLayout::NullableUnwrapped {
+                nullable_id: _,
+                other_fields: _,
+            } => worst_case_type(builder),
+        },
+        RecursivePointer => worst_case_type(builder),
+        Closure(_, lambda_set, _) => layout_spec(builder, &lambda_set.runtime_representation()),
     }
 }
 
@@ -543,69 +971,80 @@ fn builtin_spec(builder: &mut FuncDefBuilder, builtin: &Builtin) -> Result<TypeI
 
     match builtin {
         Int128 | Int64 | Int32 | Int16 | Int8 | Int1 | Usize => builder.add_tuple_type(&[]),
-        Float128 => todo!(),
-        Float64 => todo!(),
-        Float32 => todo!(),
-        Float16 => todo!(),
-        Str => todo!(),
-        Dict(_, _) => todo!(),
-        Set(_) => todo!(),
-        List(_, _) => {
-            // TODO should incorporate the element type into the name
-            Ok(builder.add_named_type(MOD_LIST, TypeName(b"List")))
+        Float128 | Float64 | Float32 | Float16 => builder.add_tuple_type(&[]),
+        Str | EmptyStr => str_type(builder),
+        Dict(key_layout, value_layout) => {
+            let value_type = layout_spec(builder, value_layout)?;
+            let key_type = layout_spec(builder, key_layout)?;
+            let element_type = builder.add_tuple_type(&[key_type, value_type])?;
+
+            let cell = builder.add_heap_cell_type();
+            let bag = builder.add_bag_type(element_type)?;
+            builder.add_tuple_type(&[cell, bag])
+        }
+        Set(key_layout) => {
+            let value_type = builder.add_tuple_type(&[])?;
+            let key_type = layout_spec(builder, key_layout)?;
+            let element_type = builder.add_tuple_type(&[key_type, value_type])?;
+
+            let cell = builder.add_heap_cell_type();
+            let bag = builder.add_bag_type(element_type)?;
+            builder.add_tuple_type(&[cell, bag])
+        }
+        List(element_layout) => {
+            let element_type = layout_spec(builder, element_layout)?;
+
+            let cell = builder.add_heap_cell_type();
+            let bag = builder.add_bag_type(element_type)?;
+
+            builder.add_tuple_type(&[cell, bag])
+        }
+        EmptyList => {
+            // TODO make sure that we consistently treat the EmptyList as a list of unit values
+            let element_type = builder.add_tuple_type(&[])?;
+
+            let cell = builder.add_heap_cell_type();
+            let bag = builder.add_bag_type(element_type)?;
+
+            builder.add_tuple_type(&[cell, bag])
+        }
+        EmptyDict | EmptySet => {
+            // TODO make sure that we consistently treat the these as a dict of unit values
+            let unit = builder.add_tuple_type(&[])?;
+            let element_type = builder.add_tuple_type(&[unit, unit])?;
+
+            let cell = builder.add_heap_cell_type();
+            let bag = builder.add_bag_type(element_type)?;
+
+            builder.add_tuple_type(&[cell, bag])
         }
-        EmptyStr => todo!(),
-        EmptyList => todo!(),
-        EmptyDict => todo!(),
-        EmptySet => todo!(),
     }
 }
 
+fn str_type<TC: TypeContext>(builder: &mut TC) -> Result<TypeId> {
+    let cell_id = builder.add_heap_cell_type();
+    builder.add_tuple_type(&[cell_id])
+}
+
 // const OK_TAG_ID: u8 = 1u8;
 // const ERR_TAG_ID: u8 = 0u8;
 
 const LIST_CELL_INDEX: u32 = 0;
 const LIST_BAG_INDEX: u32 = 1;
-const LIST_LEN_INDEX: u32 = 2;
+
+const DICT_CELL_INDEX: u32 = LIST_CELL_INDEX;
+const DICT_BAG_INDEX: u32 = LIST_BAG_INDEX;
 
 fn new_list(builder: &mut FuncDefBuilder, block: BlockId, element_type: TypeId) -> Result<ValueId> {
     let cell = builder.add_new_heap_cell(block)?;
     let bag = builder.add_empty_bag(block, element_type)?;
-    let length = new_usize(builder, block)?;
-    builder.add_make_tuple(block, &[cell, bag, length])
-}
-
-fn new_usize(builder: &mut FuncDefBuilder, block: BlockId) -> Result<ValueId> {
-    new_num(builder, block)
+    builder.add_make_tuple(block, &[cell, bag])
 }
 
 fn new_static_string(builder: &mut FuncDefBuilder, block: BlockId) -> Result<ValueId> {
-    let cell = builder.add_new_heap_cell(block)?;
+    let module = MOD_APP;
 
-    // immediately mutate the cell, so any future updates on this value are invalid
-    // updating a static string would cause a crash at runtime
-    let _ = builder.add_update(block, UpdateModeVar(&[]), cell)?;
-
-    let length = new_usize(builder, block)?;
-    builder.add_make_tuple(block, &[cell, length])
-}
-
-fn new_order(builder: &mut FuncDefBuilder, block: BlockId) -> Result<ValueId> {
-    // always generats EQ
-    let tag_id = 0;
-
-    let unit = builder.add_tuple_type(&[])?;
-    let unit_value = builder.add_make_tuple(block, &[])?;
-    builder.add_make_union(block, &[unit, unit, unit], tag_id, unit_value)
-}
-
-fn new_bool(builder: &mut FuncDefBuilder, block: BlockId) -> Result<ValueId> {
-    // always generats False
-    let tag_id = 0;
-
-    let unit = builder.add_tuple_type(&[])?;
-    let unit_value = builder.add_make_tuple(block, &[])?;
-    builder.add_make_union(block, &[unit, unit], tag_id, unit_value)
+    builder.add_const_ref(block, module, STATIC_STR_NAME)
 }
 
 fn new_num(builder: &mut FuncDefBuilder, block: BlockId) -> Result<ValueId> {
diff --git a/compiler/mono/src/borrow.rs b/compiler/mono/src/borrow.rs
index 443bfb20df..be0aa9bb1d 100644
--- a/compiler/mono/src/borrow.rs
+++ b/compiler/mono/src/borrow.rs
@@ -1,4 +1,4 @@
-use crate::ir::{Expr, JoinPointId, Param, Proc, Stmt, TopLevelFunctionLayout};
+use crate::ir::{Expr, JoinPointId, Param, Proc, ProcLayout, Stmt};
 use crate::layout::Layout;
 use bumpalo::collections::Vec;
 use bumpalo::Bump;
@@ -18,15 +18,14 @@ fn should_borrow_layout(layout: &Layout) -> bool {
 
 pub fn infer_borrow<'a>(
     arena: &'a Bump,
-    procs: &MutMap<(Symbol, TopLevelFunctionLayout<'a>), Proc<'a>>,
+    procs: &MutMap<(Symbol, ProcLayout<'a>), Proc<'a>>,
 ) -> ParamMap<'a> {
     let mut param_map = ParamMap {
         items: MutMap::default(),
     };
 
-    for ((s, top_level), proc) in procs {
-        let key = (*s, arena.alloc(*top_level).full());
-        param_map.visit_proc(arena, proc, key);
+    for (key, proc) in procs {
+        param_map.visit_proc(arena, proc, *key);
     }
 
     let mut env = BorrowInfState {
@@ -40,7 +39,7 @@ pub fn infer_borrow<'a>(
 
     // This is a fixed-point analysis
     //
-    // all functions initiall own all their paramters
+    // all functions initiall own all their parameters
     // through a series of checks and heuristics, some arguments are set to borrowed
     // when that doesn't lead to conflicts the change is kept, otherwise it may be reverted
     //
@@ -51,8 +50,7 @@ pub fn infer_borrow<'a>(
         // mutually recursive functions (or just make all their arguments owned)
 
         for (key, proc) in procs {
-            let layout = arena.alloc(key.1).full();
-            env.collect_proc(proc, layout);
+            env.collect_proc(proc, key.1);
         }
 
         if !env.modified {
@@ -69,7 +67,7 @@ pub fn infer_borrow<'a>(
 
 #[derive(Debug, PartialEq, Eq, Hash, Clone)]
 pub enum Key<'a> {
-    Declaration(Symbol, Layout<'a>),
+    Declaration(Symbol, ProcLayout<'a>),
     JoinPoint(JoinPointId),
 }
 
@@ -98,7 +96,7 @@ impl<'a> IntoIterator for &'a ParamMap<'a> {
 }
 
 impl<'a> ParamMap<'a> {
-    pub fn get_symbol(&self, symbol: Symbol, layout: Layout<'a>) -> Option<&'a [Param<'a>]> {
+    pub fn get_symbol(&self, symbol: Symbol, layout: ProcLayout<'a>) -> Option<&'a [Param<'a>]> {
         let key = Key::Declaration(symbol, layout);
 
         self.items.get(&key).copied()
@@ -153,7 +151,7 @@ impl<'a> ParamMap<'a> {
         .into_bump_slice()
     }
 
-    fn visit_proc(&mut self, arena: &'a Bump, proc: &Proc<'a>, key: (Symbol, Layout<'a>)) {
+    fn visit_proc(&mut self, arena: &'a Bump, proc: &Proc<'a>, key: (Symbol, ProcLayout<'a>)) {
         if proc.must_own_arguments {
             self.visit_proc_always_owned(arena, proc, key);
             return;
@@ -171,7 +169,7 @@ impl<'a> ParamMap<'a> {
         &mut self,
         arena: &'a Bump,
         proc: &Proc<'a>,
-        key: (Symbol, Layout<'a>),
+        key: (Symbol, ProcLayout<'a>),
     ) {
         let already_in_there = self.items.insert(
             Key::Declaration(proc.name, key.1),
@@ -193,12 +191,16 @@ impl<'a> ParamMap<'a> {
                     id: j,
                     parameters: xs,
                     remainder: v,
-                    continuation: b,
+                    body: b,
                 } => {
                     let already_in_there = self
                         .items
                         .insert(Key::JoinPoint(*j), Self::init_borrow_params(arena, xs));
-                    debug_assert!(already_in_there.is_none());
+                    debug_assert!(
+                        already_in_there.is_none(),
+                        "join point {:?} is already defined!",
+                        j
+                    );
 
                     stack.push(v);
                     stack.push(b);
@@ -220,7 +222,7 @@ impl<'a> ParamMap<'a> {
                 }
                 Refcounting(_, _) => unreachable!("these have not been introduced yet"),
 
-                Ret(_) | Rethrow | Jump(_, _) | RuntimeError(_) => {
+                Ret(_) | Resume(_) | Jump(_, _) | RuntimeError(_) => {
                     // these are terminal, do nothing
                 }
             }
@@ -348,7 +350,7 @@ impl<'a> BorrowInfState<'a> {
     /// let z = e in ...
     ///
     /// and determines whether z and which of the symbols used in e
-    /// must be taken as owned paramters
+    /// must be taken as owned parameters
     fn collect_call(&mut self, z: Symbol, e: &crate::ir::Call<'a>) {
         use crate::ir::CallType::*;
 
@@ -359,12 +361,17 @@ impl<'a> BorrowInfState<'a> {
 
         match call_type {
             ByName {
-                name, full_layout, ..
+                name,
+                ret_layout,
+                arg_layouts,
+                ..
             } => {
+                let top_level = ProcLayout::new(self.arena, arg_layouts, *ret_layout);
+
                 // get the borrow signature of the applied function
                 let ps = self
                     .param_map
-                    .get_symbol(*name, *full_layout)
+                    .get_symbol(*name, top_level)
                     .expect("function is defined");
 
                 // the return value will be owned
@@ -393,15 +400,23 @@ impl<'a> BorrowInfState<'a> {
             }
 
             HigherOrderLowLevel {
-                op, closure_layout, ..
+                op,
+                arg_layouts,
+                ret_layout,
+                ..
             } => {
                 use roc_module::low_level::LowLevel::*;
 
                 debug_assert!(op.is_higher_order());
 
+                let closure_layout = ProcLayout {
+                    arguments: arg_layouts,
+                    result: *ret_layout,
+                };
+
                 match op {
                     ListMap | ListKeepIf | ListKeepOks | ListKeepErrs => {
-                        match self.param_map.get_symbol(arguments[1], *closure_layout) {
+                        match self.param_map.get_symbol(arguments[1], closure_layout) {
                             Some(function_ps) => {
                                 // own the list if the function wants to own the element
                                 if !function_ps[0].borrow {
@@ -417,7 +432,7 @@ impl<'a> BorrowInfState<'a> {
                         }
                     }
                     ListMapWithIndex => {
-                        match self.param_map.get_symbol(arguments[1], *closure_layout) {
+                        match self.param_map.get_symbol(arguments[1], closure_layout) {
                             Some(function_ps) => {
                                 // own the list if the function wants to own the element
                                 if !function_ps[1].borrow {
@@ -432,7 +447,7 @@ impl<'a> BorrowInfState<'a> {
                             None => unreachable!(),
                         }
                     }
-                    ListMap2 => match self.param_map.get_symbol(arguments[2], *closure_layout) {
+                    ListMap2 => match self.param_map.get_symbol(arguments[2], closure_layout) {
                         Some(function_ps) => {
                             // own the lists if the function wants to own the element
                             if !function_ps[0].borrow {
@@ -450,7 +465,7 @@ impl<'a> BorrowInfState<'a> {
                         }
                         None => unreachable!(),
                     },
-                    ListMap3 => match self.param_map.get_symbol(arguments[3], *closure_layout) {
+                    ListMap3 => match self.param_map.get_symbol(arguments[3], closure_layout) {
                         Some(function_ps) => {
                             // own the lists if the function wants to own the element
                             if !function_ps[0].borrow {
@@ -471,7 +486,7 @@ impl<'a> BorrowInfState<'a> {
                         None => unreachable!(),
                     },
                     ListSortWith => {
-                        match self.param_map.get_symbol(arguments[1], *closure_layout) {
+                        match self.param_map.get_symbol(arguments[1], closure_layout) {
                             Some(function_ps) => {
                                 // always own the input list
                                 self.own_var(arguments[0]);
@@ -485,7 +500,7 @@ impl<'a> BorrowInfState<'a> {
                         }
                     }
                     ListWalk | ListWalkUntil | ListWalkBackwards | DictWalk => {
-                        match self.param_map.get_symbol(arguments[2], *closure_layout) {
+                        match self.param_map.get_symbol(arguments[2], closure_layout) {
                             Some(function_ps) => {
                                 // own the data structure if the function wants to own the element
                                 if !function_ps[0].borrow {
@@ -554,7 +569,12 @@ impl<'a> BorrowInfState<'a> {
             EmptyArray => {
                 self.own_var(z);
             }
-            AccessAtIndex { structure: x, .. } => {
+
+            Call(call) => self.collect_call(z, call),
+
+            Literal(_) | RuntimeErrorFunction(_) => {}
+
+            StructAtIndex { structure: x, .. } => {
                 // if the structure (record/tag/array) is owned, the extracted value is
                 if self.is_owned(*x) {
                     self.own_var(z);
@@ -566,9 +586,29 @@ impl<'a> BorrowInfState<'a> {
                 }
             }
 
-            Call(call) => self.collect_call(z, call),
+            UnionAtIndex { structure: x, .. } => {
+                // if the structure (record/tag/array) is owned, the extracted value is
+                if self.is_owned(*x) {
+                    self.own_var(z);
+                }
 
-            Literal(_) | RuntimeErrorFunction(_) => {}
+                // if the extracted value is owned, the structure must be too
+                if self.is_owned(z) {
+                    self.own_var(*x);
+                }
+            }
+
+            GetTagId { structure: x, .. } => {
+                // if the structure (record/tag/array) is owned, the extracted value is
+                if self.is_owned(*x) {
+                    self.own_var(z);
+                }
+
+                // if the extracted value is owned, the structure must be too
+                if self.is_owned(z) {
+                    self.own_var(*x);
+                }
+            }
         }
     }
 
@@ -579,7 +619,8 @@ impl<'a> BorrowInfState<'a> {
                 call_type:
                     crate::ir::CallType::ByName {
                         name: g,
-                        full_layout,
+                        arg_layouts,
+                        ret_layout,
                         ..
                     },
                 arguments: ys,
@@ -588,10 +629,12 @@ impl<'a> BorrowInfState<'a> {
             Stmt::Ret(z),
         ) = (v, b)
         {
+            let top_level = ProcLayout::new(self.arena, arg_layouts, *ret_layout);
+
             if self.current_proc == *g && x == *z {
                 // anonymous functions (for which the ps may not be known)
                 // can never be tail-recursive, so this is fine
-                if let Some(ps) = self.param_map.get_symbol(*g, *full_layout) {
+                if let Some(ps) = self.param_map.get_symbol(*g, top_level) {
                     self.own_params_using_args(ys, ps)
                 }
             }
@@ -618,7 +661,7 @@ impl<'a> BorrowInfState<'a> {
                 id: j,
                 parameters: ys,
                 remainder: v,
-                continuation: b,
+                body: b,
             } => {
                 let old = self.param_set.clone();
                 self.update_param_set(ys);
@@ -641,6 +684,7 @@ impl<'a> BorrowInfState<'a> {
                 layout: _,
                 pass,
                 fail,
+                exception_id: _,
             } => {
                 self.collect_stmt(pass);
                 self.collect_stmt(fail);
@@ -672,13 +716,13 @@ impl<'a> BorrowInfState<'a> {
             }
             Refcounting(_, _) => unreachable!("these have not been introduced yet"),
 
-            Ret(_) | RuntimeError(_) | Rethrow => {
+            Ret(_) | RuntimeError(_) | Resume(_) => {
                 // these are terminal, do nothing
             }
         }
     }
 
-    fn collect_proc(&mut self, proc: &Proc<'a>, layout: Layout<'a>) {
+    fn collect_proc(&mut self, proc: &Proc<'a>, layout: ProcLayout<'a>) {
         let old = self.param_set.clone();
 
         let ys = Vec::from_iter_in(proc.args.iter().map(|t| t.1), self.arena).into_bump_slice();
@@ -720,7 +764,6 @@ pub fn lowlevel_borrow_signature(arena: &Bump, op: LowLevel) -> &[bool] {
     match op {
         ListLen | StrIsEmpty | StrCountGraphemes => arena.alloc_slice_copy(&[borrowed]),
         ListSet => arena.alloc_slice_copy(&[owned, irrelevant, irrelevant]),
-        ListSetInPlace => arena.alloc_slice_copy(&[owned, irrelevant, irrelevant]),
         ListGetUnsafe => arena.alloc_slice_copy(&[borrowed, irrelevant]),
         ListConcat => arena.alloc_slice_copy(&[owned, owned]),
         StrConcat => arena.alloc_slice_copy(&[owned, borrowed]),
@@ -748,6 +791,7 @@ pub fn lowlevel_borrow_signature(arena: &Bump, op: LowLevel) -> &[bool] {
         // List.append should own its first argument
         ListAppend => arena.alloc_slice_copy(&[owned, owned]),
         ListDrop => arena.alloc_slice_copy(&[owned, irrelevant]),
+        ListSwap => arena.alloc_slice_copy(&[owned, irrelevant, irrelevant]),
 
         Eq | NotEq => arena.alloc_slice_copy(&[borrowed, borrowed]),
 
diff --git a/compiler/mono/src/decision_tree.rs b/compiler/mono/src/decision_tree.rs
index 3aa4a7f22d..ef6a508c54 100644
--- a/compiler/mono/src/decision_tree.rs
+++ b/compiler/mono/src/decision_tree.rs
@@ -1,6 +1,6 @@
 use crate::exhaustive::{Ctor, RenderAs, TagId, Union};
 use crate::ir::{
-    BranchInfo, DestructType, Env, Expr, JoinPointId, Literal, Param, Pattern, Procs, Stmt, Wrapped,
+    BranchInfo, DestructType, Env, Expr, JoinPointId, Literal, Param, Pattern, Procs, Stmt,
 };
 use crate::layout::{Builtin, Layout, LayoutCache, UnionLayout};
 use roc_collections::all::{MutMap, MutSet};
@@ -17,12 +17,13 @@ const RECORD_TAG_NAME: &str = "#Record";
 /// some normal branches and gives out a decision tree that has "labels" at all
 /// the leafs and a dictionary that maps these "labels" to the code that should
 /// run.
-pub fn compile<'a>(raw_branches: Vec<(Guard<'a>, Pattern<'a>, u64)>) -> DecisionTree<'a> {
+fn compile<'a>(raw_branches: Vec<(Guard<'a>, Pattern<'a>, u64)>) -> DecisionTree<'a> {
     let formatted = raw_branches
         .into_iter()
         .map(|(guard, pattern, index)| Branch {
             goal: index,
-            patterns: vec![(Vec::new(), guard, pattern)],
+            guard,
+            patterns: vec![(Vec::new(), pattern)],
         })
         .collect();
 
@@ -33,9 +34,8 @@ pub fn compile<'a>(raw_branches: Vec<(Guard<'a>, Pattern<'a>, u64)>) -> Decision
 pub enum Guard<'a> {
     NoGuard,
     Guard {
-        /// Symbol that stores a boolean
-        /// when true this branch is picked, otherwise skipped
-        symbol: Symbol,
+        /// pattern
+        pattern: Pattern<'a>,
         /// after assigning to symbol, the stmt jumps to this label
         id: JoinPointId,
         stmt: Stmt<'a>,
@@ -49,17 +49,35 @@ impl<'a> Guard<'a> {
 }
 
 #[derive(Clone, Debug, PartialEq)]
-pub enum DecisionTree<'a> {
+enum DecisionTree<'a> {
     Match(Label),
     Decision {
         path: Vec<PathInstruction>,
-        edges: Vec<(Test<'a>, DecisionTree<'a>)>,
+        edges: Vec<(GuardedTest<'a>, DecisionTree<'a>)>,
         default: Option<Box<DecisionTree<'a>>>,
     },
 }
 
 #[derive(Clone, Debug, PartialEq)]
-pub enum Test<'a> {
+enum GuardedTest<'a> {
+    // e.g. `_ if True -> ...`
+    GuardedNoTest {
+        /// pattern
+        pattern: Pattern<'a>,
+        /// after assigning to symbol, the stmt jumps to this label
+        id: JoinPointId,
+        /// body
+        stmt: Stmt<'a>,
+    },
+    TestNotGuarded {
+        test: Test<'a>,
+    },
+    Placeholder,
+}
+
+#[derive(Clone, Debug, PartialEq)]
+#[allow(clippy::enum_variant_names)]
+enum Test<'a> {
     IsCtor {
         tag_id: u8,
         tag_name: TagName,
@@ -75,16 +93,6 @@ pub enum Test<'a> {
         tag_id: u8,
         num_alts: usize,
     },
-    // A pattern that always succeeds (like `_`) can still have a guard
-    Guarded {
-        opt_test: Option<Box<Test<'a>>>,
-        /// Symbol that stores a boolean
-        /// when true this branch is picked, otherwise skipped
-        symbol: Symbol,
-        /// after assigning to symbol, the stmt jumps to this label
-        id: JoinPointId,
-        stmt: Stmt<'a>,
-    },
 }
 use std::hash::{Hash, Hasher};
 impl<'a> Hash for Test<'a> {
@@ -118,15 +126,23 @@ impl<'a> Hash for Test<'a> {
                 tag_id.hash(state);
                 num_alts.hash(state);
             }
-            Guarded { opt_test: None, .. } => {
-                state.write_u8(6);
+        }
+    }
+}
+
+impl<'a> Hash for GuardedTest<'a> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        match self {
+            GuardedTest::GuardedNoTest { id, .. } => {
+                state.write_u8(1);
+                id.hash(state);
             }
-            Guarded {
-                opt_test: Some(nested),
-                ..
-            } => {
-                state.write_u8(7);
-                nested.hash(state);
+            GuardedTest::TestNotGuarded { test } => {
+                state.write_u8(0);
+                test.hash(state);
+            }
+            GuardedTest::Placeholder => {
+                state.write_u8(2);
             }
         }
     }
@@ -137,59 +153,156 @@ impl<'a> Hash for Test<'a> {
 #[derive(Clone, Debug, PartialEq)]
 struct Branch<'a> {
     goal: Label,
-    patterns: Vec<(Vec<PathInstruction>, Guard<'a>, Pattern<'a>)>,
+    guard: Guard<'a>,
+    patterns: Vec<(Vec<PathInstruction>, Pattern<'a>)>,
 }
 
 fn to_decision_tree(raw_branches: Vec<Branch>) -> DecisionTree {
     let branches: Vec<_> = raw_branches.into_iter().map(flatten_patterns).collect();
 
+    debug_assert!(!branches.is_empty());
+
     match check_for_match(&branches) {
-        Some(goal) => DecisionTree::Match(goal),
-        None => {
+        Match::Exact(goal) => DecisionTree::Match(goal),
+
+        Match::GuardOnly => {
+            // the first branch has no more tests to do, but it has an if-guard
+
+            let mut branches = branches;
+            let first = branches.remove(0);
+
+            match first.guard {
+                Guard::NoGuard => unreachable!(),
+
+                Guard::Guard { id, stmt, pattern } => {
+                    let guarded_test = GuardedTest::GuardedNoTest { id, stmt, pattern };
+
+                    // the guard test does not have a path
+                    let path = vec![];
+
+                    // we expect none of the patterns need tests, those decisions should have been made already
+                    debug_assert!(first
+                        .patterns
+                        .iter()
+                        .all(|(_, pattern)| !needs_tests(pattern)));
+
+                    let default = if branches.is_empty() {
+                        None
+                    } else {
+                        Some(Box::new(to_decision_tree(branches)))
+                    };
+
+                    DecisionTree::Decision {
+                        path,
+                        edges: vec![(guarded_test, DecisionTree::Match(first.goal))],
+                        default,
+                    }
+                }
+            }
+        }
+
+        Match::None => {
             // must clone here to release the borrow on `branches`
             let path = pick_path(&branches).clone();
+
+            let bs = branches.clone();
             let (edges, fallback) = gather_edges(branches, &path);
 
             let mut decision_edges: Vec<_> = edges
                 .into_iter()
-                .map(|(a, b)| (a, to_decision_tree(b)))
+                .map(|(test, branches)| {
+                    if bs == branches {
+                        panic!();
+                    } else {
+                        (test, to_decision_tree(branches))
+                    }
+                })
                 .collect();
 
-            match (decision_edges.split_last_mut(), fallback.split_last()) {
-                (Some(((_tag, decision_tree), rest)), None) if rest.is_empty() => {
-                    // TODO remove clone
-                    decision_tree.clone()
+            match (decision_edges.as_slice(), fallback.as_slice()) {
+                ([(_test, _decision_tree)], []) => {
+                    // only one test with no fallback: we will always enter this branch
+
+                    // get the `_decision_tree` without cloning
+                    decision_edges.pop().unwrap().1
                 }
-                (_, None) => DecisionTree::Decision {
-                    path: path.clone(),
-                    edges: decision_edges,
-                    default: None,
-                },
-                (None, Some(_)) => to_decision_tree(fallback),
-                _ => DecisionTree::Decision {
-                    path: path.clone(),
-                    edges: decision_edges,
-                    default: Some(Box::new(to_decision_tree(fallback))),
-                },
+                (_, []) => break_out_guard(path, decision_edges, None),
+                ([], _) => {
+                    // should be guaranteed by the patterns
+                    debug_assert!(!fallback.is_empty());
+                    to_decision_tree(fallback)
+                }
+                (_, _) => break_out_guard(
+                    path,
+                    decision_edges,
+                    Some(Box::new(to_decision_tree(fallback))),
+                ),
             }
         }
     }
 }
 
-fn is_complete(tests: &[Test]) -> bool {
+/// Give a guard it's own Decision
+fn break_out_guard<'a>(
+    path: Vec<PathInstruction>,
+    mut edges: Vec<(GuardedTest<'a>, DecisionTree<'a>)>,
+    default: Option<Box<DecisionTree<'a>>>,
+) -> DecisionTree<'a> {
+    match edges
+        .iter()
+        .position(|(t, _)| matches!(t, GuardedTest::Placeholder))
+    {
+        None => DecisionTree::Decision {
+            path,
+            edges,
+            default,
+        },
+        Some(index) => {
+            let (a, b) = edges.split_at_mut(index + 1);
+
+            let new_default = break_out_guard(path.clone(), b.to_vec(), default);
+
+            let mut left = a.to_vec();
+            let guard = left.pop().unwrap();
+
+            let help = DecisionTree::Decision {
+                path: path.clone(),
+                edges: vec![guard],
+                default: Some(Box::new(new_default)),
+            };
+
+            DecisionTree::Decision {
+                path,
+                edges: left,
+                default: Some(Box::new(help)),
+            }
+        }
+    }
+}
+
+fn guarded_tests_are_complete(tests: &[GuardedTest]) -> bool {
     let length = tests.len();
     debug_assert!(length > 0);
-    match tests.last() {
-        None => unreachable!("should never happen"),
-        Some(v) => match v {
-            Test::IsCtor { union, .. } => length == union.alternatives.len(),
-            Test::IsByte { num_alts, .. } => length == *num_alts,
-            Test::IsBit(_) => length == 2,
-            Test::IsInt(_) => false,
-            Test::IsFloat(_) => false,
-            Test::IsStr(_) => false,
-            Test::Guarded { .. } => false,
-        },
+
+    let no_guard = tests
+        .iter()
+        .all(|t| matches!(t, GuardedTest::TestNotGuarded { .. }));
+
+    match tests.last().unwrap() {
+        GuardedTest::Placeholder => false,
+        GuardedTest::GuardedNoTest { .. } => false,
+        GuardedTest::TestNotGuarded { test } => no_guard && tests_are_complete_help(test, length),
+    }
+}
+
+fn tests_are_complete_help(last_test: &Test, number_of_tests: usize) -> bool {
+    match last_test {
+        Test::IsCtor { union, .. } => number_of_tests == union.alternatives.len(),
+        Test::IsByte { num_alts, .. } => number_of_tests == *num_alts,
+        Test::IsBit(_) => number_of_tests == 2,
+        Test::IsInt(_) => false,
+        Test::IsFloat(_) => false,
+        Test::IsStr(_) => false,
     }
 }
 
@@ -201,29 +314,23 @@ fn flatten_patterns(branch: Branch) -> Branch {
     }
 
     Branch {
-        goal: branch.goal,
         patterns: result,
+        ..branch
     }
 }
 
 fn flatten<'a>(
-    path_pattern: (Vec<PathInstruction>, Guard<'a>, Pattern<'a>),
-    path_patterns: &mut Vec<(Vec<PathInstruction>, Guard<'a>, Pattern<'a>)>,
+    path_pattern: (Vec<PathInstruction>, Pattern<'a>),
+    path_patterns: &mut Vec<(Vec<PathInstruction>, Pattern<'a>)>,
 ) {
-    match path_pattern.2 {
+    match path_pattern.1 {
         Pattern::AppliedTag {
             union,
             arguments,
             tag_id,
             tag_name,
             layout,
-        } if union.alternatives.len() == 1
-            && !matches!(
-                layout,
-                Layout::Union(UnionLayout::NullableWrapped { .. })
-                    | Layout::Union(UnionLayout::NullableUnwrapped { .. })
-            ) =>
-        {
+        } if union.alternatives.len() == 1 && !layout.is_nullable() => {
             // TODO ^ do we need to check that guard.is_none() here?
 
             let path = path_pattern.0;
@@ -233,7 +340,6 @@ fn flatten<'a>(
                 // NOTE here elm will unbox, but we don't use that
                 path_patterns.push((
                     path,
-                    path_pattern.1.clone(),
                     Pattern::AppliedTag {
                         union,
                         arguments,
@@ -250,15 +356,7 @@ fn flatten<'a>(
                         tag_id,
                     });
 
-                    flatten(
-                        (
-                            new_path,
-                            // same guard here?
-                            path_pattern.1.clone(),
-                            arg_pattern.clone(),
-                        ),
-                        path_patterns,
-                    );
+                    flatten((new_path, arg_pattern.clone()), path_patterns);
                 }
             }
         }
@@ -275,28 +373,40 @@ fn flatten<'a>(
 /// path. If that is the case we give the resulting label and a mapping from free
 /// variables to "how to get their value". So a pattern like (Just (x,_)) will give
 /// us something like ("x" => value.0.0)
-fn check_for_match(branches: &[Branch]) -> Option<Label> {
+
+enum Match {
+    Exact(Label),
+    GuardOnly,
+    None,
+}
+
+fn check_for_match(branches: &[Branch]) -> Match {
     match branches.get(0) {
-        Some(Branch { goal, patterns })
-            if patterns
-                .iter()
-                .all(|(_, guard, pattern)| guard.is_none() && !needs_tests(pattern)) =>
-        {
-            Some(*goal)
+        Some(Branch {
+            goal,
+            patterns,
+            guard,
+        }) if patterns.iter().all(|(_, pattern)| !needs_tests(pattern)) => {
+            if guard.is_none() {
+                Match::Exact(*goal)
+            } else {
+                Match::GuardOnly
+            }
         }
-        _ => None,
+        _ => Match::None,
     }
 }
 
 /// GATHER OUTGOING EDGES
 
+// my understanding: branches that we could jump to based on the pattern at the current path
 fn gather_edges<'a>(
     branches: Vec<Branch<'a>>,
     path: &[PathInstruction],
-) -> (Vec<(Test<'a>, Vec<Branch<'a>>)>, Vec<Branch<'a>>) {
+) -> (Vec<(GuardedTest<'a>, Vec<Branch<'a>>)>, Vec<Branch<'a>>) {
     let relevant_tests = tests_at_path(path, &branches);
 
-    let check = is_complete(&relevant_tests);
+    let check = guarded_tests_are_complete(&relevant_tests);
 
     // TODO remove clone
     let all_edges = relevant_tests
@@ -318,13 +428,16 @@ fn gather_edges<'a>(
 
 /// FIND RELEVANT TESTS
 
-fn tests_at_path<'a>(selected_path: &[PathInstruction], branches: &[Branch<'a>]) -> Vec<Test<'a>> {
+fn tests_at_path<'a>(
+    selected_path: &[PathInstruction],
+    branches: &[Branch<'a>],
+) -> Vec<GuardedTest<'a>> {
     // NOTE the ordering of the result is important!
 
     let mut all_tests = Vec::new();
 
     for branch in branches {
-        test_at_path(selected_path, branch, &mut all_tests);
+        all_tests.extend(test_at_path(selected_path, branch));
     }
 
     // The rust HashMap also uses equality, here we really want to use the custom hash function
@@ -355,41 +468,25 @@ fn tests_at_path<'a>(selected_path: &[PathInstruction], branches: &[Branch<'a>])
 fn test_at_path<'a>(
     selected_path: &[PathInstruction],
     branch: &Branch<'a>,
-    all_tests: &mut Vec<Test<'a>>,
-) {
+) -> Option<GuardedTest<'a>> {
     use Pattern::*;
     use Test::*;
 
     match branch
         .patterns
         .iter()
-        .find(|(path, _, _)| path == selected_path)
+        .find(|(path, _)| path == selected_path)
     {
-        None => {}
-        Some((_, guard, pattern)) => {
-            let guarded = |test| {
-                if let Guard::Guard { symbol, id, stmt } = guard {
-                    Guarded {
-                        opt_test: Some(Box::new(test)),
-                        stmt: stmt.clone(),
-                        symbol: *symbol,
-                        id: *id,
-                    }
-                } else {
-                    test
-                }
-            };
-
-            match pattern {
-                // TODO use guard!
+        None => None,
+        Some((_, pattern)) => {
+            let test = match pattern {
                 Identifier(_) | Underscore => {
-                    if let Guard::Guard { symbol, id, stmt } = guard {
-                        all_tests.push(Guarded {
-                            opt_test: None,
-                            stmt: stmt.clone(),
-                            symbol: *symbol,
-                            id: *id,
-                        });
+                    if let Guard::Guard { .. } = &branch.guard {
+                        // no tests for this pattern remain, but we cannot discard it yet
+                        // because it has a guard!
+                        return Some(GuardedTest::Placeholder);
+                    } else {
+                        return None;
                     }
                 }
 
@@ -417,12 +514,27 @@ fn test_at_path<'a>(
                         }
                     }
 
-                    all_tests.push(IsCtor {
+                    IsCtor {
                         tag_id: 0,
                         tag_name: TagName::Global(RECORD_TAG_NAME.into()),
                         union,
                         arguments,
-                    });
+                    }
+                }
+
+                NewtypeDestructure {
+                    tag_name,
+                    arguments,
+                } => {
+                    let tag_id = 0;
+                    let union = Union::newtype_wrapper(tag_name.clone(), arguments.len());
+
+                    IsCtor {
+                        tag_id,
+                        tag_name: tag_name.clone(),
+                        union,
+                        arguments: arguments.to_vec(),
+                    }
                 }
 
                 AppliedTag {
@@ -431,100 +543,96 @@ fn test_at_path<'a>(
                     arguments,
                     union,
                     ..
-                } => {
-                    all_tests.push(IsCtor {
-                        tag_id: *tag_id,
-                        tag_name: tag_name.clone(),
-                        union: union.clone(),
-                        arguments: arguments.to_vec(),
-                    });
-                }
-                BitLiteral { value, .. } => {
-                    all_tests.push(IsBit(*value));
-                }
-                EnumLiteral { tag_id, union, .. } => {
-                    all_tests.push(IsByte {
-                        tag_id: *tag_id,
-                        num_alts: union.alternatives.len(),
-                    });
-                }
-                IntLiteral(v) => {
-                    all_tests.push(guarded(IsInt(*v)));
-                }
-                FloatLiteral(v) => {
-                    all_tests.push(IsFloat(*v));
-                }
-                StrLiteral(v) => {
-                    all_tests.push(IsStr(v.clone()));
-                }
+                } => IsCtor {
+                    tag_id: *tag_id,
+                    tag_name: tag_name.clone(),
+                    union: union.clone(),
+                    arguments: arguments.to_vec(),
+                },
+                BitLiteral { value, .. } => IsBit(*value),
+                EnumLiteral { tag_id, union, .. } => IsByte {
+                    tag_id: *tag_id,
+                    num_alts: union.alternatives.len(),
+                },
+                IntLiteral(v) => IsInt(*v),
+                FloatLiteral(v) => IsFloat(*v),
+                StrLiteral(v) => IsStr(v.clone()),
             };
+
+            let guarded_test = GuardedTest::TestNotGuarded { test };
+
+            Some(guarded_test)
         }
     }
 }
 
 /// BUILD EDGES
 
+// understanding: if the test is successful, where could we go?
 fn edges_for<'a>(
     path: &[PathInstruction],
     branches: Vec<Branch<'a>>,
-    test: Test<'a>,
-) -> (Test<'a>, Vec<Branch<'a>>) {
+    test: GuardedTest<'a>,
+) -> (GuardedTest<'a>, Vec<Branch<'a>>) {
     let mut new_branches = Vec::new();
 
-    for branch in branches.iter() {
-        to_relevant_branch(&test, path, branch, &mut new_branches);
+    // if we test for a guard, skip all branches until one that has a guard
+
+    let it = match test {
+        GuardedTest::GuardedNoTest { .. } | GuardedTest::Placeholder => {
+            let index = branches
+                .iter()
+                .position(|b| !b.guard.is_none())
+                .expect("if testing for a guard, one branch must have a guard");
+
+            branches[index..].iter()
+        }
+        GuardedTest::TestNotGuarded { .. } => branches.iter(),
+    };
+
+    for branch in it {
+        new_branches.extend(to_relevant_branch(&test, path, branch));
     }
 
     (test, new_branches)
 }
 
 fn to_relevant_branch<'a>(
-    test: &Test<'a>,
+    guarded_test: &GuardedTest<'a>,
     path: &[PathInstruction],
     branch: &Branch<'a>,
-    new_branches: &mut Vec<Branch<'a>>,
-) {
+) -> Option<Branch<'a>> {
     // TODO remove clone
     match extract(path, branch.patterns.clone()) {
-        Extract::NotFound => {
-            new_branches.push(branch.clone());
-        }
+        Extract::NotFound => Some(branch.clone()),
         Extract::Found {
             start,
-            found_pattern: (guard, pattern),
+            found_pattern: pattern,
             end,
-        } => {
-            let actual_test = match test {
-                Test::Guarded {
-                    opt_test: Some(box_test),
-                    ..
-                } => box_test,
-                _ => test,
-            };
+        } => match guarded_test {
+            GuardedTest::Placeholder | GuardedTest::GuardedNoTest { .. } => {
+                // if there is no test, the pattern should not require any
+                debug_assert!(
+                    matches!(pattern, Pattern::Identifier(_) | Pattern::Underscore,),
+                    "{:?}",
+                    pattern,
+                );
 
-            if let Some(mut new_branch) =
-                to_relevant_branch_help(actual_test, path, start, end, branch, guard, pattern)
-            {
-                // guards can/should only occur at the top level. When we recurse on these
-                // branches, the guard is not relevant any more. Not setthing the guard to None
-                // leads to infinite recursion.
-                new_branch.patterns.iter_mut().for_each(|(_, guard, _)| {
-                    *guard = Guard::NoGuard;
-                });
-
-                new_branches.push(new_branch);
+                Some(branch.clone())
             }
-        }
+            GuardedTest::TestNotGuarded { test } => {
+                to_relevant_branch_help(test, path, start, end, branch, pattern)
+            }
+        },
     }
 }
 
 fn to_relevant_branch_help<'a>(
     test: &Test<'a>,
     path: &[PathInstruction],
-    mut start: Vec<(Vec<PathInstruction>, Guard<'a>, Pattern<'a>)>,
-    end: Vec<(Vec<PathInstruction>, Guard<'a>, Pattern<'a>)>,
+    mut start: Vec<(Vec<PathInstruction>, Pattern<'a>)>,
+    end: Vec<(Vec<PathInstruction>, Pattern<'a>)>,
     branch: &Branch<'a>,
-    guard: Guard<'a>,
     pattern: Pattern<'a>,
 ) -> Option<Branch<'a>> {
     use Pattern::*;
@@ -552,19 +660,58 @@ fn to_relevant_branch_help<'a>(
                         tag_id: *tag_id,
                     });
 
-                    (new_path, Guard::NoGuard, pattern)
+                    (new_path, pattern)
                 });
                 start.extend(sub_positions);
                 start.extend(end);
 
                 Some(Branch {
                     goal: branch.goal,
+                    guard: branch.guard.clone(),
                     patterns: start,
                 })
             }
             _ => None,
         },
 
+        NewtypeDestructure {
+            tag_name,
+            arguments,
+            ..
+        } => match test {
+            IsCtor {
+                tag_name: test_name,
+                tag_id: test_id,
+                ..
+            } if &tag_name == test_name => {
+                let tag_id = 0;
+                debug_assert_eq!(tag_id, *test_id);
+
+                let sub_positions =
+                    arguments
+                        .into_iter()
+                        .enumerate()
+                        .map(|(index, (pattern, _))| {
+                            let mut new_path = path.to_vec();
+                            new_path.push(PathInstruction {
+                                index: index as u64,
+                                tag_id,
+                            });
+                            (new_path, pattern)
+                        });
+                start.extend(sub_positions);
+                start.extend(end);
+
+                Some(Branch {
+                    goal: branch.goal,
+                    guard: branch.guard.clone(),
+                    patterns: start,
+                })
+            }
+
+            _ => None,
+        },
+
         AppliedTag {
             tag_name,
             tag_id,
@@ -581,59 +728,61 @@ fn to_relevant_branch_help<'a>(
                     debug_assert_eq!(tag_id, *test_id);
 
                     // the test matches the constructor of this pattern
-
-                    match Wrapped::opt_from_layout(&layout) {
-                        None => todo!(),
-                        Some(wrapped) => {
-                            match wrapped {
-                                Wrapped::SingleElementRecord => {
-                                    // Theory: Unbox doesn't have any value for us
-                                    debug_assert_eq!(arguments.len(), 1);
-                                    let arg = arguments[0].clone();
-                                    {
-                                        // NOTE here elm unboxes, but we ignore that
-                                        // Path::Unbox(Box::new(path.clone()))
-                                        start.push((path.to_vec(), guard, arg.0));
-                                        start.extend(end);
-                                    }
-                                }
-                                Wrapped::RecordOrSingleTagUnion => {
-                                    let sub_positions = arguments.into_iter().enumerate().map(
-                                        |(index, (pattern, _))| {
-                                            let mut new_path = path.to_vec();
-                                            new_path.push(PathInstruction {
-                                                index: index as u64,
-                                                tag_id,
-                                            });
-                                            (new_path, Guard::NoGuard, pattern)
-                                        },
-                                    );
-                                    start.extend(sub_positions);
-                                    start.extend(end);
-                                }
-                                Wrapped::MultiTagUnion => {
-                                    let sub_positions = arguments.into_iter().enumerate().map(
-                                        |(index, (pattern, _))| {
-                                            let mut new_path = path.to_vec();
-                                            new_path.push(PathInstruction {
-                                                index: 1 + index as u64,
-                                                tag_id,
-                                            });
-                                            (new_path, Guard::NoGuard, pattern)
-                                        },
-                                    );
-                                    start.extend(sub_positions);
-                                    start.extend(end);
-                                }
-                                Wrapped::EmptyRecord => todo!(),
+                    match layout {
+                        UnionLayout::NonRecursive([[Layout::Struct([_])]]) => {
+                            // a one-element record equivalent
+                            // Theory: Unbox doesn't have any value for us
+                            debug_assert_eq!(arguments.len(), 1);
+                            let arg = arguments[0].clone();
+                            {
+                                // NOTE here elm unboxes, but we ignore that
+                                // Path::Unbox(Box::new(path.clone()))
+                                start.push((path.to_vec(), arg.0));
+                                start.extend(end);
                             }
-
-                            Some(Branch {
-                                goal: branch.goal,
-                                patterns: start,
-                            })
+                        }
+                        UnionLayout::NonRecursive([_]) | UnionLayout::NonNullableUnwrapped(_) => {
+                            let sub_positions =
+                                arguments
+                                    .into_iter()
+                                    .enumerate()
+                                    .map(|(index, (pattern, _))| {
+                                        let mut new_path = path.to_vec();
+                                        new_path.push(PathInstruction {
+                                            index: index as u64,
+                                            tag_id,
+                                        });
+                                        (new_path, pattern)
+                                    });
+                            start.extend(sub_positions);
+                            start.extend(end);
+                        }
+                        UnionLayout::NonRecursive(_)
+                        | UnionLayout::Recursive(_)
+                        | UnionLayout::NullableWrapped { .. }
+                        | UnionLayout::NullableUnwrapped { .. } => {
+                            let sub_positions =
+                                arguments
+                                    .into_iter()
+                                    .enumerate()
+                                    .map(|(index, (pattern, _))| {
+                                        let mut new_path = path.to_vec();
+                                        new_path.push(PathInstruction {
+                                            index: index as u64,
+                                            tag_id,
+                                        });
+                                        (new_path, pattern)
+                                    });
+                            start.extend(sub_positions);
+                            start.extend(end);
                         }
                     }
+
+                    Some(Branch {
+                        goal: branch.goal,
+                        guard: branch.guard.clone(),
+                        patterns: start,
+                    })
                 }
                 _ => None,
             }
@@ -643,6 +792,7 @@ fn to_relevant_branch_help<'a>(
                 start.extend(end);
                 Some(Branch {
                     goal: branch.goal,
+                    guard: branch.guard.clone(),
                     patterns: start,
                 })
             }
@@ -654,6 +804,7 @@ fn to_relevant_branch_help<'a>(
                 start.extend(end);
                 Some(Branch {
                     goal: branch.goal,
+                    guard: branch.guard.clone(),
                     patterns: start,
                 })
             }
@@ -665,6 +816,7 @@ fn to_relevant_branch_help<'a>(
                 start.extend(end);
                 Some(Branch {
                     goal: branch.goal,
+                    guard: branch.guard.clone(),
                     patterns: start,
                 })
             }
@@ -676,6 +828,7 @@ fn to_relevant_branch_help<'a>(
                 start.extend(end);
                 Some(Branch {
                     goal: branch.goal,
+                    guard: branch.guard.clone(),
                     patterns: start,
                 })
             }
@@ -689,6 +842,7 @@ fn to_relevant_branch_help<'a>(
                 start.extend(end);
                 Some(Branch {
                     goal: branch.goal,
+                    guard: branch.guard.clone(),
                     patterns: start,
                 })
             }
@@ -701,15 +855,15 @@ fn to_relevant_branch_help<'a>(
 enum Extract<'a> {
     NotFound,
     Found {
-        start: Vec<(Vec<PathInstruction>, Guard<'a>, Pattern<'a>)>,
-        found_pattern: (Guard<'a>, Pattern<'a>),
-        end: Vec<(Vec<PathInstruction>, Guard<'a>, Pattern<'a>)>,
+        start: Vec<(Vec<PathInstruction>, Pattern<'a>)>,
+        found_pattern: Pattern<'a>,
+        end: Vec<(Vec<PathInstruction>, Pattern<'a>)>,
     },
 }
 
 fn extract<'a>(
     selected_path: &[PathInstruction],
-    path_patterns: Vec<(Vec<PathInstruction>, Guard<'a>, Pattern<'a>)>,
+    path_patterns: Vec<(Vec<PathInstruction>, Pattern<'a>)>,
 ) -> Extract<'a> {
     let mut start = Vec::new();
 
@@ -719,7 +873,7 @@ fn extract<'a>(
         if current.0 == selected_path {
             return Extract::Found {
                 start,
-                found_pattern: (current.1, current.2),
+                found_pattern: current.1,
                 end: it.collect::<Vec<_>>(),
             };
         } else {
@@ -736,10 +890,10 @@ fn is_irrelevant_to<'a>(selected_path: &[PathInstruction], branch: &Branch<'a>)
     match branch
         .patterns
         .iter()
-        .find(|(path, _, _)| path == selected_path)
+        .find(|(path, _)| path == selected_path)
     {
         None => true,
-        Some((_, guard, pattern)) => guard.is_none() && !needs_tests(pattern),
+        Some((_, pattern)) => branch.guard.is_none() && !needs_tests(pattern),
     }
 }
 
@@ -750,6 +904,7 @@ fn needs_tests(pattern: &Pattern) -> bool {
         Identifier(_) | Underscore => false,
 
         RecordDestructure(_, _)
+        | NewtypeDestructure { .. }
         | AppliedTag { .. }
         | BitLiteral { .. }
         | EnumLiteral { .. }
@@ -766,8 +921,10 @@ fn pick_path<'a>(branches: &'a [Branch]) -> &'a Vec<PathInstruction> {
 
     // is choice path
     for branch in branches {
-        for (path, guard, pattern) in &branch.patterns {
-            if !guard.is_none() || needs_tests(&pattern) {
+        for (path, pattern) in &branch.patterns {
+            // NOTE we no longer check for the guard here
+            // if !branch.guard.is_none() || needs_tests(&pattern) {
+            if needs_tests(&pattern) {
                 all_paths.push(path);
             } else {
                 // do nothing
@@ -879,6 +1036,15 @@ fn small_branching_factor(branches: &[Branch], path: &[PathInstruction]) -> usiz
 #[derive(Clone, Debug, PartialEq)]
 enum Decider<'a, T> {
     Leaf(T),
+    Guarded {
+        /// after assigning to symbol, the stmt jumps to this label
+        id: JoinPointId,
+        stmt: Stmt<'a>,
+        pattern: Pattern<'a>,
+
+        success: Box<Decider<'a, T>>,
+        failure: Box<Decider<'a, T>>,
+    },
     Chain {
         test_chain: Vec<(Vec<PathInstruction>, Test<'a>)>,
         success: Box<Decider<'a, T>>,
@@ -912,11 +1078,15 @@ pub fn optimize_when<'a>(
         .into_iter()
         .enumerate()
         .map(|(index, (pattern, guard, branch))| {
-            ((guard, pattern, index as u64), (index as u64, branch))
+            let has_guard = !guard.is_none();
+            (
+                (guard, pattern.clone(), index as u64),
+                (index as u64, branch, pattern, has_guard),
+            )
         })
         .unzip();
 
-    let indexed_branches: Vec<(u64, Stmt<'a>)> = _indexed_branches;
+    let indexed_branches: Vec<_> = _indexed_branches;
 
     let decision_tree = compile(patterns);
     let decider = tree_to_decider(decision_tree);
@@ -928,7 +1098,14 @@ pub fn optimize_when<'a>(
     let mut choices = MutMap::default();
     let mut jumps = Vec::new();
 
-    for (index, branch) in indexed_branches.into_iter() {
+    for (index, mut branch, pattern, has_guard) in indexed_branches.into_iter() {
+        // bind the fields referenced in the pattern. For guards this happens separately, so
+        // the pattern variables are defined when evaluating the guard.
+        if !has_guard {
+            branch =
+                crate::ir::store_pattern(env, procs, layout_cache, &pattern, cond_symbol, branch);
+        }
+
         let ((branch_index, choice), opt_jump) = create_choices(&target_counts, index, branch);
 
         if let Some((index, body)) = opt_jump {
@@ -956,7 +1133,7 @@ pub fn optimize_when<'a>(
         stmt = Stmt::Join {
             id,
             parameters: &[],
-            continuation: env.arena.alloc(body),
+            body: env.arena.alloc(body),
             remainder: env.arena.alloc(stmt),
         };
     }
@@ -965,7 +1142,7 @@ pub fn optimize_when<'a>(
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub struct PathInstruction {
+struct PathInstruction {
     index: u64,
     tag_id: u8,
 }
@@ -983,112 +1160,51 @@ fn path_to_expr_help<'a>(
     let mut it = instructions.iter().peekable();
 
     while let Some(PathInstruction { index, tag_id }) = it.next() {
-        match Wrapped::opt_from_layout(&layout) {
-            None => {
-                // this MUST be an index into a single-element (hence unwrapped) record
+        let index = *index;
 
-                debug_assert_eq!(*index, 0, "{:?}", &layout);
-                debug_assert_eq!(*tag_id, 0);
-                debug_assert!(it.peek().is_none());
-
-                let field_layouts = vec![layout];
-
-                debug_assert!(*index < field_layouts.len() as u64);
-
-                debug_assert_eq!(field_layouts.len(), 1);
-
-                let inner_expr = Expr::AccessAtIndex {
-                    index: *index,
-                    field_layouts: env.arena.alloc(field_layouts),
+        match &layout {
+            Layout::Union(union_layout) => {
+                let inner_expr = Expr::UnionAtIndex {
+                    tag_id: *tag_id,
                     structure: symbol,
-                    wrapped: Wrapped::SingleElementRecord,
-                };
-
-                symbol = env.unique_symbol();
-                let inner_layout = layout;
-                stores.push((symbol, inner_layout, inner_expr));
-
-                break;
-            }
-            Some(wrapped) => {
-                let index = *index;
-
-                let field_layouts = match &layout {
-                    Layout::Union(variant) => {
-                        use UnionLayout::*;
-
-                        match variant {
-                            NonRecursive(layouts) | Recursive(layouts) => layouts[*tag_id as usize],
-                            NonNullableUnwrapped(fields) => {
-                                debug_assert_eq!(*tag_id, 0);
-                                fields
-                            }
-                            NullableWrapped {
-                                nullable_id,
-                                other_tags: layouts,
-                                ..
-                            } => {
-                                use std::cmp::Ordering;
-                                match (*tag_id as usize).cmp(&(*nullable_id as usize)) {
-                                    Ordering::Equal => {
-                                        // the nullable tag is going to pretend it stores a tag id
-                                        &*env
-                                            .arena
-                                            .alloc([Layout::Builtin(crate::layout::TAG_SIZE)])
-                                    }
-                                    Ordering::Less => layouts[*tag_id as usize],
-                                    Ordering::Greater => layouts[*tag_id as usize - 1],
-                                }
-                            }
-                            NullableUnwrapped {
-                                nullable_id,
-                                other_fields,
-                            } => {
-                                let tag_id = *tag_id != 0;
-
-                                if tag_id == *nullable_id {
-                                    // the nullable tag has no fields; we can only lookup its tag id
-                                    debug_assert_eq!(index, 0);
-
-                                    // the nullable tag is going to pretend it stores a tag id
-                                    &*env.arena.alloc([Layout::Builtin(crate::layout::TAG_SIZE)])
-                                } else {
-                                    *other_fields
-                                }
-                            }
-                        }
-                    }
-
-                    Layout::Struct(layouts) => layouts,
-                    other => env.arena.alloc([*other]),
-                };
-
-                debug_assert!(
-                    index < field_layouts.len() as u64,
-                    "{} {:?} {:?} {:?}",
                     index,
-                    field_layouts,
-                    &layout,
-                    tag_id,
-                );
-
-                let inner_layout = match &field_layouts[index as usize] {
-                    Layout::RecursivePointer => layout,
-                    other => *other,
+                    union_layout: *union_layout,
                 };
 
-                let inner_expr = Expr::AccessAtIndex {
-                    index,
-                    field_layouts,
-                    structure: symbol,
-                    wrapped,
-                };
+                let inner_layout = union_layout.layout_at(*tag_id as u8, index as usize);
 
                 symbol = env.unique_symbol();
                 stores.push((symbol, inner_layout, inner_expr));
 
                 layout = inner_layout;
             }
+
+            Layout::Struct(field_layouts) => {
+                debug_assert!(field_layouts.len() > 1);
+
+                let inner_expr = Expr::StructAtIndex {
+                    index,
+                    field_layouts,
+                    structure: symbol,
+                };
+
+                let inner_layout = field_layouts[index as usize];
+
+                symbol = env.unique_symbol();
+                stores.push((symbol, inner_layout, inner_expr));
+
+                layout = inner_layout;
+            }
+
+            _ => {
+                // this MUST be an index into a single-element (hence unwrapped) record
+
+                debug_assert_eq!(index, 0, "{:?}", &layout);
+                debug_assert_eq!(*tag_id, 0);
+                debug_assert!(it.peek().is_none());
+
+                break;
+            }
         }
     }
 
@@ -1101,54 +1217,54 @@ fn test_to_equality<'a>(
     cond_layout: &Layout<'a>,
     path: &[PathInstruction],
     test: Test<'a>,
-) -> (StoresVec<'a>, Symbol, Symbol, Layout<'a>) {
-    let (rhs_symbol, mut stores, _layout) =
+) -> (
+    StoresVec<'a>,
+    Symbol,
+    Symbol,
+    Layout<'a>,
+    Option<ConstructorKnown<'a>>,
+) {
+    let (rhs_symbol, mut stores, test_layout) =
         path_to_expr_help(env, cond_symbol, &path, *cond_layout);
 
     match test {
-        Test::IsCtor {
-            tag_id,
-            union,
-            arguments,
-            ..
-        } => {
+        Test::IsCtor { tag_id, union, .. } => {
             let path_symbol = rhs_symbol;
             // the IsCtor check should never be generated for tag unions of size 1
             // (e.g. record pattern guard matches)
             debug_assert!(union.alternatives.len() > 1);
 
-            let lhs = Expr::Literal(Literal::Int(tag_id as i128));
+            match test_layout {
+                Layout::Union(union_layout) => {
+                    let lhs = Expr::Literal(Literal::Int(tag_id as i128));
 
-            let mut field_layouts =
-                bumpalo::collections::Vec::with_capacity_in(arguments.len(), env.arena);
+                    let rhs = Expr::GetTagId {
+                        structure: path_symbol,
+                        union_layout,
+                    };
 
-            // add the tag discriminant
-            field_layouts.push(Layout::Builtin(Builtin::Int64));
+                    let lhs_symbol = env.unique_symbol();
+                    let rhs_symbol = env.unique_symbol();
 
-            for (_, layout) in arguments {
-                field_layouts.push(layout);
+                    let tag_id_layout = union_layout.tag_id_layout();
+
+                    stores.push((lhs_symbol, tag_id_layout, lhs));
+                    stores.push((rhs_symbol, tag_id_layout, rhs));
+
+                    (
+                        stores,
+                        lhs_symbol,
+                        rhs_symbol,
+                        Layout::Builtin(Builtin::Int64),
+                        Some(ConstructorKnown::OnlyPass {
+                            scrutinee: path_symbol,
+                            layout: *cond_layout,
+                            tag_id,
+                        }),
+                    )
+                }
+                _ => unreachable!("{:?}", (cond_layout, union)),
             }
-            let field_layouts = field_layouts.into_bump_slice();
-
-            let rhs = Expr::AccessAtIndex {
-                index: 0,
-                field_layouts,
-                structure: path_symbol,
-                wrapped: Wrapped::MultiTagUnion,
-            };
-
-            let lhs_symbol = env.unique_symbol();
-            let rhs_symbol = env.unique_symbol();
-
-            stores.push((lhs_symbol, Layout::Builtin(Builtin::Int64), lhs));
-            stores.push((rhs_symbol, Layout::Builtin(Builtin::Int64), rhs));
-
-            (
-                stores,
-                lhs_symbol,
-                rhs_symbol,
-                Layout::Builtin(Builtin::Int64),
-            )
         }
         Test::IsInt(test_int) => {
             // TODO don't downcast i128 here
@@ -1162,6 +1278,7 @@ fn test_to_equality<'a>(
                 lhs_symbol,
                 rhs_symbol,
                 Layout::Builtin(Builtin::Int64),
+                None,
             )
         }
 
@@ -1177,6 +1294,7 @@ fn test_to_equality<'a>(
                 lhs_symbol,
                 rhs_symbol,
                 Layout::Builtin(Builtin::Float64),
+                None,
             )
         }
 
@@ -1192,6 +1310,7 @@ fn test_to_equality<'a>(
                 lhs_symbol,
                 rhs_symbol,
                 Layout::Builtin(Builtin::Int8),
+                None,
             )
         }
 
@@ -1205,6 +1324,7 @@ fn test_to_equality<'a>(
                 lhs_symbol,
                 rhs_symbol,
                 Layout::Builtin(Builtin::Int1),
+                None,
             )
         }
 
@@ -1219,10 +1339,9 @@ fn test_to_equality<'a>(
                 lhs_symbol,
                 rhs_symbol,
                 Layout::Builtin(Builtin::Str),
+                None,
             )
         }
-
-        Test::Guarded { .. } => unreachable!("should be handled elsewhere"),
     }
 }
 
@@ -1231,6 +1350,7 @@ type Tests<'a> = std::vec::Vec<(
     Symbol,
     Symbol,
     Layout<'a>,
+    Option<ConstructorKnown<'a>>,
 )>;
 
 fn stores_and_condition<'a>(
@@ -1238,82 +1358,21 @@ fn stores_and_condition<'a>(
     cond_symbol: Symbol,
     cond_layout: &Layout<'a>,
     test_chain: Vec<(Vec<PathInstruction>, Test<'a>)>,
-) -> (Tests<'a>, Option<(Symbol, JoinPointId, Stmt<'a>)>) {
-    let mut tests = Vec::with_capacity(test_chain.len());
-
-    let mut guard = None;
+) -> Tests<'a> {
+    let mut tests: Tests = Vec::with_capacity(test_chain.len());
 
     // Assumption: there is at most 1 guard, and it is the outer layer.
     for (path, test) in test_chain {
-        match test {
-            Test::Guarded {
-                opt_test,
-                id,
-                symbol,
-                stmt,
-            } => {
-                if let Some(nested) = opt_test {
-                    tests.push(test_to_equality(
-                        env,
-                        cond_symbol,
-                        &cond_layout,
-                        &path,
-                        *nested,
-                    ));
-                }
-
-                // let (stores, rhs_symbol) = path_to_expr(env, cond_symbol, &path, &cond_layout);
-
-                guard = Some((symbol, id, stmt));
-            }
-
-            _ => tests.push(test_to_equality(
-                env,
-                cond_symbol,
-                &cond_layout,
-                &path,
-                test,
-            )),
-        }
+        tests.push(test_to_equality(
+            env,
+            cond_symbol,
+            &cond_layout,
+            &path,
+            test,
+        ))
     }
 
-    (tests, guard)
-}
-
-fn compile_guard<'a>(
-    env: &mut Env<'a, '_>,
-    ret_layout: Layout<'a>,
-    id: JoinPointId,
-    stmt: &'a Stmt<'a>,
-    fail: &'a Stmt<'a>,
-    mut cond: Stmt<'a>,
-) -> Stmt<'a> {
-    // the guard is the final thing that we check, so needs to be layered on first!
-    let test_symbol = env.unique_symbol();
-    let arena = env.arena;
-
-    cond = crate::ir::cond(
-        env,
-        test_symbol,
-        Layout::Builtin(Builtin::Int1),
-        cond,
-        fail.clone(),
-        ret_layout,
-    );
-
-    // calculate the guard value
-    let param = Param {
-        symbol: test_symbol,
-        layout: Layout::Builtin(Builtin::Int1),
-        borrow: false,
-    };
-
-    Stmt::Join {
-        id,
-        parameters: arena.alloc([param]),
-        remainder: stmt,
-        continuation: arena.alloc(cond),
-    }
+    tests
 }
 
 fn compile_test<'a>(
@@ -1433,23 +1492,23 @@ fn compile_tests<'a>(
     env: &mut Env<'a, '_>,
     ret_layout: Layout<'a>,
     tests: Tests<'a>,
-    opt_guard: Option<(Symbol, JoinPointId, Stmt<'a>)>,
     fail: &'a Stmt<'a>,
     mut cond: Stmt<'a>,
 ) -> Stmt<'a> {
-    let arena = env.arena;
-
-    // the guard is the final thing that we check, so needs to be layered on first!
-    if let Some((_, id, stmt)) = opt_guard {
-        cond = compile_guard(env, ret_layout, id, arena.alloc(stmt), fail, cond);
-    }
-
-    for (new_stores, lhs, rhs, _layout) in tests.into_iter() {
-        cond = compile_test(env, ret_layout, new_stores, lhs, rhs, fail, cond);
+    for (new_stores, lhs, rhs, _layout, opt_constructor_info) in tests.into_iter() {
+        match opt_constructor_info {
+            None => {
+                cond = compile_test(env, ret_layout, new_stores, lhs, rhs, fail, cond);
+            }
+            Some(cinfo) => {
+                cond = compile_test_help(env, cinfo, ret_layout, new_stores, lhs, rhs, fail, cond);
+            }
+        }
     }
     cond
 }
 
+#[derive(Debug)]
 enum ConstructorKnown<'a> {
     Both {
         scrutinee: Symbol,
@@ -1525,6 +1584,64 @@ fn decide_to_branching<'a>(
             Stmt::Jump(jumps[index].1, &[])
         }
         Leaf(Inline(expr)) => expr,
+        Guarded {
+            id,
+            stmt,
+            pattern,
+            success,
+            failure,
+        } => {
+            // the guard is the final thing that we check, so needs to be layered on first!
+            let test_symbol = env.unique_symbol();
+            let arena = env.arena;
+
+            let pass_expr = decide_to_branching(
+                env,
+                procs,
+                layout_cache,
+                cond_symbol,
+                cond_layout,
+                ret_layout,
+                *success,
+                jumps,
+            );
+
+            let fail_expr = decide_to_branching(
+                env,
+                procs,
+                layout_cache,
+                cond_symbol,
+                cond_layout,
+                ret_layout,
+                *failure,
+                jumps,
+            );
+
+            let decide = crate::ir::cond(
+                env,
+                test_symbol,
+                Layout::Builtin(Builtin::Int1),
+                pass_expr,
+                fail_expr,
+                ret_layout,
+            );
+
+            // calculate the guard value
+            let param = Param {
+                symbol: test_symbol,
+                layout: Layout::Builtin(Builtin::Int1),
+                borrow: false,
+            };
+
+            let join = Stmt::Join {
+                id,
+                parameters: arena.alloc([param]),
+                remainder: arena.alloc(stmt),
+                body: arena.alloc(decide),
+            };
+
+            crate::ir::store_pattern(env, procs, layout_cache, &pattern, cond_symbol, join)
+        }
         Chain {
             test_chain,
             success,
@@ -1557,9 +1674,9 @@ fn decide_to_branching<'a>(
             let chain_branch_info =
                 ConstructorKnown::from_test_chain(cond_symbol, &cond_layout, &test_chain);
 
-            let (tests, guard) = stores_and_condition(env, cond_symbol, &cond_layout, test_chain);
+            let tests = stores_and_condition(env, cond_symbol, &cond_layout, test_chain);
 
-            let number_of_tests = tests.len() as i64 + guard.is_some() as i64;
+            let number_of_tests = tests.len() as i64;
 
             debug_assert!(number_of_tests > 0);
 
@@ -1567,37 +1684,30 @@ fn decide_to_branching<'a>(
             if number_of_tests == 1 {
                 // if there is just one test, compile to a simple if-then-else
 
-                if guard.is_none() {
-                    // use knowledge about constructors for optimization
-                    debug_assert_eq!(tests.len(), 1);
+                let (new_stores, lhs, rhs, _layout, _cinfo) = tests.into_iter().next().unwrap();
 
-                    let (new_stores, lhs, rhs, _layout) = tests.into_iter().next().unwrap();
-
-                    compile_test_help(
-                        env,
-                        chain_branch_info,
-                        ret_layout,
-                        new_stores,
-                        lhs,
-                        rhs,
-                        fail,
-                        pass_expr,
-                    )
-                } else {
-                    compile_tests(env, ret_layout, tests, guard, fail, pass_expr)
-                }
+                compile_test_help(
+                    env,
+                    chain_branch_info,
+                    ret_layout,
+                    new_stores,
+                    lhs,
+                    rhs,
+                    fail,
+                    pass_expr,
+                )
             } else {
                 // otherwise, we use a join point so the code for the `else` case
                 // is only generated once.
                 let fail_jp_id = JoinPointId(env.unique_symbol());
                 let jump = arena.alloc(Stmt::Jump(fail_jp_id, &[]));
 
-                let test_stmt = compile_tests(env, ret_layout, tests, guard, jump, pass_expr);
+                let test_stmt = compile_tests(env, ret_layout, tests, jump, pass_expr);
 
                 Stmt::Join {
                     id: fail_jp_id,
                     parameters: &[],
-                    continuation: fail,
+                    body: fail,
                     remainder: arena.alloc(test_stmt),
                 }
             }
@@ -1681,12 +1791,36 @@ fn decide_to_branching<'a>(
 
             // We have learned more about the exact layout of the cond (based on the path)
             // but tests are still relative to the original cond symbol
-            let mut switch = Stmt::Switch {
-                cond_layout: inner_cond_layout,
-                cond_symbol: inner_cond_symbol,
-                branches: branches.into_bump_slice(),
-                default_branch: (default_branch_info, env.arena.alloc(default_branch)),
-                ret_layout,
+            let mut switch = if let Layout::Union(union_layout) = inner_cond_layout {
+                let tag_id_symbol = env.unique_symbol();
+
+                let temp = Stmt::Switch {
+                    cond_layout: union_layout.tag_id_layout(),
+                    cond_symbol: tag_id_symbol,
+                    branches: branches.into_bump_slice(),
+                    default_branch: (default_branch_info, env.arena.alloc(default_branch)),
+                    ret_layout,
+                };
+
+                let expr = Expr::GetTagId {
+                    structure: inner_cond_symbol,
+                    union_layout,
+                };
+
+                Stmt::Let(
+                    tag_id_symbol,
+                    expr,
+                    union_layout.tag_id_layout(),
+                    env.arena.alloc(temp),
+                )
+            } else {
+                Stmt::Switch {
+                    cond_layout: inner_cond_layout,
+                    cond_symbol: inner_cond_symbol,
+                    branches: branches.into_bump_slice(),
+                    default_branch: (default_branch_info, env.arena.alloc(default_branch)),
+                    ret_layout,
+                }
             };
 
             for (symbol, layout, expr) in cond_stores_vec.into_iter().rev() {
@@ -1757,28 +1891,15 @@ fn tree_to_decider(tree: DecisionTree) -> Decider<u64> {
                 }
                 2 => {
                     let (_, failure_tree) = edges.remove(1);
-                    let (test, success_tree) = edges.remove(0);
+                    let (guarded_test, success_tree) = edges.remove(0);
 
-                    if test_always_succeeds(&test) {
-                        tree_to_decider(success_tree)
-                    } else {
-                        to_chain(path, test, success_tree, failure_tree)
-                    }
+                    chain_decider(path, guarded_test, failure_tree, success_tree)
                 }
 
                 _ => {
                     let fallback = edges.remove(edges.len() - 1).1;
 
-                    let necessary_tests = edges
-                        .into_iter()
-                        .map(|(test, decider)| (test, tree_to_decider(decider)))
-                        .collect();
-
-                    FanOut {
-                        path,
-                        tests: necessary_tests,
-                        fallback: Box::new(tree_to_decider(fallback)),
-                    }
+                    fanout_decider(path, fallback, edges)
                 }
             },
 
@@ -1786,34 +1907,88 @@ fn tree_to_decider(tree: DecisionTree) -> Decider<u64> {
                 0 => tree_to_decider(*last),
                 1 => {
                     let failure_tree = *last;
-                    let (test, success_tree) = edges.remove(0);
+                    let (guarded_test, success_tree) = edges.remove(0);
 
-                    if test_always_succeeds(&test) {
-                        tree_to_decider(success_tree)
-                    } else {
-                        to_chain(path, test, success_tree, failure_tree)
-                    }
+                    chain_decider(path, guarded_test, failure_tree, success_tree)
                 }
 
                 _ => {
                     let fallback = *last;
 
-                    let necessary_tests = edges
-                        .into_iter()
-                        .map(|(test, decider)| (test, tree_to_decider(decider)))
-                        .collect();
-
-                    FanOut {
-                        path,
-                        tests: necessary_tests,
-                        fallback: Box::new(tree_to_decider(fallback)),
-                    }
+                    fanout_decider(path, fallback, edges)
                 }
             },
         },
     }
 }
 
+fn fanout_decider<'a>(
+    path: Vec<PathInstruction>,
+    fallback: DecisionTree<'a>,
+    edges: Vec<(GuardedTest<'a>, DecisionTree<'a>)>,
+) -> Decider<'a, u64> {
+    let fallback_decider = tree_to_decider(fallback);
+    let necessary_tests = edges
+        .into_iter()
+        .map(|(test, tree)| fanout_decider_help(tree, test))
+        .collect();
+
+    Decider::FanOut {
+        path,
+        tests: necessary_tests,
+        fallback: Box::new(fallback_decider),
+    }
+}
+
+fn fanout_decider_help<'a>(
+    dectree: DecisionTree<'a>,
+    guarded_test: GuardedTest<'a>,
+) -> (Test<'a>, Decider<'a, u64>) {
+    match guarded_test {
+        GuardedTest::Placeholder | GuardedTest::GuardedNoTest { .. } => {
+            unreachable!("this would not end up in a switch")
+        }
+        GuardedTest::TestNotGuarded { test } => {
+            let decider = tree_to_decider(dectree);
+            (test, decider)
+        }
+    }
+}
+
+fn chain_decider<'a>(
+    path: Vec<PathInstruction>,
+    guarded_test: GuardedTest<'a>,
+    failure_tree: DecisionTree<'a>,
+    success_tree: DecisionTree<'a>,
+) -> Decider<'a, u64> {
+    match guarded_test {
+        GuardedTest::GuardedNoTest { id, stmt, pattern } => {
+            let failure = Box::new(tree_to_decider(failure_tree));
+            let success = Box::new(tree_to_decider(success_tree));
+
+            Decider::Guarded {
+                id,
+                stmt,
+                pattern,
+                success,
+                failure: failure.clone(),
+            }
+        }
+        GuardedTest::TestNotGuarded { test } => {
+            if test_always_succeeds(&test) {
+                tree_to_decider(success_tree)
+            } else {
+                to_chain(path, test, success_tree, failure_tree)
+            }
+        }
+
+        GuardedTest::Placeholder => {
+            // ?
+            tree_to_decider(success_tree)
+        }
+    }
+}
+
 fn to_chain<'a>(
     path: Vec<PathInstruction>,
     test: Test<'a>,
@@ -1863,6 +2038,13 @@ fn count_targets(targets: &mut bumpalo::collections::Vec<u64>, initial: &Decider
                 targets[*target as usize] += 1;
             }
 
+            Guarded {
+                success, failure, ..
+            } => {
+                stack.push(success);
+                stack.push(failure);
+            }
+
             Chain {
                 success, failure, ..
             } => {
@@ -1911,6 +2093,20 @@ fn insert_choices<'a>(
             Leaf(choice_dict[&target].clone())
         }
 
+        Guarded {
+            id,
+            stmt,
+            pattern,
+            success,
+            failure,
+        } => Guarded {
+            id,
+            stmt,
+            pattern,
+            success: Box::new(insert_choices(choice_dict, *success)),
+            failure: Box::new(insert_choices(choice_dict, *failure)),
+        },
+
         Chain {
             test_chain,
             success,
diff --git a/compiler/mono/src/exhaustive.rs b/compiler/mono/src/exhaustive.rs
index f3726960e8..14d70270bf 100644
--- a/compiler/mono/src/exhaustive.rs
+++ b/compiler/mono/src/exhaustive.rs
@@ -11,6 +11,21 @@ pub struct Union {
     pub render_as: RenderAs,
 }
 
+impl Union {
+    pub fn newtype_wrapper(tag_name: TagName, arity: usize) -> Self {
+        let alternatives = vec![Ctor {
+            name: tag_name,
+            tag_id: TagId(0),
+            arity,
+        }];
+
+        Union {
+            alternatives,
+            render_as: RenderAs::Tag,
+        }
+    }
+}
+
 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
 pub enum RenderAs {
     Tag,
@@ -84,6 +99,20 @@ fn simplify(pattern: &crate::ir::Pattern) -> Pattern {
             Ctor(union, tag_id, patterns)
         }
 
+        NewtypeDestructure {
+            arguments,
+            tag_name,
+        } => {
+            let tag_id = 0;
+            let simplified_args: std::vec::Vec<_> =
+                arguments.iter().map(|v| simplify(&v.0)).collect();
+            Ctor(
+                Union::newtype_wrapper(tag_name.clone(), arguments.len()),
+                TagId(tag_id),
+                simplified_args,
+            )
+        }
+
         AppliedTag {
             tag_id,
             arguments,
@@ -324,7 +353,7 @@ fn to_nonredundant_rows(
             vec![simplify(&loc_pat.value)]
         };
 
-        if is_useful(checked_rows.clone(), next_row.clone()) {
+        if matches!(guard, Guard::HasGuard) || is_useful(checked_rows.clone(), next_row.clone()) {
             checked_rows.push(next_row);
         } else {
             return Err(Error::Redundant {
diff --git a/compiler/mono/src/expand_rc.rs b/compiler/mono/src/expand_rc.rs
index d3c852a737..01ee612edd 100644
--- a/compiler/mono/src/expand_rc.rs
+++ b/compiler/mono/src/expand_rc.rs
@@ -1,4 +1,4 @@
-use crate::ir::{BranchInfo, Expr, ModifyRc, Stmt, Wrapped};
+use crate::ir::{BranchInfo, Expr, ModifyRc, Stmt};
 use crate::layout::{Layout, UnionLayout};
 use bumpalo::collections::Vec;
 use bumpalo::Bump;
@@ -191,7 +191,7 @@ impl<'a, 'i> Env<'a, 'i> {
 
         Symbol::new(self.home, ident_id)
     }
-
+    #[allow(dead_code)]
     fn manual_unique_symbol(home: ModuleId, ident_ids: &mut IdentIds) -> Symbol {
         let ident_id = ident_ids.gen_unique();
 
@@ -245,9 +245,6 @@ fn layout_for_constructor<'a>(
             HasFields(fields)
         }
         Closure(_arguments, _lambda_set, _result) => {
-            // TODO can this be improved again?
-            // let fpointer = Layout::FunctionPointer(arguments, result);
-            // let fields = arena.alloc([fpointer, *lambda_set.layout]);
             // HasFields(fields)
             ConstructorLayout::Unknown
         }
@@ -305,11 +302,13 @@ fn work_for_constructor<'a>(
                             // we have to extract it now, but we only extract it
                             // if at least one field is aliased.
 
+                            todo!("get the tag id");
+                            /*
                             let expr = Expr::AccessAtIndex {
                                 index: i as u64,
                                 field_layouts: constructor_layout,
                                 structure: *symbol,
-                                wrapped: Wrapped::MultiTagUnion,
+                                wrapped: todo!("get the tag id"),
                             };
 
                             // create a fresh symbol for this field
@@ -323,6 +322,7 @@ fn work_for_constructor<'a>(
 
                             env.deferred.assignments.push((alias_symbol, expr, layout));
                             result.push(alias_symbol);
+                            */
                         }
                         None => {
                             // if all refcounted fields were unaliased, generate a normal decrement
@@ -343,7 +343,7 @@ fn can_push_inc_through(stmt: &Stmt) -> bool {
     match stmt {
         Let(_, expr, _, _) => {
             // we can always delay an increment/decrement until after a field access
-            matches!(expr, Expr::AccessAtIndex { .. } | Expr::Literal(_))
+            matches!(expr, Expr::StructAtIndex { .. } | Expr::Literal(_))
         }
 
         Refcounting(ModifyRc::Inc(_, _), _) => true,
@@ -397,19 +397,15 @@ pub fn expand_and_cancel_proc<'a>(
 fn expand_and_cancel<'a>(env: &mut Env<'a, '_>, stmt: &'a Stmt<'a>) -> &'a Stmt<'a> {
     use Stmt::*;
 
-    let mut deferred_default = Deferred {
+    let mut deferred = Deferred {
         inc_dec_map: Default::default(),
         assignments: Vec::new_in(env.arena),
         decrefs: Vec::new_in(env.arena),
     };
 
-    let deferred = if can_push_inc_through(stmt) {
-        deferred_default
-    } else {
-        std::mem::swap(&mut deferred_default, &mut env.deferred);
-
-        deferred_default
-    };
+    if !can_push_inc_through(stmt) {
+        std::mem::swap(&mut deferred, &mut env.deferred);
+    }
 
     let mut result = {
         match stmt {
@@ -425,7 +421,7 @@ fn expand_and_cancel<'a>(env: &mut Env<'a, '_>, stmt: &'a Stmt<'a>) -> &'a Stmt<
 
                 while !matches!(
                     &expr,
-                    Expr::AccessAtIndex { .. } | Expr::Struct(_) | Expr::Call(_)
+                    Expr::StructAtIndex { .. } | Expr::Struct(_) | Expr::Call(_)
                 ) {
                     if let Stmt::Let(symbol1, expr1, layout1, cont1) = cont {
                         literal_stack.push((symbol, expr.clone(), *layout));
@@ -442,11 +438,10 @@ fn expand_and_cancel<'a>(env: &mut Env<'a, '_>, stmt: &'a Stmt<'a>) -> &'a Stmt<
                 let new_cont;
 
                 match &expr {
-                    Expr::AccessAtIndex {
+                    Expr::StructAtIndex {
                         structure,
                         index,
                         field_layouts,
-                        wrapped,
                     } => {
                         let entry = env
                             .alias_map
@@ -455,14 +450,8 @@ fn expand_and_cancel<'a>(env: &mut Env<'a, '_>, stmt: &'a Stmt<'a>) -> &'a Stmt<
 
                         entry.insert(*index, symbol);
 
-                        // fixes https://github.com/rtfeldman/roc/issues/1099
-                        if matches!(
-                            wrapped,
-                            Wrapped::SingleElementRecord | Wrapped::RecordOrSingleTagUnion
-                        ) {
-                            env.layout_map
-                                .insert(*structure, Layout::Struct(field_layouts));
-                        }
+                        env.layout_map
+                            .insert(*structure, Layout::Struct(field_layouts));
 
                         // if the field is a struct, we know its constructor too!
                         let field_layout = &field_layouts[*index as usize];
@@ -602,6 +591,7 @@ fn expand_and_cancel<'a>(env: &mut Env<'a, '_>, stmt: &'a Stmt<'a>) -> &'a Stmt<
                 layout,
                 pass,
                 fail,
+                exception_id,
             } => {
                 let pass = expand_and_cancel(env, pass);
                 let fail = expand_and_cancel(env, fail);
@@ -612,6 +602,7 @@ fn expand_and_cancel<'a>(env: &mut Env<'a, '_>, stmt: &'a Stmt<'a>) -> &'a Stmt<
                     layout: *layout,
                     pass,
                     fail,
+                    exception_id: *exception_id,
                 };
 
                 env.arena.alloc(stmt)
@@ -620,7 +611,7 @@ fn expand_and_cancel<'a>(env: &mut Env<'a, '_>, stmt: &'a Stmt<'a>) -> &'a Stmt<
             Join {
                 id,
                 parameters,
-                continuation,
+                body: continuation,
                 remainder,
             } => {
                 let continuation = expand_and_cancel(env, continuation);
@@ -629,14 +620,14 @@ fn expand_and_cancel<'a>(env: &mut Env<'a, '_>, stmt: &'a Stmt<'a>) -> &'a Stmt<
                 let stmt = Join {
                     id: *id,
                     parameters,
-                    continuation,
+                    body: continuation,
                     remainder,
                 };
 
                 env.arena.alloc(stmt)
             }
 
-            Rethrow | Ret(_) | Jump(_, _) | RuntimeError(_) => stmt,
+            Resume(_) | Ret(_) | Jump(_, _) | RuntimeError(_) => stmt,
         }
     };
 
diff --git a/compiler/mono/src/inc_dec.rs b/compiler/mono/src/inc_dec.rs
index 3ab421a27c..a9583856e4 100644
--- a/compiler/mono/src/inc_dec.rs
+++ b/compiler/mono/src/inc_dec.rs
@@ -1,5 +1,5 @@
 use crate::borrow::{ParamMap, BORROWED, OWNED};
-use crate::ir::{Expr, JoinPointId, ModifyRc, Param, Proc, Stmt};
+use crate::ir::{Expr, JoinPointId, ModifyRc, Param, Proc, ProcLayout, Stmt};
 use crate::layout::Layout;
 use bumpalo::collections::Vec;
 use bumpalo::Bump;
@@ -7,16 +7,16 @@ use roc_collections::all::{MutMap, MutSet};
 use roc_module::symbol::Symbol;
 
 pub fn free_variables(stmt: &Stmt<'_>) -> MutSet<Symbol> {
-    let (mut occuring, bound) = occuring_variables(stmt);
+    let (mut occurring, bound) = occurring_variables(stmt);
 
     for ref s in bound {
-        occuring.remove(s);
+        occurring.remove(s);
     }
 
-    occuring
+    occurring
 }
 
-pub fn occuring_variables(stmt: &Stmt<'_>) -> (MutSet<Symbol>, MutSet<Symbol>) {
+pub fn occurring_variables(stmt: &Stmt<'_>) -> (MutSet<Symbol>, MutSet<Symbol>) {
     let mut stack = std::vec![stmt];
     let mut result = MutSet::default();
     let mut bound_variables = MutSet::default();
@@ -26,7 +26,7 @@ pub fn occuring_variables(stmt: &Stmt<'_>) -> (MutSet<Symbol>, MutSet<Symbol>) {
 
         match stmt {
             Let(symbol, expr, _, cont) => {
-                occuring_variables_expr(expr, &mut result);
+                occurring_variables_expr(expr, &mut result);
                 result.insert(*symbol);
                 bound_variables.insert(*symbol);
                 stack.push(cont);
@@ -39,7 +39,7 @@ pub fn occuring_variables(stmt: &Stmt<'_>) -> (MutSet<Symbol>, MutSet<Symbol>) {
                 fail,
                 ..
             } => {
-                occuring_variables_call(call, &mut result);
+                occurring_variables_call(call, &mut result);
                 result.insert(*symbol);
                 bound_variables.insert(*symbol);
                 stack.push(pass);
@@ -50,7 +50,7 @@ pub fn occuring_variables(stmt: &Stmt<'_>) -> (MutSet<Symbol>, MutSet<Symbol>) {
                 result.insert(*symbol);
             }
 
-            Rethrow => {}
+            Resume(_) => {}
 
             Refcounting(modify, cont) => {
                 let symbol = modify.get_symbol();
@@ -64,7 +64,7 @@ pub fn occuring_variables(stmt: &Stmt<'_>) -> (MutSet<Symbol>, MutSet<Symbol>) {
 
             Join {
                 parameters,
-                continuation,
+                body: continuation,
                 remainder,
                 ..
             } => {
@@ -93,23 +93,23 @@ pub fn occuring_variables(stmt: &Stmt<'_>) -> (MutSet<Symbol>, MutSet<Symbol>) {
     (result, bound_variables)
 }
 
-fn occuring_variables_call(call: &crate::ir::Call<'_>, result: &mut MutSet<Symbol>) {
+fn occurring_variables_call(call: &crate::ir::Call<'_>, result: &mut MutSet<Symbol>) {
     // NOTE though the function name does occur, it is a static constant in the program
     // for liveness, it should not be included here.
     result.extend(call.arguments.iter().copied());
 }
 
-pub fn occuring_variables_expr(expr: &Expr<'_>, result: &mut MutSet<Symbol>) {
+pub fn occurring_variables_expr(expr: &Expr<'_>, result: &mut MutSet<Symbol>) {
     use Expr::*;
 
     match expr {
-        AccessAtIndex {
+        StructAtIndex {
             structure: symbol, ..
         } => {
             result.insert(*symbol);
         }
 
-        Call(call) => occuring_variables_call(call, result),
+        Call(call) => occurring_variables_call(call, result),
 
         Tag { arguments, .. }
         | Struct(arguments)
@@ -129,6 +129,18 @@ pub fn occuring_variables_expr(expr: &Expr<'_>, result: &mut MutSet<Symbol>) {
         }
 
         EmptyArray | RuntimeErrorFunction(_) | Literal(_) => {}
+
+        GetTagId {
+            structure: symbol, ..
+        } => {
+            result.insert(*symbol);
+        }
+
+        UnionAtIndex {
+            structure: symbol, ..
+        } => {
+            result.insert(*symbol);
+        }
     }
 }
 
@@ -160,13 +172,13 @@ struct Context<'a> {
 fn update_live_vars<'a>(expr: &Expr<'a>, v: &LiveVarSet) -> LiveVarSet {
     let mut v = v.clone();
 
-    occuring_variables_expr(expr, &mut v);
+    occurring_variables_expr(expr, &mut v);
 
     v
 }
 
 /// `isFirstOcc xs x i = true` if `xs[i]` is the first occurrence of `xs[i]` in `xs`
-fn is_first_occurence(xs: &[Symbol], i: usize) -> bool {
+fn is_first_occurrence(xs: &[Symbol], i: usize) -> bool {
     match xs.get(i) {
         None => unreachable!(),
         Some(s) => i == xs.iter().position(|v| s == v).unwrap(),
@@ -213,7 +225,11 @@ fn is_borrow_param(x: Symbol, ys: &[Symbol], ps: &[Param]) -> bool {
 // We do not need to consume the projection of a variable that is not consumed
 fn consume_expr(m: &VarMap, e: &Expr<'_>) -> bool {
     match e {
-        Expr::AccessAtIndex { structure: x, .. } => match m.get(x) {
+        Expr::StructAtIndex { structure: x, .. } => match m.get(x) {
+            Some(info) => info.consume,
+            None => true,
+        },
+        Expr::UnionAtIndex { structure: x, .. } => match m.get(x) {
             Some(info) => info.consume,
             None => true,
         },
@@ -319,7 +335,7 @@ impl<'a> Context<'a> {
     {
         for (i, x) in xs.iter().enumerate() {
             let info = self.get_var_info(*x);
-            if !info.reference || !is_first_occurence(xs, i) {
+            if !info.reference || !is_first_occurrence(xs, i) {
                 // do nothing
             } else {
                 let num_consumptions = get_num_consumptions(*x, xs, consume_param_pred.clone()); // number of times the argument is used
@@ -393,7 +409,7 @@ impl<'a> Context<'a> {
             // Remark: `x` may occur multiple times in the application (e.g., `f x y x`).
             // This is why we check whether it is the first occurrence.
             if self.must_consume(*x)
-                && is_first_occurence(xs, i)
+                && is_first_occurrence(xs, i)
                 && is_borrow_param(*x, xs, ps)
                 && !b_live_vars.contains(x)
             {
@@ -418,7 +434,7 @@ impl<'a> Context<'a> {
             This is why we check whether it is the first occurrence. */
 
             if self.must_consume(*x)
-                && is_first_occurence(xs, i)
+                && is_first_occurrence(xs, i)
                 && *is_borrow
                 && !b_live_vars.contains(x)
             {
@@ -454,7 +470,12 @@ impl<'a> Context<'a> {
             }
 
             HigherOrderLowLevel {
-                op, closure_layout, ..
+                op,
+                closure_env_layout,
+                specialization_id,
+                arg_layouts,
+                ret_layout,
+                ..
             } => {
                 macro_rules! create_call {
                     ($borrows:expr) => {
@@ -462,8 +483,11 @@ impl<'a> Context<'a> {
                             call_type: if let Some(OWNED) = $borrows.map(|p| p.borrow) {
                                 HigherOrderLowLevel {
                                     op: *op,
-                                    closure_layout: *closure_layout,
+                                    closure_env_layout: *closure_env_layout,
                                     function_owns_closure_data: true,
+                                    specialization_id: *specialization_id,
+                                    arg_layouts,
+                                    ret_layout: *ret_layout,
                                 }
                             } else {
                                 call_type
@@ -489,12 +513,17 @@ impl<'a> Context<'a> {
                 const FUNCTION: bool = BORROWED;
                 const CLOSURE_DATA: bool = BORROWED;
 
+                let function_layout = ProcLayout {
+                    arguments: arg_layouts,
+                    result: *ret_layout,
+                };
+
                 match op {
                     roc_module::low_level::LowLevel::ListMap
                     | roc_module::low_level::LowLevel::ListKeepIf
                     | roc_module::low_level::LowLevel::ListKeepOks
                     | roc_module::low_level::LowLevel::ListKeepErrs => {
-                        match self.param_map.get_symbol(arguments[1], *closure_layout) {
+                        match self.param_map.get_symbol(arguments[1], function_layout) {
                             Some(function_ps) => {
                                 let borrows = [function_ps[0].borrow, FUNCTION, CLOSURE_DATA];
 
@@ -516,7 +545,7 @@ impl<'a> Context<'a> {
                         }
                     }
                     roc_module::low_level::LowLevel::ListMapWithIndex => {
-                        match self.param_map.get_symbol(arguments[1], *closure_layout) {
+                        match self.param_map.get_symbol(arguments[1], function_layout) {
                             Some(function_ps) => {
                                 let borrows = [function_ps[1].borrow, FUNCTION, CLOSURE_DATA];
 
@@ -537,7 +566,7 @@ impl<'a> Context<'a> {
                         }
                     }
                     roc_module::low_level::LowLevel::ListMap2 => {
-                        match self.param_map.get_symbol(arguments[2], *closure_layout) {
+                        match self.param_map.get_symbol(arguments[2], function_layout) {
                             Some(function_ps) => {
                                 let borrows = [
                                     function_ps[0].borrow,
@@ -564,7 +593,7 @@ impl<'a> Context<'a> {
                         }
                     }
                     roc_module::low_level::LowLevel::ListMap3 => {
-                        match self.param_map.get_symbol(arguments[3], *closure_layout) {
+                        match self.param_map.get_symbol(arguments[3], function_layout) {
                             Some(function_ps) => {
                                 let borrows = [
                                     function_ps[0].borrow,
@@ -593,7 +622,7 @@ impl<'a> Context<'a> {
                         }
                     }
                     roc_module::low_level::LowLevel::ListSortWith => {
-                        match self.param_map.get_symbol(arguments[1], *closure_layout) {
+                        match self.param_map.get_symbol(arguments[1], function_layout) {
                             Some(function_ps) => {
                                 let borrows = [OWNED, FUNCTION, CLOSURE_DATA];
 
@@ -615,7 +644,7 @@ impl<'a> Context<'a> {
                     | roc_module::low_level::LowLevel::ListWalkUntil
                     | roc_module::low_level::LowLevel::ListWalkBackwards
                     | roc_module::low_level::LowLevel::DictWalk => {
-                        match self.param_map.get_symbol(arguments[2], *closure_layout) {
+                        match self.param_map.get_symbol(arguments[2], function_layout) {
                             Some(function_ps) => {
                                 // borrow data structure based on first argument of the folded function
                                 // borrow the default based on second argument of the folded function
@@ -669,12 +698,17 @@ impl<'a> Context<'a> {
             }
 
             ByName {
-                name, full_layout, ..
+                name,
+                ret_layout,
+                arg_layouts,
+                ..
             } => {
+                let top_level = ProcLayout::new(self.arena, arg_layouts, *ret_layout);
+
                 // get the borrow signature
                 let ps = self
                     .param_map
-                    .get_symbol(*name, *full_layout)
+                    .get_symbol(*name, top_level)
                     .expect("function is defined");
 
                 let v = Expr::Call(crate::ir::Call {
@@ -713,7 +747,19 @@ impl<'a> Context<'a> {
                 self.arena.alloc(Stmt::Let(z, v, l, b)),
                 &b_live_vars,
             ),
-            AccessAtIndex { structure: x, .. } => {
+
+            Call(crate::ir::Call {
+                call_type,
+                arguments,
+            }) => self.visit_call(z, call_type, arguments, l, b, b_live_vars),
+
+            EmptyArray | Literal(_) | Reset(_) | RuntimeErrorFunction(_) => {
+                // EmptyArray is always stack-allocated
+                // function pointers are persistent
+                self.arena.alloc(Stmt::Let(z, v, l, b))
+            }
+
+            StructAtIndex { structure: x, .. } => {
                 let b = self.add_dec_if_needed(x, b, b_live_vars);
                 let info_x = self.get_var_info(x);
                 let b = if info_x.consume {
@@ -725,14 +771,27 @@ impl<'a> Context<'a> {
                 self.arena.alloc(Stmt::Let(z, v, l, b))
             }
 
-            Call(crate::ir::Call {
-                call_type,
-                arguments,
-            }) => self.visit_call(z, call_type, arguments, l, b, b_live_vars),
+            GetTagId { structure: x, .. } => {
+                let b = self.add_dec_if_needed(x, b, b_live_vars);
+                let info_x = self.get_var_info(x);
+                let b = if info_x.consume {
+                    self.add_inc(z, 1, b)
+                } else {
+                    b
+                };
+
+                self.arena.alloc(Stmt::Let(z, v, l, b))
+            }
+
+            UnionAtIndex { structure: x, .. } => {
+                let b = self.add_dec_if_needed(x, b, b_live_vars);
+                let info_x = self.get_var_info(x);
+                let b = if info_x.consume {
+                    self.add_inc(z, 1, b)
+                } else {
+                    b
+                };
 
-            EmptyArray | Literal(_) | Reset(_) | RuntimeErrorFunction(_) => {
-                // EmptyArray is always stack-allocated
-                // function pointers are persistent
                 self.arena.alloc(Stmt::Let(z, v, l, b))
             }
         };
@@ -892,6 +951,7 @@ impl<'a> Context<'a> {
                 pass,
                 fail,
                 layout,
+                exception_id,
             } => {
                 // live vars of the whole expression
                 let invoke_live_vars = collect_stmt(stmt, &self.jp_live_vars, MutSet::default());
@@ -926,6 +986,7 @@ impl<'a> Context<'a> {
                     pass,
                     fail,
                     layout: *layout,
+                    exception_id: *exception_id,
                 };
 
                 let cont = self.arena.alloc(invoke);
@@ -951,12 +1012,17 @@ impl<'a> Context<'a> {
                     }
 
                     CallType::ByName {
-                        name, full_layout, ..
+                        name,
+                        ret_layout,
+                        arg_layouts,
+                        ..
                     } => {
+                        let top_level = ProcLayout::new(self.arena, arg_layouts, *ret_layout);
+
                         // get the borrow signature
                         let ps = self
                             .param_map
-                            .get_symbol(*name, *full_layout)
+                            .get_symbol(*name, top_level)
                             .expect("function is defined");
                         self.add_dec_after_application(call.arguments, ps, cont, &invoke_live_vars)
                     }
@@ -968,7 +1034,7 @@ impl<'a> Context<'a> {
                 id: j,
                 parameters: _,
                 remainder: b,
-                continuation: v,
+                body: v,
             } => {
                 // get the parameters with borrow signature
                 let xs = self.param_map.get_join_point(*j);
@@ -990,7 +1056,7 @@ impl<'a> Context<'a> {
                         id: *j,
                         parameters: xs,
                         remainder: b,
-                        continuation: v,
+                        body: v,
                     }),
                     b_live_vars,
                 )
@@ -1009,7 +1075,7 @@ impl<'a> Context<'a> {
                 }
             }
 
-            Rethrow => (stmt, MutSet::default()),
+            Resume(_) => (stmt, MutSet::default()),
 
             Jump(j, xs) => {
                 let empty = MutSet::default();
@@ -1094,7 +1160,7 @@ pub fn collect_stmt(
             vars = collect_stmt(cont, jp_live_vars, vars);
             vars.remove(symbol);
             let mut result = MutSet::default();
-            occuring_variables_expr(expr, &mut result);
+            occurring_variables_expr(expr, &mut result);
             vars.extend(result);
 
             vars
@@ -1112,7 +1178,7 @@ pub fn collect_stmt(
             vars.remove(symbol);
 
             let mut result = MutSet::default();
-            occuring_variables_call(call, &mut result);
+            occurring_variables_call(call, &mut result);
 
             vars.extend(result);
 
@@ -1133,7 +1199,7 @@ pub fn collect_stmt(
             id: j,
             parameters,
             remainder: b,
-            continuation: v,
+            body: v,
         } => {
             let mut j_live_vars = collect_stmt(v, jp_live_vars, MutSet::default());
             for param in parameters.iter() {
@@ -1175,7 +1241,7 @@ pub fn collect_stmt(
             vars
         }
 
-        Rethrow => vars,
+        Resume(_) => vars,
 
         RuntimeError(_) => vars,
     }
@@ -1210,7 +1276,7 @@ pub fn visit_proc<'a>(
     arena: &'a Bump,
     param_map: &'a ParamMap<'a>,
     proc: &mut Proc<'a>,
-    layout: Layout<'a>,
+    layout: ProcLayout<'a>,
 ) {
     let ctx = Context::new(arena, param_map);
 
diff --git a/compiler/mono/src/ir.rs b/compiler/mono/src/ir.rs
index cbf8649aaf..a693052ab9 100644
--- a/compiler/mono/src/ir.rs
+++ b/compiler/mono/src/ir.rs
@@ -3,8 +3,8 @@
 use self::InProgressProc::*;
 use crate::exhaustive::{Ctor, Guard, RenderAs, TagId};
 use crate::layout::{
-    Builtin, ClosureRepresentation, LambdaSet, Layout, LayoutCache, LayoutProblem, MemoryMode,
-    UnionLayout, WrappedVariant, TAG_SIZE,
+    Builtin, ClosureRepresentation, LambdaSet, Layout, LayoutCache, LayoutProblem,
+    RawFunctionLayout, UnionLayout, WrappedVariant,
 };
 use bumpalo::collections::Vec;
 use bumpalo::Bump;
@@ -25,14 +25,22 @@ macro_rules! return_on_layout_error {
     ($env:expr, $layout_result:expr) => {
         match $layout_result {
             Ok(cached) => cached,
-            Err(LayoutProblem::UnresolvedTypeVar(_)) => {
+            Err(error) => return_on_layout_error_help!($env, error),
+        }
+    };
+}
+
+macro_rules! return_on_layout_error_help {
+    ($env:expr, $error:expr) => {
+        match $error {
+            LayoutProblem::UnresolvedTypeVar(_) => {
                 return Stmt::RuntimeError($env.arena.alloc(format!(
                     "UnresolvedTypeVar {} line {}",
                     file!(),
                     line!()
                 )));
             }
-            Err(LayoutProblem::Erroneous) => {
+            LayoutProblem::Erroneous => {
                 return Stmt::RuntimeError($env.arena.alloc(format!(
                     "Erroneous {} line {}",
                     file!(),
@@ -43,11 +51,23 @@ macro_rules! return_on_layout_error {
     };
 }
 
+#[derive(Debug, Clone, Copy)]
+pub enum OptLevel {
+    Normal,
+    Optimize,
+}
+
 #[derive(Clone, Debug, PartialEq)]
 pub enum MonoProblem {
     PatternProblem(crate::exhaustive::Error),
 }
 
+#[derive(Debug, Clone, Copy)]
+pub struct EntryPoint<'a> {
+    pub symbol: Symbol,
+    pub layout: ProcLayout<'a>,
+}
+
 #[derive(Clone, Debug, PartialEq)]
 pub struct PartialProc<'a> {
     pub annotation: Variable,
@@ -130,7 +150,7 @@ pub enum HostExposedLayouts<'a> {
     NotHostExposed,
     HostExposed {
         rigids: BumpMap<Lowercase, Layout<'a>>,
-        aliases: BumpMap<Symbol, (Symbol, TopLevelFunctionLayout<'a>, Layout<'a>)>,
+        aliases: BumpMap<Symbol, (Symbol, ProcLayout<'a>, RawFunctionLayout<'a>)>,
     },
 }
 
@@ -198,12 +218,12 @@ impl<'a> Proc<'a> {
 
     pub fn insert_refcount_operations(
         arena: &'a Bump,
-        procs: &mut MutMap<(Symbol, TopLevelFunctionLayout<'a>), Proc<'a>>,
+        procs: &mut MutMap<(Symbol, ProcLayout<'a>), Proc<'a>>,
     ) {
         let borrow_params = arena.alloc(crate::borrow::infer_borrow(arena, procs));
 
         for (key, proc) in procs.iter_mut() {
-            crate::inc_dec::visit_proc(arena, borrow_params, proc, arena.alloc(key.1).full());
+            crate::inc_dec::visit_proc(arena, borrow_params, proc, key.1);
         }
     }
 
@@ -287,8 +307,8 @@ pub struct Procs<'a> {
     pub imported_module_thunks: BumpSet<Symbol>,
     pub module_thunks: BumpSet<Symbol>,
     pub pending_specializations:
-        Option<BumpMap<Symbol, MutMap<TopLevelFunctionLayout<'a>, PendingSpecialization<'a>>>>,
-    pub specialized: BumpMap<(Symbol, TopLevelFunctionLayout<'a>), InProgressProc<'a>>,
+        Option<BumpMap<Symbol, MutMap<ProcLayout<'a>, PendingSpecialization<'a>>>>,
+    pub specialized: BumpMap<(Symbol, ProcLayout<'a>), InProgressProc<'a>>,
     pub runtime_errors: BumpMap<Symbol, &'a str>,
     pub call_by_pointer_wrappers: BumpMap<Symbol, Symbol>,
     pub externals_we_need: BumpMap<ModuleId, ExternalSpecializations<'a>>,
@@ -319,7 +339,7 @@ impl<'a> Procs<'a> {
     pub fn get_specialized_procs_without_rc(
         self,
         arena: &'a Bump,
-    ) -> MutMap<(Symbol, TopLevelFunctionLayout<'a>), Proc<'a>> {
+    ) -> MutMap<(Symbol, ProcLayout<'a>), Proc<'a>> {
         let mut result = MutMap::with_capacity_and_hasher(self.specialized.len(), default_hasher());
 
         let cloned = self.specialized.clone();
@@ -367,7 +387,7 @@ impl<'a> Procs<'a> {
     pub fn get_specialized_procs(
         self,
         arena: &'a Bump,
-    ) -> MutMap<(Symbol, TopLevelFunctionLayout<'a>), Proc<'a>> {
+    ) -> MutMap<(Symbol, ProcLayout<'a>), Proc<'a>> {
         let mut result = MutMap::with_capacity_and_hasher(self.specialized.len(), default_hasher());
 
         for ((s, toplevel), in_prog_proc) in self.specialized.into_iter() {
@@ -398,8 +418,7 @@ impl<'a> Procs<'a> {
         let borrow_params = arena.alloc(crate::borrow::infer_borrow(arena, &result));
 
         for (key, proc) in result.iter_mut() {
-            let layout = arena.alloc(key.1).full();
-            crate::inc_dec::visit_proc(arena, borrow_params, proc, layout);
+            crate::inc_dec::visit_proc(arena, borrow_params, proc, key.1);
         }
 
         result
@@ -409,7 +428,7 @@ impl<'a> Procs<'a> {
         self,
         arena: &'a Bump,
     ) -> (
-        MutMap<(Symbol, TopLevelFunctionLayout<'a>), Proc<'a>>,
+        MutMap<(Symbol, ProcLayout<'a>), Proc<'a>>,
         &'a crate::borrow::ParamMap<'a>,
     ) {
         let mut result = MutMap::with_capacity_and_hasher(self.specialized.len(), default_hasher());
@@ -442,8 +461,7 @@ impl<'a> Procs<'a> {
         let borrow_params = arena.alloc(crate::borrow::infer_borrow(arena, &result));
 
         for (key, proc) in result.iter_mut() {
-            let layout = arena.alloc(key.1).full();
-            crate::inc_dec::visit_proc(arena, borrow_params, proc, layout);
+            crate::inc_dec::visit_proc(arena, borrow_params, proc, key.1);
         }
 
         (result, borrow_params)
@@ -517,7 +535,7 @@ impl<'a> Procs<'a> {
         captured_symbols: CapturedSymbols<'a>,
         ret_var: Variable,
         layout_cache: &mut LayoutCache<'a>,
-    ) -> Result<TopLevelFunctionLayout<'a>, RuntimeError> {
+    ) -> Result<ProcLayout<'a>, RuntimeError> {
         // anonymous functions cannot reference themselves, therefore cannot be tail-recursive
         let is_self_recursive = false;
 
@@ -525,7 +543,7 @@ impl<'a> Procs<'a> {
             .from_var(env.arena, annotation, env.subs)
             .unwrap_or_else(|err| panic!("TODO turn fn_var into a RuntimeError {:?}", err));
 
-        let top_level = TopLevelFunctionLayout::from_layout(env.arena, layout);
+        let top_level = ProcLayout::from_layout(env.arena, layout);
 
         match patterns_to_when(env, layout_cache, loc_args, ret_var, loc_body) {
             Ok((_, pattern_symbols, body)) => {
@@ -590,8 +608,7 @@ impl<'a> Procs<'a> {
                             match specialize(env, self, symbol, layout_cache, pending, partial_proc)
                             {
                                 Ok((proc, layout)) => {
-                                    let top_level =
-                                        TopLevelFunctionLayout::from_layout(env.arena, layout);
+                                    let top_level = ProcLayout::from_raw(env.arena, layout);
 
                                     debug_assert_eq!(outside_layout, top_level);
 
@@ -619,7 +636,7 @@ impl<'a> Procs<'a> {
     pub fn insert_exposed(
         &mut self,
         name: Symbol,
-        layout: TopLevelFunctionLayout<'a>,
+        layout: ProcLayout<'a>,
         arena: &'a Bump,
         subs: &Subs,
         opt_annotation: Option<roc_can::def::Annotation>,
@@ -663,7 +680,7 @@ impl<'a> Procs<'a> {
         env: &mut Env<'a, '_>,
         fn_var: Variable,
         name: Symbol,
-        layout: TopLevelFunctionLayout<'a>,
+        layout: ProcLayout<'a>,
         layout_cache: &mut LayoutCache<'a>,
     ) {
         let tuple = (name, layout);
@@ -727,10 +744,10 @@ impl<'a> Procs<'a> {
 fn add_pending<'a>(
     pending_specializations: &mut BumpMap<
         Symbol,
-        MutMap<TopLevelFunctionLayout<'a>, PendingSpecialization<'a>>,
+        MutMap<ProcLayout<'a>, PendingSpecialization<'a>>,
     >,
     symbol: Symbol,
-    layout: TopLevelFunctionLayout<'a>,
+    layout: ProcLayout<'a>,
     pending: PendingSpecialization<'a>,
 ) {
     let all_pending = pending_specializations
@@ -855,7 +872,11 @@ pub enum Stmt<'a> {
         layout: Layout<'a>,
         pass: &'a Stmt<'a>,
         fail: &'a Stmt<'a>,
+        exception_id: ExceptionId,
     },
+    /// after cleanup, rethrow the exception object (stored in the exception id)
+    /// so it bubbles up
+    Resume(ExceptionId),
     Switch {
         /// This *must* stand for an integer, because Switch potentially compiles to a jump table.
         cond_symbol: Symbol,
@@ -869,15 +890,15 @@ pub enum Stmt<'a> {
         ret_layout: Layout<'a>,
     },
     Ret(Symbol),
-    Rethrow,
     Refcounting(ModifyRc, &'a Stmt<'a>),
     /// a join point `join f <params> = <continuation> in remainder`
     Join {
         id: JoinPointId,
         parameters: &'a [Param<'a>],
-        /// does not contain jumps to this id
-        continuation: &'a Stmt<'a>,
-        /// the "body" of the join point, contains the jumps to this id
+        /// body of the join point
+        /// what happens after _jumping to_ the join point
+        body: &'a Stmt<'a>,
+        /// what happens after _defining_ the join point
         remainder: &'a Stmt<'a>,
     },
     Jump(JoinPointId, &'a [Symbol]),
@@ -916,7 +937,14 @@ impl<'a> BranchInfo<'a> {
                 .append(", tag_id: ")
                 .append(format!("{}", tag_id))
                 .append("} "),
-            _ => alloc.text(""),
+
+            _ => {
+                if PRETTY_PRINT_IR_SYMBOLS {
+                    alloc.text(" <no branch info>")
+                } else {
+                    alloc.text("")
+                }
+            }
         }
     }
 }
@@ -929,9 +957,9 @@ pub enum ModifyRc {
 }
 
 impl ModifyRc {
-    pub fn to_doc<'b, D, A>(&'b self, alloc: &'b D) -> DocBuilder<'b, D, A>
+    pub fn to_doc<'a, D, A>(self, alloc: &'a D) -> DocBuilder<'a, D, A>
     where
-        D: DocAllocator<'b, A>,
+        D: DocAllocator<'a, A>,
         D::Doc: Clone,
         A: Clone,
     {
@@ -940,20 +968,20 @@ impl ModifyRc {
         match self {
             Inc(symbol, 1) => alloc
                 .text("inc ")
-                .append(symbol_to_doc(alloc, *symbol))
+                .append(symbol_to_doc(alloc, symbol))
                 .append(";"),
             Inc(symbol, n) => alloc
                 .text("inc ")
                 .append(alloc.text(format!("{}", n)))
-                .append(symbol_to_doc(alloc, *symbol))
+                .append(symbol_to_doc(alloc, symbol))
                 .append(";"),
             Dec(symbol) => alloc
                 .text("dec ")
-                .append(symbol_to_doc(alloc, *symbol))
+                .append(symbol_to_doc(alloc, symbol))
                 .append(";"),
             DecRef(symbol) => alloc
                 .text("decref ")
-                .append(symbol_to_doc(alloc, *symbol))
+                .append(symbol_to_doc(alloc, symbol))
                 .append(";"),
         }
     }
@@ -986,55 +1014,6 @@ pub enum Literal<'a> {
     Byte(u8),
 }
 
-#[derive(Clone, Copy, Debug, PartialEq)]
-pub enum Wrapped {
-    EmptyRecord,
-    SingleElementRecord,
-    RecordOrSingleTagUnion,
-    MultiTagUnion,
-}
-
-impl Wrapped {
-    pub fn from_layout(layout: &Layout<'_>) -> Self {
-        match Self::opt_from_layout(layout) {
-            Some(result) => result,
-            None => unreachable!("not an indexable type {:?}", layout),
-        }
-    }
-
-    pub fn opt_from_layout(layout: &Layout<'_>) -> Option<Self> {
-        match layout {
-            Layout::Struct(fields) => match fields.len() {
-                0 => Some(Wrapped::EmptyRecord),
-                1 => Some(Wrapped::SingleElementRecord),
-                _ => Some(Wrapped::RecordOrSingleTagUnion),
-            },
-
-            Layout::Union(variant) => {
-                use UnionLayout::*;
-
-                match variant {
-                    Recursive(tags) | NonRecursive(tags) => match tags {
-                        [] => todo!("how to handle empty tag unions?"),
-                        [single] => match single.len() {
-                            0 => Some(Wrapped::EmptyRecord),
-                            1 => Some(Wrapped::SingleElementRecord),
-                            _ => Some(Wrapped::RecordOrSingleTagUnion),
-                        },
-                        _ => Some(Wrapped::MultiTagUnion),
-                    },
-                    NonNullableUnwrapped(_) => Some(Wrapped::RecordOrSingleTagUnion),
-
-                    NullableWrapped { .. } | NullableUnwrapped { .. } => {
-                        Some(Wrapped::MultiTagUnion)
-                    }
-                }
-            }
-            _ => None,
-        }
-    }
-}
-
 #[derive(Clone, Debug, PartialEq)]
 pub struct Call<'a> {
     pub call_type: CallType<'a>,
@@ -1113,7 +1092,6 @@ impl UpdateModeId {
 pub enum CallType<'a> {
     ByName {
         name: Symbol,
-        full_layout: Layout<'a>,
         ret_layout: Layout<'a>,
         arg_layouts: &'a [Layout<'a>],
         specialization_id: CallSpecId,
@@ -1129,9 +1107,14 @@ pub enum CallType<'a> {
     HigherOrderLowLevel {
         op: LowLevel,
         /// the layout of the closure argument, if any
-        closure_layout: Layout<'a>,
+        closure_env_layout: Option<Layout<'a>>,
+        /// specialization id of the function argument
+        specialization_id: CallSpecId,
         /// does the function need to own the closure data
         function_owns_closure_data: bool,
+        /// function layout
+        arg_layouts: &'a [Layout<'a>],
+        ret_layout: Layout<'a>,
     },
 }
 
@@ -1143,7 +1126,7 @@ pub enum Expr<'a> {
     Call(Call<'a>),
 
     Tag {
-        tag_layout: Layout<'a>,
+        tag_layout: UnionLayout<'a>,
         tag_name: TagName,
         tag_id: u8,
         union_size: u8,
@@ -1151,11 +1134,22 @@ pub enum Expr<'a> {
     },
     Struct(&'a [Symbol]),
 
-    AccessAtIndex {
+    StructAtIndex {
         index: u64,
         field_layouts: &'a [Layout<'a>],
         structure: Symbol,
-        wrapped: Wrapped,
+    },
+
+    GetTagId {
+        structure: Symbol,
+        union_layout: UnionLayout<'a>,
+    },
+
+    UnionAtIndex {
+        structure: Symbol,
+        tag_id: u8,
+        union_layout: UnionLayout<'a>,
+        index: u64,
     },
 
     Array {
@@ -1303,13 +1297,26 @@ impl<'a> Expr<'a> {
             }
             EmptyArray => alloc.text("Array []"),
 
-            AccessAtIndex {
+            StructAtIndex {
                 index, structure, ..
             } => alloc
-                .text(format!("Index {} ", index))
+                .text(format!("StructAtIndex {} ", index))
                 .append(symbol_to_doc(alloc, *structure)),
 
             RuntimeErrorFunction(s) => alloc.text(format!("ErrorFunction {}", s)),
+
+            GetTagId { structure, .. } => alloc
+                .text("GetTagId ")
+                .append(symbol_to_doc(alloc, *structure)),
+
+            UnionAtIndex {
+                tag_id,
+                structure,
+                index,
+                ..
+            } => alloc
+                .text(format!("UnionAtIndex (Id {}) (Index {}) ", tag_id, index))
+                .append(symbol_to_doc(alloc, *structure)),
         }
     }
 }
@@ -1354,7 +1361,7 @@ impl<'a> Stmt<'a> {
                 symbol,
                 call,
                 pass,
-                fail: Stmt::Rethrow,
+                fail: Stmt::Resume(_),
                 ..
             } => alloc
                 .text("let ")
@@ -1387,7 +1394,7 @@ impl<'a> Stmt<'a> {
                 .append(symbol_to_doc(alloc, *symbol))
                 .append(";"),
 
-            Rethrow => alloc.text("unreachable;"),
+            Resume(_) => alloc.text("unreachable;"),
 
             Switch {
                 cond_symbol,
@@ -1449,7 +1456,7 @@ impl<'a> Stmt<'a> {
             Join {
                 id,
                 parameters,
-                continuation,
+                body: continuation,
                 remainder,
             } => {
                 let it = parameters.iter().map(|p| symbol_to_doc(alloc, p.symbol));
@@ -1757,10 +1764,15 @@ pub fn specialize_all<'a>(
                 ) {
                     Ok((proc, layout)) => {
                         // TODO thiscode is duplicated elsewhere
-                        let top_level = TopLevelFunctionLayout::from_layout(env.arena, layout);
+                        let top_level = ProcLayout::from_raw(env.arena, layout);
 
                         if procs.module_thunks.contains(&proc.name) {
-                            debug_assert!(top_level.arguments.is_empty(), "{:?}", name);
+                            debug_assert!(
+                                top_level.arguments.is_empty(),
+                                "{:?} from {:?}",
+                                name,
+                                layout
+                            );
                         }
 
                         debug_assert_eq!(outside_layout, top_level, " in {:?}", name);
@@ -1771,8 +1783,8 @@ pub fn specialize_all<'a>(
                     }) => {
                         let proc = generate_runtime_error_function(env, name, attempted_layout);
 
-                        let top_level =
-                            TopLevelFunctionLayout::from_layout(env.arena, attempted_layout);
+                        let top_level = ProcLayout::from_raw(env.arena, attempted_layout);
+
                         procs.specialized.insert((name, top_level), Done(proc));
                     }
                 }
@@ -1832,7 +1844,7 @@ fn specialize_all_help<'a>(
             partial_proc,
         ) {
             Ok((proc, layout)) => {
-                let top_level = TopLevelFunctionLayout::from_layout(env.arena, layout);
+                let top_level = ProcLayout::from_raw(env.arena, layout);
 
                 if procs.module_thunks.contains(&name) {
                     debug_assert!(top_level.arguments.is_empty());
@@ -1846,7 +1858,8 @@ fn specialize_all_help<'a>(
             }) => {
                 let proc = generate_runtime_error_function(env, name, attempted_layout);
 
-                let top_level = TopLevelFunctionLayout::from_layout(env.arena, attempted_layout);
+                let top_level = ProcLayout::from_raw(env.arena, attempted_layout);
+
                 procs.specialized.insert((name, top_level), Done(proc));
             }
         }
@@ -1856,19 +1869,8 @@ fn specialize_all_help<'a>(
 fn generate_runtime_error_function<'a>(
     env: &mut Env<'a, '_>,
     name: Symbol,
-    layout: Layout<'a>,
+    layout: RawFunctionLayout<'a>,
 ) -> Proc<'a> {
-    let (arg_layouts, ret_layout) = match layout {
-        Layout::FunctionPointer(a, r) => (a, *r),
-        _ => (&[] as &[_], layout),
-    };
-
-    let mut args = Vec::with_capacity_in(arg_layouts.len(), env.arena);
-
-    for arg in arg_layouts {
-        args.push((*arg, env.unique_symbol()));
-    }
-
     let mut msg = bumpalo::collections::string::String::with_capacity_in(80, env.arena);
     use std::fmt::Write;
     write!(
@@ -1882,9 +1884,24 @@ fn generate_runtime_error_function<'a>(
 
     let runtime_error = Stmt::RuntimeError(msg.into_bump_str());
 
+    let (args, ret_layout) = match layout {
+        RawFunctionLayout::Function(arg_layouts, lambda_set, ret_layout) => {
+            let mut args = Vec::with_capacity_in(arg_layouts.len(), env.arena);
+
+            for arg in arg_layouts {
+                args.push((*arg, env.unique_symbol()));
+            }
+
+            args.push((lambda_set.runtime_representation(), Symbol::ARG_CLOSURE));
+
+            (args.into_bump_slice(), *ret_layout)
+        }
+        RawFunctionLayout::ZeroArgumentThunk(ret_layout) => (&[] as &[_], ret_layout),
+    };
+
     Proc {
         name,
-        args: args.into_bump_slice(),
+        args,
         body: runtime_error,
         closure_data_layout: None,
         ret_layout,
@@ -1943,13 +1960,13 @@ fn specialize_external<'a>(
 
         for (symbol, variable) in host_exposed_variables {
             let layout = layout_cache
-                .from_var(env.arena, *variable, env.subs)
+                .raw_from_var(env.arena, *variable, env.subs)
                 .unwrap();
 
             let name = env.unique_symbol();
 
             match layout {
-                Layout::Closure(argument_layouts, lambda_set, return_layout) => {
+                RawFunctionLayout::Function(argument_layouts, lambda_set, return_layout) => {
                     let assigned = env.unique_symbol();
                     let unit = env.unique_symbol();
 
@@ -1981,10 +1998,11 @@ fn specialize_external<'a>(
                         host_exposed_layouts: HostExposedLayouts::NotHostExposed,
                     };
 
-                    let top_level = TopLevelFunctionLayout {
-                        arguments: env.arena.alloc([lambda_set.runtime_representation()]),
-                        result: *return_layout,
-                    };
+                    let top_level = ProcLayout::new(
+                        env.arena,
+                        env.arena.alloc([lambda_set.runtime_representation()]),
+                        *return_layout,
+                    );
 
                     procs
                         .specialized
@@ -1992,7 +2010,7 @@ fn specialize_external<'a>(
 
                     aliases.insert(*symbol, (name, top_level, layout));
                 }
-                _ => todo!(),
+                RawFunctionLayout::ZeroArgumentThunk(_) => unreachable!("so far"),
             }
         }
 
@@ -2058,26 +2076,24 @@ fn specialize_external<'a>(
                 (Some(closure_layout), CapturedSymbols::Captured(captured)) => {
                     // debug_assert!(!captured.is_empty());
 
-                    let wrapped = closure_layout.get_wrapped();
-
                     match closure_layout.layout_for_member(proc_name) {
                         ClosureRepresentation::Union {
                             tag_layout: field_layouts,
+                            union_layout,
+                            tag_id,
                             ..
                         } => {
-                            debug_assert_eq!(field_layouts.len() - 1, captured.len());
-                            // TODO check for field_layouts.len() == 1 and do a rename in that case?
-                            for (mut index, (symbol, _variable)) in captured.iter().enumerate() {
-                                // the field layouts do store the tag, but the tag value is
-                                // not captured. So we drop the layout of the tag ID here
-                                index += 1;
+                            debug_assert!(matches!(union_layout, UnionLayout::NonRecursive(_)));
+                            debug_assert_eq!(field_layouts.len(), captured.len());
 
-                                // TODO therefore should the wrapped here not be RecordOrSingleTagUnion?
-                                let expr = Expr::AccessAtIndex {
-                                    index: index as _,
-                                    field_layouts,
+                            for (index, (symbol, _variable)) in captured.iter().enumerate() {
+                                let expr = Expr::UnionAtIndex {
+                                    tag_id,
                                     structure: Symbol::ARG_CLOSURE,
-                                    wrapped,
+                                    // union at index still expects the index to be +1; it thinks
+                                    // the tag id is stored
+                                    index: index as u64,
+                                    union_layout,
                                 };
 
                                 let layout = field_layouts[index];
@@ -2102,11 +2118,10 @@ fn specialize_external<'a>(
                                 );
 
                                 for (index, (symbol, _variable)) in captured.iter().enumerate() {
-                                    let expr = Expr::AccessAtIndex {
+                                    let expr = Expr::StructAtIndex {
                                         index: index as _,
                                         field_layouts,
                                         structure: Symbol::ARG_CLOSURE,
-                                        wrapped: Wrapped::RecordOrSingleTagUnion,
                                     };
 
                                     let layout = field_layouts[index];
@@ -2196,11 +2211,8 @@ fn build_specialized_proc_from_var<'a>(
     pattern_symbols: &[Symbol],
     fn_var: Variable,
 ) -> Result<SpecializedLayout<'a>, LayoutProblem> {
-    match layout_cache.from_var(env.arena, fn_var, env.subs) {
-        Ok(Layout::FunctionPointer(_, _)) => {
-            unreachable!(r"layout generated by from_var should never by a function pointer")
-        }
-        Ok(Layout::Closure(pattern_layouts, closure_layout, ret_layout)) => {
+    match layout_cache.raw_from_var(env.arena, fn_var, env.subs)? {
+        RawFunctionLayout::Function(pattern_layouts, closure_layout, ret_layout) => {
             let mut pattern_layouts_vec = Vec::with_capacity_in(pattern_layouts.len(), env.arena);
             pattern_layouts_vec.extend_from_slice(pattern_layouts);
 
@@ -2213,72 +2225,19 @@ fn build_specialized_proc_from_var<'a>(
                 *ret_layout,
             )
         }
-        _ => {
-            match env.subs.get_without_compacting(fn_var).content {
-                Content::Structure(FlatType::Func(pattern_vars, closure_var, ret_var)) => {
-                    let closure_layout = LambdaSet::from_var(env.arena, env.subs, closure_var)?;
-                    build_specialized_proc_adapter(
-                        env,
-                        layout_cache,
-                        proc_name,
-                        pattern_symbols,
-                        &pattern_vars,
-                        Some(closure_layout),
-                        ret_var,
-                    )
-                }
-                Content::Alias(_, _, actual) => build_specialized_proc_from_var(
-                    env,
-                    layout_cache,
-                    proc_name,
-                    pattern_symbols,
-                    actual,
-                ),
-                _ => {
-                    // a top-level constant 0-argument thunk
-                    build_specialized_proc_adapter(
-                        env,
-                        layout_cache,
-                        proc_name,
-                        pattern_symbols,
-                        &[],
-                        None,
-                        fn_var,
-                    )
-                }
-            }
+        RawFunctionLayout::ZeroArgumentThunk(ret_layout) => {
+            // a top-level constant 0-argument thunk
+            build_specialized_proc(
+                env.arena,
+                proc_name,
+                pattern_symbols,
+                Vec::new_in(env.arena),
+                None,
+                ret_layout,
+            )
         }
     }
 }
-#[allow(clippy::type_complexity)]
-fn build_specialized_proc_adapter<'a>(
-    env: &mut Env<'a, '_>,
-    layout_cache: &mut LayoutCache<'a>,
-    proc_name: Symbol,
-    pattern_symbols: &[Symbol],
-    pattern_vars: &[Variable],
-    closure_layout: Option<LambdaSet<'a>>,
-    ret_var: Variable,
-) -> Result<SpecializedLayout<'a>, LayoutProblem> {
-    let mut arg_layouts = Vec::with_capacity_in(pattern_vars.len(), &env.arena);
-
-    for arg_var in pattern_vars {
-        let layout = layout_cache.from_var(&env.arena, *arg_var, env.subs)?;
-
-        arg_layouts.push(layout);
-    }
-
-    let ret_layout = layout_cache.from_var(&env.arena, ret_var, env.subs)?;
-
-    build_specialized_proc(
-        env.arena,
-        proc_name,
-        pattern_symbols,
-        arg_layouts,
-        closure_layout,
-        ret_layout,
-    )
-}
 
 #[allow(clippy::type_complexity)]
 fn build_specialized_proc<'a>(
@@ -2423,11 +2382,13 @@ fn build_specialized_proc<'a>(
 #[derive(Debug)]
 struct SpecializeFailure<'a> {
     /// The layout we attempted to create
-    attempted_layout: Layout<'a>,
+    attempted_layout: RawFunctionLayout<'a>,
     /// The problem we ran into while creating it
     problem: LayoutProblem,
 }
 
+type SpecializeSuccess<'a> = (Proc<'a>, RawFunctionLayout<'a>);
+
 fn specialize<'a>(
     env: &mut Env<'a, '_>,
     procs: &mut Procs<'a>,
@@ -2435,7 +2396,7 @@ fn specialize<'a>(
     layout_cache: &mut LayoutCache<'a>,
     pending: PendingSpecialization,
     partial_proc: PartialProc<'a>,
-) -> Result<(Proc<'a>, Layout<'a>), SpecializeFailure<'a>> {
+) -> Result<SpecializeSuccess<'a>, SpecializeFailure<'a>> {
     let PendingSpecialization {
         solved_type,
         host_exposed_aliases,
@@ -2480,7 +2441,7 @@ fn specialize_solved_type<'a>(
     solved_type: SolvedType,
     host_exposed_aliases: BumpMap<Symbol, SolvedType>,
     partial_proc: PartialProc<'a>,
-) -> Result<(Proc<'a>, Layout<'a>), SpecializeFailure<'a>> {
+) -> Result<SpecializeSuccess<'a>, SpecializeFailure<'a>> {
     // add the specializations that other modules require of us
     use roc_solve::solve::instantiate_rigids;
 
@@ -2489,10 +2450,22 @@ fn specialize_solved_type<'a>(
 
     let fn_var = introduce_solved_type_to_subs(env, &solved_type);
 
+    // for debugging only
     let attempted_layout = layout_cache
         .from_var(&env.arena, fn_var, env.subs)
         .unwrap_or_else(|err| panic!("TODO handle invalid function {:?}", err));
 
+    let raw = match attempted_layout {
+        Layout::Closure(a, lambda_set, c) => {
+            if procs.module_thunks.contains(&proc_name) {
+                RawFunctionLayout::ZeroArgumentThunk(lambda_set.runtime_representation())
+            } else {
+                RawFunctionLayout::Function(a, lambda_set, c)
+            }
+        }
+        _ => RawFunctionLayout::ZeroArgumentThunk(attempted_layout),
+    };
+
     // make sure rigid variables in the annotation are converted to flex variables
     instantiate_rigids(env.subs, partial_proc.annotation);
 
@@ -2527,10 +2500,7 @@ fn specialize_solved_type<'a>(
             env.subs.rollback_to(snapshot);
             layout_cache.rollback_to(cache_snapshot);
 
-            Ok((
-                proc,
-                cleanup_attempted_type(env, procs, proc_name, attempted_layout),
-            ))
+            Ok((proc, raw))
         }
         Err(error) => {
             env.subs.rollback_to(snapshot);
@@ -2538,32 +2508,19 @@ fn specialize_solved_type<'a>(
 
             Err(SpecializeFailure {
                 problem: error,
-                attempted_layout: cleanup_attempted_type(env, procs, proc_name, attempted_layout),
+                attempted_layout: raw,
             })
         }
     }
 }
 
-fn cleanup_attempted_type<'a>(
-    env: &mut Env<'a, '_>,
-    procs: &mut Procs<'a>,
-    proc_name: Symbol,
-    layout: Layout<'a>,
-) -> Layout<'a> {
-    if procs.module_thunks.contains(&proc_name) {
-        Layout::FunctionPointer(&[], env.arena.alloc(layout))
-    } else {
-        layout
-    }
-}
-
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
-pub struct TopLevelFunctionLayout<'a> {
-    arguments: &'a [Layout<'a>],
-    result: Layout<'a>,
+pub struct ProcLayout<'a> {
+    pub arguments: &'a [Layout<'a>],
+    pub result: Layout<'a>,
 }
 
-impl<'a> TopLevelFunctionLayout<'a> {
+impl<'a> ProcLayout<'a> {
     pub fn new(arena: &'a Bump, old_arguments: &'a [Layout<'a>], result: Layout<'a>) -> Self {
         let mut arguments = Vec::with_capacity_in(old_arguments.len(), arena);
 
@@ -2577,29 +2534,37 @@ impl<'a> TopLevelFunctionLayout<'a> {
             }
         }
 
-        TopLevelFunctionLayout {
+        let new_result = match result {
+            Layout::Closure(_, lambda_set, _) => lambda_set.runtime_representation(),
+            other => other,
+        };
+
+        ProcLayout {
             arguments: arguments.into_bump_slice(),
-            result,
+            result: new_result,
         }
     }
     pub fn from_layout(arena: &'a Bump, layout: Layout<'a>) -> Self {
         match layout {
-            Layout::FunctionPointer(old_arguments, result) => {
-                Self::new(arena, old_arguments, *result)
-            }
             Layout::Closure(arguments, lambda_set, result) => {
-                let full = lambda_set.extend_function_layout(arena, arguments, result);
-                TopLevelFunctionLayout::from_layout(arena, full)
+                let arguments = lambda_set.extend_argument_list(arena, arguments);
+                ProcLayout::new(arena, arguments, *result)
             }
-            _ => TopLevelFunctionLayout {
+            _ => ProcLayout {
                 arguments: &[],
                 result: layout,
             },
         }
     }
 
-    pub fn full(&'a self) -> Layout<'a> {
-        Layout::FunctionPointer(self.arguments, &self.result)
+    fn from_raw(arena: &'a Bump, raw: RawFunctionLayout<'a>) -> Self {
+        match raw {
+            RawFunctionLayout::Function(arguments, lambda_set, result) => {
+                let arguments = lambda_set.extend_argument_list(arena, arguments);
+                ProcLayout::new(arena, arguments, *result)
+            }
+            RawFunctionLayout::ZeroArgumentThunk(result) => ProcLayout::new(arena, &[], result),
+        }
     }
 }
 
@@ -2632,9 +2597,6 @@ fn specialize_naked_symbol<'a>(
     } else if env.is_imported_symbol(symbol) {
         match layout_cache.from_var(env.arena, variable, env.subs) {
             Err(e) => panic!("invalid layout {:?}", e),
-            Ok(Layout::FunctionPointer(_, _)) => {
-                unreachable!(r"layout generated by from_var should never by a function pointer")
-            }
             Ok(_) => {
                 // this is a 0-arity thunk
                 let result = call_by_name(
@@ -2678,34 +2640,39 @@ macro_rules! match_on_closure_argument {
     ($env:expr, $procs:expr, $layout_cache:expr, $closure_data_symbol:expr, $closure_data_var:expr, $op:expr, [$($x:expr),* $(,)?], $layout: expr, $assigned:expr, $hole:expr) => {{
         let closure_data_layout = return_on_layout_error!(
             $env,
-            $layout_cache.from_var($env.arena, $closure_data_var, $env.subs)
+            $layout_cache.raw_from_var($env.arena, $closure_data_var, $env.subs)
         );
 
-        let top_level = TopLevelFunctionLayout::from_layout($env.arena, closure_data_layout);
+        let top_level = ProcLayout::from_raw($env.arena, closure_data_layout);
 
         let arena = $env.arena;
 
+        let arg_layouts = top_level.arguments;
+        let ret_layout = top_level.result;
+
         match closure_data_layout {
-            Layout::Closure(_, lambda_set, _) => {
+            RawFunctionLayout::Function(_, lambda_set, _) =>  {
                 lowlevel_match_on_lambda_set(
                     $env,
                     lambda_set,
                     $closure_data_symbol,
-                    |top_level_function, closure_data, function_layout| self::Call {
+                    |top_level_function, closure_data, closure_env_layout, specialization_id| self::Call {
                         call_type: CallType::HigherOrderLowLevel {
                             op: $op,
-                            closure_layout: function_layout,
-                            function_owns_closure_data: false
+                            closure_env_layout,
+                            specialization_id,
+                            function_owns_closure_data: false,
+                            arg_layouts,
+                            ret_layout,
                         },
                         arguments: arena.alloc([$($x,)* top_level_function, closure_data]),
                     },
-                    arena.alloc(top_level).full(),
                     $layout,
                     $assigned,
                     $hole,
                 )
             }
-            _ => unreachable!(),
+            RawFunctionLayout::ZeroArgumentThunk(_) => unreachable!(),
         }
     }};
 }
@@ -3183,118 +3150,123 @@ pub fn with_hole<'a>(
             branches,
             final_else,
         } => {
-            let ret_layout = layout_cache
-                .from_var(env.arena, branch_var, env.subs)
-                .expect("invalid ret_layout");
-            let cond_layout = layout_cache
-                .from_var(env.arena, cond_var, env.subs)
-                .expect("invalid cond_layout");
+            match (
+                layout_cache.from_var(env.arena, branch_var, env.subs),
+                layout_cache.from_var(env.arena, cond_var, env.subs),
+            ) {
+                (Ok(ret_layout), Ok(cond_layout)) => {
+                    // if the hole is a return, then we don't need to merge the two
+                    // branches together again, we can just immediately return
+                    let is_terminated = matches!(hole, Stmt::Ret(_));
 
-            // if the hole is a return, then we don't need to merge the two
-            // branches together again, we can just immediately return
-            let is_terminated = matches!(hole, Stmt::Ret(_));
+                    if is_terminated {
+                        let terminator = hole;
 
-            if is_terminated {
-                let terminator = hole;
+                        let mut stmt = with_hole(
+                            env,
+                            final_else.value,
+                            branch_var,
+                            procs,
+                            layout_cache,
+                            assigned,
+                            terminator,
+                        );
 
-                let mut stmt = with_hole(
-                    env,
-                    final_else.value,
-                    branch_var,
-                    procs,
-                    layout_cache,
-                    assigned,
-                    terminator,
-                );
+                        for (loc_cond, loc_then) in branches.into_iter().rev() {
+                            let branching_symbol = env.unique_symbol();
 
-                for (loc_cond, loc_then) in branches.into_iter().rev() {
-                    let branching_symbol = env.unique_symbol();
+                            let then = with_hole(
+                                env,
+                                loc_then.value,
+                                branch_var,
+                                procs,
+                                layout_cache,
+                                assigned,
+                                terminator,
+                            );
 
-                    let then = with_hole(
-                        env,
-                        loc_then.value,
-                        branch_var,
-                        procs,
-                        layout_cache,
-                        assigned,
-                        terminator,
-                    );
+                            stmt = cond(env, branching_symbol, cond_layout, then, stmt, ret_layout);
 
-                    stmt = cond(env, branching_symbol, cond_layout, then, stmt, ret_layout);
+                            // add condition
+                            stmt = with_hole(
+                                env,
+                                loc_cond.value,
+                                cond_var,
+                                procs,
+                                layout_cache,
+                                branching_symbol,
+                                env.arena.alloc(stmt),
+                            );
+                        }
+                        stmt
+                    } else {
+                        let assigned_in_jump = env.unique_symbol();
+                        let id = JoinPointId(env.unique_symbol());
 
-                    // add condition
-                    stmt = with_hole(
-                        env,
-                        loc_cond.value,
-                        cond_var,
-                        procs,
-                        layout_cache,
-                        branching_symbol,
-                        env.arena.alloc(stmt),
-                    );
-                }
-                stmt
-            } else {
-                let assigned_in_jump = env.unique_symbol();
-                let id = JoinPointId(env.unique_symbol());
-
-                let terminator = env
-                    .arena
-                    .alloc(Stmt::Jump(id, env.arena.alloc([assigned_in_jump])));
-
-                let mut stmt = with_hole(
-                    env,
-                    final_else.value,
-                    branch_var,
-                    procs,
-                    layout_cache,
-                    assigned_in_jump,
-                    terminator,
-                );
-
-                for (loc_cond, loc_then) in branches.into_iter().rev() {
-                    let branching_symbol = possible_reuse_symbol(env, procs, &loc_cond.value);
-
-                    let then = with_hole(
-                        env,
-                        loc_then.value,
-                        branch_var,
-                        procs,
-                        layout_cache,
-                        assigned_in_jump,
-                        terminator,
-                    );
-
-                    stmt = cond(env, branching_symbol, cond_layout, then, stmt, ret_layout);
-
-                    // add condition
-                    stmt = assign_to_symbol(
-                        env,
-                        procs,
-                        layout_cache,
-                        cond_var,
-                        loc_cond,
-                        branching_symbol,
-                        stmt,
-                    );
-                }
-
-                let layout = layout_cache
-                    .from_var(env.arena, branch_var, env.subs)
-                    .unwrap_or_else(|err| panic!("TODO turn fn_var into a RuntimeError {:?}", err));
-
-                let param = Param {
-                    symbol: assigned,
-                    layout,
-                    borrow: false,
-                };
-
-                Stmt::Join {
-                    id,
-                    parameters: env.arena.alloc([param]),
-                    remainder: env.arena.alloc(stmt),
-                    continuation: hole,
+                        let terminator = env
+                            .arena
+                            .alloc(Stmt::Jump(id, env.arena.alloc([assigned_in_jump])));
+
+                        let mut stmt = with_hole(
+                            env,
+                            final_else.value,
+                            branch_var,
+                            procs,
+                            layout_cache,
+                            assigned_in_jump,
+                            terminator,
+                        );
+
+                        for (loc_cond, loc_then) in branches.into_iter().rev() {
+                            let branching_symbol =
+                                possible_reuse_symbol(env, procs, &loc_cond.value);
+
+                            let then = with_hole(
+                                env,
+                                loc_then.value,
+                                branch_var,
+                                procs,
+                                layout_cache,
+                                assigned_in_jump,
+                                terminator,
+                            );
+
+                            stmt = cond(env, branching_symbol, cond_layout, then, stmt, ret_layout);
+
+                            // add condition
+                            stmt = assign_to_symbol(
+                                env,
+                                procs,
+                                layout_cache,
+                                cond_var,
+                                loc_cond,
+                                branching_symbol,
+                                stmt,
+                            );
+                        }
+
+                        let layout = layout_cache
+                            .from_var(env.arena, branch_var, env.subs)
+                            .unwrap_or_else(|err| {
+                                panic!("TODO turn fn_var into a RuntimeError {:?}", err)
+                            });
+
+                        let param = Param {
+                            symbol: assigned,
+                            layout,
+                            borrow: false,
+                        };
+
+                        Stmt::Join {
+                            id,
+                            parameters: env.arena.alloc([param]),
+                            remainder: env.arena.alloc(stmt),
+                            body: hole,
+                        }
+                    }
                 }
+                (Err(_), _) => Stmt::RuntimeError("invalid ret_layout"),
+                (_, Err(_)) => Stmt::RuntimeError("invalid cond_layout"),
             }
         }
 
@@ -3346,7 +3318,7 @@ pub fn with_hole<'a>(
                 id,
                 parameters: env.arena.alloc([param]),
                 remainder: env.arena.alloc(stmt),
-                continuation: env.arena.alloc(hole),
+                body: env.arena.alloc(hole),
             }
         }
 
@@ -3364,10 +3336,7 @@ pub fn with_hole<'a>(
                     Stmt::Let(
                         assigned,
                         expr,
-                        Layout::Builtin(Builtin::List(
-                            MemoryMode::Refcounted,
-                            env.arena.alloc(elem_layout),
-                        )),
+                        Layout::Builtin(Builtin::List(env.arena.alloc(elem_layout))),
                         hole,
                     )
                 }
@@ -3380,7 +3349,6 @@ pub fn with_hole<'a>(
         }
 
         List {
-            list_var: _,
             elem_var,
             loc_elems,
         } => {
@@ -3399,12 +3367,10 @@ pub fn with_hole<'a>(
                 elems: arg_symbols,
             };
 
-            let mode = MemoryMode::Refcounted;
-
             let stmt = Stmt::Let(
                 assigned,
                 expr,
-                Layout::Builtin(Builtin::List(mode, env.arena.alloc(elem_layout))),
+                Layout::Builtin(Builtin::List(env.arena.alloc(elem_layout))),
                 hole,
             );
 
@@ -3450,33 +3416,30 @@ pub fn with_hole<'a>(
 
             let record_symbol = possible_reuse_symbol(env, procs, &loc_expr.value);
 
-            let wrapped = {
-                let record_layout = layout_cache
-                    .from_var(env.arena, record_var, env.subs)
-                    .unwrap_or_else(|err| panic!("TODO turn fn_var into a RuntimeError {:?}", err));
+            let mut stmt = match field_layouts.as_slice() {
+                [_] => {
+                    let mut hole = hole.clone();
+                    substitute_in_exprs(env.arena, &mut hole, assigned, record_symbol);
 
-                match Wrapped::opt_from_layout(&record_layout) {
-                    Some(result) => result,
-                    None => {
-                        debug_assert_eq!(field_layouts.len(), 1);
-                        Wrapped::SingleElementRecord
-                    }
+                    hole
+                }
+                _ => {
+                    let expr = Expr::StructAtIndex {
+                        index: index.expect("field not in its own type") as u64,
+                        field_layouts: field_layouts.into_bump_slice(),
+                        structure: record_symbol,
+                    };
+
+                    let layout = layout_cache
+                        .from_var(env.arena, field_var, env.subs)
+                        .unwrap_or_else(|err| {
+                            panic!("TODO turn fn_var into a RuntimeError {:?}", err)
+                        });
+
+                    Stmt::Let(assigned, expr, layout, hole)
                 }
             };
 
-            let expr = Expr::AccessAtIndex {
-                index: index.expect("field not in its own type") as u64,
-                field_layouts: field_layouts.into_bump_slice(),
-                structure: record_symbol,
-                wrapped,
-            };
-
-            let layout = layout_cache
-                .from_var(env.arena, field_var, env.subs)
-                .unwrap_or_else(|err| panic!("TODO turn fn_var into a RuntimeError {:?}", err));
-
-            let mut stmt = Stmt::Let(assigned, expr, layout, hole);
-
             stmt = assign_to_symbol(
                 env,
                 procs,
@@ -3533,16 +3496,16 @@ pub fn with_hole<'a>(
                 layout_cache,
             ) {
                 Ok(_) => {
-                    let full_layout = return_on_layout_error!(
+                    let raw_layout = return_on_layout_error!(
                         env,
-                        layout_cache.from_var(env.arena, function_var, env.subs)
+                        layout_cache.raw_from_var(env.arena, function_var, env.subs)
                     );
 
-                    match full_layout {
-                        Layout::Closure(_, lambda_set, _) => {
+                    match raw_layout {
+                        RawFunctionLayout::Function(_, lambda_set, _) => {
                             construct_closure_data(env, lambda_set, name, &[], assigned, hole)
                         }
-                        _ => unreachable!(),
+                        RawFunctionLayout::ZeroArgumentThunk(_) => unreachable!(),
                     }
                 }
 
@@ -3614,26 +3577,16 @@ pub fn with_hole<'a>(
                 other => arena.alloc([*other]),
             };
 
-            let wrapped = if field_layouts.len() == 1 {
-                Wrapped::SingleElementRecord
-            } else {
-                Wrapped::RecordOrSingleTagUnion
-            };
-
-            let mut stmt = if symbols.len() == 1 {
-                let mut hole = hole.clone();
-                substitute_in_exprs(env.arena, &mut hole, assigned, symbols[0]);
-                hole
-            } else {
-                let expr = Expr::Struct(symbols);
-                Stmt::Let(assigned, expr, record_layout, hole)
-            };
-
             debug_assert_eq!(field_layouts.len(), symbols.len());
             debug_assert_eq!(fields.len(), symbols.len());
-            let it = field_layouts.iter().zip(symbols.iter()).zip(fields);
 
-            for ((field_layout, symbol), what_to_do) in it {
+            if symbols.len() == 1 {
+                // TODO we can probably special-case this more, skippiing the generation of
+                // UpdateExisting
+                let mut stmt = hole.clone();
+
+                let what_to_do = &fields[0];
+
                 match what_to_do {
                     UpdateExisting(field) => {
                         stmt = assign_to_symbol(
@@ -3642,23 +3595,50 @@ pub fn with_hole<'a>(
                             layout_cache,
                             field.var,
                             *field.loc_expr.clone(),
-                            *symbol,
+                            assigned,
                             stmt,
                         );
                     }
-                    CopyExisting(index) => {
-                        let access_expr = Expr::AccessAtIndex {
-                            structure,
-                            index,
-                            field_layouts,
-                            wrapped,
-                        };
-                        stmt = Stmt::Let(*symbol, access_expr, *field_layout, arena.alloc(stmt));
+                    CopyExisting(_) => {
+                        unreachable!(
+                            r"when a record has just one field and is updated, it must update that one field"
+                        );
                     }
                 }
-            }
 
-            stmt
+                stmt
+            } else {
+                let expr = Expr::Struct(symbols);
+                let mut stmt = Stmt::Let(assigned, expr, record_layout, hole);
+
+                let it = field_layouts.iter().zip(symbols.iter()).zip(fields);
+
+                for ((field_layout, symbol), what_to_do) in it {
+                    match what_to_do {
+                        UpdateExisting(field) => {
+                            stmt = assign_to_symbol(
+                                env,
+                                procs,
+                                layout_cache,
+                                field.var,
+                                *field.loc_expr.clone(),
+                                *symbol,
+                                stmt,
+                            );
+                        }
+                        CopyExisting(index) => {
+                            let access_expr = Expr::StructAtIndex {
+                                structure,
+                                index,
+                                field_layouts,
+                            };
+                            stmt =
+                                Stmt::Let(*symbol, access_expr, *field_layout, arena.alloc(stmt));
+                        }
+                    }
+                }
+                stmt
+            }
         }
 
         Closure {
@@ -3672,9 +3652,13 @@ pub fn with_hole<'a>(
         } => {
             let loc_body = *boxed_body;
 
-            match layout_cache.from_var(env.arena, function_type, env.subs) {
-                Err(e) => panic!("invalid layout {:?}", e),
-                Ok(Layout::Closure(_argument_layouts, lambda_set, _ret_layout)) => {
+            let raw = layout_cache.raw_from_var(env.arena, function_type, env.subs);
+
+            match return_on_layout_error!(env, raw) {
+                RawFunctionLayout::ZeroArgumentThunk(_) => {
+                    unreachable!("a closure syntactically always must have at least one argument")
+                }
+                RawFunctionLayout::Function(_argument_layouts, lambda_set, _ret_layout) => {
                     let mut captured_symbols = Vec::from_iter_in(captured_symbols, env.arena);
                     captured_symbols.sort();
                     let captured_symbols = captured_symbols.into_bump_slice();
@@ -3709,49 +3693,11 @@ pub fn with_hole<'a>(
 
                     construct_closure_data(env, lambda_set, name, symbols, assigned, hole)
                 }
-                Ok(Layout::FunctionPointer(_, _)) => {
-                    // CLEANUP
-                    unreachable!("closures should never have a function pointer layout");
-
-                    //                    match procs.insert_anonymous(
-                    //                        env,
-                    //                        name,
-                    //                        function_type,
-                    //                        arguments,
-                    //                        loc_body,
-                    //                        CapturedSymbols::None,
-                    //                        return_type,
-                    //                        layout_cache,
-                    //                    ) {
-                    //                        Ok(layout) => {
-                    //                            bg!(name);
-                    //                            // TODO should the let have layout Pointer?
-                    //                            // Stmt::Let(assigned, Expr::Struct(&[]), Layout::Struct(&[]), hole)
-                    //                            if false {
-                    //                                let mut hole = hole.clone();
-                    //                                substitute_in_exprs(env.arena, &mut hole, assigned, name);
-                    //                                hole
-                    //                            } else {
-                    //                                Stmt::Let(
-                    //                                    assigned,
-                    //                                    call_by_pointer(env, procs, name, layout),
-                    //                                    layout,
-                    //                                    hole,
-                    //                                )
-                    //                            }
-                    //                        }
-                    //
-                    //                        Err(_error) => Stmt::RuntimeError(
-                    //                            "TODO convert anonymous function error to a RuntimeError string",
-                    //                        ),
-                    //                    }
-                }
-                Ok(_) => unreachable!(),
             }
         }
 
         Call(boxed, loc_args, _) => {
-            let (fn_var, loc_expr, _lambda_set_var, ret_var) = *boxed;
+            let (fn_var, loc_expr, _lambda_set_var, _ret_var) = *boxed;
 
             // even if a call looks like it's by name, it may in fact be by-pointer.
             // E.g. in `(\f, x -> f x)` the call is in fact by pointer.
@@ -3804,18 +3750,7 @@ pub fn with_hole<'a>(
 
                     let full_layout = return_on_layout_error!(
                         env,
-                        layout_cache.from_var(env.arena, fn_var, env.subs)
-                    );
-
-                    let arg_layouts = match full_layout {
-                        Layout::FunctionPointer(args, _) => args,
-                        Layout::Closure(args, _, _) => args,
-                        _ => unreachable!("function has layout that is not function pointer"),
-                    };
-
-                    let ret_layout = return_on_layout_error!(
-                        env,
-                        layout_cache.from_var(env.arena, ret_var, env.subs)
+                        layout_cache.raw_from_var(env.arena, fn_var, env.subs)
                     );
 
                     // if the function expression (loc_expr) is already a symbol,
@@ -3829,8 +3764,8 @@ pub fn with_hole<'a>(
                         Imported(_) => {
                             unreachable!("an imported value is never an anonymous function")
                         }
-                        Value(function_symbol) => {
-                            if let Layout::Closure(_, lambda_set, _) = full_layout {
+                        Value(function_symbol) => match full_layout {
+                            RawFunctionLayout::Function(arg_layouts, lambda_set, ret_layout) => {
                                 let closure_data_symbol = function_symbol;
 
                                 result = match_on_lambda_set(
@@ -3839,20 +3774,25 @@ pub fn with_hole<'a>(
                                     closure_data_symbol,
                                     arg_symbols,
                                     arg_layouts,
-                                    ret_layout,
+                                    *ret_layout,
                                     assigned,
                                     hole,
                                 );
-                            } else {
+                            }
+                            RawFunctionLayout::ZeroArgumentThunk(_) => {
                                 unreachable!("calling a non-closure layout")
                             }
-                        }
+                        },
                         NotASymbol => {
                             // the expression is not a symbol. That means it's an expression
                             // evaluating to a function value.
 
                             match full_layout {
-                                Layout::Closure(_, lambda_set, _) => {
+                                RawFunctionLayout::Function(
+                                    arg_layouts,
+                                    lambda_set,
+                                    ret_layout,
+                                ) => {
                                     let closure_data_symbol = env.unique_symbol();
 
                                     result = match_on_lambda_set(
@@ -3861,7 +3801,7 @@ pub fn with_hole<'a>(
                                         closure_data_symbol,
                                         arg_symbols,
                                         arg_layouts,
-                                        ret_layout,
+                                        *ret_layout,
                                         assigned,
                                         hole,
                                     );
@@ -3876,40 +3816,11 @@ pub fn with_hole<'a>(
                                         env.arena.alloc(result),
                                     );
                                 }
-                                Layout::FunctionPointer(_, _) => {
-                                    // CLEANUP
-                                    unreachable!("should not be a function pointer");
-
-                                    //                                    let function_symbol = env.unique_symbol();
-                                    //                                    let closure_data_symbol = function_symbol;
-                                    //
-                                    //                                    // layout of the closure record
-                                    //                                    let closure_data_layout = Layout::Struct(&[]);
-                                    //
-                                    //                                    result = lambda_set_to_switch_make_branch_help(
-                                    //                                        env,
-                                    //                                        function_symbol,
-                                    //                                        closure_data_symbol,
-                                    //                                        closure_data_layout,
-                                    //                                        arg_symbols,
-                                    //                                        arg_layouts,
-                                    //                                        ret_layout,
-                                    //                                        assigned,
-                                    //                                        hole,
-                                    //                                    );
-                                    //
-                                    //                                    result = with_hole(
-                                    //                                        env,
-                                    //                                        loc_expr.value,
-                                    //                                        fn_var,
-                                    //                                        procs,
-                                    //                                        layout_cache,
-                                    //                                        closure_data_symbol,
-                                    //                                        env.arena.alloc(result),
-                                    //                                    );
-                                }
-                                _ => {
-                                    todo!("{:?}", full_layout)
+                                RawFunctionLayout::ZeroArgumentThunk(_) => {
+                                    unreachable!(
+                                        "{:?} cannot be called in the source language",
+                                        full_layout
+                                    )
                                 }
                             }
                         }
@@ -4127,30 +4038,22 @@ fn construct_closure_data<'a>(
             tag_layout: _,
             union_size,
             tag_name,
+            union_layout,
         } => {
-            let tag_id_symbol = env.unique_symbol();
-            let mut tag_symbols = Vec::with_capacity_in(symbols.len() + 1, env.arena);
-            tag_symbols.push(tag_id_symbol);
-            tag_symbols.extend(symbols);
-
-            let expr1 = Expr::Literal(Literal::Int(tag_id as i128));
-            let expr2 = Expr::Tag {
+            let expr = Expr::Tag {
                 tag_id,
-                tag_layout: lambda_set.runtime_representation(),
+                tag_layout: union_layout,
                 union_size,
                 tag_name,
-                arguments: tag_symbols.into_bump_slice(),
+                arguments: symbols,
             };
 
-            let hole = Stmt::Let(
+            Stmt::Let(
                 assigned,
-                expr2,
+                expr,
                 lambda_set.runtime_representation(),
                 env.arena.alloc(hole),
-            );
-
-            let hole = env.arena.alloc(hole);
-            Stmt::Let(tag_id_symbol, expr1, Layout::Builtin(Builtin::Int64), hole)
+            )
         }
         ClosureRepresentation::Other(Layout::Struct(field_layouts)) => {
             debug_assert_eq!(field_layouts.len(), symbols.len());
@@ -4228,20 +4131,23 @@ fn convert_tag_union<'a>(
             hole,
         ),
         ByteUnion(tag_names) => {
-            let tag_id = tag_names
-                .iter()
-                .position(|key| key == &tag_name)
-                .expect("tag must be in its own type");
+            let opt_tag_id = tag_names.iter().position(|key| key == &tag_name);
 
-            Stmt::Let(
-                assigned,
-                Expr::Literal(Literal::Byte(tag_id as u8)),
-                Layout::Builtin(Builtin::Int8),
-                hole,
-            )
+            match opt_tag_id {
+                Some(tag_id) => Stmt::Let(
+                    assigned,
+                    Expr::Literal(Literal::Byte(tag_id as u8)),
+                    Layout::Builtin(Builtin::Int8),
+                    hole,
+                ),
+                None => Stmt::RuntimeError("tag must be in its own type"),
+            }
         }
 
-        Unwrapped(_, field_layouts) => {
+        Newtype {
+            arguments: field_layouts,
+            ..
+        } => {
             let field_symbols_temp = sorted_field_symbols(env, procs, layout_cache, args);
 
             let mut field_symbols = Vec::with_capacity_in(field_layouts.len(), env.arena);
@@ -4275,15 +4181,13 @@ fn convert_tag_union<'a>(
             let opt_tag_id_symbol;
 
             use WrappedVariant::*;
-            let (tag, layout) = match variant {
+            let (tag, union_layout) = match variant {
                 Recursive { sorted_tag_layouts } => {
                     debug_assert!(sorted_tag_layouts.len() > 1);
-                    let tag_id_symbol = env.unique_symbol();
-                    opt_tag_id_symbol = Some(tag_id_symbol);
+                    opt_tag_id_symbol = None;
 
                     field_symbols = {
                         let mut temp = Vec::with_capacity_in(field_symbols_temp.len() + 1, arena);
-                        temp.push(tag_id_symbol);
 
                         temp.extend(field_symbols_temp.iter().map(|r| r.1));
 
@@ -4298,17 +4202,17 @@ fn convert_tag_union<'a>(
                     }
 
                     debug_assert!(layouts.len() > 1);
-                    let layout = Layout::Union(UnionLayout::Recursive(layouts.into_bump_slice()));
+                    let union_layout = UnionLayout::Recursive(layouts.into_bump_slice());
 
                     let tag = Expr::Tag {
-                        tag_layout: layout,
+                        tag_layout: union_layout,
                         tag_name,
                         tag_id: tag_id as u8,
                         union_size,
                         arguments: field_symbols,
                     };
 
-                    (tag, layout)
+                    (tag, union_layout)
                 }
                 NonNullableUnwrapped {
                     fields,
@@ -4326,25 +4230,23 @@ fn convert_tag_union<'a>(
                         temp.into_bump_slice()
                     };
 
-                    let layout = Layout::Union(UnionLayout::NonNullableUnwrapped(fields));
+                    let union_layout = UnionLayout::NonNullableUnwrapped(fields);
 
                     let tag = Expr::Tag {
-                        tag_layout: layout,
+                        tag_layout: union_layout,
                         tag_name,
                         tag_id: tag_id as u8,
                         union_size,
                         arguments: field_symbols,
                     };
 
-                    (tag, layout)
+                    (tag, union_layout)
                 }
                 NonRecursive { sorted_tag_layouts } => {
-                    let tag_id_symbol = env.unique_symbol();
-                    opt_tag_id_symbol = Some(tag_id_symbol);
+                    opt_tag_id_symbol = None;
 
                     field_symbols = {
-                        let mut temp = Vec::with_capacity_in(field_symbols_temp.len() + 1, arena);
-                        temp.push(tag_id_symbol);
+                        let mut temp = Vec::with_capacity_in(field_symbols_temp.len(), arena);
 
                         temp.extend(field_symbols_temp.iter().map(|r| r.1));
 
@@ -4358,30 +4260,27 @@ fn convert_tag_union<'a>(
                         layouts.push(arg_layouts);
                     }
 
-                    let layout =
-                        Layout::Union(UnionLayout::NonRecursive(layouts.into_bump_slice()));
+                    let union_layout = UnionLayout::NonRecursive(layouts.into_bump_slice());
 
                     let tag = Expr::Tag {
-                        tag_layout: layout,
+                        tag_layout: union_layout,
                         tag_name,
                         tag_id: tag_id as u8,
                         union_size,
                         arguments: field_symbols,
                     };
 
-                    (tag, layout)
+                    (tag, union_layout)
                 }
                 NullableWrapped {
                     nullable_id,
                     nullable_name: _,
                     sorted_tag_layouts,
                 } => {
-                    let tag_id_symbol = env.unique_symbol();
-                    opt_tag_id_symbol = Some(tag_id_symbol);
+                    opt_tag_id_symbol = None;
 
                     field_symbols = {
                         let mut temp = Vec::with_capacity_in(field_symbols_temp.len() + 1, arena);
-                        temp.push(tag_id_symbol);
 
                         temp.extend(field_symbols_temp.iter().map(|r| r.1));
 
@@ -4395,20 +4294,20 @@ fn convert_tag_union<'a>(
                         layouts.push(arg_layouts);
                     }
 
-                    let layout = Layout::Union(UnionLayout::NullableWrapped {
+                    let union_layout = UnionLayout::NullableWrapped {
                         nullable_id,
                         other_tags: layouts.into_bump_slice(),
-                    });
+                    };
 
                     let tag = Expr::Tag {
-                        tag_layout: layout,
+                        tag_layout: union_layout,
                         tag_name,
                         tag_id: tag_id as u8,
                         union_size,
                         arguments: field_symbols,
                     };
 
-                    (tag, layout)
+                    (tag, union_layout)
                 }
                 NullableUnwrapped {
                     nullable_id,
@@ -4422,32 +4321,30 @@ fn convert_tag_union<'a>(
 
                     field_symbols = {
                         let mut temp = Vec::with_capacity_in(field_symbols_temp.len() + 1, arena);
-                        // FIXME drop tag
-                        temp.push(tag_id_symbol);
 
                         temp.extend(field_symbols_temp.iter().map(|r| r.1));
 
                         temp.into_bump_slice()
                     };
 
-                    let layout = Layout::Union(UnionLayout::NullableUnwrapped {
+                    let union_layout = UnionLayout::NullableUnwrapped {
                         nullable_id,
                         other_fields,
-                    });
+                    };
 
                     let tag = Expr::Tag {
-                        tag_layout: layout,
+                        tag_layout: union_layout,
                         tag_name,
                         tag_id: tag_id as u8,
                         union_size,
                         arguments: field_symbols,
                     };
 
-                    (tag, layout)
+                    (tag, union_layout)
                 }
             };
 
-            let mut stmt = Stmt::Let(assigned, tag, layout, hole);
+            let mut stmt = Stmt::Let(assigned, tag, Layout::Union(union_layout), hole);
             let iter = field_symbols_temp
                 .into_iter()
                 .map(|x| x.2 .0)
@@ -4461,7 +4358,7 @@ fn convert_tag_union<'a>(
                 stmt = Stmt::Let(
                     tag_id_symbol,
                     Expr::Literal(Literal::Int(tag_id as i128)),
-                    Layout::Builtin(TAG_SIZE),
+                    union_layout.tag_id_layout(),
                     arena.alloc(stmt),
                 );
             }
@@ -4520,14 +4417,16 @@ fn tag_union_to_function<'a>(
     match inserted {
         Ok(_layout) => {
             // only need to construct closure data
-            let full_layout =
-                return_on_layout_error!(env, layout_cache.from_var(env.arena, whole_var, env.subs));
+            let raw_layout = return_on_layout_error!(
+                env,
+                layout_cache.raw_from_var(env.arena, whole_var, env.subs)
+            );
 
-            match full_layout {
-                Layout::Closure(_, lambda_set, _) => {
+            match raw_layout {
+                RawFunctionLayout::Function(_, lambda_set, _) => {
                     construct_closure_data(env, lambda_set, proc_symbol, &[], assigned, hole)
                 }
-                _ => unreachable!(),
+                RawFunctionLayout::ZeroArgumentThunk(_) => unreachable!(),
             }
         }
 
@@ -4678,12 +4577,14 @@ pub fn from_can<'a>(
                 arguments,
             };
 
+            let exception_id = ExceptionId(env.unique_symbol());
             let rest = Stmt::Invoke {
                 symbol: env.unique_symbol(),
                 call,
                 layout: bool_layout,
                 pass: env.arena.alloc(rest),
-                fail: env.arena.alloc(Stmt::Rethrow),
+                fail: env.arena.alloc(Stmt::Resume(exception_id)),
+                exception_id,
             };
 
             with_hole(
@@ -4769,28 +4670,36 @@ pub fn from_can<'a>(
 
                             // does this function capture any local values?
                             let function_layout =
-                                layout_cache.from_var(env.arena, function_type, env.subs);
+                                layout_cache.raw_from_var(env.arena, function_type, env.subs);
 
-                            let captured_symbols = if let Ok(Layout::Closure(_, lambda_set, _)) =
-                                &function_layout
-                            {
-                                if let Layout::Struct(&[]) = lambda_set.runtime_representation() {
-                                    CapturedSymbols::None
-                                } else {
-                                    let mut temp = Vec::from_iter_in(captured_symbols, env.arena);
-                                    temp.sort();
-                                    CapturedSymbols::Captured(temp.into_bump_slice())
+                            let captured_symbols = match function_layout {
+                                Ok(RawFunctionLayout::Function(_, lambda_set, _)) => {
+                                    if let Layout::Struct(&[]) = lambda_set.runtime_representation()
+                                    {
+                                        CapturedSymbols::None
+                                    } else {
+                                        let mut temp =
+                                            Vec::from_iter_in(captured_symbols, env.arena);
+                                        temp.sort();
+                                        CapturedSymbols::Captured(temp.into_bump_slice())
+                                    }
+                                }
+                                Ok(RawFunctionLayout::ZeroArgumentThunk(_)) => {
+                                    // top-level thunks cannot capture any variables
+                                    debug_assert!(
+                                        captured_symbols.is_empty(),
+                                        "{:?} with layout {:?} {:?} {:?}",
+                                        &captured_symbols,
+                                        function_layout,
+                                        env.subs,
+                                        (function_type, closure_type, closure_ext_var),
+                                    );
+                                    CapturedSymbols::None
+                                }
+                                Err(_) => {
+                                    debug_assert!(captured_symbols.is_empty());
+                                    CapturedSymbols::None
                                 }
-                            } else {
-                                debug_assert!(
-                                    captured_symbols.is_empty(),
-                                    "{:?} with layout {:?} {:?} {:?}",
-                                    &captured_symbols,
-                                    function_layout,
-                                    env.subs,
-                                    (function_type, closure_type, closure_ext_var),
-                                );
-                                CapturedSymbols::None
                             };
 
                             procs.insert_named(
@@ -5187,21 +5096,17 @@ fn from_can_when<'a>(
                     jump,
                 );
 
-                let new_guard_stmt =
-                    store_pattern(env, procs, layout_cache, &pattern, cond_symbol, guard_stmt);
                 (
-                    pattern,
+                    pattern.clone(),
                     Guard::Guard {
                         id,
-                        symbol,
-                        stmt: new_guard_stmt,
+                        pattern,
+                        stmt: guard_stmt,
                     },
                     branch_stmt,
                 )
             } else {
-                let new_branch_stmt =
-                    store_pattern(env, procs, layout_cache, &pattern, cond_symbol, branch_stmt);
-                (pattern, Guard::NoGuard, new_branch_stmt)
+                (pattern, Guard::NoGuard, branch_stmt)
             }
         });
     let mono_branches = Vec::from_iter_in(it, arena);
@@ -5265,6 +5170,7 @@ fn substitute_in_stmt_help<'a>(
             layout,
             pass,
             fail,
+            exception_id,
         } => {
             let opt_call = substitute_in_call(arena, call, subs);
             let opt_pass = substitute_in_stmt_help(arena, pass, subs);
@@ -5281,6 +5187,7 @@ fn substitute_in_stmt_help<'a>(
                     layout: *layout,
                     pass,
                     fail,
+                    exception_id: *exception_id,
                 }))
             } else {
                 None
@@ -5290,7 +5197,7 @@ fn substitute_in_stmt_help<'a>(
             id,
             parameters,
             remainder,
-            continuation,
+            body: continuation,
         } => {
             let opt_remainder = substitute_in_stmt_help(arena, remainder, subs);
             let opt_continuation = substitute_in_stmt_help(arena, continuation, subs);
@@ -5303,7 +5210,7 @@ fn substitute_in_stmt_help<'a>(
                     id: *id,
                     parameters,
                     remainder,
-                    continuation,
+                    body: continuation,
                 }))
             } else {
                 None
@@ -5400,7 +5307,7 @@ fn substitute_in_stmt_help<'a>(
             }
         }
 
-        Rethrow => None,
+        Resume(_) => None,
 
         RuntimeError(_) => None,
     }
@@ -5421,13 +5328,11 @@ fn substitute_in_call<'a>(
             name,
             arg_layouts,
             ret_layout,
-            full_layout,
             specialization_id,
         } => substitute(subs, *name).map(|new| CallType::ByName {
             name: new,
             arg_layouts,
             ret_layout: *ret_layout,
-            full_layout: *full_layout,
             specialization_id: *specialization_id,
         }),
         CallType::Foreign { .. } => None,
@@ -5559,25 +5464,49 @@ fn substitute_in_expr<'a>(
             }
         }
 
-        AccessAtIndex {
+        StructAtIndex {
             index,
             structure,
             field_layouts,
-            wrapped,
         } => match substitute(subs, *structure) {
-            Some(structure) => Some(AccessAtIndex {
+            Some(structure) => Some(StructAtIndex {
                 index: *index,
                 field_layouts: *field_layouts,
-                wrapped: *wrapped,
                 structure,
             }),
             None => None,
         },
+
+        GetTagId {
+            structure,
+            union_layout,
+        } => match substitute(subs, *structure) {
+            Some(structure) => Some(GetTagId {
+                structure,
+                union_layout: *union_layout,
+            }),
+            None => None,
+        },
+
+        UnionAtIndex {
+            structure,
+            tag_id,
+            index,
+            union_layout,
+        } => match substitute(subs, *structure) {
+            Some(structure) => Some(UnionAtIndex {
+                structure,
+                tag_id: *tag_id,
+                index: *index,
+                union_layout: *union_layout,
+            }),
+            None => None,
+        },
     }
 }
 
 #[allow(clippy::too_many_arguments)]
-fn store_pattern<'a>(
+pub fn store_pattern<'a>(
     env: &mut Env<'a, '_>,
     procs: &mut Procs<'a>,
     layout_cache: &mut LayoutCache<'a>,
@@ -5625,81 +5554,64 @@ fn store_pattern_help<'a>(
         | StrLiteral(_) => {
             return StorePattern::NotProductive(stmt);
         }
+        NewtypeDestructure { arguments, .. } => match arguments.as_slice() {
+            [single] => {
+                return store_pattern_help(env, procs, layout_cache, &single.0, outer_symbol, stmt);
+            }
+            _ => {
+                let mut fields = Vec::with_capacity_in(arguments.len(), env.arena);
+                fields.extend(arguments.iter().map(|x| x.1));
+
+                let layout = Layout::Struct(fields.into_bump_slice());
+
+                return store_newtype_pattern(
+                    env,
+                    procs,
+                    layout_cache,
+                    outer_symbol,
+                    &layout,
+                    &arguments,
+                    stmt,
+                );
+            }
+        },
         AppliedTag {
-            arguments, layout, ..
+            arguments,
+            layout,
+            tag_id,
+            ..
         } => {
-            let wrapped = Wrapped::from_layout(layout);
-            let write_tag = wrapped == Wrapped::MultiTagUnion;
-
-            let mut arg_layouts = Vec::with_capacity_in(arguments.len(), env.arena);
-            let mut is_productive = false;
-
-            if write_tag {
-                // add an element for the tag discriminant
-                arg_layouts.push(Layout::Builtin(TAG_SIZE));
-            }
-
-            for (_, layout) in arguments {
-                arg_layouts.push(*layout);
-            }
-
-            for (index, (argument, arg_layout)) in arguments.iter().enumerate().rev() {
-                let index = if write_tag { index + 1 } else { index };
-
-                let mut arg_layout = arg_layout;
-
-                if let Layout::RecursivePointer = arg_layout {
-                    arg_layout = layout;
-                }
-
-                let load = Expr::AccessAtIndex {
-                    wrapped,
-                    index: index as u64,
-                    field_layouts: arg_layouts.clone().into_bump_slice(),
-                    structure: outer_symbol,
-                };
-
-                match argument {
-                    Identifier(symbol) => {
-                        // store immediately in the given symbol
-                        stmt = Stmt::Let(*symbol, load, *arg_layout, env.arena.alloc(stmt));
-                        is_productive = true;
+            return store_tag_pattern(
+                env,
+                procs,
+                layout_cache,
+                outer_symbol,
+                *layout,
+                &arguments,
+                *tag_id,
+                stmt,
+            );
+        }
+        RecordDestructure(destructs, [_single_field]) => {
+            for destruct in destructs {
+                match &destruct.typ {
+                    DestructType::Required(symbol) => {
+                        substitute_in_exprs(env.arena, &mut stmt, *symbol, outer_symbol);
                     }
-                    Underscore => {
-                        // ignore
-                    }
-                    IntLiteral(_)
-                    | FloatLiteral(_)
-                    | EnumLiteral { .. }
-                    | BitLiteral { .. }
-                    | StrLiteral(_) => {}
-                    _ => {
-                        // store the field in a symbol, and continue matching on it
-                        let symbol = env.unique_symbol();
-
-                        // first recurse, continuing to unpack symbol
-                        match store_pattern_help(env, procs, layout_cache, argument, symbol, stmt) {
-                            StorePattern::Productive(new) => {
-                                is_productive = true;
-                                stmt = new;
-                                // only if we bind one of its (sub)fields to a used name should we
-                                // extract the field
-                                stmt = Stmt::Let(symbol, load, *arg_layout, env.arena.alloc(stmt));
-                            }
-                            StorePattern::NotProductive(new) => {
-                                // do nothing
-                                stmt = new;
-                            }
-                        }
+                    DestructType::Guard(guard_pattern) => {
+                        return store_pattern_help(
+                            env,
+                            procs,
+                            layout_cache,
+                            guard_pattern,
+                            outer_symbol,
+                            stmt,
+                        );
                     }
                 }
             }
-
-            if !is_productive {
-                return StorePattern::NotProductive(stmt);
-            }
         }
-        RecordDestructure(destructs, Layout::Struct(sorted_fields)) => {
+        RecordDestructure(destructs, sorted_fields) => {
             let mut is_productive = false;
             for (index, destruct) in destructs.iter().enumerate().rev() {
                 match store_record_destruct(
@@ -5726,15 +5638,158 @@ fn store_pattern_help<'a>(
                 return StorePattern::NotProductive(stmt);
             }
         }
-
-        RecordDestructure(_, _) => {
-            unreachable!("a record destructure must always occur on a struct layout");
-        }
     }
 
     StorePattern::Productive(stmt)
 }
 
+#[allow(clippy::too_many_arguments)]
+fn store_tag_pattern<'a>(
+    env: &mut Env<'a, '_>,
+    procs: &mut Procs<'a>,
+    layout_cache: &mut LayoutCache<'a>,
+    structure: Symbol,
+    union_layout: UnionLayout<'a>,
+    arguments: &[(Pattern<'a>, Layout<'a>)],
+    tag_id: u8,
+    mut stmt: Stmt<'a>,
+) -> StorePattern<'a> {
+    use Pattern::*;
+
+    let mut is_productive = false;
+
+    for (index, (argument, arg_layout)) in arguments.iter().enumerate().rev() {
+        let mut arg_layout = *arg_layout;
+
+        if let Layout::RecursivePointer = arg_layout {
+            arg_layout = Layout::Union(union_layout);
+        }
+
+        let load = Expr::UnionAtIndex {
+            index: index as u64,
+            structure,
+            tag_id,
+            union_layout,
+        };
+
+        match argument {
+            Identifier(symbol) => {
+                // store immediately in the given symbol
+                stmt = Stmt::Let(*symbol, load, arg_layout, env.arena.alloc(stmt));
+                is_productive = true;
+            }
+            Underscore => {
+                // ignore
+            }
+            IntLiteral(_)
+            | FloatLiteral(_)
+            | EnumLiteral { .. }
+            | BitLiteral { .. }
+            | StrLiteral(_) => {}
+            _ => {
+                // store the field in a symbol, and continue matching on it
+                let symbol = env.unique_symbol();
+
+                // first recurse, continuing to unpack symbol
+                match store_pattern_help(env, procs, layout_cache, argument, symbol, stmt) {
+                    StorePattern::Productive(new) => {
+                        is_productive = true;
+                        stmt = new;
+                        // only if we bind one of its (sub)fields to a used name should we
+                        // extract the field
+                        stmt = Stmt::Let(symbol, load, arg_layout, env.arena.alloc(stmt));
+                    }
+                    StorePattern::NotProductive(new) => {
+                        // do nothing
+                        stmt = new;
+                    }
+                }
+            }
+        }
+    }
+
+    if is_productive {
+        StorePattern::Productive(stmt)
+    } else {
+        StorePattern::NotProductive(stmt)
+    }
+}
+
+#[allow(clippy::too_many_arguments)]
+fn store_newtype_pattern<'a>(
+    env: &mut Env<'a, '_>,
+    procs: &mut Procs<'a>,
+    layout_cache: &mut LayoutCache<'a>,
+    structure: Symbol,
+    layout: &Layout<'a>,
+    arguments: &[(Pattern<'a>, Layout<'a>)],
+    mut stmt: Stmt<'a>,
+) -> StorePattern<'a> {
+    use Pattern::*;
+
+    let mut arg_layouts = Vec::with_capacity_in(arguments.len(), env.arena);
+    let mut is_productive = false;
+
+    for (_, layout) in arguments {
+        arg_layouts.push(*layout);
+    }
+
+    for (index, (argument, arg_layout)) in arguments.iter().enumerate().rev() {
+        let mut arg_layout = *arg_layout;
+
+        if let Layout::RecursivePointer = arg_layout {
+            arg_layout = *layout;
+        }
+
+        let load = Expr::StructAtIndex {
+            index: index as u64,
+            field_layouts: arg_layouts.clone().into_bump_slice(),
+            structure,
+        };
+
+        match argument {
+            Identifier(symbol) => {
+                // store immediately in the given symbol
+                stmt = Stmt::Let(*symbol, load, arg_layout, env.arena.alloc(stmt));
+                is_productive = true;
+            }
+            Underscore => {
+                // ignore
+            }
+            IntLiteral(_)
+            | FloatLiteral(_)
+            | EnumLiteral { .. }
+            | BitLiteral { .. }
+            | StrLiteral(_) => {}
+            _ => {
+                // store the field in a symbol, and continue matching on it
+                let symbol = env.unique_symbol();
+
+                // first recurse, continuing to unpack symbol
+                match store_pattern_help(env, procs, layout_cache, argument, symbol, stmt) {
+                    StorePattern::Productive(new) => {
+                        is_productive = true;
+                        stmt = new;
+                        // only if we bind one of its (sub)fields to a used name should we
+                        // extract the field
+                        stmt = Stmt::Let(symbol, load, arg_layout, env.arena.alloc(stmt));
+                    }
+                    StorePattern::NotProductive(new) => {
+                        // do nothing
+                        stmt = new;
+                    }
+                }
+            }
+        }
+    }
+
+    if is_productive {
+        StorePattern::Productive(stmt)
+    } else {
+        StorePattern::NotProductive(stmt)
+    }
+}
+
 #[allow(clippy::too_many_arguments)]
 fn store_record_destruct<'a>(
     env: &mut Env<'a, '_>,
@@ -5748,14 +5803,10 @@ fn store_record_destruct<'a>(
 ) -> StorePattern<'a> {
     use Pattern::*;
 
-    let wrapped = Wrapped::from_layout(&Layout::Struct(sorted_fields));
-
-    // TODO wrapped could be SingleElementRecord
-    let load = Expr::AccessAtIndex {
+    let load = Expr::StructAtIndex {
         index,
         field_layouts: sorted_fields,
         structure: outer_symbol,
-        wrapped,
     };
 
     match &destruct.typ {
@@ -5890,20 +5941,17 @@ fn force_thunk<'a>(
     assigned: Symbol,
     hole: &'a Stmt<'a>,
 ) -> Stmt<'a> {
-    let full_layout = Layout::FunctionPointer(&[], env.arena.alloc(layout));
-
     let call = self::Call {
         call_type: CallType::ByName {
             name: thunk_name,
             ret_layout: layout,
-            full_layout,
             arg_layouts: &[],
             specialization_id: env.next_call_specialization_id(),
         },
         arguments: &[],
     };
 
-    Stmt::Let(assigned, Expr::Call(call), layout, env.arena.alloc(hole))
+    build_call(env, call, assigned, layout, env.arena.alloc(hole))
 }
 
 fn let_empty_struct<'a>(assigned: Symbol, hole: &'a Stmt<'a>) -> Stmt<'a> {
@@ -5922,35 +5970,48 @@ fn reuse_function_symbol<'a>(
 ) -> Stmt<'a> {
     match procs.partial_procs.get(&original) {
         None => {
-            let is_imported = env.is_imported_symbol(original);
-
             match arg_var {
-                Some(arg_var) if is_imported => {
+                Some(arg_var) if env.is_imported_symbol(original) => {
                     let layout = layout_cache
                         .from_var(env.arena, arg_var, env.subs)
                         .expect("creating layout does not fail");
 
-                    let top_level = TopLevelFunctionLayout::from_layout(env.arena, layout);
+                    if procs.imported_module_thunks.contains(&original) {
+                        let top_level = ProcLayout::new(env.arena, &[], layout);
+                        procs.insert_passed_by_name(
+                            env,
+                            arg_var,
+                            original,
+                            top_level,
+                            layout_cache,
+                        );
 
-                    procs.insert_passed_by_name(env, arg_var, original, top_level, layout_cache);
+                        force_thunk(env, original, layout, symbol, env.arena.alloc(result))
+                    } else {
+                        let top_level = ProcLayout::from_layout(env.arena, layout);
+                        procs.insert_passed_by_name(
+                            env,
+                            arg_var,
+                            original,
+                            top_level,
+                            layout_cache,
+                        );
 
-                    // an imported symbol is either a function, or a top-level 0-argument thunk
-                    // it never has closure data, so we use the empty struct
-                    return let_empty_struct(symbol, env.arena.alloc(result));
+                        let_empty_struct(symbol, env.arena.alloc(result))
+                    }
                 }
 
                 _ => {
                     // danger: a foreign symbol may not be specialized!
                     debug_assert!(
-                        !is_imported,
+                        !env.is_imported_symbol(original),
                         "symbol {:?} while processing module {:?}",
                         original,
                         (env.home, &arg_var),
                     );
+                    result
                 }
             }
-
-            result
         }
 
         Some(partial_proc) => {
@@ -5958,7 +6019,10 @@ fn reuse_function_symbol<'a>(
             // this symbol is a function, that is used by-name (e.g. as an argument to another
             // function). Register it with the current variable, then create a function pointer
             // to it in the IR.
-            let res_layout = layout_cache.from_var(env.arena, arg_var, env.subs);
+            let res_layout = return_on_layout_error!(
+                env,
+                layout_cache.raw_from_var(env.arena, arg_var, env.subs)
+            );
 
             // we have three kinds of functions really. Plain functions, closures by capture,
             // and closures by unification. Here we record whether this function captures
@@ -5967,10 +6031,9 @@ fn reuse_function_symbol<'a>(
             let captured = partial_proc.captured_symbols.clone();
 
             match res_layout {
-                Ok(Layout::Closure(argument_layouts, lambda_set, ret_layout)) => {
+                RawFunctionLayout::Function(argument_layouts, lambda_set, ret_layout) => {
                     // define the function pointer
-                    let function_ptr_layout =
-                        TopLevelFunctionLayout::from_layout(env.arena, res_layout.unwrap());
+                    let function_ptr_layout = ProcLayout::from_raw(env.arena, res_layout);
 
                     if captures {
                         // this is a closure by capture, meaning it itself captures local variables.
@@ -6003,7 +6066,7 @@ fn reuse_function_symbol<'a>(
                     } else if procs.module_thunks.contains(&original) {
                         // this is a 0-argument thunk
                         let layout = Layout::Closure(argument_layouts, lambda_set, ret_layout);
-                        let top_level = TopLevelFunctionLayout::new(env.arena, &[], layout);
+                        let top_level = ProcLayout::new(env.arena, &[], layout);
                         procs.insert_passed_by_name(
                             env,
                             arg_var,
@@ -6027,23 +6090,12 @@ fn reuse_function_symbol<'a>(
                         return let_empty_struct(symbol, env.arena.alloc(result));
                     }
                 }
-                Ok(layout) => {
+                RawFunctionLayout::ZeroArgumentThunk(ret_layout) => {
                     // this is a 0-argument thunk
-                    let top_level = TopLevelFunctionLayout::new(env.arena, &[], layout);
+                    let top_level = ProcLayout::new(env.arena, &[], ret_layout);
                     procs.insert_passed_by_name(env, arg_var, original, top_level, layout_cache);
 
-                    force_thunk(env, original, layout, symbol, env.arena.alloc(result))
-                }
-                Err(LayoutProblem::Erroneous) => {
-                    let message = format!("The {:?} symbol has an erroneous type", symbol);
-                    Stmt::RuntimeError(env.arena.alloc(message))
-                }
-                Err(LayoutProblem::UnresolvedTypeVar(v)) => {
-                    let message = format!(
-                        "The {:?} symbol contains a unresolved type var {:?}",
-                        symbol, v
-                    );
-                    Stmt::RuntimeError(env.arena.alloc(message))
+                    force_thunk(env, original, ret_layout, symbol, env.arena.alloc(result))
                 }
             }
         }
@@ -6154,24 +6206,38 @@ fn can_throw_exception(call: &Call) -> bool {
     }
 }
 
+/// Symbol that links an Invoke with a Rethrow
+/// we'll assign the exception object to this symbol
+/// so we can later rethrow the exception
+#[derive(Copy, Clone, PartialEq, Debug)]
+pub struct ExceptionId(Symbol);
+
+impl ExceptionId {
+    pub fn into_inner(self) -> Symbol {
+        self.0
+    }
+}
+
 fn build_call<'a>(
     env: &mut Env<'a, '_>,
     call: Call<'a>,
     assigned: Symbol,
-    layout: Layout<'a>,
+    return_layout: Layout<'a>,
     hole: &'a Stmt<'a>,
 ) -> Stmt<'a> {
     if can_throw_exception(&call) {
-        let fail = env.arena.alloc(Stmt::Rethrow);
+        let id = ExceptionId(env.unique_symbol());
+        let fail = env.arena.alloc(Stmt::Resume(id));
         Stmt::Invoke {
             symbol: assigned,
             call,
-            layout,
+            layout: return_layout,
             fail,
             pass: hole,
+            exception_id: id,
         }
     } else {
-        Stmt::Let(assigned, Expr::Call(call), layout, hole)
+        Stmt::Let(assigned, Expr::Call(call), return_layout, hole)
     }
 }
 
@@ -6187,7 +6253,7 @@ fn call_by_name<'a>(
     hole: &'a Stmt<'a>,
 ) -> Stmt<'a> {
     // Register a pending_specialization for this function
-    match layout_cache.from_var(env.arena, fn_var, env.subs) {
+    match layout_cache.raw_from_var(env.arena, fn_var, env.subs) {
         Err(LayoutProblem::UnresolvedTypeVar(var)) => {
             let msg = format!(
                 "Hit an unresolved type variable {:?} when creating a layout for {:?} (var {:?})",
@@ -6202,110 +6268,105 @@ fn call_by_name<'a>(
             );
             Stmt::RuntimeError(env.arena.alloc(msg))
         }
-        Ok(layout) if procs.module_thunks.contains(&proc_name) => {
-            // here we turn a call to a module thunk into  forcing of that thunk
-            if loc_args.is_empty() {
-                call_by_name_module_thunk(
-                    env,
-                    procs,
-                    fn_var,
-                    proc_name,
-                    env.arena.alloc(layout),
-                    layout_cache,
-                    assigned,
-                    hole,
-                )
-            } else if let Layout::Closure(arg_layouts, lambda_set, ret_layout) = layout {
-                // here we turn a call to a module thunk into forcing of that thunk
-                // the thunk represents the closure environment for the body, so we then match
-                // on the closure environment to perform the call that the body represents.
-                //
-                // Example:
-                //
-                // > main = parseA  "foo" "bar"
-                // > parseA = Str.concat
+        Ok(RawFunctionLayout::Function(arg_layouts, lambda_set, ret_layout)) => {
+            if procs.module_thunks.contains(&proc_name) {
+                if loc_args.is_empty() {
+                    call_by_name_module_thunk(
+                        env,
+                        procs,
+                        fn_var,
+                        proc_name,
+                        env.arena.alloc(lambda_set.runtime_representation()),
+                        layout_cache,
+                        assigned,
+                        hole,
+                    )
+                } else {
+                    // here we turn a call to a module thunk into forcing of that thunk
+                    // the thunk represents the closure environment for the body, so we then match
+                    // on the closure environment to perform the call that the body represents.
+                    //
+                    // Example:
+                    //
+                    // > main = parseA  "foo" "bar"
+                    // > parseA = Str.concat
 
-                let closure_data_symbol = env.unique_symbol();
+                    let closure_data_symbol = env.unique_symbol();
 
-                let arena = env.arena;
-                let arg_symbols = Vec::from_iter_in(
-                    loc_args
-                        .iter()
-                        .map(|(_, arg_expr)| possible_reuse_symbol(env, procs, &arg_expr.value)),
-                    arena,
-                )
-                .into_bump_slice();
+                    let arena = env.arena;
+                    let arg_symbols = Vec::from_iter_in(
+                        loc_args.iter().map(|(_, arg_expr)| {
+                            possible_reuse_symbol(env, procs, &arg_expr.value)
+                        }),
+                        arena,
+                    )
+                    .into_bump_slice();
 
-                let result = match_on_lambda_set(
-                    env,
-                    lambda_set,
-                    closure_data_symbol,
-                    arg_symbols,
-                    arg_layouts,
-                    *ret_layout,
-                    assigned,
-                    hole,
-                );
+                    debug_assert_eq!(arg_symbols.len(), arg_layouts.len());
 
-                let result = call_by_name_module_thunk(
-                    env,
-                    procs,
-                    fn_var,
-                    proc_name,
-                    env.arena.alloc(layout),
-                    layout_cache,
-                    closure_data_symbol,
-                    env.arena.alloc(result),
-                );
+                    let result = match_on_lambda_set(
+                        env,
+                        lambda_set,
+                        closure_data_symbol,
+                        arg_symbols,
+                        arg_layouts,
+                        *ret_layout,
+                        assigned,
+                        hole,
+                    );
 
-                let iter = loc_args.into_iter().rev().zip(arg_symbols.iter().rev());
-                assign_to_symbols(env, procs, layout_cache, iter, result)
+                    let result = call_by_name_module_thunk(
+                        env,
+                        procs,
+                        fn_var,
+                        proc_name,
+                        env.arena.alloc(lambda_set.runtime_representation()),
+                        layout_cache,
+                        closure_data_symbol,
+                        env.arena.alloc(result),
+                    );
+
+                    let iter = loc_args.into_iter().rev().zip(arg_symbols.iter().rev());
+                    assign_to_symbols(env, procs, layout_cache, iter, result)
+                }
             } else {
-                unreachable!("calling a non-closure layout")
-            }
-        }
-        Ok(Layout::FunctionPointer(argument_layouts, ret_layout)) => call_by_name_help(
-            env,
-            procs,
-            fn_var,
-            proc_name,
-            loc_args,
-            Layout::FunctionPointer(argument_layouts, ret_layout),
-            argument_layouts,
-            ret_layout,
-            layout_cache,
-            assigned,
-            hole,
-        ),
-        Ok(Layout::Closure(c_argument_layouts, lambda_set, c_ret_layout)) => {
-            match lambda_set.extend_function_layout(env.arena, c_argument_layouts, c_ret_layout) {
-                Layout::FunctionPointer(argument_layouts, ret_layout) => call_by_name_help(
+                let argument_layouts = lambda_set.extend_argument_list(env.arena, arg_layouts);
+
+                call_by_name_help(
                     env,
                     procs,
                     fn_var,
                     proc_name,
                     loc_args,
-                    Layout::Closure(c_argument_layouts, lambda_set, c_ret_layout),
+                    lambda_set,
                     argument_layouts,
                     ret_layout,
                     layout_cache,
                     assigned,
                     hole,
-                ),
-                _ => unreachable!(),
+                )
             }
         }
-        Ok(other) if loc_args.is_empty() => {
-            // this is a 0-argument thunk
-            if env.is_imported_symbol(proc_name) {
+        Ok(RawFunctionLayout::ZeroArgumentThunk(ret_layout)) => {
+            if procs.module_thunks.contains(&proc_name) {
+                // here we turn a call to a module thunk into  forcing of that thunk
+                call_by_name_module_thunk(
+                    env,
+                    procs,
+                    fn_var,
+                    proc_name,
+                    env.arena.alloc(ret_layout),
+                    layout_cache,
+                    assigned,
+                    hole,
+                )
+            } else if env.is_imported_symbol(proc_name) {
                 add_needed_external(procs, env, fn_var, proc_name);
+                force_thunk(env, proc_name, ret_layout, assigned, hole)
+            } else {
+                panic!("most likely we're trying to call something that is not a function");
             }
-            force_thunk(env, proc_name, other, assigned, hole)
         }
-        other => panic!(
-            "calling {:?}, which is not a function but received arguments, and is a {:?}",
-            proc_name, other,
-        ),
     }
 }
 
@@ -6316,7 +6377,7 @@ fn call_by_name_help<'a>(
     fn_var: Variable,
     proc_name: Symbol,
     loc_args: std::vec::Vec<(Variable, Located<roc_can::expr::Expr>)>,
-    maybe_closure_layout: Layout<'a>,
+    lambda_set: LambdaSet<'a>,
     argument_layouts: &'a [Layout<'a>],
     ret_layout: &'a Layout<'a>,
     layout_cache: &mut LayoutCache<'a>,
@@ -6328,8 +6389,7 @@ fn call_by_name_help<'a>(
 
     // debug_assert!(!procs.module_thunks.contains(&proc_name), "{:?}", proc_name);
 
-    let top_level_layout = TopLevelFunctionLayout::new(env.arena, argument_layouts, *ret_layout);
-    let function_layout = env.arena.alloc(top_level_layout).full();
+    let top_level_layout = ProcLayout::new(env.arena, argument_layouts, *ret_layout);
 
     // the arguments given to the function, stored in symbols
     let field_symbols = Vec::from_iter_in(
@@ -6371,7 +6431,6 @@ fn call_by_name_help<'a>(
             call_type: CallType::ByName {
                 name: proc_name,
                 ret_layout: *ret_layout,
-                full_layout: function_layout,
                 arg_layouts: argument_layouts,
                 specialization_id: env.next_call_specialization_id(),
             },
@@ -6387,7 +6446,13 @@ fn call_by_name_help<'a>(
 
         debug_assert_ne!(proc_name.module_id(), ModuleId::ATTR);
         if procs.imported_module_thunks.contains(&proc_name) {
-            force_thunk(env, proc_name, maybe_closure_layout, assigned, hole)
+            force_thunk(
+                env,
+                proc_name,
+                lambda_set.runtime_representation(),
+                assigned,
+                hole,
+            )
         } else {
             debug_assert!(
                 !field_symbols.is_empty(),
@@ -6404,7 +6469,6 @@ fn call_by_name_help<'a>(
                 call_type: CallType::ByName {
                     name: proc_name,
                     ret_layout: *ret_layout,
-                    full_layout: function_layout,
                     arg_layouts: argument_layouts,
                     specialization_id: env.next_call_specialization_id(),
                 },
@@ -6457,7 +6521,6 @@ fn call_by_name_help<'a>(
                     call_type: CallType::ByName {
                         name: proc_name,
                         ret_layout: *ret_layout,
-                        full_layout: function_layout,
                         arg_layouts: argument_layouts,
                         specialization_id: env.next_call_specialization_id(),
                     },
@@ -6486,28 +6549,18 @@ fn call_by_name_help<'a>(
 
                         match specialize(env, procs, proc_name, layout_cache, pending, partial_proc)
                         {
-                            Ok((proc, layout)) => {
-                                // NOTE we cannot make the below assertion any more; figure out why
-
-                                debug_assert_eq!(
-                                    &maybe_closure_layout, &layout,
-                                    "\nProcedure {:?}\n\n{:?}\n\n{:?}",
-                                    proc_name, maybe_closure_layout, layout
-                                );
-
-                                call_specialized_proc(
-                                    env,
-                                    procs,
-                                    proc_name,
-                                    proc,
-                                    layout,
-                                    field_symbols,
-                                    loc_args,
-                                    layout_cache,
-                                    assigned,
-                                    hole,
-                                )
-                            }
+                            Ok((proc, layout)) => call_specialized_proc(
+                                env,
+                                procs,
+                                proc_name,
+                                proc,
+                                layout,
+                                field_symbols,
+                                loc_args,
+                                layout_cache,
+                                assigned,
+                                hole,
+                            ),
                             Err(SpecializeFailure {
                                 attempted_layout,
                                 problem: _,
@@ -6558,14 +6611,10 @@ fn call_by_name_module_thunk<'a>(
 
     // debug_assert!(!procs.module_thunks.contains(&proc_name), "{:?}", proc_name);
 
-    let top_level_layout = TopLevelFunctionLayout::new(env.arena, &[], *ret_layout);
+    let top_level_layout = ProcLayout::new(env.arena, &[], *ret_layout);
 
-    // the layout without the `FunctionPointer(&[], ...)` wrapper
     let inner_layout = *ret_layout;
 
-    // the layout with the wrapper
-    let module_thunk_layout = Layout::FunctionPointer(&[], ret_layout);
-
     // If we've already specialized this one, no further work is needed.
     if procs
         .specialized
@@ -6622,8 +6671,7 @@ fn call_by_name_module_thunk<'a>(
                         match specialize(env, procs, proc_name, layout_cache, pending, partial_proc)
                         {
                             Ok((proc, layout)) => {
-                                // NOTE we cannot make the below assertion any more; figure out why
-                                debug_assert_eq!(layout, module_thunk_layout);
+                                debug_assert!(layout.is_zero_argument_thunk());
 
                                 let was_present =
                                     procs.specialized.remove(&(proc_name, top_level_layout));
@@ -6639,12 +6687,10 @@ fn call_by_name_module_thunk<'a>(
                                 attempted_layout,
                                 problem: _,
                             }) => {
-                                debug_assert_eq!(attempted_layout, module_thunk_layout);
-
                                 let proc = generate_runtime_error_function(
                                     env,
                                     proc_name,
-                                    module_thunk_layout,
+                                    attempted_layout,
                                 );
 
                                 let was_present =
@@ -6675,22 +6721,20 @@ fn call_specialized_proc<'a>(
     procs: &mut Procs<'a>,
     proc_name: Symbol,
     proc: Proc<'a>,
-    layout: Layout<'a>,
+    layout: RawFunctionLayout<'a>,
     field_symbols: &'a [Symbol],
     loc_args: std::vec::Vec<(Variable, Located<roc_can::expr::Expr>)>,
     layout_cache: &mut LayoutCache<'a>,
     assigned: Symbol,
     hole: &'a Stmt<'a>,
 ) -> Stmt<'a> {
-    let function_layout = env
-        .arena
-        .alloc(TopLevelFunctionLayout::from_layout(env.arena, layout));
+    let function_layout = ProcLayout::from_raw(env.arena, layout);
 
-    procs.specialized.remove(&(proc_name, *function_layout));
+    procs.specialized.remove(&(proc_name, function_layout));
 
     procs
         .specialized
-        .insert((proc_name, *function_layout), Done(proc));
+        .insert((proc_name, function_layout), Done(proc));
 
     if field_symbols.is_empty() {
         debug_assert!(loc_args.is_empty());
@@ -6703,12 +6747,12 @@ fn call_specialized_proc<'a>(
         // there are no arguments. This confuses our IR,
         // and we have to fix it here.
         match layout {
-            Layout::Closure(_, closure_layout, _) => {
+            RawFunctionLayout::Function(_, lambda_set, _) => {
+                // when the body is a closure, the function will return the closure environment
                 let call = self::Call {
                     call_type: CallType::ByName {
                         name: proc_name,
                         ret_layout: function_layout.result,
-                        full_layout: function_layout.full(),
                         arg_layouts: function_layout.arguments,
                         specialization_id: env.next_call_specialization_id(),
                     },
@@ -6719,23 +6763,12 @@ fn call_specialized_proc<'a>(
                     env,
                     call,
                     assigned,
-                    closure_layout.runtime_representation(),
+                    lambda_set.runtime_representation(),
                     hole,
                 )
             }
-            _ => {
-                let call = self::Call {
-                    call_type: CallType::ByName {
-                        name: proc_name,
-                        ret_layout: function_layout.result,
-                        full_layout: function_layout.full(),
-                        arg_layouts: function_layout.arguments,
-                        specialization_id: env.next_call_specialization_id(),
-                    },
-                    arguments: field_symbols,
-                };
-
-                build_call(env, call, assigned, function_layout.full(), hole)
+            RawFunctionLayout::ZeroArgumentThunk(_) => {
+                unreachable!()
             }
         }
     } else {
@@ -6752,7 +6785,6 @@ fn call_specialized_proc<'a>(
             call_type: CallType::ByName {
                 name: proc_name,
                 ret_layout: function_layout.result,
-                full_layout: function_layout.full(),
                 arg_layouts: function_layout.arguments,
                 specialization_id: env.next_call_specialization_id(),
             },
@@ -6787,12 +6819,16 @@ pub enum Pattern<'a> {
     },
     StrLiteral(Box<str>),
 
-    RecordDestructure(Vec<'a, RecordDestruct<'a>>, Layout<'a>),
+    RecordDestructure(Vec<'a, RecordDestruct<'a>>, &'a [Layout<'a>]),
+    NewtypeDestructure {
+        tag_name: TagName,
+        arguments: Vec<'a, (Pattern<'a>, Layout<'a>)>,
+    },
     AppliedTag {
         tag_name: TagName,
         tag_id: u8,
         arguments: Vec<'a, (Pattern<'a>, Layout<'a>)>,
-        layout: Layout<'a>,
+        layout: UnionLayout<'a>,
         union: crate::exhaustive::Union,
     },
 }
@@ -6949,16 +6985,10 @@ fn from_can_pattern_help<'a>(
                         union,
                     }
                 }
-                Unwrapped(_, field_layouts) => {
-                    let union = crate::exhaustive::Union {
-                        render_as: RenderAs::Tag,
-                        alternatives: vec![Ctor {
-                            tag_id: TagId(0),
-                            name: tag_name.clone(),
-                            arity: field_layouts.len(),
-                        }],
-                    };
-
+                Newtype {
+                    arguments: field_layouts,
+                    ..
+                } => {
                     let mut arguments = arguments.clone();
 
                     arguments.sort_by(|arg1, arg2| {
@@ -6983,14 +7013,9 @@ fn from_can_pattern_help<'a>(
                         ));
                     }
 
-                    let layout = Layout::Struct(field_layouts.into_bump_slice());
-
-                    Pattern::AppliedTag {
+                    Pattern::NewtypeDestructure {
                         tag_name: tag_name.clone(),
-                        tag_id: 0,
                         arguments: mono_args,
-                        union,
-                        layout,
                     }
                 }
                 Wrapped(variant) => {
@@ -7027,8 +7052,7 @@ fn from_can_pattern_help<'a>(
                                 ctors.push(Ctor {
                                     tag_id: TagId(i as u8),
                                     name: tag_name.clone(),
-                                    // don't include tag discriminant in arity
-                                    arity: args.len() - 1,
+                                    arity: args.len(),
                                 })
                             }
 
@@ -7041,13 +7065,13 @@ fn from_can_pattern_help<'a>(
 
                             debug_assert_eq!(
                                 arguments.len(),
-                                argument_layouts[1..].len(),
+                                argument_layouts.len(),
                                 "The {:?} tag got {} arguments, but its layout expects {}!",
                                 tag_name,
                                 arguments.len(),
-                                argument_layouts[1..].len(),
+                                argument_layouts.len(),
                             );
-                            let it = argument_layouts[1..].iter();
+                            let it = argument_layouts.iter();
 
                             for ((_, loc_pat), layout) in arguments.iter().zip(it) {
                                 mono_args.push((
@@ -7071,8 +7095,7 @@ fn from_can_pattern_help<'a>(
                                 temp
                             };
 
-                            let layout =
-                                Layout::Union(UnionLayout::NonRecursive(layouts.into_bump_slice()));
+                            let layout = UnionLayout::NonRecursive(layouts.into_bump_slice());
 
                             Pattern::AppliedTag {
                                 tag_name: tag_name.clone(),
@@ -7104,8 +7127,8 @@ fn from_can_pattern_help<'a>(
 
                             let mut mono_args = Vec::with_capacity_in(arguments.len(), env.arena);
 
-                            debug_assert_eq!(arguments.len(), argument_layouts[1..].len());
-                            let it = argument_layouts[1..].iter();
+                            debug_assert_eq!(arguments.len(), argument_layouts.len());
+                            let it = argument_layouts.iter();
 
                             for ((_, loc_pat), layout) in arguments.iter().zip(it) {
                                 mono_args.push((
@@ -7130,8 +7153,7 @@ fn from_can_pattern_help<'a>(
                             };
 
                             debug_assert!(layouts.len() > 1);
-                            let layout =
-                                Layout::Union(UnionLayout::Recursive(layouts.into_bump_slice()));
+                            let layout = UnionLayout::Recursive(layouts.into_bump_slice());
 
                             Pattern::AppliedTag {
                                 tag_name: tag_name.clone(),
@@ -7176,7 +7198,7 @@ fn from_can_pattern_help<'a>(
                                 ));
                             }
 
-                            let layout = Layout::Union(UnionLayout::NonNullableUnwrapped(fields));
+                            let layout = UnionLayout::NonNullableUnwrapped(fields);
 
                             Pattern::AppliedTag {
                                 tag_name: tag_name.clone(),
@@ -7236,7 +7258,7 @@ fn from_can_pattern_help<'a>(
                             let it = if tag_name == &nullable_name {
                                 [].iter()
                             } else {
-                                argument_layouts[1..].iter()
+                                argument_layouts.iter()
                             };
 
                             for ((_, loc_pat), layout) in arguments.iter().zip(it) {
@@ -7261,10 +7283,10 @@ fn from_can_pattern_help<'a>(
                                 temp
                             };
 
-                            let layout = Layout::Union(UnionLayout::NullableWrapped {
+                            let layout = UnionLayout::NullableWrapped {
                                 nullable_id,
                                 other_tags: layouts.into_bump_slice(),
-                            });
+                            };
 
                             Pattern::AppliedTag {
                                 tag_name: tag_name.clone(),
@@ -7307,7 +7329,7 @@ fn from_can_pattern_help<'a>(
                                 [].iter()
                             } else {
                                 // FIXME drop tag
-                                argument_layouts[1..].iter()
+                                argument_layouts.iter()
                             };
 
                             for ((_, loc_pat), layout) in arguments.iter().zip(it) {
@@ -7322,10 +7344,10 @@ fn from_can_pattern_help<'a>(
                                 ));
                             }
 
-                            let layout = Layout::Union(UnionLayout::NullableUnwrapped {
+                            let layout = UnionLayout::NullableUnwrapped {
                                 nullable_id,
                                 other_fields,
-                            });
+                            };
 
                             Pattern::AppliedTag {
                                 tag_name: tag_name.clone(),
@@ -7459,7 +7481,7 @@ fn from_can_pattern_help<'a>(
 
             Ok(Pattern::RecordDestructure(
                 mono_destructs,
-                Layout::Struct(field_layouts.into_bump_slice()),
+                field_layouts.into_bump_slice(),
             ))
         }
     }
@@ -7623,7 +7645,7 @@ pub fn num_argument_to_int_or_float(
                 4 => IntOrFloat::UnsignedIntType(IntPrecision::I32),
                 8 => IntOrFloat::UnsignedIntType(IntPrecision::I64),
                 _ => panic!(
-                    "Invalid target for Num type arguement: Roc does't support compiling to {}-bit systems.",
+                    "Invalid target for Num type argument: Roc does't support compiling to {}-bit systems.",
                     ptr_bytes * 8
                 ),
             }
@@ -7644,60 +7666,55 @@ fn lowlevel_match_on_lambda_set<'a, ToLowLevelCall>(
     lambda_set: LambdaSet<'a>,
     closure_data_symbol: Symbol,
     to_lowlevel_call: ToLowLevelCall,
-    function_layout: Layout<'a>,
     return_layout: Layout<'a>,
     assigned: Symbol,
     hole: &'a Stmt<'a>,
 ) -> Stmt<'a>
 where
-    ToLowLevelCall: Fn(Symbol, Symbol, Layout<'a>) -> Call<'a> + Copy,
+    ToLowLevelCall: Fn(Symbol, Symbol, Option<Layout<'a>>, CallSpecId) -> Call<'a> + Copy,
 {
     match lambda_set.runtime_representation() {
-        Layout::Union(_) => {
+        Layout::Union(union_layout) => {
             let closure_tag_id_symbol = env.unique_symbol();
 
             let result = lowlevel_union_lambda_set_to_switch(
                 env,
                 lambda_set.set,
                 closure_tag_id_symbol,
-                Layout::Builtin(crate::layout::TAG_SIZE),
+                union_layout.tag_id_layout(),
                 closure_data_symbol,
+                lambda_set.is_represented(),
                 to_lowlevel_call,
-                function_layout,
                 return_layout,
                 assigned,
                 hole,
             );
 
             // extract & assign the closure_tag_id_symbol
-            let expr = Expr::AccessAtIndex {
-                index: 0,
-                field_layouts: env.arena.alloc([Layout::Builtin(Builtin::Int64)]),
+            let expr = Expr::GetTagId {
                 structure: closure_data_symbol,
-                wrapped: Wrapped::MultiTagUnion,
+                union_layout,
             };
 
             Stmt::Let(
                 closure_tag_id_symbol,
                 expr,
-                Layout::Builtin(Builtin::Int64),
+                union_layout.tag_id_layout(),
                 env.arena.alloc(result),
             )
         }
         Layout::Struct(_) => {
             let function_symbol = lambda_set.set[0].0;
 
-            // build the call
-            Stmt::Let(
-                assigned,
-                Expr::Call(to_lowlevel_call(
-                    function_symbol,
-                    closure_data_symbol,
-                    function_layout,
-                )),
-                return_layout,
-                env.arena.alloc(hole),
-            )
+            let call_spec_id = env.next_call_specialization_id();
+            let call = to_lowlevel_call(
+                function_symbol,
+                closure_data_symbol,
+                lambda_set.is_represented(),
+                call_spec_id,
+            );
+
+            build_call(env, call, assigned, return_layout, env.arena.alloc(hole))
         }
         Layout::Builtin(Builtin::Int1) => {
             let closure_tag_id_symbol = closure_data_symbol;
@@ -7708,8 +7725,8 @@ where
                 closure_tag_id_symbol,
                 Layout::Builtin(Builtin::Int1),
                 closure_data_symbol,
+                lambda_set.is_represented(),
                 to_lowlevel_call,
-                function_layout,
                 return_layout,
                 assigned,
                 hole,
@@ -7724,8 +7741,8 @@ where
                 closure_tag_id_symbol,
                 Layout::Builtin(Builtin::Int8),
                 closure_data_symbol,
+                lambda_set.is_represented(),
                 to_lowlevel_call,
-                function_layout,
                 return_layout,
                 assigned,
                 hole,
@@ -7742,14 +7759,14 @@ fn lowlevel_union_lambda_set_to_switch<'a, ToLowLevelCall>(
     closure_tag_id_symbol: Symbol,
     closure_tag_id_layout: Layout<'a>,
     closure_data_symbol: Symbol,
+    closure_env_layout: Option<Layout<'a>>,
     to_lowlevel_call: ToLowLevelCall,
-    function_layout: Layout<'a>,
     return_layout: Layout<'a>,
     assigned: Symbol,
     hole: &'a Stmt<'a>,
 ) -> Stmt<'a>
 where
-    ToLowLevelCall: Fn(Symbol, Symbol, Layout<'a>) -> Call<'a> + Copy,
+    ToLowLevelCall: Fn(Symbol, Symbol, Option<Layout<'a>>, CallSpecId) -> Call<'a> + Copy,
 {
     debug_assert!(!lambda_set.is_empty());
 
@@ -7762,17 +7779,14 @@ where
 
         let hole = Stmt::Jump(join_point_id, env.arena.alloc([assigned]));
 
-        // build the call
-        let stmt = Stmt::Let(
-            assigned,
-            Expr::Call(to_lowlevel_call(
-                *function_symbol,
-                closure_data_symbol,
-                function_layout,
-            )),
-            return_layout,
-            env.arena.alloc(hole),
+        let call_spec_id = env.next_call_specialization_id();
+        let call = to_lowlevel_call(
+            *function_symbol,
+            closure_data_symbol,
+            closure_env_layout,
+            call_spec_id,
         );
+        let stmt = build_call(env, call, assigned, return_layout, env.arena.alloc(hole));
 
         branches.push((i as u64, BranchInfo::None, stmt));
     }
@@ -7800,7 +7814,7 @@ where
     Stmt::Join {
         id: join_point_id,
         parameters: &*env.arena.alloc([param]),
-        continuation: hole,
+        body: hole,
         remainder: env.arena.alloc(switch),
     }
 }
@@ -7818,7 +7832,7 @@ fn match_on_lambda_set<'a>(
     hole: &'a Stmt<'a>,
 ) -> Stmt<'a> {
     match lambda_set.runtime_representation() {
-        Layout::Union(_) => {
+        Layout::Union(union_layout) => {
             let closure_tag_id_symbol = env.unique_symbol();
 
             let result = union_lambda_set_to_switch(
@@ -7826,7 +7840,7 @@ fn match_on_lambda_set<'a>(
                 lambda_set.set,
                 lambda_set.runtime_representation(),
                 closure_tag_id_symbol,
-                Layout::Builtin(crate::layout::TAG_SIZE),
+                union_layout.tag_id_layout(),
                 closure_data_symbol,
                 argument_symbols,
                 argument_layouts,
@@ -7836,17 +7850,15 @@ fn match_on_lambda_set<'a>(
             );
 
             // extract & assign the closure_tag_id_symbol
-            let expr = Expr::AccessAtIndex {
-                index: 0,
-                field_layouts: env.arena.alloc([Layout::Builtin(Builtin::Int64)]),
+            let expr = Expr::GetTagId {
                 structure: closure_data_symbol,
-                wrapped: Wrapped::MultiTagUnion,
+                union_layout,
             };
 
             Stmt::Let(
                 closure_tag_id_symbol,
                 expr,
-                Layout::Builtin(Builtin::Int64),
+                union_layout.tag_id_layout(),
                 env.arena.alloc(result),
             )
         }
@@ -7958,7 +7970,7 @@ fn union_lambda_set_to_switch<'a>(
     Stmt::Join {
         id: join_point_id,
         parameters: &*env.arena.alloc([param]),
-        continuation: hole,
+        body: hole,
         remainder: env.arena.alloc(switch),
     }
 }
@@ -8027,24 +8039,18 @@ fn union_lambda_set_branch_help<'a>(
         }
     };
 
-    let full_layout = Layout::FunctionPointer(argument_layouts, env.arena.alloc(return_layout));
-
     // build the call
-    Stmt::Let(
-        assigned,
-        Expr::Call(self::Call {
-            call_type: CallType::ByName {
-                name: function_symbol,
-                full_layout,
-                ret_layout: return_layout,
-                arg_layouts: argument_layouts,
-                specialization_id: env.next_call_specialization_id(),
-            },
-            arguments: argument_symbols,
-        }),
-        return_layout,
-        hole,
-    )
+    let call = self::Call {
+        call_type: CallType::ByName {
+            name: function_symbol,
+            ret_layout: return_layout,
+            arg_layouts: argument_layouts,
+            specialization_id: env.next_call_specialization_id(),
+        },
+        arguments: argument_symbols,
+    };
+
+    build_call(env, call, assigned, return_layout, hole)
 }
 
 #[allow(clippy::too_many_arguments)]
@@ -8105,7 +8111,7 @@ fn enum_lambda_set_to_switch<'a>(
     Stmt::Join {
         id: join_point_id,
         parameters: &*env.arena.alloc([param]),
-        continuation: hole,
+        body: hole,
         remainder: env.arena.alloc(switch),
     }
 }
@@ -8151,24 +8157,16 @@ fn enum_lambda_set_branch<'a>(
         }
     };
 
-    let full_layout = Layout::FunctionPointer(argument_layouts, env.arena.alloc(return_layout));
-
-    // build the call
-    Stmt::Let(
-        assigned,
-        Expr::Call(self::Call {
-            call_type: CallType::ByName {
-                name: function_symbol,
-                full_layout,
-                ret_layout: return_layout,
-                arg_layouts: argument_layouts,
-                specialization_id: env.next_call_specialization_id(),
-            },
-            arguments: argument_symbols,
-        }),
-        return_layout,
-        env.arena.alloc(hole),
-    )
+    let call = self::Call {
+        call_type: CallType::ByName {
+            name: function_symbol,
+            ret_layout: return_layout,
+            arg_layouts: argument_layouts,
+            specialization_id: env.next_call_specialization_id(),
+        },
+        arguments: argument_symbols,
+    };
+    build_call(env, call, assigned, return_layout, env.arena.alloc(hole))
 }
 
 #[allow(clippy::too_many_arguments)]
@@ -8178,14 +8176,14 @@ fn lowlevel_enum_lambda_set_to_switch<'a, ToLowLevelCall>(
     closure_tag_id_symbol: Symbol,
     closure_tag_id_layout: Layout<'a>,
     closure_data_symbol: Symbol,
+    closure_env_layout: Option<Layout<'a>>,
     to_lowlevel_call: ToLowLevelCall,
-    function_layout: Layout<'a>,
     return_layout: Layout<'a>,
     assigned: Symbol,
     hole: &'a Stmt<'a>,
 ) -> Stmt<'a>
 where
-    ToLowLevelCall: Fn(Symbol, Symbol, Layout<'a>) -> Call<'a> + Copy,
+    ToLowLevelCall: Fn(Symbol, Symbol, Option<Layout<'a>>, CallSpecId) -> Call<'a> + Copy,
 {
     debug_assert!(!lambda_set.is_empty());
 
@@ -8198,14 +8196,17 @@ where
 
         let hole = Stmt::Jump(join_point_id, env.arena.alloc([result_symbol]));
 
-        // build the call
-        let stmt = Stmt::Let(
+        let call_spec_id = env.next_call_specialization_id();
+        let call = to_lowlevel_call(
+            *function_symbol,
+            closure_data_symbol,
+            closure_env_layout,
+            call_spec_id,
+        );
+        let stmt = build_call(
+            env,
+            call,
             result_symbol,
-            Expr::Call(to_lowlevel_call(
-                *function_symbol,
-                closure_data_symbol,
-                function_layout,
-            )),
             return_layout,
             env.arena.alloc(hole),
         );
@@ -8236,7 +8237,7 @@ where
     Stmt::Join {
         id: join_point_id,
         parameters: &*env.arena.alloc([param]),
-        continuation: hole,
+        body: hole,
         remainder: env.arena.alloc(switch),
     }
 }
diff --git a/compiler/mono/src/layout.rs b/compiler/mono/src/layout.rs
index 37f8669418..42e8793b78 100644
--- a/compiler/mono/src/layout.rs
+++ b/compiler/mono/src/layout.rs
@@ -14,14 +14,6 @@ const GENERATE_NULLABLE: bool = true;
 
 /// If a (Num *) gets translated to a Layout, this is the numeric type it defaults to.
 const DEFAULT_NUM_BUILTIN: Builtin<'_> = Builtin::Int64;
-pub const TAG_SIZE: Builtin<'_> = Builtin::Int64;
-
-#[derive(Copy, Clone)]
-#[repr(u8)]
-pub enum InPlace {
-    InPlace,
-    Clone,
-}
 
 #[derive(Debug, Clone)]
 pub enum LayoutProblem {
@@ -29,6 +21,18 @@ pub enum LayoutProblem {
     Erroneous,
 }
 
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+pub enum RawFunctionLayout<'a> {
+    Function(&'a [Layout<'a>], LambdaSet<'a>, &'a Layout<'a>),
+    ZeroArgumentThunk(Layout<'a>),
+}
+
+impl RawFunctionLayout<'_> {
+    pub fn is_zero_argument_thunk(&self) -> bool {
+        matches!(self, RawFunctionLayout::ZeroArgumentThunk(_))
+    }
+}
+
 /// Types for code gen must be monomorphic. No type variables allowed!
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
 pub enum Layout<'a> {
@@ -36,30 +40,12 @@ pub enum Layout<'a> {
     /// A layout that is empty (turns into the empty struct in LLVM IR
     /// but for our purposes, not zero-sized, so it does not get dropped from data structures
     /// this is important for closures that capture zero-sized values
-    PhantomEmptyStruct,
     Struct(&'a [Layout<'a>]),
     Union(UnionLayout<'a>),
     RecursivePointer,
-    /// A function. The types of its arguments, then the type of its return value.
-    FunctionPointer(&'a [Layout<'a>], &'a Layout<'a>),
-    Closure(&'a [Layout<'a>], LambdaSet<'a>, &'a Layout<'a>),
-    Pointer(&'a Layout<'a>),
-}
 
-impl<'a> Layout<'a> {
-    pub fn in_place(&self) -> InPlace {
-        match self {
-            Layout::Builtin(Builtin::EmptyList) => InPlace::InPlace,
-            Layout::Builtin(Builtin::List(memory_mode, _)) => match memory_mode {
-                MemoryMode::Unique => InPlace::InPlace,
-                MemoryMode::Refcounted => InPlace::Clone,
-            },
-            Layout::Builtin(Builtin::EmptyStr) => InPlace::InPlace,
-            Layout::Builtin(Builtin::Str) => InPlace::Clone,
-            Layout::Builtin(Builtin::Int1) => InPlace::Clone,
-            _ => unreachable!("Layout {:?} does not have an inplace", self),
-        }
-    }
+    /// A function. The types of its arguments, then the type of its return value.
+    Closure(&'a [Layout<'a>], LambdaSet<'a>, &'a Layout<'a>),
 }
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
@@ -89,9 +75,9 @@ pub enum UnionLayout<'a> {
 }
 
 impl<'a> UnionLayout<'a> {
-    pub fn to_doc<'b, D, A>(&'b self, alloc: &'b D, _parens: Parens) -> DocBuilder<'b, D, A>
+    pub fn to_doc<D, A>(self, alloc: &'a D, _parens: Parens) -> DocBuilder<'a, D, A>
     where
-        D: DocAllocator<'b, A>,
+        D: DocAllocator<'a, A>,
         D::Doc: Clone,
         A: Clone,
     {
@@ -114,6 +100,101 @@ impl<'a> UnionLayout<'a> {
             _ => alloc.text("TODO"),
         }
     }
+
+    pub fn layout_at(self, tag_id: u8, index: usize) -> Layout<'a> {
+        let result = match self {
+            UnionLayout::NonRecursive(tag_layouts) => {
+                let field_layouts = tag_layouts[tag_id as usize];
+
+                // this cannot be recursive; return immediately
+                return field_layouts[index];
+            }
+            UnionLayout::Recursive(tag_layouts) => {
+                let field_layouts = tag_layouts[tag_id as usize];
+
+                field_layouts[index]
+            }
+            UnionLayout::NonNullableUnwrapped(field_layouts) => field_layouts[index],
+            UnionLayout::NullableWrapped {
+                nullable_id,
+                other_tags,
+            } => {
+                debug_assert_ne!(nullable_id, tag_id as i64);
+
+                let tag_index = if (tag_id as i64) < nullable_id {
+                    tag_id
+                } else {
+                    tag_id - 1
+                };
+
+                let field_layouts = other_tags[tag_index as usize];
+                field_layouts[index]
+            }
+
+            UnionLayout::NullableUnwrapped {
+                nullable_id,
+                other_fields,
+            } => {
+                debug_assert_ne!(nullable_id, tag_id != 0);
+
+                other_fields[index as usize]
+            }
+        };
+
+        if let Layout::RecursivePointer = result {
+            Layout::Union(self)
+        } else {
+            result
+        }
+    }
+
+    fn tag_id_builtin_help(union_size: usize) -> Builtin<'a> {
+        if union_size <= u8::MAX as usize {
+            Builtin::Int8
+        } else if union_size <= u16::MAX as usize {
+            Builtin::Int16
+        } else {
+            panic!("tag union is too big")
+        }
+    }
+
+    pub fn tag_id_builtin(&self) -> Builtin<'a> {
+        match self {
+            UnionLayout::NonRecursive(tags) | UnionLayout::Recursive(tags) => {
+                let union_size = tags.len();
+
+                Self::tag_id_builtin_help(union_size)
+            }
+
+            UnionLayout::NullableWrapped { other_tags, .. } => {
+                Self::tag_id_builtin_help(other_tags.len() + 1)
+            }
+            UnionLayout::NonNullableUnwrapped(_) => Builtin::Int1,
+            UnionLayout::NullableUnwrapped { .. } => Builtin::Int1,
+        }
+    }
+
+    pub fn tag_id_layout(&self) -> Layout<'a> {
+        Layout::Builtin(self.tag_id_builtin())
+    }
+
+    pub fn stores_tag_id(&self) -> bool {
+        match self {
+            UnionLayout::NonRecursive(_)
+            | UnionLayout::Recursive(_)
+            | UnionLayout::NullableWrapped { .. } => true,
+            UnionLayout::NonNullableUnwrapped(_) | UnionLayout::NullableUnwrapped { .. } => false,
+        }
+    }
+
+    pub fn is_nullable(&self) -> bool {
+        match self {
+            UnionLayout::NonRecursive(_)
+            | UnionLayout::Recursive(_)
+            | UnionLayout::NonNullableUnwrapped { .. } => false,
+            UnionLayout::NullableWrapped { .. } | UnionLayout::NullableUnwrapped { .. } => true,
+        }
+    }
 }
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
@@ -133,6 +214,7 @@ pub enum ClosureRepresentation<'a> {
         tag_name: TagName,
         tag_id: u8,
         union_size: u8,
+        union_layout: UnionLayout<'a>,
     },
     /// the representation is anything but a union
     Other(Layout<'a>),
@@ -143,6 +225,14 @@ impl<'a> LambdaSet<'a> {
         *self.representation
     }
 
+    pub fn is_represented(&self) -> Option<Layout<'a>> {
+        if let Layout::Struct(&[]) = self.representation {
+            None
+        } else {
+            Some(*self.representation)
+        }
+    }
+
     pub fn layout_for_member(&self, function_symbol: Symbol) -> ClosureRepresentation<'a> {
         debug_assert!(
             self.set.iter().any(|(s, _)| *s == function_symbol),
@@ -166,6 +256,7 @@ impl<'a> LambdaSet<'a> {
                             tag_id: index as u8,
                             tag_layout: tags[index],
                             tag_name: TagName::Closure(function_symbol),
+                            union_layout: *union,
                         }
                     }
                     UnionLayout::Recursive(_) => todo!("recursive closures"),
@@ -184,32 +275,31 @@ impl<'a> LambdaSet<'a> {
         }
     }
 
-    pub fn extend_function_layout(
+    pub fn extend_argument_list(
         &self,
         arena: &'a Bump,
         argument_layouts: &'a [Layout<'a>],
-        ret_layout: &'a Layout<'a>,
-    ) -> Layout<'a> {
+    ) -> &'a [Layout<'a>] {
         if let [] = self.set {
             // TERRIBLE HACK for builting functions
-            Layout::FunctionPointer(argument_layouts, ret_layout)
+            argument_layouts
         } else {
             match self.representation {
                 Layout::Struct(&[]) => {
                     // this function does not have anything in its closure, and the lambda set is a
                     // singleton, so we pass no extra argument
-                    Layout::FunctionPointer(argument_layouts, ret_layout)
+                    argument_layouts
                 }
                 Layout::Builtin(Builtin::Int1) | Layout::Builtin(Builtin::Int8) => {
                     // we don't pass this along either
-                    Layout::FunctionPointer(argument_layouts, ret_layout)
+                    argument_layouts
                 }
                 _ => {
                     let mut arguments = Vec::with_capacity_in(argument_layouts.len() + 1, arena);
                     arguments.extend(argument_layouts);
                     arguments.push(self.runtime_representation());
 
-                    Layout::FunctionPointer(arguments.into_bump_slice(), ret_layout)
+                    arguments.into_bump_slice()
                 }
             }
         }
@@ -277,10 +367,13 @@ impl<'a> LambdaSet<'a> {
 
         use UnionVariant::*;
         match variant {
-            Never | Unit | UnitWithArguments => Layout::Struct(&[]),
+            Never => Layout::Union(UnionLayout::NonRecursive(&[])),
+            Unit | UnitWithArguments => Layout::Struct(&[]),
             BoolUnion { .. } => Layout::Builtin(Builtin::Int1),
             ByteUnion(_) => Layout::Builtin(Builtin::Int8),
-            Unwrapped(_tag_name, layouts) => Layout::Struct(layouts.into_bump_slice()),
+            Newtype {
+                arguments: layouts, ..
+            } => Layout::Struct(layouts.into_bump_slice()),
             Wrapped(variant) => {
                 use WrappedVariant::*;
 
@@ -307,17 +400,6 @@ impl<'a> LambdaSet<'a> {
         }
     }
 
-    pub fn get_wrapped(&self) -> crate::ir::Wrapped {
-        use crate::ir::Wrapped;
-
-        match self.representation {
-            Layout::Struct(fields) if fields.len() == 1 => Wrapped::SingleElementRecord,
-            Layout::Struct(_) => Wrapped::RecordOrSingleTagUnion,
-            Layout::Union(_) => Wrapped::MultiTagUnion,
-            _ => Wrapped::SingleElementRecord,
-        }
-    }
-
     pub fn stack_size(&self, pointer_size: u32) -> u32 {
         self.representation.stack_size(pointer_size)
     }
@@ -333,12 +415,6 @@ impl<'a> LambdaSet<'a> {
     }
 }
 
-#[derive(Clone, Debug, PartialEq, Eq, Hash, Copy)]
-pub enum MemoryMode {
-    Unique,
-    Refcounted,
-}
-
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
 pub enum Builtin<'a> {
     Int128,
@@ -355,7 +431,7 @@ pub enum Builtin<'a> {
     Str,
     Dict(&'a Layout<'a>, &'a Layout<'a>),
     Set(&'a Layout<'a>),
-    List(MemoryMode, &'a Layout<'a>),
+    List(&'a Layout<'a>),
     EmptyStr,
     EmptyList,
     EmptyDict,
@@ -479,7 +555,6 @@ impl<'a> Layout<'a> {
 
         match self {
             Builtin(builtin) => builtin.safe_to_memcpy(),
-            PhantomEmptyStruct => true,
             Struct(fields) => fields
                 .iter()
                 .all(|field_layout| field_layout.safe_to_memcpy()),
@@ -499,15 +574,7 @@ impl<'a> Layout<'a> {
                     }
                 }
             }
-            FunctionPointer(_, _) => {
-                // Function pointers are immutable and can always be safely copied
-                true
-            }
             Closure(_, closure_layout, _) => closure_layout.safe_to_memcpy(),
-            Pointer(_) => {
-                // We cannot memcpy pointers, because then we would have the same pointer in multiple places!
-                false
-            }
             RecursivePointer => {
                 // We cannot memcpy pointers, because then we would have the same pointer in multiple places!
                 false
@@ -519,12 +586,7 @@ impl<'a> Layout<'a> {
         // For this calculation, we don't need an accurate
         // stack size, we just need to know whether it's zero,
         // so it's fine to use a pointer size of 1.
-        if let Layout::PhantomEmptyStruct = self {
-            false
-        } else {
-            // self.stack_size(1) == 0
-            false
-        }
+        false
     }
 
     pub fn stack_size(&self, pointer_size: u32) -> u32 {
@@ -532,7 +594,6 @@ impl<'a> Layout<'a> {
 
         match self {
             Builtin(builtin) => builtin.stack_size(pointer_size),
-            PhantomEmptyStruct => 0,
             Struct(fields) => {
                 let mut sum = 0;
 
@@ -546,16 +607,20 @@ impl<'a> Layout<'a> {
                 use UnionLayout::*;
 
                 match variant {
-                    NonRecursive(fields) => fields
-                        .iter()
-                        .map(|tag_layout| {
-                            tag_layout
-                                .iter()
-                                .map(|field| field.stack_size(pointer_size))
-                                .sum()
-                        })
-                        .max()
-                        .unwrap_or_default(),
+                    NonRecursive(fields) => {
+                        fields
+                            .iter()
+                            .map(|tag_layout| {
+                                tag_layout
+                                    .iter()
+                                    .map(|field| field.stack_size(pointer_size))
+                                    .sum::<u32>()
+                            })
+                            .max()
+                            .unwrap_or_default()
+                            // the size of the tag_id
+                            + pointer_size
+                    }
 
                     Recursive(_)
                     | NullableWrapped { .. }
@@ -564,9 +629,7 @@ impl<'a> Layout<'a> {
                 }
             }
             Closure(_, lambda_set, _) => lambda_set.stack_size(pointer_size),
-            FunctionPointer(_, _) => pointer_size,
             RecursivePointer => pointer_size,
-            Pointer(_) => pointer_size,
         }
     }
 
@@ -596,10 +659,7 @@ impl<'a> Layout<'a> {
                 }
             }
             Layout::Builtin(builtin) => builtin.alignment_bytes(pointer_size),
-            Layout::PhantomEmptyStruct => 0,
             Layout::RecursivePointer => pointer_size,
-            Layout::FunctionPointer(_, _) => pointer_size,
-            Layout::Pointer(_) => pointer_size,
             Layout::Closure(_, captured, _) => {
                 pointer_size.max(captured.alignment_bytes(pointer_size))
             }
@@ -622,7 +682,7 @@ impl<'a> Layout<'a> {
 
             RecursivePointer => true,
 
-            Builtin(List(MemoryMode::Refcounted, _)) | Builtin(Str) => true,
+            Builtin(List(_)) | Builtin(Str) => true,
 
             _ => false,
         }
@@ -636,7 +696,6 @@ impl<'a> Layout<'a> {
 
         match self {
             Builtin(builtin) => builtin.is_refcounted(),
-            PhantomEmptyStruct => false,
             Struct(fields) => fields.iter().any(|f| f.contains_refcounted()),
             Union(variant) => {
                 use UnionLayout::*;
@@ -655,13 +714,12 @@ impl<'a> Layout<'a> {
             }
             RecursivePointer => true,
             Closure(_, closure_layout, _) => closure_layout.contains_refcounted(),
-            FunctionPointer(_, _) | Pointer(_) => false,
         }
     }
 
-    pub fn to_doc<'b, D, A>(&'b self, alloc: &'b D, parens: Parens) -> DocBuilder<'b, D, A>
+    pub fn to_doc<D, A>(self, alloc: &'a D, parens: Parens) -> DocBuilder<'a, D, A>
     where
-        D: DocAllocator<'b, A>,
+        D: DocAllocator<'a, A>,
         D::Doc: Clone,
         A: Clone,
     {
@@ -669,7 +727,6 @@ impl<'a> Layout<'a> {
 
         match self {
             Builtin(builtin) => builtin.to_doc(alloc, parens),
-            PhantomEmptyStruct => alloc.text("{}"),
             Struct(fields) => {
                 let fields_doc = fields.iter().map(|x| x.to_doc(alloc, parens));
 
@@ -680,14 +737,6 @@ impl<'a> Layout<'a> {
             }
             Union(union_layout) => union_layout.to_doc(alloc, parens),
             RecursivePointer => alloc.text("*self"),
-            FunctionPointer(args, result) => {
-                let args_doc = args.iter().map(|x| x.to_doc(alloc, Parens::InFunction));
-
-                alloc
-                    .intersperse(args_doc, ", ")
-                    .append(alloc.text(" -> "))
-                    .append(result.to_doc(alloc, Parens::InFunction))
-            }
             Closure(args, closure_layout, result) => {
                 let args_doc = args.iter().map(|x| x.to_doc(alloc, Parens::InFunction));
 
@@ -702,7 +751,6 @@ impl<'a> Layout<'a> {
                     .append(" |} -> ")
                     .append(result.to_doc(alloc, Parens::InFunction))
             }
-            Pointer(_) => todo!(),
         }
     }
 }
@@ -737,8 +785,6 @@ impl<'a> CachedVariable<'a> {
     }
 }
 
-// use ven_ena::unify::{InPlace, Snapshot, UnificationTable, UnifyKey};
-
 impl<'a> ven_ena::unify::UnifyKey for CachedVariable<'a> {
     type Value = CachedLayout<'a>;
 
@@ -803,6 +849,37 @@ impl<'a> LayoutCache<'a> {
         }
     }
 
+    pub fn raw_from_var(
+        &mut self,
+        arena: &'a Bump,
+        var: Variable,
+        subs: &Subs,
+    ) -> Result<RawFunctionLayout<'a>, LayoutProblem> {
+        // Store things according to the root Variable, to avoid duplicate work.
+        let var = subs.get_root_key_without_compacting(var);
+
+        let cached_var = CachedVariable::new(var);
+
+        self.expand_to_fit(cached_var);
+
+        use CachedLayout::*;
+        match self.layouts.probe_value(cached_var) {
+            Problem(problem) => Err(problem),
+            Cached(_) | NotCached => {
+                let mut env = Env {
+                    arena,
+                    subs,
+                    seen: MutSet::default(),
+                };
+
+                Layout::from_var(&mut env, var).map(|l| match l {
+                    Layout::Closure(a, b, c) => RawFunctionLayout::Function(a, b, c),
+                    other => RawFunctionLayout::ZeroArgumentThunk(other),
+                })
+            }
+        }
+    }
+
     fn expand_to_fit(&mut self, var: CachedVariable<'a>) {
         use ven_ena::unify::UnifyKey;
 
@@ -874,7 +951,7 @@ impl<'a> Builtin<'a> {
             Str | EmptyStr => Builtin::STR_WORDS * pointer_size,
             Dict(_, _) | EmptyDict => Builtin::DICT_WORDS * pointer_size,
             Set(_) | EmptySet => Builtin::SET_WORDS * pointer_size,
-            List(_, _) | EmptyList => Builtin::LIST_WORDS * pointer_size,
+            List(_) | EmptyList => Builtin::LIST_WORDS * pointer_size,
         }
     }
 
@@ -900,7 +977,7 @@ impl<'a> Builtin<'a> {
             Str | EmptyStr => pointer_size,
             Dict(_, _) | EmptyDict => pointer_size,
             Set(_) | EmptySet => pointer_size,
-            List(_, _) | EmptyList => pointer_size,
+            List(_) | EmptyList => pointer_size,
         }
     }
 
@@ -910,7 +987,7 @@ impl<'a> Builtin<'a> {
         match self {
             Int128 | Int64 | Int32 | Int16 | Int8 | Int1 | Usize | Float128 | Float64 | Float32
             | Float16 | EmptyStr | EmptyDict | EmptyList | EmptySet => true,
-            Str | Dict(_, _) | Set(_) | List(_, _) => false,
+            Str | Dict(_, _) | Set(_) | List(_) => false,
         }
     }
 
@@ -921,18 +998,15 @@ impl<'a> Builtin<'a> {
         match self {
             Int128 | Int64 | Int32 | Int16 | Int8 | Int1 | Usize | Float128 | Float64 | Float32
             | Float16 | EmptyStr | EmptyDict | EmptyList | EmptySet => false,
-            List(mode, element_layout) => match mode {
-                MemoryMode::Refcounted => true,
-                MemoryMode::Unique => element_layout.contains_refcounted(),
-            },
+            List(_) => true,
 
             Str | Dict(_, _) | Set(_) => true,
         }
     }
 
-    pub fn to_doc<'b, D, A>(&'b self, alloc: &'b D, _parens: Parens) -> DocBuilder<'b, D, A>
+    pub fn to_doc<D, A>(self, alloc: &'a D, _parens: Parens) -> DocBuilder<'a, D, A>
     where
-        D: DocAllocator<'b, A>,
+        D: DocAllocator<'a, A>,
         D::Doc: Clone,
         A: Clone,
     {
@@ -957,7 +1031,7 @@ impl<'a> Builtin<'a> {
             EmptySet => alloc.text("EmptySet"),
 
             Str => alloc.text("Str"),
-            List(_, layout) => alloc
+            List(layout) => alloc
                 .text("List ")
                 .append(layout.to_doc(alloc, Parens::InTypeParam)),
             Set(layout) => alloc
@@ -1139,7 +1213,6 @@ fn layout_from_flat_type<'a>(
             // That means none of the optimizations for enums or single tag tag unions apply
 
             let rec_var = subs.get_root_key_without_compacting(rec_var);
-            env.insert_seen(rec_var);
             let mut tag_layouts = Vec::with_capacity_in(tags.len(), arena);
 
             // VERY IMPORTANT: sort the tags
@@ -1157,17 +1230,15 @@ fn layout_from_flat_type<'a>(
                 }
             }
 
+            env.insert_seen(rec_var);
             for (index, (_name, variables)) in tags_vec.into_iter().enumerate() {
                 if matches!(nullable, Some(i) if i == index as i64) {
-                    // don't add the
+                    // don't add the nullable case
                     continue;
                 }
 
                 let mut tag_layout = Vec::with_capacity_in(variables.len() + 1, arena);
 
-                // store the discriminant
-                tag_layout.push(Layout::Builtin(TAG_SIZE));
-
                 for var in variables {
                     // TODO does this cause problems with mutually recursive unions?
                     if rec_var == subs.get_root_key_without_compacting(var) {
@@ -1189,6 +1260,7 @@ fn layout_from_flat_type<'a>(
 
                 tag_layouts.push(tag_layout.into_bump_slice());
             }
+            env.remove_seen(rec_var);
 
             let union_layout = if let Some(tag_id) = nullable {
                 match tag_layouts.into_bump_slice() {
@@ -1207,13 +1279,11 @@ fn layout_from_flat_type<'a>(
                 }
             } else if tag_layouts.len() == 1 {
                 // drop the tag id
-                UnionLayout::NonNullableUnwrapped(&tag_layouts.pop().unwrap()[1..])
+                UnionLayout::NonNullableUnwrapped(&tag_layouts.pop().unwrap())
             } else {
                 UnionLayout::Recursive(tag_layouts.into_bump_slice())
             };
 
-            env.remove_seen(rec_var);
-
             Ok(Layout::Union(union_layout))
         }
         EmptyTagUnion => {
@@ -1292,9 +1362,15 @@ pub enum UnionVariant<'a> {
     Never,
     Unit,
     UnitWithArguments,
-    BoolUnion { ttrue: TagName, ffalse: TagName },
+    BoolUnion {
+        ttrue: TagName,
+        ffalse: TagName,
+    },
     ByteUnion(Vec<'a, TagName>),
-    Unwrapped(TagName, Vec<'a, Layout<'a>>),
+    Newtype {
+        tag_name: TagName,
+        arguments: Vec<'a, Layout<'a>>,
+    },
     Wrapped(WrappedVariant<'a>),
 }
 
@@ -1448,10 +1524,6 @@ pub fn union_sorted_tags_help<'a>(
         seen: MutSet::default(),
     };
 
-    if let Some(rec_var) = opt_rec_var {
-        env.insert_seen(rec_var);
-    }
-
     match tags_vec.len() {
         0 => {
             // trying to instantiate a type with no values
@@ -1516,7 +1588,10 @@ pub fn union_sorted_tags_help<'a>(
                     fields: layouts.into_bump_slice(),
                 })
             } else {
-                UnionVariant::Unwrapped(tag_name, layouts)
+                UnionVariant::Newtype {
+                    tag_name,
+                    arguments: layouts,
+                }
             }
         }
         num_tags => {
@@ -1546,9 +1621,6 @@ pub fn union_sorted_tags_help<'a>(
 
                 let mut arg_layouts = Vec::with_capacity_in(arguments.len() + 1, arena);
 
-                // add the tag discriminant (size currently always hardcoded to i64)
-                arg_layouts.push(Layout::Builtin(TAG_SIZE));
-
                 for var in arguments {
                     match Layout::from_var(&mut env, var) {
                         Ok(layout) => {
@@ -1668,7 +1740,10 @@ pub fn layout_from_tag_union<'a>(
                 Unit | UnitWithArguments => Layout::Struct(&[]),
                 BoolUnion { .. } => Layout::Builtin(Builtin::Int1),
                 ByteUnion(_) => Layout::Builtin(Builtin::Int8),
-                Unwrapped(_, mut field_layouts) => {
+                Newtype {
+                    arguments: mut field_layouts,
+                    ..
+                } => {
                     if field_layouts.len() == 1 {
                         field_layouts.pop().unwrap()
                     } else {
@@ -1897,11 +1972,7 @@ pub fn list_layout_from_elem<'a>(
         _ => {
             let elem_layout = Layout::from_var(env, elem_var)?;
 
-            // This is a normal list.
-            Ok(Layout::Builtin(Builtin::List(
-                MemoryMode::Refcounted,
-                env.arena.alloc(elem_layout),
-            )))
+            Ok(Layout::Builtin(Builtin::List(env.arena.alloc(elem_layout))))
         }
     }
 }
@@ -1921,6 +1992,7 @@ impl LayoutId {
 
 struct IdsByLayout<'a> {
     by_id: MutMap<Layout<'a>, u32>,
+    toplevels_by_id: MutMap<crate::ir::ProcLayout<'a>, u32>,
     next_id: u32,
 }
 
@@ -1937,6 +2009,7 @@ impl<'a> LayoutIds<'a> {
         // There's probably a nicer way to write it that still works.
         let ids = self.by_symbol.entry(symbol).or_insert_with(|| IdsByLayout {
             by_id: HashMap::with_capacity_and_hasher(1, default_hasher()),
+            toplevels_by_id: Default::default(),
             next_id: 1,
         });
 
@@ -1953,6 +2026,39 @@ impl<'a> LayoutIds<'a> {
 
         LayoutId(answer)
     }
+
+    /// Returns a LayoutId which is unique for the given symbol and layout.
+    /// If given the same symbol and same layout, returns the same LayoutId.
+    pub fn get_toplevel<'b>(
+        &mut self,
+        symbol: Symbol,
+        layout: &'b crate::ir::ProcLayout<'a>,
+    ) -> LayoutId {
+        // Note: this function does some weird stuff to satisfy the borrow checker.
+        // There's probably a nicer way to write it that still works.
+        let ids = self.by_symbol.entry(symbol).or_insert_with(|| IdsByLayout {
+            by_id: Default::default(),
+            toplevels_by_id: HashMap::with_capacity_and_hasher(1, default_hasher()),
+            next_id: 1,
+        });
+
+        // Get the id associated with this layout, or default to next_id.
+        let answer = ids
+            .toplevels_by_id
+            .get(&layout)
+            .copied()
+            .unwrap_or(ids.next_id);
+
+        // If we had to default to next_id, it must not have been found;
+        // store the ID we're going to return and increment next_id.
+        if answer == ids.next_id {
+            ids.toplevels_by_id.insert(*layout, ids.next_id);
+
+            ids.next_id += 1;
+        }
+
+        LayoutId(answer)
+    }
 }
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
@@ -1967,7 +2073,7 @@ impl<'a> std::convert::TryFrom<&Layout<'a>> for ListLayout<'a> {
     fn try_from(value: &Layout<'a>) -> Result<Self, Self::Error> {
         match value {
             Layout::Builtin(Builtin::EmptyList) => Ok(ListLayout::EmptyList),
-            Layout::Builtin(Builtin::List(_, element)) => Ok(ListLayout::List(element)),
+            Layout::Builtin(Builtin::List(element)) => Ok(ListLayout::List(element)),
             _ => Err(()),
         }
     }
diff --git a/compiler/mono/src/tail_recursion.rs b/compiler/mono/src/tail_recursion.rs
index 4bb77a90e4..0eae4df30a 100644
--- a/compiler/mono/src/tail_recursion.rs
+++ b/compiler/mono/src/tail_recursion.rs
@@ -60,7 +60,7 @@ pub fn make_tail_recursive<'a>(
                 id,
                 remainder: jump,
                 parameters: params,
-                continuation: new,
+                body: new,
             }
         }
     }
@@ -102,9 +102,10 @@ fn insert_jumps<'a>(
                 },
             fail,
             pass: Stmt::Ret(rsym),
+            exception_id,
             ..
         } if needle == *fsym && symbol == rsym => {
-            debug_assert_eq!(fail, &&Stmt::Rethrow);
+            debug_assert_eq!(fail, &&Stmt::Resume(*exception_id));
 
             // replace the call and return with a jump
 
@@ -131,6 +132,7 @@ fn insert_jumps<'a>(
             fail,
             pass,
             layout,
+            exception_id,
         } => {
             let opt_pass = insert_jumps(arena, pass, goal_id, needle);
             let opt_fail = insert_jumps(arena, fail, goal_id, needle);
@@ -145,6 +147,7 @@ fn insert_jumps<'a>(
                     layout: *layout,
                     pass,
                     fail,
+                    exception_id: *exception_id,
                 };
 
                 Some(arena.alloc(stmt))
@@ -157,7 +160,7 @@ fn insert_jumps<'a>(
             id,
             parameters,
             remainder,
-            continuation,
+            body: continuation,
         } => {
             let opt_remainder = insert_jumps(arena, remainder, goal_id, needle);
             let opt_continuation = insert_jumps(arena, continuation, goal_id, needle);
@@ -170,7 +173,7 @@ fn insert_jumps<'a>(
                     id: *id,
                     parameters,
                     remainder,
-                    continuation,
+                    body: continuation,
                 }))
             } else {
                 None
@@ -238,7 +241,7 @@ fn insert_jumps<'a>(
             None => None,
         },
 
-        Rethrow => None,
+        Resume(_) => None,
         Ret(_) => None,
         Jump(_, _) => None,
         RuntimeError(_) => None,
diff --git a/compiler/mono/tests/helpers/mod.rs b/compiler/mono/tests/helpers/mod.rs
deleted file mode 100644
index ef782e17a1..0000000000
--- a/compiler/mono/tests/helpers/mod.rs
+++ /dev/null
@@ -1,41 +0,0 @@
-extern crate bumpalo;
-
-/// Used in the with_larger_debug_stack() function, for tests that otherwise
-/// run out of stack space in debug builds (but don't in --release builds)
-#[allow(dead_code)]
-const EXPANDED_STACK_SIZE: usize = 8 * 1024 * 1024;
-
-/// Without this, some tests pass in `cargo test --release` but fail without
-/// the --release flag because they run out of stack space. This increases
-/// stack size for debug builds only, while leaving the stack space at the default
-/// amount for release builds.
-#[allow(dead_code)]
-#[cfg(debug_assertions)]
-pub fn with_larger_debug_stack<F>(run_test: F)
-where
-    F: FnOnce(),
-    F: Send,
-    F: 'static,
-{
-    std::thread::Builder::new()
-        .stack_size(EXPANDED_STACK_SIZE)
-        .spawn(run_test)
-        .expect("Error while spawning expanded dev stack size thread")
-        .join()
-        .expect("Error while joining expanded dev stack size thread")
-}
-
-/// In --release builds, don't increase the stack size. Run the test normally.
-/// This way, we find out if any of our tests are blowing the stack even after
-/// optimizations in release builds.
-#[allow(dead_code)]
-#[cfg(not(debug_assertions))]
-#[inline(always)]
-pub fn with_larger_debug_stack<F>(run_test: F)
-where
-    F: FnOnce() -> (),
-    F: Send,
-    F: 'static,
-{
-    run_test()
-}
diff --git a/compiler/mono/tests/test_mono.rs b/compiler/mono/tests/test_mono.rs
deleted file mode 100644
index ce1f0bac7e..0000000000
--- a/compiler/mono/tests/test_mono.rs
+++ /dev/null
@@ -1,2600 +0,0 @@
-#[macro_use]
-extern crate pretty_assertions;
-
-#[macro_use]
-extern crate indoc;
-
-extern crate bumpalo;
-extern crate roc_mono;
-
-mod helpers;
-
-// Test monomorphization
-#[cfg(test)]
-mod test_mono {
-    use roc_can::builtins::builtin_defs_map;
-    use roc_collections::all::MutMap;
-    use roc_module::symbol::Symbol;
-    use roc_mono::ir::Proc;
-
-    use roc_mono::ir::TopLevelFunctionLayout;
-
-    fn promote_expr_to_module(src: &str) -> String {
-        let mut buffer =
-            String::from("app \"test\" provides [ main ] to \"./platform\"\n\nmain =\n");
-
-        for line in src.lines() {
-            // indent the body!
-            buffer.push_str("    ");
-            buffer.push_str(line);
-            buffer.push('\n');
-        }
-
-        buffer
-    }
-
-    fn compiles_to_ir(src: &str, expected: &str) {
-        use bumpalo::Bump;
-        use std::path::{Path, PathBuf};
-
-        let arena = &Bump::new();
-
-        // let stdlib = roc_builtins::unique::uniq_stdlib();
-        let stdlib = roc_builtins::std::standard_stdlib();
-        let filename = PathBuf::from("Test.roc");
-        let src_dir = Path::new("fake/test/path");
-
-        let module_src;
-        let temp;
-        if src.starts_with("app") {
-            // this is already a module
-            module_src = src;
-        } else {
-            // this is an expression, promote it to a module
-            temp = promote_expr_to_module(src);
-            module_src = &temp;
-        }
-
-        let exposed_types = MutMap::default();
-
-        let loaded = roc_load::file::load_and_monomorphize_from_str(
-            arena,
-            filename,
-            &module_src,
-            &stdlib,
-            src_dir,
-            exposed_types,
-            8,
-            builtin_defs_map,
-        );
-
-        let mut loaded = match loaded {
-            Ok(x) => x,
-            Err(roc_load::file::LoadingProblem::FormattedReport(report)) => {
-                println!("{}", report);
-                panic!();
-            }
-            Err(e) => panic!("{:?}", e),
-        };
-
-        use roc_load::file::MonomorphizedModule;
-        let MonomorphizedModule {
-            module_id: home,
-            procedures,
-            exposed_to_host,
-            ..
-        } = loaded;
-
-        let can_problems = loaded.can_problems.remove(&home).unwrap_or_default();
-        let type_problems = loaded.type_problems.remove(&home).unwrap_or_default();
-        let mono_problems = loaded.mono_problems.remove(&home).unwrap_or_default();
-
-        if !can_problems.is_empty() {
-            println!("Ignoring {} canonicalization problems", can_problems.len());
-        }
-
-        assert_eq!(type_problems, Vec::new());
-        assert_eq!(mono_problems, Vec::new());
-
-        debug_assert_eq!(exposed_to_host.len(), 1);
-
-        let main_fn_symbol = exposed_to_host.keys().copied().next().unwrap();
-
-        verify_procedures(expected, procedures, main_fn_symbol);
-    }
-
-    #[cfg(debug_assertions)]
-    fn verify_procedures(
-        expected: &str,
-        procedures: MutMap<(Symbol, TopLevelFunctionLayout<'_>), Proc<'_>>,
-        main_fn_symbol: Symbol,
-    ) {
-        let index = procedures
-            .keys()
-            .position(|(s, _)| *s == main_fn_symbol)
-            .unwrap();
-
-        let mut procs_string = procedures
-            .values()
-            .map(|proc| proc.to_pretty(200))
-            .collect::<Vec<_>>();
-
-        let main_fn = procs_string.swap_remove(index);
-
-        procs_string.sort();
-        procs_string.push(main_fn);
-
-        let result = procs_string.join("\n");
-
-        let the_same = result == expected;
-
-        if !the_same {
-            let expected_lines = expected.split('\n').collect::<Vec<&str>>();
-            let result_lines = result.split('\n').collect::<Vec<&str>>();
-
-            for line in &result_lines {
-                if !line.is_empty() {
-                    println!("                {}", line);
-                } else {
-                    println!();
-                }
-            }
-
-            assert_eq!(expected_lines, result_lines);
-            assert_eq!(0, 1);
-        }
-    }
-
-    // NOTE because the Show instance of module names is different in --release mode,
-    // these tests would all fail. In the future, when we do interesting optimizations,
-    // we'll likely want some tests for --release too.
-    #[cfg(not(debug_assertions))]
-    fn verify_procedures(
-        _expected: &str,
-        _procedures: MutMap<(Symbol, TopLevelFunctionLayout<'_>), Proc<'_>>,
-        _main_fn_symbol: Symbol,
-    ) {
-        // Do nothing
-    }
-
-    #[test]
-    fn ir_int_literal() {
-        compiles_to_ir(
-            r#"
-            5
-            "#,
-            indoc!(
-                r#"
-                procedure Test.0 ():
-                    let Test.1 = 5i64;
-                    ret Test.1;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn ir_int_add() {
-        compiles_to_ir(
-            r#"
-            x = [ 1,2 ]
-            5 + 4 + 3 + List.len x
-            "#,
-            indoc!(
-                r#"
-                procedure List.7 (#Attr.2):
-                    let Test.6 = lowlevel ListLen #Attr.2;
-                    ret Test.6;
-
-                procedure Num.24 (#Attr.2, #Attr.3):
-                    let Test.5 = lowlevel NumAdd #Attr.2 #Attr.3;
-                    ret Test.5;
-
-                procedure Test.0 ():
-                    let Test.11 = 1i64;
-                    let Test.12 = 2i64;
-                    let Test.1 = Array [Test.11, Test.12];
-                    let Test.9 = 5i64;
-                    let Test.10 = 4i64;
-                    invoke Test.7 = CallByName Num.24 Test.9 Test.10 catch
-                        dec Test.1;
-                        unreachable;
-                    let Test.8 = 3i64;
-                    invoke Test.3 = CallByName Num.24 Test.7 Test.8 catch
-                        dec Test.1;
-                        unreachable;
-                    let Test.4 = CallByName List.7 Test.1;
-                    dec Test.1;
-                    let Test.2 = CallByName Num.24 Test.3 Test.4;
-                    ret Test.2;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn ir_assignment() {
-        compiles_to_ir(
-            r#"
-            x = 5
-
-            x
-            "#,
-            indoc!(
-                r#"
-                procedure Test.0 ():
-                    let Test.1 = 5i64;
-                    ret Test.1;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn ir_when_maybe() {
-        compiles_to_ir(
-            r#"
-            when Just 3 is
-                Just n -> n
-                Nothing -> 0
-            "#,
-            indoc!(
-                r#"
-                procedure Test.0 ():
-                    let Test.10 = 0i64;
-                    let Test.9 = 3i64;
-                    let Test.3 = Just Test.10 Test.9;
-                    let Test.6 = 0i64;
-                    let Test.7 = Index 0 Test.3;
-                    let Test.8 = lowlevel Eq Test.6 Test.7;
-                    if Test.8 then
-                        let Test.2 = Index 1 Test.3;
-                        ret Test.2;
-                    else
-                        let Test.5 = 0i64;
-                        ret Test.5;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn ir_when_these() {
-        compiles_to_ir(
-            r#"
-            when These 1 2 is
-                This x -> x
-                That y -> y
-                These x _ -> x
-            "#,
-            indoc!(
-                r#"
-                procedure Test.0 ():
-                    let Test.11 = 1i64;
-                    let Test.9 = 1i64;
-                    let Test.10 = 2i64;
-                    let Test.5 = These Test.11 Test.9 Test.10;
-                    switch Test.5:
-                        case 2:
-                            let Test.2 = Index 1 Test.5;
-                            ret Test.2;
-                    
-                        case 0:
-                            let Test.3 = Index 1 Test.5;
-                            ret Test.3;
-                    
-                        default:
-                            let Test.4 = Index 1 Test.5;
-                            ret Test.4;
-                    
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn ir_when_record() {
-        compiles_to_ir(
-            r#"
-            when { x: 1, y: 3.14 } is
-                { x } -> x
-            "#,
-            indoc!(
-                r#"
-                procedure Test.0 ():
-                    let Test.4 = 1i64;
-                    let Test.5 = 3.14f64;
-                    let Test.2 = Struct {Test.4, Test.5};
-                    let Test.1 = Index 0 Test.2;
-                    ret Test.1;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn ir_plus() {
-        compiles_to_ir(
-            r#"
-            1 + 2
-            "#,
-            indoc!(
-                r#"
-                procedure Num.24 (#Attr.2, #Attr.3):
-                    let Test.4 = lowlevel NumAdd #Attr.2 #Attr.3;
-                    ret Test.4;
-
-                procedure Test.0 ():
-                    let Test.2 = 1i64;
-                    let Test.3 = 2i64;
-                    let Test.1 = CallByName Num.24 Test.2 Test.3;
-                    ret Test.1;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn ir_round() {
-        compiles_to_ir(
-            r#"
-            Num.round 3.6
-            "#,
-            indoc!(
-                r#"
-                procedure Num.47 (#Attr.2):
-                    let Test.3 = lowlevel NumRound #Attr.2;
-                    ret Test.3;
-
-                procedure Test.0 ():
-                    let Test.2 = 3.6f64;
-                    let Test.1 = CallByName Num.47 Test.2;
-                    ret Test.1;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn ir_when_idiv() {
-        compiles_to_ir(
-            r#"
-            when 1000 // 10 is
-                Ok val -> val
-                Err _ -> -1
-            "#,
-            indoc!(
-                r#"
-                procedure Num.42 (#Attr.2, #Attr.3):
-                    let Test.17 = 0i64;
-                    let Test.13 = lowlevel NotEq #Attr.3 Test.17;
-                    if Test.13 then
-                        let Test.16 = 1i64;
-                        let Test.15 = lowlevel NumDivUnchecked #Attr.2 #Attr.3;
-                        let Test.14 = Ok Test.16 Test.15;
-                        ret Test.14;
-                    else
-                        let Test.12 = 0i64;
-                        let Test.11 = Struct {};
-                        let Test.10 = Err Test.12 Test.11;
-                        ret Test.10;
-
-                procedure Test.0 ():
-                    let Test.8 = 1000i64;
-                    let Test.9 = 10i64;
-                    let Test.2 = CallByName Num.42 Test.8 Test.9;
-                    let Test.5 = 1i64;
-                    let Test.6 = Index 0 Test.2;
-                    let Test.7 = lowlevel Eq Test.5 Test.6;
-                    if Test.7 then
-                        let Test.1 = Index 1 Test.2;
-                        ret Test.1;
-                    else
-                        let Test.4 = -1i64;
-                        ret Test.4;
-            "#
-            ),
-        )
-    }
-
-    #[test]
-    fn ir_two_defs() {
-        compiles_to_ir(
-            r#"
-            x = 3
-            y = 4
-
-            x + y
-            "#,
-            indoc!(
-                r#"
-                procedure Num.24 (#Attr.2, #Attr.3):
-                    let Test.4 = lowlevel NumAdd #Attr.2 #Attr.3;
-                    ret Test.4;
-
-                procedure Test.0 ():
-                    let Test.1 = 3i64;
-                    let Test.2 = 4i64;
-                    let Test.3 = CallByName Num.24 Test.1 Test.2;
-                    ret Test.3;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn ir_when_just() {
-        compiles_to_ir(
-            r#"
-            x : [ Nothing, Just I64 ]
-            x = Just 41
-
-            when x is
-                Just v -> v + 0x1
-                Nothing -> 0x1
-            "#,
-            indoc!(
-                r#"
-                procedure Num.24 (#Attr.2, #Attr.3):
-                    let Test.6 = lowlevel NumAdd #Attr.2 #Attr.3;
-                    ret Test.6;
-
-                procedure Test.0 ():
-                    let Test.12 = 0i64;
-                    let Test.11 = 41i64;
-                    let Test.1 = Just Test.12 Test.11;
-                    let Test.8 = 0i64;
-                    let Test.9 = Index 0 Test.1;
-                    let Test.10 = lowlevel Eq Test.8 Test.9;
-                    if Test.10 then
-                        let Test.3 = Index 1 Test.1;
-                        let Test.5 = 1i64;
-                        let Test.4 = CallByName Num.24 Test.3 Test.5;
-                        ret Test.4;
-                    else
-                        let Test.7 = 1i64;
-                        ret Test.7;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn one_element_tag() {
-        compiles_to_ir(
-            r#"
-            x : [ Pair I64 ]
-            x = Pair 2
-
-            x
-            "#,
-            indoc!(
-                r#"
-                procedure Test.0 ():
-                    let Test.4 = 2i64;
-                    ret Test.4;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn guard_pattern_true() {
-        compiles_to_ir(
-            r#"
-            wrapper = \{} ->
-                when 2 is
-                    2 if False -> 42
-                    _ -> 0
-
-            wrapper {}
-            "#,
-            indoc!(
-                r#"
-                procedure Test.1 (Test.3):
-                    let Test.6 = 2i64;
-                    joinpoint Test.12:
-                        let Test.10 = 0i64;
-                        ret Test.10;
-                    in
-                    let Test.11 = 2i64;
-                    let Test.14 = lowlevel Eq Test.11 Test.6;
-                    if Test.14 then
-                        joinpoint Test.8 Test.13:
-                            if Test.13 then
-                                let Test.7 = 42i64;
-                                ret Test.7;
-                            else
-                                jump Test.12;
-                        in
-                        let Test.9 = false;
-                        jump Test.8 Test.9;
-                    else
-                        jump Test.12;
-
-                procedure Test.0 ():
-                    let Test.5 = Struct {};
-                    let Test.4 = CallByName Test.1 Test.5;
-                    ret Test.4;
-               "#
-            ),
-        )
-    }
-
-    #[test]
-    fn when_on_record() {
-        compiles_to_ir(
-            r#"
-            when { x: 0x2 } is
-                { x } -> x + 3
-            "#,
-            indoc!(
-                r#"
-                procedure Num.24 (#Attr.2, #Attr.3):
-                    let Test.5 = lowlevel NumAdd #Attr.2 #Attr.3;
-                    ret Test.5;
-
-                procedure Test.0 ():
-                    let Test.6 = 2i64;
-                    let Test.1 = Index 0 Test.6;
-                    let Test.4 = 3i64;
-                    let Test.3 = CallByName Num.24 Test.1 Test.4;
-                    ret Test.3;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn when_nested_maybe() {
-        compiles_to_ir(
-            r#"
-            Maybe a : [ Nothing, Just a ]
-
-            x : Maybe (Maybe I64)
-            x = Just (Just 41)
-
-            when x is
-                Just (Just v) -> v + 0x1
-                _ -> 0x1
-            "#,
-            indoc!(
-                r#"
-                procedure Num.24 (#Attr.2, #Attr.3):
-                    let Test.8 = lowlevel NumAdd #Attr.2 #Attr.3;
-                    ret Test.8;
-
-                procedure Test.0 ():
-                    let Test.20 = 0i64;
-                    let Test.22 = 0i64;
-                    let Test.21 = 41i64;
-                    let Test.19 = Just Test.22 Test.21;
-                    let Test.2 = Just Test.20 Test.19;
-                    joinpoint Test.16:
-                        let Test.10 = 1i64;
-                        ret Test.10;
-                    in
-                    let Test.14 = 0i64;
-                    let Test.15 = Index 0 Test.2;
-                    let Test.18 = lowlevel Eq Test.14 Test.15;
-                    if Test.18 then
-                        let Test.11 = Index 1 Test.2;
-                        let Test.12 = 0i64;
-                        let Test.13 = Index 0 Test.11;
-                        let Test.17 = lowlevel Eq Test.12 Test.13;
-                        if Test.17 then
-                            let Test.9 = Index 1 Test.2;
-                            let Test.5 = Index 1 Test.9;
-                            let Test.7 = 1i64;
-                            let Test.6 = CallByName Num.24 Test.5 Test.7;
-                            ret Test.6;
-                        else
-                            jump Test.16;
-                    else
-                        jump Test.16;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn when_on_two_values() {
-        compiles_to_ir(
-            r#"
-            when Pair 2 3 is
-                Pair 4 3 -> 9
-                Pair a b -> a + b
-            "#,
-            indoc!(
-                r#"
-                procedure Num.24 (#Attr.2, #Attr.3):
-                    let Test.7 = lowlevel NumAdd #Attr.2 #Attr.3;
-                    ret Test.7;
-
-                procedure Test.0 ():
-                    let Test.16 = 3i64;
-                    let Test.15 = 2i64;
-                    let Test.4 = Struct {Test.15, Test.16};
-                    joinpoint Test.12:
-                        let Test.2 = Index 0 Test.4;
-                        let Test.3 = Index 1 Test.4;
-                        let Test.6 = CallByName Num.24 Test.2 Test.3;
-                        ret Test.6;
-                    in
-                    let Test.10 = Index 1 Test.4;
-                    let Test.11 = 3i64;
-                    let Test.14 = lowlevel Eq Test.11 Test.10;
-                    if Test.14 then
-                        let Test.8 = Index 0 Test.4;
-                        let Test.9 = 4i64;
-                        let Test.13 = lowlevel Eq Test.9 Test.8;
-                        if Test.13 then
-                            let Test.5 = 9i64;
-                            ret Test.5;
-                        else
-                            jump Test.12;
-                    else
-                        jump Test.12;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn dict() {
-        compiles_to_ir(
-            r#"
-            Dict.len Dict.empty
-            "#,
-            indoc!(
-                r#"
-                procedure Dict.2 ():
-                    let Test.4 = lowlevel DictEmpty ;
-                    ret Test.4;
-
-                procedure Dict.8 (#Attr.2):
-                    let Test.3 = lowlevel DictSize #Attr.2;
-                    ret Test.3;
-
-                procedure Test.0 ():
-                    let Test.2 = CallByName Dict.2;
-                    let Test.1 = CallByName Dict.8 Test.2;
-                    ret Test.1;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn list_append_closure() {
-        compiles_to_ir(
-            r#"
-            myFunction = \l -> List.append l 42
-
-            myFunction [ 1, 2 ]
-            "#,
-            indoc!(
-                r#"
-                procedure List.5 (#Attr.2, #Attr.3):
-                    let Test.7 = lowlevel ListAppend #Attr.2 #Attr.3;
-                    ret Test.7;
-
-                procedure Test.1 (Test.2):
-                    let Test.6 = 42i64;
-                    let Test.5 = CallByName List.5 Test.2 Test.6;
-                    ret Test.5;
-
-                procedure Test.0 ():
-                    let Test.8 = 1i64;
-                    let Test.9 = 2i64;
-                    let Test.4 = Array [Test.8, Test.9];
-                    let Test.3 = CallByName Test.1 Test.4;
-                    ret Test.3;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn list_append() {
-        // TODO this leaks at the moment
-        // ListAppend needs to decrement its arguments
-        compiles_to_ir(
-            r#"
-            List.append [1] 2
-            "#,
-            indoc!(
-                r#"
-                procedure List.5 (#Attr.2, #Attr.3):
-                    let Test.4 = lowlevel ListAppend #Attr.2 #Attr.3;
-                    ret Test.4;
-
-                procedure Test.0 ():
-                    let Test.5 = 1i64;
-                    let Test.2 = Array [Test.5];
-                    let Test.3 = 2i64;
-                    let Test.1 = CallByName List.5 Test.2 Test.3;
-                    ret Test.1;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn list_len() {
-        compiles_to_ir(
-            r#"
-            x = [1,2,3]
-            y = [ 1.0 ]
-
-            List.len x + List.len y
-            "#,
-            indoc!(
-                r#"
-                procedure List.7 (#Attr.2):
-                    let Test.7 = lowlevel ListLen #Attr.2;
-                    ret Test.7;
-
-                procedure List.7 (#Attr.2):
-                    let Test.8 = lowlevel ListLen #Attr.2;
-                    ret Test.8;
-
-                procedure Num.24 (#Attr.2, #Attr.3):
-                    let Test.6 = lowlevel NumAdd #Attr.2 #Attr.3;
-                    ret Test.6;
-
-                procedure Test.0 ():
-                    let Test.10 = 1i64;
-                    let Test.11 = 2i64;
-                    let Test.12 = 3i64;
-                    let Test.1 = Array [Test.10, Test.11, Test.12];
-                    let Test.9 = 1f64;
-                    let Test.2 = Array [Test.9];
-                    let Test.4 = CallByName List.7 Test.1;
-                    dec Test.1;
-                    let Test.5 = CallByName List.7 Test.2;
-                    dec Test.2;
-                    let Test.3 = CallByName Num.24 Test.4 Test.5;
-                    ret Test.3;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn when_joinpoint() {
-        compiles_to_ir(
-            r#"
-            wrapper = \{} ->
-                x : [ Red, White, Blue ]
-                x = Blue
-
-                y =
-                    when x is
-                        Red -> 1
-                        White -> 2
-                        Blue -> 3
-
-                y
-
-            wrapper {}
-            "#,
-            indoc!(
-                r#"
-                procedure Test.1 (Test.5):
-                    let Test.2 = 0u8;
-                    joinpoint Test.9 Test.3:
-                        ret Test.3;
-                    in
-                    switch Test.2:
-                        case 1:
-                            let Test.10 = 1i64;
-                            jump Test.9 Test.10;
-                    
-                        case 2:
-                            let Test.11 = 2i64;
-                            jump Test.9 Test.11;
-                    
-                        default:
-                            let Test.12 = 3i64;
-                            jump Test.9 Test.12;
-                    
-
-                procedure Test.0 ():
-                    let Test.7 = Struct {};
-                    let Test.6 = CallByName Test.1 Test.7;
-                    ret Test.6;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn simple_if() {
-        compiles_to_ir(
-            r#"
-            if True then
-                1
-            else
-                2
-            "#,
-            indoc!(
-                r#"
-                procedure Test.0 ():
-                    let Test.3 = true;
-                    if Test.3 then
-                        let Test.4 = 1i64;
-                        ret Test.4;
-                    else
-                        let Test.2 = 2i64;
-                        ret Test.2;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn if_multi_branch() {
-        compiles_to_ir(
-            r#"
-            if True then
-                1
-            else if False then
-                2
-            else
-                3
-            "#,
-            indoc!(
-                r#"
-                procedure Test.0 ():
-                    let Test.6 = true;
-                    if Test.6 then
-                        let Test.7 = 1i64;
-                        ret Test.7;
-                    else
-                        let Test.4 = false;
-                        if Test.4 then
-                            let Test.5 = 2i64;
-                            ret Test.5;
-                        else
-                            let Test.3 = 3i64;
-                            ret Test.3;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn when_on_result() {
-        compiles_to_ir(
-            r#"
-            wrapper = \{} ->
-                x : Result I64 I64
-                x = Ok 2
-
-                y =
-                    when x is
-                        Ok 3 -> 1
-                        Ok _ -> 2
-                        Err _ -> 3
-                y
-
-            wrapper {}
-            "#,
-            indoc!(
-                r#"
-                procedure Test.1 (Test.5):
-                    let Test.20 = 1i64;
-                    let Test.19 = 2i64;
-                    let Test.2 = Ok Test.20 Test.19;
-                    joinpoint Test.9 Test.3:
-                        ret Test.3;
-                    in
-                    let Test.16 = 1i64;
-                    let Test.17 = Index 0 Test.2;
-                    let Test.18 = lowlevel Eq Test.16 Test.17;
-                    if Test.18 then
-                        let Test.13 = Index 1 Test.2;
-                        let Test.14 = 3i64;
-                        let Test.15 = lowlevel Eq Test.14 Test.13;
-                        if Test.15 then
-                            let Test.10 = 1i64;
-                            jump Test.9 Test.10;
-                        else
-                            let Test.11 = 2i64;
-                            jump Test.9 Test.11;
-                    else
-                        let Test.12 = 3i64;
-                        jump Test.9 Test.12;
-
-                procedure Test.0 ():
-                    let Test.7 = Struct {};
-                    let Test.6 = CallByName Test.1 Test.7;
-                    ret Test.6;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn let_with_record_pattern() {
-        compiles_to_ir(
-            r#"
-            { x } = { x: 0x2, y: 3.14 }
-
-            x
-            "#,
-            indoc!(
-                r#"
-                procedure Test.0 ():
-                    let Test.4 = 2i64;
-                    let Test.5 = 3.14f64;
-                    let Test.3 = Struct {Test.4, Test.5};
-                    let Test.1 = Index 0 Test.3;
-                    ret Test.1;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn let_with_record_pattern_list() {
-        compiles_to_ir(
-            r#"
-            { x } = { x: [ 1, 3, 4 ], y: 3.14 }
-
-            x
-            "#,
-            indoc!(
-                r#"
-                procedure Test.0 ():
-                    let Test.6 = 1i64;
-                    let Test.7 = 3i64;
-                    let Test.8 = 4i64;
-                    let Test.4 = Array [Test.6, Test.7, Test.8];
-                    let Test.5 = 3.14f64;
-                    let Test.3 = Struct {Test.4, Test.5};
-                    let Test.1 = Index 0 Test.3;
-                    decref Test.3;
-                    ret Test.1;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn if_guard_bind_variable_false() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                wrapper = \{} ->
-                    when 10 is
-                        x if x == 5 -> 0
-                        _ -> 42
-
-                wrapper {}
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure Bool.5 (#Attr.2, #Attr.3):
-                    let Test.11 = lowlevel Eq #Attr.2 #Attr.3;
-                    ret Test.11;
-
-                procedure Test.1 (Test.3):
-                    let Test.6 = 10i64;
-                    joinpoint Test.8 Test.13:
-                        if Test.13 then
-                            let Test.7 = 0i64;
-                            ret Test.7;
-                        else
-                            let Test.12 = 42i64;
-                            ret Test.12;
-                    in
-                    let Test.10 = 5i64;
-                    let Test.9 = CallByName Bool.5 Test.6 Test.10;
-                    jump Test.8 Test.9;
-
-                procedure Test.0 ():
-                    let Test.5 = Struct {};
-                    let Test.4 = CallByName Test.1 Test.5;
-                    ret Test.4;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn alias_variable() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                x = 5
-                y = x
-
-                3
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure Test.0 ():
-                    let Test.1 = 5i64;
-                    let Test.3 = 3i64;
-                    ret Test.3;
-                "#
-            ),
-        );
-
-        compiles_to_ir(
-            indoc!(
-                r#"
-                x = 5
-                y = x
-
-                y
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure Test.0 ():
-                    let Test.1 = 5i64;
-                    ret Test.1;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn branch_store_variable() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                when 0 is
-                    1 -> 12
-                    a -> a
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure Test.0 ():
-                    let Test.2 = 0i64;
-                    let Test.5 = 1i64;
-                    let Test.6 = lowlevel Eq Test.5 Test.2;
-                    if Test.6 then
-                        let Test.3 = 12i64;
-                        ret Test.3;
-                    else
-                        ret Test.2;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn list_pass_to_function() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                x : List I64
-                x = [1,2,3]
-
-                id : List I64 -> List I64
-                id = \y -> List.set y 0 0
-
-                id x
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure List.4 (#Attr.2, #Attr.3, #Attr.4):
-                    let Test.11 = lowlevel ListLen #Attr.2;
-                    let Test.9 = lowlevel NumLt #Attr.3 Test.11;
-                    if Test.9 then
-                        let Test.10 = lowlevel ListSet #Attr.2 #Attr.3 #Attr.4;
-                        ret Test.10;
-                    else
-                        ret #Attr.2;
-
-                procedure Test.2 (Test.3):
-                    let Test.6 = 0i64;
-                    let Test.7 = 0i64;
-                    let Test.5 = CallByName List.4 Test.3 Test.6 Test.7;
-                    ret Test.5;
-
-                procedure Test.0 ():
-                    let Test.12 = 1i64;
-                    let Test.13 = 2i64;
-                    let Test.14 = 3i64;
-                    let Test.1 = Array [Test.12, Test.13, Test.14];
-                    let Test.4 = CallByName Test.2 Test.1;
-                    ret Test.4;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn record_optional_field_let_no_use_default() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                f = \r ->
-                    { x ? 10, y } = r
-                    x + y
-
-
-                f { x: 4, y: 9 }
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure Num.24 (#Attr.2, #Attr.3):
-                    let Test.8 = lowlevel NumAdd #Attr.2 #Attr.3;
-                    ret Test.8;
-
-                procedure Test.1 (Test.2):
-                    let Test.3 = Index 0 Test.2;
-                    let Test.4 = Index 1 Test.2;
-                    let Test.7 = CallByName Num.24 Test.3 Test.4;
-                    ret Test.7;
-
-                procedure Test.0 ():
-                    let Test.9 = 4i64;
-                    let Test.10 = 9i64;
-                    let Test.6 = Struct {Test.9, Test.10};
-                    let Test.5 = CallByName Test.1 Test.6;
-                    ret Test.5;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn record_optional_field_let_use_default() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                f = \r ->
-                    { x ? 10, y } = r
-                    x + y
-
-
-                f { y: 9 }
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure Num.24 (#Attr.2, #Attr.3):
-                    let Test.8 = lowlevel NumAdd #Attr.2 #Attr.3;
-                    ret Test.8;
-
-                procedure Test.1 (Test.2):
-                    let Test.4 = Index 0 Test.2;
-                    let Test.3 = 10i64;
-                    let Test.7 = CallByName Num.24 Test.3 Test.4;
-                    ret Test.7;
-
-                procedure Test.0 ():
-                    let Test.9 = 9i64;
-                    let Test.5 = CallByName Test.1 Test.9;
-                    ret Test.5;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn record_optional_field_function_no_use_default() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                f = \{ x ? 10, y } -> x + y
-
-
-                f { x: 4, y: 9 }
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure Num.24 (#Attr.2, #Attr.3):
-                    let Test.8 = lowlevel NumAdd #Attr.2 #Attr.3;
-                    ret Test.8;
-
-                procedure Test.1 (Test.4):
-                    let Test.2 = Index 0 Test.4;
-                    let Test.3 = Index 1 Test.4;
-                    let Test.2 = 10i64;
-                    let Test.7 = CallByName Num.24 Test.2 Test.3;
-                    ret Test.7;
-
-                procedure Test.0 ():
-                    let Test.9 = 4i64;
-                    let Test.10 = 9i64;
-                    let Test.6 = Struct {Test.9, Test.10};
-                    let Test.5 = CallByName Test.1 Test.6;
-                    ret Test.5;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn record_optional_field_function_use_default() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                f = \{ x ? 10, y } -> x + y
-
-
-                f { y: 9 }
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure Num.24 (#Attr.2, #Attr.3):
-                    let Test.8 = lowlevel NumAdd #Attr.2 #Attr.3;
-                    ret Test.8;
-
-                procedure Test.1 (Test.4):
-                    let Test.3 = Index 0 Test.4;
-                    let Test.2 = 10i64;
-                    let Test.2 = 10i64;
-                    let Test.7 = CallByName Num.24 Test.2 Test.3;
-                    ret Test.7;
-
-                procedure Test.0 ():
-                    let Test.9 = 9i64;
-                    let Test.5 = CallByName Test.1 Test.9;
-                    ret Test.5;
-                "#
-            ),
-        )
-    }
-
-    #[ignore]
-    #[test]
-    fn quicksort_help() {
-        crate::helpers::with_larger_debug_stack(|| {
-            compiles_to_ir(
-                indoc!(
-                    r#"
-                quicksortHelp : List (Num a), I64, I64 -> List (Num a)
-                quicksortHelp = \list, low, high ->
-                    if low < high then
-                        (Pair partitionIndex partitioned) = Pair 0 []
-
-                        partitioned
-                            |> quicksortHelp low (partitionIndex - 1)
-                            |> quicksortHelp (partitionIndex + 1) high
-                    else
-                        list
-
-                quicksortHelp [] 0 0
-                "#
-                ),
-                indoc!(
-                    r#"
-                    procedure List.3 (#Attr.2, #Attr.3):
-                        let Test.38 = lowlevel ListLen #Attr.2;
-                        let Test.34 = lowlevel NumLt #Attr.3 Test.38;
-                        if Test.34 then
-                            let Test.36 = 1i64;
-                            let Test.37 = lowlevel ListGetUnsafe #Attr.2 #Attr.3;
-                            let Test.35 = Ok Test.36 Test.37;
-                            ret Test.35;
-                        else
-                            let Test.32 = 0i64;
-                            let Test.33 = Struct {};
-                            let Test.31 = Err Test.32 Test.33;
-                            ret Test.31;
-
-                    procedure List.4 (#Attr.2, #Attr.3, #Attr.4):
-                        let Test.14 = lowlevel ListLen #Attr.2;
-                        let Test.12 = lowlevel NumLt #Attr.3 Test.14;
-                        if Test.12 then
-                            let Test.13 = lowlevel ListSet #Attr.2 #Attr.3 #Attr.4;
-                            ret Test.13;
-                        else
-                            ret #Attr.2;
-
-                    procedure Test.1 (Test.2):
-                        let Test.39 = 0i64;
-                        let Test.28 = CallByName List.3 Test.2 Test.39;
-                        let Test.30 = 0i64;
-                        let Test.29 = CallByName List.3 Test.2 Test.30;
-                        let Test.7 = Struct {Test.28, Test.29};
-                        joinpoint Test.25:
-                            let Test.18 = Array [];
-                            ret Test.18;
-                        in
-                        let Test.19 = Index 0 Test.7;
-                        let Test.20 = 1i64;
-                        let Test.21 = Index 0 Test.19;
-                        let Test.27 = lowlevel Eq Test.20 Test.21;
-                        if Test.27 then
-                            let Test.22 = Index 1 Test.7;
-                            let Test.23 = 1i64;
-                            let Test.24 = Index 0 Test.22;
-                            let Test.26 = lowlevel Eq Test.23 Test.24;
-                            if Test.26 then
-                                let Test.17 = Index 0 Test.7;
-                                let Test.3 = Index 1 Test.17;
-                                let Test.16 = Index 1 Test.7;
-                                let Test.4 = Index 1 Test.16;
-                                let Test.15 = 0i64;
-                                let Test.9 = CallByName List.4 Test.2 Test.15 Test.4;
-                                let Test.10 = 0i64;
-                                let Test.8 = CallByName List.4 Test.9 Test.10 Test.3;
-                                ret Test.8;
-                            else
-                                dec Test.2;
-                                jump Test.25;
-                        else
-                            dec Test.2;
-                            jump Test.25;
-
-                    procedure Test.0 ():
-                        let Test.40 = 1i64;
-                        let Test.41 = 2i64;
-                        let Test.6 = Array [Test.40, Test.41];
-                        let Test.5 = CallByName Test.1 Test.6;
-                        ret Test.5;
-                "#
-                ),
-            )
-        })
-    }
-
-    #[test]
-    fn quicksort_swap() {
-        crate::helpers::with_larger_debug_stack(|| {
-            compiles_to_ir(
-                indoc!(
-                    r#"
-                    app "test" provides [ main ] to "./platform"
-
-                    swap = \list ->
-                        when Pair (List.get list 0) (List.get list 0) is
-                            Pair (Ok atI) (Ok atJ) ->
-                                list
-                                    |> List.set 0 atJ
-                                    |> List.set 0 atI
-
-                            _ ->
-                                []
-
-                    main =
-                        swap [ 1, 2 ]
-                "#
-                ),
-                indoc!(
-                    r#"
-                procedure List.3 (#Attr.2, #Attr.3):
-                    let Test.39 = lowlevel ListLen #Attr.2;
-                    let Test.35 = lowlevel NumLt #Attr.3 Test.39;
-                    if Test.35 then
-                        let Test.38 = 1i64;
-                        let Test.37 = lowlevel ListGetUnsafe #Attr.2 #Attr.3;
-                        let Test.36 = Ok Test.38 Test.37;
-                        ret Test.36;
-                    else
-                        let Test.34 = 0i64;
-                        let Test.33 = Struct {};
-                        let Test.32 = Err Test.34 Test.33;
-                        ret Test.32;
-
-                procedure List.4 (#Attr.2, #Attr.3, #Attr.4):
-                    let Test.15 = lowlevel ListLen #Attr.2;
-                    let Test.13 = lowlevel NumLt #Attr.3 Test.15;
-                    if Test.13 then
-                        let Test.14 = lowlevel ListSet #Attr.2 #Attr.3 #Attr.4;
-                        ret Test.14;
-                    else
-                        ret #Attr.2;
-
-                procedure Test.1 (Test.2):
-                    let Test.40 = 0i64;
-                    let Test.30 = CallByName List.3 Test.2 Test.40;
-                    let Test.31 = 0i64;
-                    let Test.29 = CallByName List.3 Test.2 Test.31;
-                    let Test.8 = Struct {Test.29, Test.30};
-                    joinpoint Test.26:
-                        let Test.19 = Array [];
-                        ret Test.19;
-                    in
-                    let Test.23 = Index 1 Test.8;
-                    let Test.24 = 1i64;
-                    let Test.25 = Index 0 Test.23;
-                    let Test.28 = lowlevel Eq Test.24 Test.25;
-                    if Test.28 then
-                        let Test.20 = Index 0 Test.8;
-                        let Test.21 = 1i64;
-                        let Test.22 = Index 0 Test.20;
-                        let Test.27 = lowlevel Eq Test.21 Test.22;
-                        if Test.27 then
-                            let Test.18 = Index 0 Test.8;
-                            let Test.4 = Index 1 Test.18;
-                            let Test.17 = Index 1 Test.8;
-                            let Test.5 = Index 1 Test.17;
-                            let Test.16 = 0i64;
-                            let Test.10 = CallByName List.4 Test.2 Test.16 Test.5;
-                            let Test.11 = 0i64;
-                            let Test.9 = CallByName List.4 Test.10 Test.11 Test.4;
-                            ret Test.9;
-                        else
-                            dec Test.2;
-                            jump Test.26;
-                    else
-                        dec Test.2;
-                        jump Test.26;
-
-                procedure Test.0 ():
-                    let Test.41 = 1i64;
-                    let Test.42 = 2i64;
-                    let Test.7 = Array [Test.41, Test.42];
-                    let Test.6 = CallByName Test.1 Test.7;
-                    ret Test.6;
-                "#
-                ),
-            )
-        })
-    }
-
-    #[ignore]
-    #[test]
-    fn quicksort_partition_help() {
-        crate::helpers::with_larger_debug_stack(|| {
-            compiles_to_ir(
-                indoc!(
-                    r#"
-                    app "test" provides [ main ] to "./platform"
-
-                    partitionHelp : I64, I64, List (Num a), I64, (Num a) -> [ Pair I64 (List (Num a)) ]
-                    partitionHelp = \i, j, list, high, pivot ->
-                        if j < high then
-                            when List.get list j is
-                                Ok value ->
-                                    if value <= pivot then
-                                        partitionHelp (i + 1) (j + 1) (swap (i + 1) j list) high pivot
-                                    else
-                                        partitionHelp i (j + 1) list high pivot
-
-                                Err _ ->
-                                    Pair i list
-                        else
-                            Pair i list
-
-                    main =
-                        partitionHelp 0 0 [] 0 0
-                    "#
-                ),
-                indoc!(
-                    r#"
-                "#
-                ),
-            )
-        })
-    }
-
-    #[ignore]
-    #[test]
-    fn quicksort_full() {
-        crate::helpers::with_larger_debug_stack(|| {
-            compiles_to_ir(
-                indoc!(
-                    r#"
-                    app "test" provides [ main ] to "./platform"
-
-                    quicksortHelp : List (Num a), I64, I64 -> List (Num a)
-                    quicksortHelp = \list, low, high ->
-                        if low < high then
-                            (Pair partitionIndex partitioned) = partition low high list
-
-                            partitioned
-                                |> quicksortHelp low (partitionIndex - 1)
-                                |> quicksortHelp (partitionIndex + 1) high
-                        else
-                            list
-
-
-                    swap : I64, I64, List a -> List a
-                    swap = \i, j, list ->
-                        when Pair (List.get list i) (List.get list j) is
-                            Pair (Ok atI) (Ok atJ) ->
-                                list
-                                    |> List.set i atJ
-                                    |> List.set j atI
-
-                            _ ->
-                                []
-
-                    partition : I64, I64, List (Num a) -> [ Pair I64 (List (Num a)) ]
-                    partition = \low, high, initialList ->
-                        when List.get initialList high is
-                            Ok pivot ->
-                                when partitionHelp (low - 1) low initialList high pivot is
-                                    Pair newI newList ->
-                                        Pair (newI + 1) (swap (newI + 1) high newList)
-
-                            Err _ ->
-                                Pair (low - 1) initialList
-
-
-                    partitionHelp : I64, I64, List (Num a), I64, (Num a) -> [ Pair I64 (List (Num a)) ]
-                    partitionHelp = \i, j, list, high, pivot ->
-                        if j < high then
-                            when List.get list j is
-                                Ok value ->
-                                    if value <= pivot then
-                                        partitionHelp (i + 1) (j + 1) (swap (i + 1) j list) high pivot
-                                    else
-                                        partitionHelp i (j + 1) list high pivot
-
-                                Err _ ->
-                                    Pair i list
-                        else
-                            Pair i list
-
-
-
-                    quicksort = \originalList ->
-                        n = List.len originalList
-                        quicksortHelp originalList 0 (n - 1)
-
-                    main =
-                        quicksort [1,2,3]
-                "#
-                ),
-                indoc!(
-                    r#"
-                "#
-                ),
-            )
-        })
-    }
-
-    #[test]
-    fn factorial() {
-        compiles_to_ir(
-            r#"
-            factorial = \n, accum ->
-                when n is
-                    0 ->
-                        accum
-
-                    _ ->
-                        factorial (n - 1) (n * accum)
-
-            factorial 10 1
-            "#,
-            indoc!(
-                r#"
-                procedure Num.25 (#Attr.2, #Attr.3):
-                    let Test.14 = lowlevel NumSub #Attr.2 #Attr.3;
-                    ret Test.14;
-
-                procedure Num.26 (#Attr.2, #Attr.3):
-                    let Test.12 = lowlevel NumMul #Attr.2 #Attr.3;
-                    ret Test.12;
-
-                procedure Test.1 (Test.2, Test.3):
-                    joinpoint Test.7 Test.2 Test.3:
-                        let Test.15 = 0i64;
-                        let Test.16 = lowlevel Eq Test.15 Test.2;
-                        if Test.16 then
-                            ret Test.3;
-                        else
-                            let Test.13 = 1i64;
-                            let Test.10 = CallByName Num.25 Test.2 Test.13;
-                            let Test.11 = CallByName Num.26 Test.2 Test.3;
-                            jump Test.7 Test.10 Test.11;
-                    in
-                    jump Test.7 Test.2 Test.3;
-
-                procedure Test.0 ():
-                    let Test.5 = 10i64;
-                    let Test.6 = 1i64;
-                    let Test.4 = CallByName Test.1 Test.5 Test.6;
-                    ret Test.4;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    #[ignore]
-    fn is_nil() {
-        compiles_to_ir(
-            r#"
-            ConsList a : [ Cons a (ConsList a), Nil ]
-
-            isNil : ConsList a -> Bool
-            isNil = \list ->
-                when list is
-                    Nil -> True
-                    Cons _ _ -> False
-
-            isNil (Cons 0x2 Nil)
-            "#,
-            indoc!(
-                r#"
-                procedure Test.1 (Test.3):
-                    let Test.13 = true;
-                    let Test.15 = Index 0 Test.3;
-                    let Test.14 = 1i64;
-                    let Test.16 = lowlevel Eq Test.14 Test.15;
-                    let Test.12 = lowlevel And Test.16 Test.13;
-                    if Test.12 then
-                        let Test.10 = true;
-                        ret Test.10;
-                    else
-                        let Test.11 = false;
-                        ret Test.11;
-
-                let Test.6 = 0i64;
-                let Test.7 = 2i64;
-                let Test.9 = 1i64;
-                let Test.8 = Nil Test.9;
-                let Test.5 = Cons Test.6 Test.7 Test.8;
-                let Test.4 = CallByName Test.1 Test.5;
-                ret Test.4;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    #[ignore]
-    fn has_none() {
-        compiles_to_ir(
-            r#"
-            Maybe a : [ Just a, Nothing ]
-            ConsList a : [ Cons a (ConsList a), Nil ]
-
-            hasNone : ConsList (Maybe a) -> Bool
-            hasNone = \list ->
-                when list is
-                    Nil -> False
-                    Cons Nothing _ -> True
-                    Cons (Just _) xs -> hasNone xs
-
-            hasNone (Cons (Just 3) Nil)
-            "#,
-            indoc!(
-                r#"
-                procedure Test.1 (Test.3):
-                    let Test.13 = true;
-                    let Test.15 = Index 0 Test.3;
-                    let Test.14 = 1i64;
-                    let Test.16 = lowlevel Eq Test.14 Test.15;
-                    let Test.12 = lowlevel And Test.16 Test.13;
-                    if Test.12 then
-                        let Test.10 = true;
-                        ret Test.10;
-                    else
-                        let Test.11 = false;
-                        ret Test.11;
-
-                let Test.6 = 0i64;
-                let Test.7 = 2i64;
-                let Test.9 = 1i64;
-                let Test.8 = Nil Test.9;
-                let Test.5 = Cons Test.6 Test.7 Test.8;
-                let Test.4 = CallByName Test.1 Test.5;
-                ret Test.4;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn mk_pair_of() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                app "test" provides [ main ] to "./platform"
-
-                mkPairOf = \x -> Pair x x
-
-                main =
-                    mkPairOf [1,2,3]
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure Test.1 (Test.2):
-                    inc Test.2;
-                    let Test.6 = Struct {Test.2, Test.2};
-                    ret Test.6;
-
-                procedure Test.0 ():
-                    let Test.7 = 1i64;
-                    let Test.8 = 2i64;
-                    let Test.9 = 3i64;
-                    let Test.5 = Array [Test.7, Test.8, Test.9];
-                    let Test.4 = CallByName Test.1 Test.5;
-                    ret Test.4;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn fst() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                app "test" provides [ main ] to "./platform"
-
-                fst = \x, _ -> x
-
-                main =
-                    fst [1,2,3] [3,2,1]
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure Test.1 (Test.2, Test.3):
-                    inc Test.2;
-                    ret Test.2;
-
-                procedure Test.0 ():
-                    let Test.11 = 1i64;
-                    let Test.12 = 2i64;
-                    let Test.13 = 3i64;
-                    let Test.5 = Array [Test.11, Test.12, Test.13];
-                    let Test.8 = 3i64;
-                    let Test.9 = 2i64;
-                    let Test.10 = 1i64;
-                    let Test.6 = Array [Test.8, Test.9, Test.10];
-                    let Test.4 = CallByName Test.1 Test.5 Test.6;
-                    dec Test.6;
-                    dec Test.5;
-                    ret Test.4;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn list_cannot_update_inplace() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                app "test" provides [ main ] to "./platform"
-
-                x : List I64
-                x = [1,2,3]
-
-                add : List I64 -> List I64
-                add = \y -> List.set y 0 0
-
-                main =
-                    List.len (add x) + List.len x
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure List.4 (#Attr.2, #Attr.3, #Attr.4):
-                    let Test.22 = lowlevel ListLen #Attr.2;
-                    let Test.20 = lowlevel NumLt #Attr.3 Test.22;
-                    if Test.20 then
-                        let Test.21 = lowlevel ListSet #Attr.2 #Attr.3 #Attr.4;
-                        ret Test.21;
-                    else
-                        ret #Attr.2;
-
-                procedure List.7 (#Attr.2):
-                    let Test.9 = lowlevel ListLen #Attr.2;
-                    ret Test.9;
-
-                procedure Num.24 (#Attr.2, #Attr.3):
-                    let Test.7 = lowlevel NumAdd #Attr.2 #Attr.3;
-                    ret Test.7;
-
-                procedure Test.1 ():
-                    let Test.11 = 1i64;
-                    let Test.12 = 2i64;
-                    let Test.13 = 3i64;
-                    let Test.10 = Array [Test.11, Test.12, Test.13];
-                    ret Test.10;
-
-                procedure Test.2 (Test.3):
-                    let Test.17 = 0i64;
-                    let Test.18 = 0i64;
-                    let Test.16 = CallByName List.4 Test.3 Test.17 Test.18;
-                    ret Test.16;
-
-                procedure Test.0 ():
-                    let Test.15 = CallByName Test.1;
-                    let Test.14 = CallByName Test.2 Test.15;
-                    let Test.5 = CallByName List.7 Test.14;
-                    dec Test.14;
-                    let Test.8 = CallByName Test.1;
-                    let Test.6 = CallByName List.7 Test.8;
-                    dec Test.8;
-                    let Test.4 = CallByName Num.24 Test.5 Test.6;
-                    ret Test.4;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn list_get() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                wrapper = \{} ->
-                    List.get [1,2,3] 0
-
-                wrapper {}
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure List.3 (#Attr.2, #Attr.3):
-                    let Test.15 = lowlevel ListLen #Attr.2;
-                    let Test.11 = lowlevel NumLt #Attr.3 Test.15;
-                    if Test.11 then
-                        let Test.14 = 1i64;
-                        let Test.13 = lowlevel ListGetUnsafe #Attr.2 #Attr.3;
-                        let Test.12 = Ok Test.14 Test.13;
-                        ret Test.12;
-                    else
-                        let Test.10 = 0i64;
-                        let Test.9 = Struct {};
-                        let Test.8 = Err Test.10 Test.9;
-                        ret Test.8;
-
-                procedure Test.1 (Test.2):
-                    let Test.16 = 1i64;
-                    let Test.17 = 2i64;
-                    let Test.18 = 3i64;
-                    let Test.6 = Array [Test.16, Test.17, Test.18];
-                    let Test.7 = 0i64;
-                    let Test.5 = CallByName List.3 Test.6 Test.7;
-                    dec Test.6;
-                    ret Test.5;
-
-                procedure Test.0 ():
-                    let Test.4 = Struct {};
-                    let Test.3 = CallByName Test.1 Test.4;
-                    ret Test.3;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn peano() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                Peano : [ S Peano, Z ]
-
-                three : Peano
-                three = S (S (S Z))
-
-                three
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure Test.0 ():
-                    let Test.9 = 0i64;
-                    let Test.11 = 0i64;
-                    let Test.13 = 0i64;
-                    let Test.14 = 1i64;
-                    let Test.12 = Z Test.14;
-                    let Test.10 = S Test.13 Test.12;
-                    let Test.8 = S Test.11 Test.10;
-                    let Test.2 = S Test.9 Test.8;
-                    ret Test.2;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn peano1() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                Peano : [ S Peano, Z ]
-
-                three : Peano
-                three = S (S (S Z))
-
-                when three is
-                    Z -> 0
-                    S _ -> 1
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure Test.0 ():
-                    let Test.13 = 0i64;
-                    let Test.15 = 0i64;
-                    let Test.17 = 0i64;
-                    let Test.18 = 1i64;
-                    let Test.16 = Z Test.18;
-                    let Test.14 = S Test.17 Test.16;
-                    let Test.12 = S Test.15 Test.14;
-                    let Test.2 = S Test.13 Test.12;
-                    let Test.9 = 1i64;
-                    let Test.10 = Index 0 Test.2;
-                    dec Test.2;
-                    let Test.11 = lowlevel Eq Test.9 Test.10;
-                    if Test.11 then
-                        let Test.7 = 0i64;
-                        ret Test.7;
-                    else
-                        let Test.8 = 1i64;
-                        ret Test.8;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn peano2() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                Peano : [ S Peano, Z ]
-
-                three : Peano
-                three = S (S (S Z))
-
-                when three is
-                    S (S _) -> 1
-                    S (_) -> 0
-                    Z -> 0
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure Test.0 ():
-                    let Test.19 = 0i64;
-                    let Test.21 = 0i64;
-                    let Test.23 = 0i64;
-                    let Test.24 = 1i64;
-                    let Test.22 = Z Test.24;
-                    let Test.20 = S Test.23 Test.22;
-                    let Test.18 = S Test.21 Test.20;
-                    let Test.2 = S Test.19 Test.18;
-                    let Test.15 = 0i64;
-                    let Test.16 = Index 0 Test.2;
-                    let Test.17 = lowlevel Eq Test.15 Test.16;
-                    if Test.17 then
-                        let Test.11 = Index 1 Test.2;
-                        let Test.12 = 0i64;
-                        let Test.13 = Index 0 Test.11;
-                        dec Test.11;
-                        decref Test.2;
-                        let Test.14 = lowlevel Eq Test.12 Test.13;
-                        if Test.14 then
-                            let Test.7 = 1i64;
-                            ret Test.7;
-                        else
-                            let Test.9 = 0i64;
-                            ret Test.9;
-                    else
-                        let Test.10 = 0i64;
-                        ret Test.10;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn optional_when() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                f = \r ->
-                    when r is
-                        { x: Blue, y ? 3 } -> y
-                        { x: Red, y ? 5 } -> y
-
-                a = f { x: Blue, y: 7 }
-                b = f { x: Blue }
-                c = f { x: Red, y: 11 }
-                d = f { x: Red }
-
-                a * b * c * d
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure Num.26 (#Attr.2, #Attr.3):
-                    let Test.17 = lowlevel NumMul #Attr.2 #Attr.3;
-                    ret Test.17;
-
-                procedure Test.1 (Test.6):
-                    let Test.22 = Index 1 Test.6;
-                    let Test.23 = false;
-                    let Test.24 = lowlevel Eq Test.23 Test.22;
-                    if Test.24 then
-                        let Test.8 = Index 0 Test.6;
-                        ret Test.8;
-                    else
-                        let Test.10 = Index 0 Test.6;
-                        ret Test.10;
-
-                procedure Test.1 (Test.6):
-                    let Test.33 = Index 0 Test.6;
-                    let Test.34 = false;
-                    let Test.35 = lowlevel Eq Test.34 Test.33;
-                    if Test.35 then
-                        let Test.8 = 3i64;
-                        ret Test.8;
-                    else
-                        let Test.10 = 5i64;
-                        ret Test.10;
-
-                procedure Test.0 ():
-                    let Test.38 = true;
-                    let Test.5 = CallByName Test.1 Test.38;
-                    let Test.36 = false;
-                    let Test.3 = CallByName Test.1 Test.36;
-                    let Test.28 = 11i64;
-                    let Test.29 = true;
-                    let Test.27 = Struct {Test.28, Test.29};
-                    let Test.4 = CallByName Test.1 Test.27;
-                    let Test.25 = 7i64;
-                    let Test.26 = false;
-                    let Test.19 = Struct {Test.25, Test.26};
-                    let Test.2 = CallByName Test.1 Test.19;
-                    let Test.18 = CallByName Num.26 Test.2 Test.3;
-                    let Test.16 = CallByName Num.26 Test.18 Test.4;
-                    let Test.15 = CallByName Num.26 Test.16 Test.5;
-                    ret Test.15;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn nested_pattern_match() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                Maybe a : [ Nothing, Just a ]
-
-                x : Maybe (Maybe I64)
-                x = Just (Just 41)
-
-                when x is
-                    Just (Just v) -> v + 0x1
-                    _ -> 0x1
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure Num.24 (#Attr.2, #Attr.3):
-                    let Test.8 = lowlevel NumAdd #Attr.2 #Attr.3;
-                    ret Test.8;
-
-                procedure Test.0 ():
-                    let Test.20 = 0i64;
-                    let Test.22 = 0i64;
-                    let Test.21 = 41i64;
-                    let Test.19 = Just Test.22 Test.21;
-                    let Test.2 = Just Test.20 Test.19;
-                    joinpoint Test.16:
-                        let Test.10 = 1i64;
-                        ret Test.10;
-                    in
-                    let Test.14 = 0i64;
-                    let Test.15 = Index 0 Test.2;
-                    let Test.18 = lowlevel Eq Test.14 Test.15;
-                    if Test.18 then
-                        let Test.11 = Index 1 Test.2;
-                        let Test.12 = 0i64;
-                        let Test.13 = Index 0 Test.11;
-                        let Test.17 = lowlevel Eq Test.12 Test.13;
-                        if Test.17 then
-                            let Test.9 = Index 1 Test.2;
-                            let Test.5 = Index 1 Test.9;
-                            let Test.7 = 1i64;
-                            let Test.6 = CallByName Num.24 Test.5 Test.7;
-                            ret Test.6;
-                        else
-                            jump Test.16;
-                    else
-                        jump Test.16;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    #[ignore]
-    fn linked_list_length_twice() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                LinkedList a : [ Nil, Cons a (LinkedList a) ]
-
-                nil : LinkedList I64
-                nil = Nil
-
-                length : LinkedList a -> I64
-                length = \list ->
-                    when list is
-                        Nil -> 0
-                        Cons _ rest -> 1 + length rest
-
-                length nil + length nil
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure Num.14 (#Attr.2, #Attr.3):
-                    let Test.9 = lowlevel NumAdd #Attr.2 #Attr.3;
-                    ret Test.9;
-
-                procedure Test.3 (Test.4):
-                    let Test.15 = true;
-                    let Test.16 = 1i64;
-                    let Test.17 = Index 0 Test.4;
-                    let Test.18 = lowlevel Eq Test.16 Test.17;
-                    let Test.14 = lowlevel And Test.18 Test.15;
-                    if Test.14 then
-                        dec Test.4;
-                        let Test.10 = 0i64;
-                        ret Test.10;
-                    else
-                        let Test.5 = Index 2 Test.4;
-                        dec Test.4;
-                        let Test.12 = 1i64;
-                        let Test.13 = CallByName Test.3 Test.5;
-                        let Test.11 = CallByName Num.14 Test.12 Test.13;
-                        ret Test.11;
-
-                procedure Test.0 ():
-                    let Test.20 = 1i64;
-                    let Test.2 = Nil Test.20;
-                    let Test.7 = CallByName Test.3 Test.2;
-                    let Test.8 = CallByName Test.3 Test.2;
-                    let Test.6 = CallByName Num.14 Test.7 Test.8;
-                    ret Test.6;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn rigids() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                app "test" provides [ main ] to "./platform"
-
-                swap : Nat, Nat, List a -> List a
-                swap = \i, j, list ->
-                    when Pair (List.get list i) (List.get list j) is
-                        Pair (Ok atI) (Ok atJ) ->
-                            foo = atJ
-
-                            list
-                                |> List.set i foo
-                                |> List.set j atI
-
-                        _ ->
-                            []
-
-                main =
-                    swap 0 0 [0x1]
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure List.3 (#Attr.2, #Attr.3):
-                    let Test.41 = lowlevel ListLen #Attr.2;
-                    let Test.37 = lowlevel NumLt #Attr.3 Test.41;
-                    if Test.37 then
-                        let Test.40 = 1i64;
-                        let Test.39 = lowlevel ListGetUnsafe #Attr.2 #Attr.3;
-                        let Test.38 = Ok Test.40 Test.39;
-                        ret Test.38;
-                    else
-                        let Test.36 = 0i64;
-                        let Test.35 = Struct {};
-                        let Test.34 = Err Test.36 Test.35;
-                        ret Test.34;
-
-                procedure List.4 (#Attr.2, #Attr.3, #Attr.4):
-                    let Test.19 = lowlevel ListLen #Attr.2;
-                    let Test.17 = lowlevel NumLt #Attr.3 Test.19;
-                    if Test.17 then
-                        let Test.18 = lowlevel ListSet #Attr.2 #Attr.3 #Attr.4;
-                        ret Test.18;
-                    else
-                        ret #Attr.2;
-
-                procedure Test.1 (Test.2, Test.3, Test.4):
-                    let Test.33 = CallByName List.3 Test.4 Test.3;
-                    let Test.32 = CallByName List.3 Test.4 Test.2;
-                    let Test.13 = Struct {Test.32, Test.33};
-                    joinpoint Test.29:
-                        let Test.22 = Array [];
-                        ret Test.22;
-                    in
-                    let Test.26 = Index 1 Test.13;
-                    let Test.27 = 1i64;
-                    let Test.28 = Index 0 Test.26;
-                    let Test.31 = lowlevel Eq Test.27 Test.28;
-                    if Test.31 then
-                        let Test.23 = Index 0 Test.13;
-                        let Test.24 = 1i64;
-                        let Test.25 = Index 0 Test.23;
-                        let Test.30 = lowlevel Eq Test.24 Test.25;
-                        if Test.30 then
-                            let Test.21 = Index 0 Test.13;
-                            let Test.6 = Index 1 Test.21;
-                            let Test.20 = Index 1 Test.13;
-                            let Test.7 = Index 1 Test.20;
-                            let Test.15 = CallByName List.4 Test.4 Test.2 Test.7;
-                            let Test.14 = CallByName List.4 Test.15 Test.3 Test.6;
-                            ret Test.14;
-                        else
-                            dec Test.4;
-                            jump Test.29;
-                    else
-                        dec Test.4;
-                        jump Test.29;
-
-                procedure Test.0 ():
-                    let Test.10 = 0i64;
-                    let Test.11 = 0i64;
-                    let Test.42 = 1i64;
-                    let Test.12 = Array [Test.42];
-                    let Test.9 = CallByName Test.1 Test.10 Test.11 Test.12;
-                    ret Test.9;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn let_x_in_x() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                x = 5
-
-                answer =
-                    1337
-
-                unused =
-                    nested = 17
-                    nested
-
-                answer
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure Test.0 ():
-                    let Test.1 = 5i64;
-                    let Test.4 = 17i64;
-                    let Test.2 = 1337i64;
-                    ret Test.2;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn let_x_in_x_indirect() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                x = 5
-
-                answer =
-                    1337
-
-                unused =
-                    nested = 17
-
-                    i = 1
-
-                    nested
-
-                { answer, unused }.answer
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure Test.0 ():
-                    let Test.1 = 5i64;
-                    let Test.4 = 17i64;
-                    let Test.5 = 1i64;
-                    let Test.2 = 1337i64;
-                    let Test.7 = Struct {Test.2, Test.4};
-                    let Test.6 = Index 0 Test.7;
-                    ret Test.6;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn nested_closure() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                app "test" provides [ main ] to "./platform"
-
-                foo = \{} ->
-                    x = 42
-                    f = \{} -> x
-                    f
-
-                main =
-                    f = foo {}
-                    f {}
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure Test.1 (Test.5):
-                    let Test.2 = 42i64;
-                    let Test.3 = Struct {Test.2};
-                    ret Test.3;
-
-                procedure Test.3 (Test.9, #Attr.12):
-                    let Test.2 = Index 0 #Attr.12;
-                    ret Test.2;
-
-                procedure Test.0 ():
-                    let Test.8 = Struct {};
-                    let Test.4 = CallByName Test.1 Test.8;
-                    let Test.7 = Struct {};
-                    let Test.6 = CallByName Test.3 Test.7 Test.4;
-                    ret Test.6;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn closure_in_list() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                app "test" provides [ main ] to "./platform"
-
-                foo = \{} ->
-                    x = 41
-
-                    f = \{} -> x
-
-                    [ f ]
-
-                main =
-                    items = foo {}
-
-                    List.len items
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure List.7 (#Attr.2):
-                    let Test.7 = lowlevel ListLen #Attr.2;
-                    ret Test.7;
-
-                procedure Test.1 (Test.5):
-                    let Test.2 = 41i64;
-                    let Test.11 = Struct {Test.2};
-                    let Test.10 = Array [Test.11];
-                    ret Test.10;
-
-                procedure Test.3 (Test.9, #Attr.12):
-                    let Test.2 = Index 0 #Attr.12;
-                    ret Test.2;
-
-                procedure Test.0 ():
-                    let Test.8 = Struct {};
-                    let Test.4 = CallByName Test.1 Test.8;
-                    let Test.6 = CallByName List.7 Test.4;
-                    dec Test.4;
-                    ret Test.6;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    #[ignore]
-    fn somehow_drops_definitions() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                app "test" provides [ main ] to "./platform"
-
-                one : I64 
-                one = 1
-
-                two : I64
-                two = 2
-
-                increment : I64 -> I64
-                increment = \x -> x + one
-
-                double : I64 -> I64
-                double = \x -> x * two
-
-                apply : (a -> a), a -> a
-                apply = \f, x -> f x
-
-                main =
-                    apply (if True then increment else double) 42
-                "#
-            ),
-            indoc!(
-                r#"
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    fn specialize_closures() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                app "test" provides [ main ] to "./platform"
-
-
-                apply : (a -> a), a -> a
-                apply = \f, x -> f x
-
-                main =
-                    one : I64 
-                    one = 1
-
-                    two : I64
-                    two = 2
-
-                    b : Bool
-                    b = True
-
-                    increment : I64 -> I64
-                    increment = \x -> x + one
-
-                    double : I64 -> I64
-                    double = \x -> if b then x * two else x
-
-                    apply (if True then increment else double) 42
-                "#
-            ),
-            indoc!(
-                r#"
-                procedure Num.24 (#Attr.2, #Attr.3):
-                    let Test.29 = lowlevel NumAdd #Attr.2 #Attr.3;
-                    ret Test.29;
-
-                procedure Num.26 (#Attr.2, #Attr.3):
-                    let Test.25 = lowlevel NumMul #Attr.2 #Attr.3;
-                    ret Test.25;
-
-                procedure Test.1 (Test.2, Test.3):
-                    let Test.17 = Index 0 Test.2;
-                    joinpoint Test.18 Test.16:
-                        ret Test.16;
-                    in
-                    switch Test.17:
-                        case 0:
-                            let Test.19 = CallByName Test.7 Test.3 Test.2;
-                            jump Test.18 Test.19;
-                    
-                        default:
-                            let Test.20 = CallByName Test.8 Test.3 Test.2;
-                            jump Test.18 Test.20;
-                    
-
-                procedure Test.7 (Test.10, #Attr.12):
-                    let Test.4 = Index 1 #Attr.12;
-                    let Test.28 = CallByName Num.24 Test.10 Test.4;
-                    ret Test.28;
-
-                procedure Test.8 (Test.11, #Attr.12):
-                    let Test.6 = Index 2 #Attr.12;
-                    let Test.5 = Index 1 #Attr.12;
-                    if Test.6 then
-                        let Test.24 = CallByName Num.26 Test.11 Test.5;
-                        ret Test.24;
-                    else
-                        ret Test.11;
-
-                procedure Test.0 ():
-                    let Test.6 = true;
-                    let Test.4 = 1i64;
-                    let Test.5 = 2i64;
-                    joinpoint Test.22 Test.14:
-                        let Test.15 = 42i64;
-                        let Test.13 = CallByName Test.1 Test.14 Test.15;
-                        ret Test.13;
-                    in
-                    let Test.27 = true;
-                    if Test.27 then
-                        let Test.30 = 0i64;
-                        let Test.7 = ClosureTag(Test.7) Test.30 Test.4;
-                        jump Test.22 Test.7;
-                    else
-                        let Test.26 = 1i64;
-                        let Test.8 = ClosureTag(Test.8) Test.26 Test.5 Test.6;
-                        jump Test.22 Test.8;
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    #[ignore]
-    fn specialize_lowlevel() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                app "test" provides [ main ] to "./platform"
-
-
-                apply : (a -> a), a -> a
-                apply = \f, x -> f x
-
-                main =
-                    one : I64 
-                    one = 1
-
-                    two : I64
-                    two = 2
-
-                    increment : I64 -> I64
-                    increment = \x -> x + 1
-
-                    double : I64 -> I64
-                    double = \x -> x * two
-
-                    when 3 is 
-                        1 -> increment 0
-                        2 -> double 0
-                        _ -> List.map [] (if True then increment else double) |> List.len 
-                "#
-            ),
-            indoc!(
-                r#"
-                "#
-            ),
-        )
-    }
-
-    #[test]
-    #[ignore]
-    fn static_str_closure() {
-        compiles_to_ir(
-            indoc!(
-                r#"
-                app "test" provides [ main ] to "./platform"
-
-                main : Str
-                main =
-                    x = "long string that is malloced"
-
-                    f : {} -> Str
-                    f = (\_ -> x)
-
-                    f {}
-                "#
-            ),
-            indoc!(
-                r#"
-                "#
-            ),
-        )
-    }
-}
diff --git a/compiler/parse/src/ast.rs b/compiler/parse/src/ast.rs
index 5a0ade9458..c9bb622e47 100644
--- a/compiler/parse/src/ast.rs
+++ b/compiler/parse/src/ast.rs
@@ -44,7 +44,7 @@ pub enum EscapedChar {
 
 impl EscapedChar {
     /// Returns the char that would have been originally parsed to
-    pub fn to_parsed_char(&self) -> char {
+    pub fn to_parsed_char(self) -> char {
         use EscapedChar::*;
 
         match self {
diff --git a/compiler/parse/src/expr.rs b/compiler/parse/src/expr.rs
index 5483d2c278..5f8f972d43 100644
--- a/compiler/parse/src/expr.rs
+++ b/compiler/parse/src/expr.rs
@@ -418,7 +418,7 @@ impl<'a> ExprState<'a> {
         F: Fn(Region, Row, Col) -> EExpr<'a>,
     {
         if !self.operators.is_empty() {
-            // this `=` or `<-` likely occured inline; treat it as an invalid operator
+            // this `=` or `<-` likely occurred inline; treat it as an invalid operator
             let opchar = match loc_op.value {
                 BinOp::Assignment => arena.alloc([b'=']) as &[_],
                 BinOp::Backpassing => arena.alloc([b'<', b'-']) as &[_],
@@ -451,7 +451,7 @@ impl<'a> ExprState<'a> {
         debug_assert_eq!(loc_op.value, BinOp::HasType);
 
         if !self.operators.is_empty() {
-            // this `:` likely occured inline; treat it as an invalid operator
+            // this `:` likely occurred inline; treat it as an invalid operator
             let opchar = arena.alloc([b':']) as &[_];
 
             let fail =
@@ -1000,7 +1000,7 @@ fn parse_expr_operator<'a>(
                         (&*arena.alloc(Located::at(alias_region, alias)), state)
                     }
                     Err(_) => {
-                        // this `=` likely occured inline; treat it as an invalid operator
+                        // this `=` likely occurred inline; treat it as an invalid operator
                         let fail = EExpr::BadOperator(
                             arena.alloc([b'=']),
                             loc_op.region.start_line,
@@ -1044,7 +1044,7 @@ fn parse_expr_operator<'a>(
                         (Located::at(expr_region, good), ann_type, state)
                     }
                     Err(_) => {
-                        // this `=` likely occured inline; treat it as an invalid operator
+                        // this `=` likely occurred inline; treat it as an invalid operator
                         let fail = EExpr::BadOperator(
                             arena.alloc([b'=']),
                             loc_op.region.start_line,
@@ -1153,7 +1153,7 @@ fn parse_expr_operator<'a>(
                             }
                         }
                         Err(_) => {
-                            // this `:` likely occured inline; treat it as an invalid operator
+                            // this `:` likely occurred inline; treat it as an invalid operator
                             let fail = EExpr::BadOperator(
                                 arena.alloc([b':']),
                                 loc_op.region.start_line,
diff --git a/compiler/parse/tests/test_parse.rs b/compiler/parse/tests/test_parse.rs
index 79ecfca92f..58ca1d2dee 100644
--- a/compiler/parse/tests/test_parse.rs
+++ b/compiler/parse/tests/test_parse.rs
@@ -3385,7 +3385,7 @@ mod test_parse {
         // Reproducing this bug requires a bizarre set of things to all be true:
         //
         // * Must be parsing a *module* def (nested expr defs don't repro this)
-        // * That top-level module def conatins a def inside it
+        // * That top-level module def contains a def inside it
         // * That inner def is defining a function
         // * The name of the inner def begins with a keyword (`if`, `then`, `else`, `when`, `is`)
         //
diff --git a/compiler/reporting/src/error/parse.rs b/compiler/reporting/src/error/parse.rs
index 0d1491711b..646f675580 100644
--- a/compiler/reporting/src/error/parse.rs
+++ b/compiler/reporting/src/error/parse.rs
@@ -1973,7 +1973,7 @@ fn to_type_report<'a>(
                 let region = Region::from_row_col(*row, *col);
 
                 let doc = alloc.stack(vec![
-                    alloc.reflow(r"I just started parsing a function argument type, but I encounterd two commas in a row:"),
+                    alloc.reflow(r"I just started parsing a function argument type, but I encountered two commas in a row:"),
                     alloc.region_with_subregion(surroundings, region),
                     alloc.concat(vec![alloc.reflow("Try removing one of them.")]),
                 ]);
diff --git a/compiler/reporting/src/error/type.rs b/compiler/reporting/src/error/type.rs
index 5e03b5ef6a..f4f3723257 100644
--- a/compiler/reporting/src/error/type.rs
+++ b/compiler/reporting/src/error/type.rs
@@ -1046,7 +1046,7 @@ fn to_pattern_report<'b>(
             let doc = alloc.stack(vec![
                 alloc.text("This pattern is being used in an unexpected way:"),
                 alloc.region(expr_region),
-                pattern_type_comparision(
+                pattern_type_comparison(
                     alloc,
                     found,
                     expected_type,
@@ -1078,7 +1078,7 @@ fn to_pattern_report<'b>(
                         .append(name.clone())
                         .append(alloc.text(" is weird:")),
                     alloc.region(region),
-                    pattern_type_comparision(
+                    pattern_type_comparison(
                         alloc,
                         found,
                         expected_type,
@@ -1112,7 +1112,7 @@ fn to_pattern_report<'b>(
                             .append(alloc.keyword("when"))
                             .append(alloc.text(" is causing a mismatch:")),
                         alloc.region(region),
-                        pattern_type_comparision(
+                        pattern_type_comparison(
                             alloc,
                             found,
                             expected_type,
@@ -1144,7 +1144,7 @@ fn to_pattern_report<'b>(
                             .append(alloc.keyword("when"))
                             .append(alloc.text(" does not match the previous ones:")),
                         alloc.region(region),
-                        pattern_type_comparision(
+                        pattern_type_comparison(
                             alloc,
                             found,
                             expected_type,
@@ -1175,7 +1175,7 @@ fn to_pattern_report<'b>(
     }
 }
 
-fn pattern_type_comparision<'b>(
+fn pattern_type_comparison<'b>(
     alloc: &'b RocDocAllocator<'b>,
     actual: ErrorType,
     expected: ErrorType,
diff --git a/compiler/reporting/src/report.rs b/compiler/reporting/src/report.rs
index 6349bb8708..a96480fa78 100644
--- a/compiler/reporting/src/report.rs
+++ b/compiler/reporting/src/report.rs
@@ -450,11 +450,11 @@ impl<'a> RocDocAllocator<'a> {
                 } else {
                     ERROR_UNDERLINE.repeat((sub_region2.end_col - sub_region2.start_col) as usize)
                 };
-                let inbetween = " "
+                let in_between = " "
                     .repeat((sub_region2.start_col.saturating_sub(sub_region1.end_col)) as usize);
 
                 self.text(highlight1)
-                    .append(self.text(inbetween))
+                    .append(self.text(in_between))
                     .append(self.text(highlight2))
             };
 
diff --git a/compiler/reporting/tests/test_reporting.rs b/compiler/reporting/tests/test_reporting.rs
index 6f1ba3f687..56b309a30a 100644
--- a/compiler/reporting/tests/test_reporting.rs
+++ b/compiler/reporting/tests/test_reporting.rs
@@ -101,7 +101,8 @@ mod test_reporting {
                 ident_ids: &mut ident_ids,
                 ptr_bytes: 8,
                 update_mode_counter: 0,
-                call_specialization_counter: 0,
+                // call_specialization_counter=0 is reserved
+                call_specialization_counter: 1,
             };
             let _mono_expr = Stmt::new(
                 &mut mono_env,
@@ -4774,7 +4775,7 @@ mod test_reporting {
                 r#"
                 ── DOUBLE COMMA ────────────────────────────────────────────────────────────────
 
-                I just started parsing a function argument type, but I encounterd two
+                I just started parsing a function argument type, but I encountered two
                 commas in a row:
 
                 1│  f : I64,,I64 -> I64
diff --git a/compiler/solve/src/module.rs b/compiler/solve/src/module.rs
index f888e2208e..f921b20709 100644
--- a/compiler/solve/src/module.rs
+++ b/compiler/solve/src/module.rs
@@ -11,6 +11,7 @@ use roc_types::types::Alias;
 pub struct SolvedModule {
     pub solved_types: MutMap<Symbol, SolvedType>,
     pub aliases: MutMap<Symbol, Alias>,
+    pub exposed_symbols: Vec<Symbol>,
     pub exposed_vars_by_symbol: MutMap<Symbol, Variable>,
     pub problems: Vec<solve::TypeError>,
 }
diff --git a/compiler/solve/src/solve.rs b/compiler/solve/src/solve.rs
index 4e54259f5a..a683be04f4 100644
--- a/compiler/solve/src/solve.rs
+++ b/compiler/solve/src/solve.rs
@@ -793,7 +793,7 @@ fn type_to_variable(
         Alias(symbol, args, alias_type) => {
             // TODO cache in uniqueness inference gives problems! all Int's get the same uniqueness var!
             // Cache aliases without type arguments. Commonly used aliases like `Int` would otherwise get O(n)
-            // different variables (once for each occurence). The recursion restriction is required
+            // different variables (once for each occurrence). The recursion restriction is required
             // for uniqueness types only: recursive aliases "introduce" an unbound uniqueness
             // attribute in the body, when
             //
diff --git a/compiler/solve/tests/solve_expr.rs b/compiler/solve/tests/solve_expr.rs
index fcafde91af..fe3f3322eb 100644
--- a/compiler/solve/tests/solve_expr.rs
+++ b/compiler/solve/tests/solve_expr.rs
@@ -118,14 +118,11 @@ mod solve_expr {
             subs.get(variable).content
         };
 
-        let actual_str = content_to_string(content, &mut subs, home, &interns);
+        let actual_str = content_to_string(content, &subs, home, &interns);
 
         // Disregard UnusedDef problems, because those are unavoidable when
         // returning a function from the test expression.
-        can_problems.retain(|prob| match prob {
-            roc_problem::can::Problem::UnusedDef(_, _) => false,
-            _ => true,
-        });
+        can_problems.retain(|prob| !matches!(prob, roc_problem::can::Problem::UnusedDef(_, _)));
 
         Ok((type_problems, can_problems, actual_str))
     }
diff --git a/compiler/str/README.md b/compiler/str/README.md
index b49935e8b2..2ab0c3c25d 100644
--- a/compiler/str/README.md
+++ b/compiler/str/README.md
@@ -82,7 +82,7 @@ When calling `List.append list1 list2` on a unique `list1`, first we'll check to
 
 If there is not enough capacity to fit both lists, then we can try to call [`realloc`](https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/realloc?view=vs-2019) to hopefully extend the size of our allocated memory. If `realloc` succeeds (meaning there happened to be enough free memory right after our current allocation), then we update `capacity` to reflect the new amount of space, and move on.
 
-> **Note:** The reason we store capacity right after the last element in the list is becasue of how memory cache lines work. Whenever we need to access `capacity`, it's because we're about to increase the length of the list, which means that we will most certainly be writing to the memory location right after its last element. That in turn means that we'll need to have that memory location in cache, which in turn means that looking up the `capacity` there is guaranteed not to cause a cache miss. (It's possible that writing the new capacity value to a later address could cause a cache miss, but this strategy minimizes the chance of that happening.) An alternate design would be where we store the capacity right before the first element in the list. In that design we wouldn't have to re-write the capacity value at the end of the list every time we grew it, but we'd be much more likely to incur more cache misses that way - because we're working at the end of the list, not at the beginning. Cache misses are many times more expensive than an extra write to a memory address that's in cache already, not to mention the potential extra load instruction to add the length to the memory address of the first element (instead of subtracting 1 from that address), so we optimize for minimizing the highly expensive cache misses by always paying a tiny additional cost when increasing the length of the list, as well as a potential even tinier cost (zero, if the length already happens to be in a register) when looking up its capacity or refcount.
+> **Note:** The reason we store capacity right after the last element in the list is because of how memory cache lines work. Whenever we need to access `capacity`, it's because we're about to increase the length of the list, which means that we will most certainly be writing to the memory location right after its last element. That in turn means that we'll need to have that memory location in cache, which in turn means that looking up the `capacity` there is guaranteed not to cause a cache miss. (It's possible that writing the new capacity value to a later address could cause a cache miss, but this strategy minimizes the chance of that happening.) An alternate design would be where we store the capacity right before the first element in the list. In that design we wouldn't have to re-write the capacity value at the end of the list every time we grew it, but we'd be much more likely to incur more cache misses that way - because we're working at the end of the list, not at the beginning. Cache misses are many times more expensive than an extra write to a memory address that's in cache already, not to mention the potential extra load instruction to add the length to the memory address of the first element (instead of subtracting 1 from that address), so we optimize for minimizing the highly expensive cache misses by always paying a tiny additional cost when increasing the length of the list, as well as a potential even tinier cost (zero, if the length already happens to be in a register) when looking up its capacity or refcount.
 
 If `realloc` fails, then we have to fall back on the same "allocate new memory and copy everything" strategy that we do with shared lists.
 
@@ -107,7 +107,7 @@ Since each bool value is a byte, it's okay for them to be packed side-by-side ev
 
 Note that unlike in the `List Str` example before, there wouldn't be any unused memory between the refcount (or capacity, depending on whether the list was shared or unique) and the first element in the list. That will always be the case when the size of the refcount is no bigger than the alignment of the list's elements.
 
-## Distinguishing bewteen refcount and capacity in the host
+## Distinguishing between refcount and capacity in the host
 
 If I'm a platform author, and I receive a `List` from the application, it's important that I be able to tell whether I'm dealing with a refcount or a capacity. (The uniqueness type information will have been erased by this time, because some applications will return a Unique list while others return a Shared list, so I need to be able to tell using runtime information only which is which.) This way, I can know to either increment the refcount, or to feel free to mutate it in-place using the capacity value.
 
diff --git a/compiler/test_gen/Cargo.toml b/compiler/test_gen/Cargo.toml
index 61058dd120..c34a5c2a17 100644
--- a/compiler/test_gen/Cargo.toml
+++ b/compiler/test_gen/Cargo.toml
@@ -6,7 +6,7 @@ license = "UPL-1.0"
 edition = "2018"
 
 [dependencies]
-roc_gen = { path = "../gen" }
+roc_gen_llvm = { path = "../gen_llvm" }
 roc_collections = { path = "../collections" }
 roc_region = { path = "../region" }
 roc_module = { path = "../module" }
@@ -30,24 +30,7 @@ inlinable_string = "0.1"
 either = "1.6.1"
 indoc = "0.3.3"
 libc = "0.2"
-# NOTE: rtfeldman/inkwell is a fork of TheDan64/inkwell which does not change anything.
-#
-# The reason for this fork is that the way Inkwell is designed, you have to use
-# a particular branch (e.g. "llvm8-0") in Cargo.toml. That would be fine, except that
-# breaking changes get pushed directly to that branch, which breaks our build
-# without warning.
-#
-# We tried referencing a specific rev on TheDan64/inkwell directly (instead of branch),
-# but although that worked locally, it did not work on GitHub Actions. (After a few
-# hours of investigation, gave up trying to figure out why.) So this is the workaround:
-# having an immutable tag on the rtfeldman/inkwell fork which points to
-# a particular "release" of Inkwell.
-#
-# When we want to update Inkwell, we can sync up rtfeldman/inkwell to the latest
-# commit of TheDan64/inkwell, push a new tag which points to the latest commit,
-# change the tag value in this Cargo.toml to point to that tag, and `cargo update`.
-# This way, GitHub Actions works and nobody's builds get broken.
-inkwell = { git = "https://github.com/rtfeldman/inkwell", tag = "llvm10-0.release4", features = [ "llvm10-0" ] }
+inkwell = { path = "../../vendor/inkwell" }
 target-lexicon = "0.10"
 libloading = "0.6"
 
diff --git a/compiler/test_gen/src/gen_compare.rs b/compiler/test_gen/src/gen_compare.rs
index 99bd2649a6..ffe42bb52f 100644
--- a/compiler/test_gen/src/gen_compare.rs
+++ b/compiler/test_gen/src/gen_compare.rs
@@ -442,4 +442,67 @@ mod gen_compare {
         assert_evals_to!("[[1]] != [[1]]", false, bool);
         assert_evals_to!("[[2]] != [[1]]", true, bool);
     }
+
+    #[test]
+    fn compare_union_same_content() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+            Foo : [ A I64, B I64 ]
+
+            a : Foo
+            a = A 42
+
+            b : Foo
+            b = B 42
+
+            a == b
+            "#
+            ),
+            false,
+            bool
+        );
+    }
+
+    #[test]
+    fn compare_recursive_union_same_content() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                Expr : [ Add Expr Expr, Mul Expr Expr, Val1 I64, Val2 I64 ]
+
+                v1 : Expr 
+                v1 = Val1 42
+
+                v2 : Expr 
+                v2 = Val2 42
+
+                v1 == v2
+            "#
+            ),
+            false,
+            bool
+        );
+    }
+
+    #[test]
+    fn compare_nullable_recursive_union_same_content() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                Expr : [ Add Expr Expr, Mul Expr Expr, Val1 I64, Val2 I64, Empty ]
+
+                v1 : Expr 
+                v1 = Val1 42
+
+                v2 : Expr 
+                v2 = Val2 42
+
+                v1 == v2
+            "#
+            ),
+            false,
+            bool
+        );
+    }
 }
diff --git a/compiler/test_gen/src/gen_hash.rs b/compiler/test_gen/src/gen_hash.rs
index 45614a033f..616b377f10 100644
--- a/compiler/test_gen/src/gen_hash.rs
+++ b/compiler/test_gen/src/gen_hash.rs
@@ -37,7 +37,7 @@ fn hash_record() {
 fn hash_result() {
     assert_evals_to!(
         "Dict.hashTestOnly 0 (List.get [ 0x1 ] 0) ",
-        2878521786781103245,
+        6707068610910845221,
         u64
     );
 }
@@ -85,10 +85,13 @@ fn hash_expr() {
                 x : Expr 
                 x = Val 1
 
-                Dict.hashTestOnly 0 (Add x x) 
+                add : Expr
+                add = Add x x
+
+                Dict.hashTestOnly 0 add
             "#
         ),
-        18264046914072177411,
+        10825806964604997723,
         u64
     );
 }
@@ -103,10 +106,13 @@ fn hash_nullable_expr() {
                 x : Expr 
                 x = Val 1
 
-                Dict.hashTestOnly 0 (Add x x) 
+                add : Expr
+                add = Add x x
+
+                Dict.hashTestOnly 0 add
             "#
         ),
-        11103255846683455235,
+        1907558799788307114,
         u64
     );
 }
@@ -129,6 +135,72 @@ fn hash_rosetree() {
     );
 }
 
+#[test]
+fn hash_union_same_content() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            Foo : [ A I64, B I64 ]
+
+            a : Foo
+            a = A 42
+
+            b : Foo
+            b = B 42
+
+            { a: Dict.hashTestOnly 0 a, b : Dict.hashTestOnly 0 b }
+            "#
+        ),
+        true,
+        (i64, i64),
+        |(a, b)| a != b
+    );
+}
+
+#[test]
+fn hash_recursive_union_same_content() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+                Expr : [ Add Expr Expr, Mul Expr Expr, Val1 I64, Val2 I64 ]
+
+                v1 : Expr 
+                v1 = Val1 42
+
+                v2 : Expr 
+                v2 = Val2 42
+
+                { a: Dict.hashTestOnly 0 v1, b : Dict.hashTestOnly 0 v2 }
+            "#
+        ),
+        true,
+        (i64, i64),
+        |(a, b)| a != b
+    );
+}
+
+#[test]
+fn hash_nullable_recursive_union_same_content() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+                Expr : [ Add Expr Expr, Mul Expr Expr, Val1 I64, Val2 I64, Empty ]
+
+                v1 : Expr 
+                v1 = Val1 42
+
+                v2 : Expr 
+                v2 = Val2 42
+
+                { a: Dict.hashTestOnly 0 v1, b : Dict.hashTestOnly 0 v2 }
+            "#
+        ),
+        true,
+        (i64, i64),
+        |(a, b)| a != b
+    );
+}
+
 #[test]
 fn hash_list() {
     assert_evals_to!(
diff --git a/compiler/test_gen/src/gen_list.rs b/compiler/test_gen/src/gen_list.rs
index 98ccb25d32..9a964c3213 100644
--- a/compiler/test_gen/src/gen_list.rs
+++ b/compiler/test_gen/src/gen_list.rs
@@ -179,6 +179,41 @@ fn list_drop() {
     assert_evals_to!("List.drop [1,2] 5", RocList::from_slice(&[]), RocList<i64>);
 }
 
+#[test]
+fn list_swap() {
+    assert_evals_to!("List.swap [] 0 1", RocList::from_slice(&[]), RocList<i64>);
+    assert_evals_to!(
+        "List.swap [ 0 ] 1 2",
+        RocList::from_slice(&[0]),
+        RocList<i64>
+    );
+    assert_evals_to!(
+        "List.swap [ 1, 2 ] 0 1",
+        RocList::from_slice(&[2, 1]),
+        RocList<i64>
+    );
+    assert_evals_to!(
+        "List.swap [ 1, 2 ] 1 0",
+        RocList::from_slice(&[2, 1]),
+        RocList<i64>
+    );
+    assert_evals_to!(
+        "List.swap [ 0, 1, 2, 3, 4, 5 ] 2 4",
+        RocList::from_slice(&[0, 1, 4, 3, 2, 5]),
+        RocList<i64>
+    );
+    assert_evals_to!(
+        "List.swap [ 0, 1, 2 ] 1 3",
+        RocList::from_slice(&[0, 1, 2]),
+        RocList<i64>
+    );
+    assert_evals_to!(
+        "List.swap [ 1, 2, 3 ] 1 1",
+        RocList::from_slice(&[1, 2, 3]),
+        RocList<i64>
+    );
+}
+
 #[test]
 fn list_append_to_empty_list() {
     assert_evals_to!("List.append [] 3", RocList::from_slice(&[3]), RocList<i64>);
@@ -348,7 +383,7 @@ fn list_walk_with_str() {
 }
 
 #[test]
-fn list_walk_substraction() {
+fn list_walk_subtraction() {
     assert_evals_to!(r#"List.walk [ 1, 2 ] Num.sub 1"#, 2, i64);
 }
 
@@ -1904,3 +1939,23 @@ fn list_sort_with() {
         RocList<i64>
     );
 }
+
+#[test]
+#[should_panic(expected = r#"Roc failed with message: "invalid ret_layout""#)]
+fn lists_with_incompatible_type_param_in_if() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            list1 = [ {} ]
+
+            list2 = [ "" ]
+
+            x = if True then list1 else list2
+
+            ""
+            "#
+        ),
+        RocStr::empty(),
+        RocStr
+    );
+}
diff --git a/compiler/test_gen/src/gen_primitives.rs b/compiler/test_gen/src/gen_primitives.rs
index 13706cab7a..3c932db913 100644
--- a/compiler/test_gen/src/gen_primitives.rs
+++ b/compiler/test_gen/src/gen_primitives.rs
@@ -878,6 +878,29 @@ fn when_peano() {
     );
 }
 
+#[test]
+#[should_panic(expected = "Roc failed with message: ")]
+fn overflow_frees_list() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            myList = [1,2,3]
+
+            # integer overflow; must use the list so it is defined before the overflow
+            # the list will then be freed in a cleanup block
+            n : I64
+            n = 9_223_372_036_854_775_807 + (Num.intCast (List.len myList))
+
+            index = Num.intCast n
+
+            List.get myList index
+                 "#
+        ),
+        3,
+        i64
+    );
+}
+
 #[test]
 #[should_panic(expected = "Roc failed with message: ")]
 fn undefined_variable() {
@@ -1660,11 +1683,11 @@ fn binary_tree_double_pattern_match() {
             foo = \btree ->
                 when btree is
                     Node (Node (Leaf x) _) _ -> x
-                    _ -> 0
+                    _ -> 1
 
             main : I64
             main =
-                foo (Node (Node (Leaf 32) (Leaf 0)) (Leaf 0))
+                foo (Node (Node (Leaf 32) (Leaf 2)) (Leaf 3))
             "#
         ),
         32,
@@ -2465,3 +2488,44 @@ fn hit_unresolved_type_variable() {
         RocStr
     );
 }
+
+#[test]
+fn pattern_match_empty_record() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+                app "test" provides [ main ] to "./platform"
+
+                main : I64
+                main =
+                    when {} is
+                        {} -> 0
+
+            "#
+        ),
+        0,
+        i64
+    );
+}
+
+#[test]
+fn pattern_match_unit_tag() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+                app "test" provides [ main ] to "./platform"
+
+                unit : [ Unit ]
+                unit = Unit
+
+                main : I64
+                main =
+                    when unit is
+                        Unit -> 0
+
+            "#
+        ),
+        0,
+        i64
+    );
+}
diff --git a/compiler/test_gen/src/gen_tags.rs b/compiler/test_gen/src/gen_tags.rs
index a16ba1a6af..74b8da70ec 100644
--- a/compiler/test_gen/src/gen_tags.rs
+++ b/compiler/test_gen/src/gen_tags.rs
@@ -19,8 +19,8 @@ fn applied_tag_nothing_ir() {
                 "#
         ),
         1,
-        (i64, i64),
-        |(tag, _)| tag
+        (i64, u8),
+        |(_, tag)| tag
     );
 }
 
@@ -38,8 +38,8 @@ fn applied_tag_nothing() {
                 "#
         ),
         1,
-        (i64, i64),
-        |(tag, _)| tag
+        (i64, u8),
+        |(_, tag)| tag
     );
 }
 
@@ -56,8 +56,8 @@ fn applied_tag_just() {
                 y
                 "#
         ),
-        (0, 0x4),
-        (i64, i64)
+        (0x4, 0),
+        (i64, u8)
     );
 }
 
@@ -74,8 +74,8 @@ fn applied_tag_just_ir() {
                 y
                 "#
         ),
-        (0, 0x4),
-        (i64, i64)
+        (0x4, 0),
+        (i64, u8)
     );
 }
 
@@ -96,8 +96,8 @@ fn applied_tag_just_enum() {
                 y
                 "#
         ),
-        (0, 2),
-        (i64, u8)
+        (2, 0),
+        (u8, i64)
     );
 }
 
@@ -468,6 +468,118 @@ fn nested_pattern_match() {
         i64
     );
 }
+
+#[test]
+fn if_guard_vanilla() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+                when "fooz" is
+                    s if s == "foo" -> 0
+                    s -> List.len (Str.toBytes s)
+                "#
+        ),
+        4,
+        i64
+    );
+}
+
+#[test]
+#[ignore]
+fn when_on_single_value_tag() {
+    // this fails because the switched-on symbol is not defined
+    assert_evals_to!(
+        indoc!(
+            r#"
+            when Identity 0 is
+                Identity 0 -> 0
+                Identity s -> s
+            "#
+        ),
+        6,
+        i64
+    );
+}
+
+#[test]
+fn if_guard_multiple() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            f = \n ->
+                when Identity n 0 is
+                        Identity x _ if x == 0 -> x + 0
+                        Identity x _ if x == 1 -> x + 0
+                        Identity x _ if x == 2 -> x + 0
+                        Identity x _ -> x - x
+
+            { a: f 0, b: f 1, c: f 2, d: f 4 }
+                "#
+        ),
+        (0, 1, 2, 0),
+        (i64, i64, i64, i64)
+    );
+}
+
+#[test]
+fn if_guard_constructor_switch() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            when Identity 32 0 is
+                    Identity 41 _ -> 0
+                    Identity s 0 if s == 32 -> 3
+                    # Identity s 0 -> s
+                    Identity z _ -> z
+                "#
+        ),
+        3,
+        i64
+    );
+
+    assert_evals_to!(
+        indoc!(
+            r#"
+            when Identity 42 "" is
+                    Identity 41 _ -> 0
+                    Identity 42 _ if 3 == 3 -> 1
+                    Identity z _ -> z
+                "#
+        ),
+        1,
+        i64
+    );
+
+    assert_evals_to!(
+        indoc!(
+            r#"
+            when Identity 42 "" is
+                    Identity 41 _ -> 0
+                    Identity 42 _ if 3 != 3 -> 1
+                    Identity z _ -> z
+                "#
+        ),
+        42,
+        i64
+    );
+}
+
+#[test]
+fn if_guard_constructor_chain() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            when Identity 43 0 is
+                    Identity 42 _ if 3 == 3 -> 43
+                    # Identity 42 _ -> 1
+                    Identity z _ -> z
+                "#
+        ),
+        43,
+        i64
+    );
+}
+
 #[test]
 fn if_guard_pattern_false() {
     assert_evals_to!(
@@ -486,6 +598,24 @@ fn if_guard_pattern_false() {
     );
 }
 
+#[test]
+fn if_guard_switch() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+                wrapper = \{} ->
+                    when 2 is
+                        2 | 3 if False -> 0
+                        _ -> 42
+
+                wrapper {}
+                "#
+        ),
+        42,
+        i64
+    );
+}
+
 #[test]
 fn if_guard_pattern_true() {
     assert_evals_to!(
@@ -633,8 +763,8 @@ fn nested_tag_union() {
                     x
                 "#
         ),
-        (0, (0, 41)),
-        (i64, (i64, i64))
+        ((41, 0), 0),
+        ((i64, u8), u8)
     );
 }
 #[test]
@@ -805,8 +935,8 @@ fn alignment_in_multi_tag_construction() {
 
                 #"
         ),
-        (1, 32i64, true),
-        (i64, i64, bool)
+        ((32i64, true), 1),
+        ((i64, bool), u8)
     );
 
     assert_evals_to!(
@@ -818,8 +948,8 @@ fn alignment_in_multi_tag_construction() {
                 x
                 #"
         ),
-        (1, 32i64, true, 2u8),
-        (i64, i64, bool, u8)
+        ((32i64, true, 2u8), 1),
+        ((i64, bool, u8), u8)
     );
 }
 
@@ -948,8 +1078,8 @@ fn nested_recursive_literal() {
                 #"
         ),
         0,
-        &i64,
-        |x: &i64| *x
+        &(i64, i64, u8),
+        |x: &(i64, i64, u8)| x.2
     );
 }
 
@@ -1003,14 +1133,14 @@ fn applied_tag_function_result() {
             x : List (Result Str *)
             x = List.map [ "a", "b" ] Ok
 
-            x
+            List.keepOks x (\y -> y)
             "#
         ),
         RocList::from_slice(&[
-            (1, RocStr::from_slice("a".as_bytes())),
-            (1, RocStr::from_slice("b".as_bytes()))
+            (RocStr::from_slice("a".as_bytes())),
+            (RocStr::from_slice("b".as_bytes()))
         ]),
-        RocList<(i64, RocStr)>
+        RocList<RocStr>
     );
 }
 
@@ -1033,3 +1163,20 @@ fn applied_tag_function_linked_list() {
         i64
     );
 }
+
+#[test]
+#[should_panic(expected = "")]
+fn tag_must_be_its_own_type() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            z : [ A, B, C ]
+            z = Z
+
+            z
+            "#
+        ),
+        1,
+        i64
+    );
+}
diff --git a/compiler/test_gen/src/helpers/eval.rs b/compiler/test_gen/src/helpers/eval.rs
index aa7e6b786b..713f1b8a30 100644
--- a/compiler/test_gen/src/helpers/eval.rs
+++ b/compiler/test_gen/src/helpers/eval.rs
@@ -4,8 +4,9 @@ use roc_build::program::FunctionIterator;
 use roc_can::builtins::builtin_defs_map;
 use roc_can::def::Def;
 use roc_collections::all::{MutMap, MutSet};
-use roc_gen::llvm::externs::add_default_roc_externs;
+use roc_gen_llvm::llvm::externs::add_default_roc_externs;
 use roc_module::symbol::Symbol;
+use roc_mono::ir::OptLevel;
 use roc_types::subs::VarStore;
 
 fn promote_expr_to_module(src: &str) -> String {
@@ -36,7 +37,6 @@ pub fn helper<'a>(
     ignore_problems: bool,
     context: &'a inkwell::context::Context,
 ) -> (&'static str, String, Library) {
-    use roc_gen::llvm::build::{build_proc, build_proc_header, Scope};
     use std::path::{Path, PathBuf};
 
     let filename = PathBuf::from("Test.roc");
@@ -79,15 +79,12 @@ pub fn helper<'a>(
 
     use roc_load::file::MonomorphizedModule;
     let MonomorphizedModule {
-        mut procedures,
+        procedures,
+        entry_point,
         interns,
-        exposed_to_host,
         ..
     } = loaded;
 
-    debug_assert_eq!(exposed_to_host.len(), 1);
-    let main_fn_symbol = exposed_to_host.keys().copied().next().unwrap();
-
     let mut lines = Vec::new();
     // errors whose reporting we delay (so we can see that code gen generates runtime errors)
     let mut delayed_errors = Vec::new();
@@ -171,17 +168,8 @@ pub fn helper<'a>(
         }
     }
 
-    let (_, main_fn_layout) = match procedures.keys().find(|(s, _)| *s == main_fn_symbol) {
-        Some(found) => *found,
-        None => panic!(
-            "The main function symbol {:?} does not have a procedure in {:?}",
-            main_fn_symbol,
-            &procedures.keys()
-        ),
-    };
-
     let builder = context.create_builder();
-    let module = roc_gen::llvm::build::module_from_builtins(context, "app");
+    let module = roc_gen_llvm::llvm::build::module_from_builtins(context, "app");
 
     // Add roc_alloc, roc_realloc, and roc_dealloc, since the repl has no
     // platform to provide them.
@@ -191,16 +179,16 @@ pub fn helper<'a>(
     module.strip_debug_info();
 
     let opt_level = if cfg!(debug_assertions) {
-        roc_gen::llvm::build::OptLevel::Normal
+        OptLevel::Normal
     } else {
-        roc_gen::llvm::build::OptLevel::Optimize
+        OptLevel::Optimize
     };
 
     let module = arena.alloc(module);
     let (module_pass, function_pass) =
-        roc_gen::llvm::build::construct_optimization_passes(module, opt_level);
+        roc_gen_llvm::llvm::build::construct_optimization_passes(module, opt_level);
 
-    let (dibuilder, compile_unit) = roc_gen::llvm::build::Env::new_debug_info(module);
+    let (dibuilder, compile_unit) = roc_gen_llvm::llvm::build::Env::new_debug_info(module);
 
     // mark our zig-defined builtins as internal
     use inkwell::attributes::{Attribute, AttributeLoc};
@@ -222,7 +210,7 @@ pub fn helper<'a>(
     }
 
     // Compile and add all the Procs before adding main
-    let env = roc_gen::llvm::build::Env {
+    let env = roc_gen_llvm::llvm::build::Env {
         arena: &arena,
         builder: &builder,
         dibuilder: &dibuilder,
@@ -236,65 +224,11 @@ pub fn helper<'a>(
         exposed_to_host: MutSet::default(),
     };
 
-    let mut layout_ids = roc_mono::layout::LayoutIds::default();
-    let mut headers = Vec::with_capacity(procedures.len());
-
-    // Add all the Proc headers to the module.
-    // We have to do this in a separate pass first,
-    // because their bodies may reference each other.
-    let mut scope = Scope::default();
-    for ((symbol, layout), proc) in procedures.drain() {
-        let fn_val = build_proc_header(&env, &mut layout_ids, symbol, layout, &proc);
-
-        if proc.args.is_empty() {
-            // this is a 0-argument thunk, i.e. a top-level constant definition
-            // it must be in-scope everywhere in the module!
-            scope.insert_top_level_thunk(symbol, arena.alloc(layout), fn_val);
-        }
-
-        headers.push((proc, fn_val));
-    }
-
-    // Build each proc using its header info.
-    for (proc, fn_val) in headers {
-        let mut current_scope = scope.clone();
-
-        // only have top-level thunks for this proc's module in scope
-        // this retain is not needed for correctness, but will cause less confusion when debugging
-        let home = proc.name.module_id();
-        current_scope.retain_top_level_thunks_for_module(home);
-
-        build_proc(&env, &mut layout_ids, scope.clone(), proc, fn_val);
-
-        // call finalize() before any code generation/verification
-        env.dibuilder.finalize();
-
-        if fn_val.verify(true) {
-            function_pass.run_on(&fn_val);
-        } else {
-            let mode = "NON-OPTIMIZED";
-
-            eprintln!(
-                "\n\nFunction {:?} failed LLVM verification in {} build. Its content was:\n",
-                fn_val.get_name().to_str().unwrap(),
-                mode,
-            );
-
-            fn_val.print_to_stderr();
-            // module.print_to_stderr();
-
-            panic!(
-                "The preceding code was from {:?}, which failed LLVM verification in {} build.",
-                fn_val.get_name().to_str().unwrap(),
-                mode,
-            );
-        }
-    }
-    let (main_fn_name, main_fn) = roc_gen::llvm::build::promote_to_main_function(
+    let (main_fn_name, main_fn) = roc_gen_llvm::llvm::build::build_procedures_return_main(
         &env,
-        &mut layout_ids,
-        main_fn_symbol,
-        main_fn_layout,
+        opt_level,
+        procedures,
+        entry_point,
     );
 
     env.dibuilder.finalize();
@@ -312,7 +246,7 @@ pub fn helper<'a>(
 
     // Verify the module
     if let Err(errors) = env.module.verify() {
-        panic!("Errors defining module: {:?}", errors);
+        panic!("Errors defining module:\n\n{}", errors.to_string());
     }
 
     // Uncomment this to see the module's optimized LLVM instruction output:
@@ -329,7 +263,7 @@ macro_rules! assert_llvm_evals_to {
     ($src:expr, $expected:expr, $ty:ty, $transform:expr, $leak:expr, $ignore_problems:expr) => {
         use bumpalo::Bump;
         use inkwell::context::Context;
-        use roc_gen::run_jit_function;
+        use roc_gen_llvm::run_jit_function;
 
         let arena = Bump::new();
         let context = Context::create();
diff --git a/compiler/test_gen/src/lib.rs b/compiler/test_gen/src/lib.rs
index 8f1630351a..8aaaf93648 100644
--- a/compiler/test_gen/src/lib.rs
+++ b/compiler/test_gen/src/lib.rs
@@ -2,7 +2,7 @@
 // See github.com/rtfeldman/roc/issues/800 for discussion of the large_enum_variant check.
 #![allow(clippy::large_enum_variant)]
 // we actually want to compare against the literal float bits
-#![allow(clippy::clippy::float_cmp)]
+#![allow(clippy::float_cmp)]
 
 pub mod gen_compare;
 pub mod gen_dict;
diff --git a/compiler/test_mono/Cargo.toml b/compiler/test_mono/Cargo.toml
new file mode 100644
index 0000000000..60f3c78a33
--- /dev/null
+++ b/compiler/test_mono/Cargo.toml
@@ -0,0 +1,40 @@
+[package]
+name = "test_mono"
+version = "0.1.0"
+authors = ["The Roc Contributors"]
+license = "UPL-1.0"
+edition = "2018"
+
+[dependencies]
+roc_collections = { path = "../collections" }
+roc_region = { path = "../region" }
+roc_module = { path = "../module" }
+roc_problem = { path = "../problem" }
+roc_types = { path = "../types" }
+roc_builtins = { path = "../builtins" }
+roc_constrain = { path = "../constrain" }
+roc_unify = { path = "../unify" }
+roc_solve = { path = "../solve" }
+roc_reporting = { path = "../reporting" }
+roc_load = { path = "../load" }
+roc_can = { path = "../can" }
+roc_parse = { path = "../parse" }
+roc_build = { path = "../build" }
+roc_mono = { path = "../mono" }
+test_mono_macros = { path = "../test_mono_macros" }
+im    = "14" # im and im-rc should always have the same version!
+im-rc = "14" # im and im-rc should always have the same version!
+bumpalo = { version = "3.6.1", features = ["collections"] }
+inlinable_string = "0.1"
+either = "1.6.1"
+indoc = "0.3.3"
+libc = "0.2"
+target-lexicon = "0.10"
+libloading = "0.6"
+
+[dev-dependencies]
+pretty_assertions = "0.5.1"
+indoc = "0.3.3"
+quickcheck = "0.8"
+quickcheck_macros = "0.8"
+bumpalo = { version = "3.6.1", features = ["collections"] }
diff --git a/compiler/test_mono/generated/alias_variable.txt b/compiler/test_mono/generated/alias_variable.txt
new file mode 100644
index 0000000000..30f430e2d7
--- /dev/null
+++ b/compiler/test_mono/generated/alias_variable.txt
@@ -0,0 +1,4 @@
+procedure Test.0 ():
+    let Test.1 = 5i64;
+    let Test.3 = 3i64;
+    ret Test.3;
diff --git a/compiler/test_mono/generated/alias_variable_and_return_it.txt b/compiler/test_mono/generated/alias_variable_and_return_it.txt
new file mode 100644
index 0000000000..755d390183
--- /dev/null
+++ b/compiler/test_mono/generated/alias_variable_and_return_it.txt
@@ -0,0 +1,3 @@
+procedure Test.0 ():
+    let Test.1 = 5i64;
+    ret Test.1;
diff --git a/compiler/test_mono/generated/branch_store_variable.txt b/compiler/test_mono/generated/branch_store_variable.txt
new file mode 100644
index 0000000000..59f4d90045
--- /dev/null
+++ b/compiler/test_mono/generated/branch_store_variable.txt
@@ -0,0 +1,9 @@
+procedure Test.0 ():
+    let Test.2 = 0i64;
+    let Test.5 = 1i64;
+    let Test.6 = lowlevel Eq Test.5 Test.2;
+    if Test.6 then
+        let Test.3 = 12i64;
+        ret Test.3;
+    else
+        ret Test.2;
diff --git a/compiler/test_mono/generated/closure_in_list.txt b/compiler/test_mono/generated/closure_in_list.txt
new file mode 100644
index 0000000000..41c1e43828
--- /dev/null
+++ b/compiler/test_mono/generated/closure_in_list.txt
@@ -0,0 +1,20 @@
+procedure List.7 (#Attr.2):
+    let Test.7 = lowlevel ListLen #Attr.2;
+    ret Test.7;
+
+procedure Test.1 (Test.5):
+    let Test.2 = 41i64;
+    let Test.11 = Struct {Test.2};
+    let Test.10 = Array [Test.11];
+    ret Test.10;
+
+procedure Test.3 (Test.9, #Attr.12):
+    let Test.2 = StructAtIndex 0 #Attr.12;
+    ret Test.2;
+
+procedure Test.0 ():
+    let Test.8 = Struct {};
+    let Test.4 = CallByName Test.1 Test.8;
+    let Test.6 = CallByName List.7 Test.4;
+    dec Test.4;
+    ret Test.6;
diff --git a/compiler/test_mono/generated/dict.txt b/compiler/test_mono/generated/dict.txt
new file mode 100644
index 0000000000..7711398541
--- /dev/null
+++ b/compiler/test_mono/generated/dict.txt
@@ -0,0 +1,12 @@
+procedure Dict.2 ():
+    let Test.4 = lowlevel DictEmpty ;
+    ret Test.4;
+
+procedure Dict.8 (#Attr.2):
+    let Test.3 = lowlevel DictSize #Attr.2;
+    ret Test.3;
+
+procedure Test.0 ():
+    let Test.2 = CallByName Dict.2;
+    let Test.1 = CallByName Dict.8 Test.2;
+    ret Test.1;
diff --git a/compiler/test_mono/generated/factorial.txt b/compiler/test_mono/generated/factorial.txt
new file mode 100644
index 0000000000..1c6b0e2d39
--- /dev/null
+++ b/compiler/test_mono/generated/factorial.txt
@@ -0,0 +1,27 @@
+procedure Num.25 (#Attr.2, #Attr.3):
+    let Test.15 = lowlevel NumSub #Attr.2 #Attr.3;
+    ret Test.15;
+
+procedure Num.26 (#Attr.2, #Attr.3):
+    let Test.12 = lowlevel NumMul #Attr.2 #Attr.3;
+    ret Test.12;
+
+procedure Test.1 (Test.2, Test.3):
+    joinpoint Test.7 Test.2 Test.3:
+        let Test.17 = 0i64;
+        let Test.18 = lowlevel Eq Test.17 Test.2;
+        if Test.18 then
+            ret Test.3;
+        else
+            let Test.14 = 1i64;
+            let Test.10 = CallByName Num.25 Test.2 Test.14;
+            let Test.11 = CallByName Num.26 Test.2 Test.3;
+            jump Test.7 Test.10 Test.11;
+    in
+    jump Test.7 Test.2 Test.3;
+
+procedure Test.0 ():
+    let Test.5 = 10i64;
+    let Test.6 = 1i64;
+    let Test.4 = CallByName Test.1 Test.5 Test.6;
+    ret Test.4;
diff --git a/compiler/test_mono/generated/fst.txt b/compiler/test_mono/generated/fst.txt
new file mode 100644
index 0000000000..13e6954e29
--- /dev/null
+++ b/compiler/test_mono/generated/fst.txt
@@ -0,0 +1,17 @@
+procedure Test.1 (Test.2, Test.3):
+    inc Test.2;
+    ret Test.2;
+
+procedure Test.0 ():
+    let Test.11 = 1i64;
+    let Test.12 = 2i64;
+    let Test.13 = 3i64;
+    let Test.5 = Array [Test.11, Test.12, Test.13];
+    let Test.8 = 3i64;
+    let Test.9 = 2i64;
+    let Test.10 = 1i64;
+    let Test.6 = Array [Test.8, Test.9, Test.10];
+    let Test.4 = CallByName Test.1 Test.5 Test.6;
+    dec Test.6;
+    dec Test.5;
+    ret Test.4;
diff --git a/compiler/test_mono/generated/guard_pattern_true.txt b/compiler/test_mono/generated/guard_pattern_true.txt
new file mode 100644
index 0000000000..c0ee2f046d
--- /dev/null
+++ b/compiler/test_mono/generated/guard_pattern_true.txt
@@ -0,0 +1,25 @@
+procedure Test.1 (Test.3):
+    let Test.6 = 2i64;
+    joinpoint Test.11:
+        let Test.10 = 0i64;
+        ret Test.10;
+    in
+    let Test.13 = 2i64;
+    let Test.14 = lowlevel Eq Test.13 Test.6;
+    if Test.14 then
+        joinpoint Test.8 Test.12:
+            if Test.12 then
+                let Test.7 = 42i64;
+                ret Test.7;
+            else
+                jump Test.11;
+        in
+        let Test.9 = false;
+        jump Test.8 Test.9;
+    else
+        jump Test.11;
+
+procedure Test.0 ():
+    let Test.5 = Struct {};
+    let Test.4 = CallByName Test.1 Test.5;
+    ret Test.4;
diff --git a/compiler/test_mono/generated/has_none.txt b/compiler/test_mono/generated/has_none.txt
new file mode 100644
index 0000000000..cfbfaadfae
--- /dev/null
+++ b/compiler/test_mono/generated/has_none.txt
@@ -0,0 +1,32 @@
+procedure Test.3 (Test.4):
+    joinpoint Test.13 Test.4:
+        let Test.23 = 1i64;
+        let Test.24 = GetTagId Test.4;
+        let Test.25 = lowlevel Eq Test.23 Test.24;
+        if Test.25 then
+            let Test.14 = false;
+            ret Test.14;
+        else
+            let Test.19 = UnionAtIndex (Id 0) (Index 0) Test.4;
+            let Test.20 = 1i64;
+            let Test.21 = GetTagId Test.19;
+            let Test.22 = lowlevel Eq Test.20 Test.21;
+            if Test.22 then
+                let Test.15 = true;
+                ret Test.15;
+            else
+                let Test.7 = UnionAtIndex (Id 0) (Index 1) Test.4;
+                jump Test.13 Test.7;
+    in
+    jump Test.13 Test.4;
+
+procedure Test.0 ():
+    let Test.28 = 0i64;
+    let Test.30 = 3i64;
+    let Test.26 = Just Test.30;
+    let Test.29 = 1i64;
+    let Test.27 = Nil ;
+    let Test.12 = Cons Test.26 Test.27;
+    let Test.11 = CallByName Test.3 Test.12;
+    dec Test.12;
+    ret Test.11;
diff --git a/compiler/test_mono/generated/if_guard_bind_variable_false.txt b/compiler/test_mono/generated/if_guard_bind_variable_false.txt
new file mode 100644
index 0000000000..36630718ca
--- /dev/null
+++ b/compiler/test_mono/generated/if_guard_bind_variable_false.txt
@@ -0,0 +1,22 @@
+procedure Bool.5 (#Attr.2, #Attr.3):
+    let Test.11 = lowlevel Eq #Attr.2 #Attr.3;
+    ret Test.11;
+
+procedure Test.1 (Test.3):
+    let Test.6 = 10i64;
+    joinpoint Test.8 Test.13:
+        if Test.13 then
+            let Test.7 = 0i64;
+            ret Test.7;
+        else
+            let Test.12 = 42i64;
+            ret Test.12;
+    in
+    let Test.10 = 5i64;
+    let Test.9 = CallByName Bool.5 Test.6 Test.10;
+    jump Test.8 Test.9;
+
+procedure Test.0 ():
+    let Test.5 = Struct {};
+    let Test.4 = CallByName Test.1 Test.5;
+    ret Test.4;
diff --git a/compiler/test_mono/generated/if_multi_branch.txt b/compiler/test_mono/generated/if_multi_branch.txt
new file mode 100644
index 0000000000..c4953af3c3
--- /dev/null
+++ b/compiler/test_mono/generated/if_multi_branch.txt
@@ -0,0 +1,13 @@
+procedure Test.0 ():
+    let Test.6 = true;
+    if Test.6 then
+        let Test.7 = 1i64;
+        ret Test.7;
+    else
+        let Test.4 = false;
+        if Test.4 then
+            let Test.5 = 2i64;
+            ret Test.5;
+        else
+            let Test.3 = 3i64;
+            ret Test.3;
diff --git a/compiler/test_mono/generated/ir_assignment.txt b/compiler/test_mono/generated/ir_assignment.txt
new file mode 100644
index 0000000000..755d390183
--- /dev/null
+++ b/compiler/test_mono/generated/ir_assignment.txt
@@ -0,0 +1,3 @@
+procedure Test.0 ():
+    let Test.1 = 5i64;
+    ret Test.1;
diff --git a/compiler/test_mono/generated/ir_int_add.txt b/compiler/test_mono/generated/ir_int_add.txt
new file mode 100644
index 0000000000..0adc761dc1
--- /dev/null
+++ b/compiler/test_mono/generated/ir_int_add.txt
@@ -0,0 +1,25 @@
+procedure List.7 (#Attr.2):
+    let Test.7 = lowlevel ListLen #Attr.2;
+    ret Test.7;
+
+procedure Num.24 (#Attr.2, #Attr.3):
+    let Test.5 = lowlevel NumAdd #Attr.2 #Attr.3;
+    ret Test.5;
+
+procedure Test.0 ():
+    let Test.14 = 1i64;
+    let Test.15 = 2i64;
+    let Test.1 = Array [Test.14, Test.15];
+    let Test.11 = 5i64;
+    let Test.12 = 4i64;
+    invoke Test.8 = CallByName Num.24 Test.11 Test.12 catch
+        dec Test.1;
+        unreachable;
+    let Test.9 = 3i64;
+    invoke Test.3 = CallByName Num.24 Test.8 Test.9 catch
+        dec Test.1;
+        unreachable;
+    let Test.4 = CallByName List.7 Test.1;
+    dec Test.1;
+    let Test.2 = CallByName Num.24 Test.3 Test.4;
+    ret Test.2;
diff --git a/compiler/test_mono/generated/ir_int_literal.txt b/compiler/test_mono/generated/ir_int_literal.txt
new file mode 100644
index 0000000000..755d390183
--- /dev/null
+++ b/compiler/test_mono/generated/ir_int_literal.txt
@@ -0,0 +1,3 @@
+procedure Test.0 ():
+    let Test.1 = 5i64;
+    ret Test.1;
diff --git a/compiler/test_mono/generated/ir_plus.txt b/compiler/test_mono/generated/ir_plus.txt
new file mode 100644
index 0000000000..1fa959e684
--- /dev/null
+++ b/compiler/test_mono/generated/ir_plus.txt
@@ -0,0 +1,9 @@
+procedure Num.24 (#Attr.2, #Attr.3):
+    let Test.4 = lowlevel NumAdd #Attr.2 #Attr.3;
+    ret Test.4;
+
+procedure Test.0 ():
+    let Test.2 = 1i64;
+    let Test.3 = 2i64;
+    let Test.1 = CallByName Num.24 Test.2 Test.3;
+    ret Test.1;
diff --git a/compiler/test_mono/generated/ir_round.txt b/compiler/test_mono/generated/ir_round.txt
new file mode 100644
index 0000000000..5448d22900
--- /dev/null
+++ b/compiler/test_mono/generated/ir_round.txt
@@ -0,0 +1,8 @@
+procedure Num.47 (#Attr.2):
+    let Test.3 = lowlevel NumRound #Attr.2;
+    ret Test.3;
+
+procedure Test.0 ():
+    let Test.2 = 3.6f64;
+    let Test.1 = CallByName Num.47 Test.2;
+    ret Test.1;
diff --git a/compiler/test_mono/generated/ir_two_defs.txt b/compiler/test_mono/generated/ir_two_defs.txt
new file mode 100644
index 0000000000..04a64113c5
--- /dev/null
+++ b/compiler/test_mono/generated/ir_two_defs.txt
@@ -0,0 +1,9 @@
+procedure Num.24 (#Attr.2, #Attr.3):
+    let Test.4 = lowlevel NumAdd #Attr.2 #Attr.3;
+    ret Test.4;
+
+procedure Test.0 ():
+    let Test.1 = 3i64;
+    let Test.2 = 4i64;
+    let Test.3 = CallByName Num.24 Test.1 Test.2;
+    ret Test.3;
diff --git a/compiler/test_mono/generated/ir_when_idiv.txt b/compiler/test_mono/generated/ir_when_idiv.txt
new file mode 100644
index 0000000000..a7463bbad2
--- /dev/null
+++ b/compiler/test_mono/generated/ir_when_idiv.txt
@@ -0,0 +1,25 @@
+procedure Num.42 (#Attr.2, #Attr.3):
+    let Test.15 = 0i64;
+    let Test.12 = lowlevel NotEq #Attr.3 Test.15;
+    if Test.12 then
+        let Test.14 = lowlevel NumDivUnchecked #Attr.2 #Attr.3;
+        let Test.13 = Ok Test.14;
+        ret Test.13;
+    else
+        let Test.11 = Struct {};
+        let Test.10 = Err Test.11;
+        ret Test.10;
+
+procedure Test.0 ():
+    let Test.8 = 1000i64;
+    let Test.9 = 10i64;
+    let Test.2 = CallByName Num.42 Test.8 Test.9;
+    let Test.5 = 1i64;
+    let Test.6 = GetTagId Test.2;
+    let Test.7 = lowlevel Eq Test.5 Test.6;
+    if Test.7 then
+        let Test.1 = UnionAtIndex (Id 1) (Index 0) Test.2;
+        ret Test.1;
+    else
+        let Test.4 = -1i64;
+        ret Test.4;
diff --git a/compiler/test_mono/generated/ir_when_just.txt b/compiler/test_mono/generated/ir_when_just.txt
new file mode 100644
index 0000000000..4ef134c7eb
--- /dev/null
+++ b/compiler/test_mono/generated/ir_when_just.txt
@@ -0,0 +1,18 @@
+procedure Num.24 (#Attr.2, #Attr.3):
+    let Test.6 = lowlevel NumAdd #Attr.2 #Attr.3;
+    ret Test.6;
+
+procedure Test.0 ():
+    let Test.12 = 41i64;
+    let Test.1 = Just Test.12;
+    let Test.9 = 0i64;
+    let Test.10 = GetTagId Test.1;
+    let Test.11 = lowlevel Eq Test.9 Test.10;
+    if Test.11 then
+        let Test.3 = UnionAtIndex (Id 0) (Index 0) Test.1;
+        let Test.5 = 1i64;
+        let Test.4 = CallByName Num.24 Test.3 Test.5;
+        ret Test.4;
+    else
+        let Test.8 = 1i64;
+        ret Test.8;
diff --git a/compiler/test_mono/generated/ir_when_maybe.txt b/compiler/test_mono/generated/ir_when_maybe.txt
new file mode 100644
index 0000000000..1cc90f1f16
--- /dev/null
+++ b/compiler/test_mono/generated/ir_when_maybe.txt
@@ -0,0 +1,12 @@
+procedure Test.0 ():
+    let Test.9 = 3i64;
+    let Test.3 = Just Test.9;
+    let Test.6 = 0i64;
+    let Test.7 = GetTagId Test.3;
+    let Test.8 = lowlevel Eq Test.6 Test.7;
+    if Test.8 then
+        let Test.2 = UnionAtIndex (Id 0) (Index 0) Test.3;
+        ret Test.2;
+    else
+        let Test.5 = 0i64;
+        ret Test.5;
diff --git a/compiler/test_mono/generated/ir_when_record.txt b/compiler/test_mono/generated/ir_when_record.txt
new file mode 100644
index 0000000000..ed777a23d0
--- /dev/null
+++ b/compiler/test_mono/generated/ir_when_record.txt
@@ -0,0 +1,6 @@
+procedure Test.0 ():
+    let Test.4 = 1i64;
+    let Test.5 = 3.14f64;
+    let Test.2 = Struct {Test.4, Test.5};
+    let Test.1 = StructAtIndex 0 Test.2;
+    ret Test.1;
diff --git a/compiler/test_mono/generated/ir_when_these.txt b/compiler/test_mono/generated/ir_when_these.txt
new file mode 100644
index 0000000000..bcba501785
--- /dev/null
+++ b/compiler/test_mono/generated/ir_when_these.txt
@@ -0,0 +1,18 @@
+procedure Test.0 ():
+    let Test.10 = 1i64;
+    let Test.11 = 2i64;
+    let Test.5 = These Test.10 Test.11;
+    let Test.9 = GetTagId Test.5;
+    switch Test.9:
+        case 2:
+            let Test.2 = UnionAtIndex (Id 2) (Index 0) Test.5;
+            ret Test.2;
+    
+        case 0:
+            let Test.3 = UnionAtIndex (Id 0) (Index 0) Test.5;
+            ret Test.3;
+    
+        default:
+            let Test.4 = UnionAtIndex (Id 1) (Index 0) Test.5;
+            ret Test.4;
+    
diff --git a/compiler/test_mono/generated/is_nil.txt b/compiler/test_mono/generated/is_nil.txt
new file mode 100644
index 0000000000..ec6a558c3a
--- /dev/null
+++ b/compiler/test_mono/generated/is_nil.txt
@@ -0,0 +1,20 @@
+procedure Test.2 (Test.3):
+    let Test.12 = 1i64;
+    let Test.13 = GetTagId Test.3;
+    let Test.14 = lowlevel Eq Test.12 Test.13;
+    if Test.14 then
+        let Test.10 = true;
+        ret Test.10;
+    else
+        let Test.11 = false;
+        ret Test.11;
+
+procedure Test.0 ():
+    let Test.17 = 0i64;
+    let Test.15 = 2i64;
+    let Test.18 = 1i64;
+    let Test.16 = Nil ;
+    let Test.9 = Cons Test.15 Test.16;
+    let Test.8 = CallByName Test.2 Test.9;
+    dec Test.9;
+    ret Test.8;
diff --git a/compiler/test_mono/generated/let_with_record_pattern.txt b/compiler/test_mono/generated/let_with_record_pattern.txt
new file mode 100644
index 0000000000..51df825ebf
--- /dev/null
+++ b/compiler/test_mono/generated/let_with_record_pattern.txt
@@ -0,0 +1,6 @@
+procedure Test.0 ():
+    let Test.4 = 2i64;
+    let Test.5 = 3.14f64;
+    let Test.3 = Struct {Test.4, Test.5};
+    let Test.1 = StructAtIndex 0 Test.3;
+    ret Test.1;
diff --git a/compiler/test_mono/generated/let_with_record_pattern_list.txt b/compiler/test_mono/generated/let_with_record_pattern_list.txt
new file mode 100644
index 0000000000..e2ed3831d0
--- /dev/null
+++ b/compiler/test_mono/generated/let_with_record_pattern_list.txt
@@ -0,0 +1,11 @@
+procedure Test.0 ():
+    let Test.6 = 1i64;
+    let Test.7 = 3i64;
+    let Test.8 = 4i64;
+    let Test.4 = Array [Test.6, Test.7, Test.8];
+    let Test.5 = 3.14f64;
+    let Test.3 = Struct {Test.4, Test.5};
+    let Test.1 = StructAtIndex 0 Test.3;
+    inc Test.1;
+    dec Test.3;
+    ret Test.1;
diff --git a/compiler/test_mono/generated/let_x_in_x.txt b/compiler/test_mono/generated/let_x_in_x.txt
new file mode 100644
index 0000000000..567439fa80
--- /dev/null
+++ b/compiler/test_mono/generated/let_x_in_x.txt
@@ -0,0 +1,5 @@
+procedure Test.0 ():
+    let Test.1 = 5i64;
+    let Test.4 = 17i64;
+    let Test.2 = 1337i64;
+    ret Test.2;
diff --git a/compiler/test_mono/generated/let_x_in_x_indirect.txt b/compiler/test_mono/generated/let_x_in_x_indirect.txt
new file mode 100644
index 0000000000..4fe43f886b
--- /dev/null
+++ b/compiler/test_mono/generated/let_x_in_x_indirect.txt
@@ -0,0 +1,8 @@
+procedure Test.0 ():
+    let Test.1 = 5i64;
+    let Test.4 = 17i64;
+    let Test.5 = 1i64;
+    let Test.2 = 1337i64;
+    let Test.7 = Struct {Test.2, Test.4};
+    let Test.6 = StructAtIndex 0 Test.7;
+    ret Test.6;
diff --git a/compiler/test_mono/generated/linked_list_length_twice.txt b/compiler/test_mono/generated/linked_list_length_twice.txt
new file mode 100644
index 0000000000..92aa6d9c20
--- /dev/null
+++ b/compiler/test_mono/generated/linked_list_length_twice.txt
@@ -0,0 +1,26 @@
+procedure Num.24 (#Attr.2, #Attr.3):
+    let Test.10 = lowlevel NumAdd #Attr.2 #Attr.3;
+    ret Test.10;
+
+procedure Test.3 (Test.5):
+    let Test.18 = 1i64;
+    let Test.19 = GetTagId Test.5;
+    let Test.20 = lowlevel Eq Test.18 Test.19;
+    if Test.20 then
+        let Test.13 = 0i64;
+        ret Test.13;
+    else
+        let Test.6 = UnionAtIndex (Id 0) (Index 1) Test.5;
+        let Test.15 = 1i64;
+        let Test.16 = CallByName Test.3 Test.6;
+        let Test.14 = CallByName Num.24 Test.15 Test.16;
+        ret Test.14;
+
+procedure Test.0 ():
+    let Test.21 = 1i64;
+    let Test.2 = Nil ;
+    let Test.8 = CallByName Test.3 Test.2;
+    let Test.9 = CallByName Test.3 Test.2;
+    dec Test.2;
+    let Test.7 = CallByName Num.24 Test.8 Test.9;
+    ret Test.7;
diff --git a/compiler/test_mono/generated/list_append.txt b/compiler/test_mono/generated/list_append.txt
new file mode 100644
index 0000000000..49c502afd3
--- /dev/null
+++ b/compiler/test_mono/generated/list_append.txt
@@ -0,0 +1,10 @@
+procedure List.5 (#Attr.2, #Attr.3):
+    let Test.4 = lowlevel ListAppend #Attr.2 #Attr.3;
+    ret Test.4;
+
+procedure Test.0 ():
+    let Test.5 = 1i64;
+    let Test.2 = Array [Test.5];
+    let Test.3 = 2i64;
+    let Test.1 = CallByName List.5 Test.2 Test.3;
+    ret Test.1;
diff --git a/compiler/test_mono/generated/list_append_closure.txt b/compiler/test_mono/generated/list_append_closure.txt
new file mode 100644
index 0000000000..cd33ef5754
--- /dev/null
+++ b/compiler/test_mono/generated/list_append_closure.txt
@@ -0,0 +1,15 @@
+procedure List.5 (#Attr.2, #Attr.3):
+    let Test.7 = lowlevel ListAppend #Attr.2 #Attr.3;
+    ret Test.7;
+
+procedure Test.1 (Test.2):
+    let Test.6 = 42i64;
+    let Test.5 = CallByName List.5 Test.2 Test.6;
+    ret Test.5;
+
+procedure Test.0 ():
+    let Test.8 = 1i64;
+    let Test.9 = 2i64;
+    let Test.4 = Array [Test.8, Test.9];
+    let Test.3 = CallByName Test.1 Test.4;
+    ret Test.3;
diff --git a/compiler/test_mono/generated/list_cannot_update_inplace.txt b/compiler/test_mono/generated/list_cannot_update_inplace.txt
new file mode 100644
index 0000000000..0c9ff5de3f
--- /dev/null
+++ b/compiler/test_mono/generated/list_cannot_update_inplace.txt
@@ -0,0 +1,40 @@
+procedure List.4 (#Attr.2, #Attr.3, #Attr.4):
+    let Test.23 = lowlevel ListLen #Attr.2;
+    let Test.21 = lowlevel NumLt #Attr.3 Test.23;
+    if Test.21 then
+        let Test.22 = lowlevel ListSet #Attr.2 #Attr.3 #Attr.4;
+        ret Test.22;
+    else
+        ret #Attr.2;
+
+procedure List.7 (#Attr.2):
+    let Test.10 = lowlevel ListLen #Attr.2;
+    ret Test.10;
+
+procedure Num.24 (#Attr.2, #Attr.3):
+    let Test.7 = lowlevel NumAdd #Attr.2 #Attr.3;
+    ret Test.7;
+
+procedure Test.1 ():
+    let Test.12 = 1i64;
+    let Test.13 = 2i64;
+    let Test.14 = 3i64;
+    let Test.11 = Array [Test.12, Test.13, Test.14];
+    ret Test.11;
+
+procedure Test.2 (Test.3):
+    let Test.18 = 0i64;
+    let Test.19 = 0i64;
+    let Test.17 = CallByName List.4 Test.3 Test.18 Test.19;
+    ret Test.17;
+
+procedure Test.0 ():
+    let Test.16 = CallByName Test.1;
+    let Test.15 = CallByName Test.2 Test.16;
+    let Test.5 = CallByName List.7 Test.15;
+    dec Test.15;
+    let Test.9 = CallByName Test.1;
+    let Test.6 = CallByName List.7 Test.9;
+    dec Test.9;
+    let Test.4 = CallByName Num.24 Test.5 Test.6;
+    ret Test.4;
diff --git a/compiler/test_mono/generated/list_get.txt b/compiler/test_mono/generated/list_get.txt
new file mode 100644
index 0000000000..e2da3a0e9b
--- /dev/null
+++ b/compiler/test_mono/generated/list_get.txt
@@ -0,0 +1,26 @@
+procedure List.3 (#Attr.2, #Attr.3):
+    let Test.13 = lowlevel ListLen #Attr.2;
+    let Test.10 = lowlevel NumLt #Attr.3 Test.13;
+    if Test.10 then
+        let Test.12 = lowlevel ListGetUnsafe #Attr.2 #Attr.3;
+        let Test.11 = Ok Test.12;
+        ret Test.11;
+    else
+        let Test.9 = Struct {};
+        let Test.8 = Err Test.9;
+        ret Test.8;
+
+procedure Test.1 (Test.2):
+    let Test.14 = 1i64;
+    let Test.15 = 2i64;
+    let Test.16 = 3i64;
+    let Test.6 = Array [Test.14, Test.15, Test.16];
+    let Test.7 = 0i64;
+    let Test.5 = CallByName List.3 Test.6 Test.7;
+    dec Test.6;
+    ret Test.5;
+
+procedure Test.0 ():
+    let Test.4 = Struct {};
+    let Test.3 = CallByName Test.1 Test.4;
+    ret Test.3;
diff --git a/compiler/test_mono/generated/list_len.txt b/compiler/test_mono/generated/list_len.txt
new file mode 100644
index 0000000000..1f50df081b
--- /dev/null
+++ b/compiler/test_mono/generated/list_len.txt
@@ -0,0 +1,25 @@
+procedure List.7 (#Attr.2):
+    let Test.8 = lowlevel ListLen #Attr.2;
+    ret Test.8;
+
+procedure List.7 (#Attr.2):
+    let Test.9 = lowlevel ListLen #Attr.2;
+    ret Test.9;
+
+procedure Num.24 (#Attr.2, #Attr.3):
+    let Test.6 = lowlevel NumAdd #Attr.2 #Attr.3;
+    ret Test.6;
+
+procedure Test.0 ():
+    let Test.11 = 1i64;
+    let Test.12 = 2i64;
+    let Test.13 = 3i64;
+    let Test.1 = Array [Test.11, Test.12, Test.13];
+    let Test.10 = 1f64;
+    let Test.2 = Array [Test.10];
+    let Test.4 = CallByName List.7 Test.1;
+    dec Test.1;
+    let Test.5 = CallByName List.7 Test.2;
+    dec Test.2;
+    let Test.3 = CallByName Num.24 Test.4 Test.5;
+    ret Test.3;
diff --git a/compiler/test_mono/generated/list_pass_to_function.txt b/compiler/test_mono/generated/list_pass_to_function.txt
new file mode 100644
index 0000000000..b33881bde8
--- /dev/null
+++ b/compiler/test_mono/generated/list_pass_to_function.txt
@@ -0,0 +1,22 @@
+procedure List.4 (#Attr.2, #Attr.3, #Attr.4):
+    let Test.11 = lowlevel ListLen #Attr.2;
+    let Test.9 = lowlevel NumLt #Attr.3 Test.11;
+    if Test.9 then
+        let Test.10 = lowlevel ListSet #Attr.2 #Attr.3 #Attr.4;
+        ret Test.10;
+    else
+        ret #Attr.2;
+
+procedure Test.2 (Test.3):
+    let Test.6 = 0i64;
+    let Test.7 = 0i64;
+    let Test.5 = CallByName List.4 Test.3 Test.6 Test.7;
+    ret Test.5;
+
+procedure Test.0 ():
+    let Test.12 = 1i64;
+    let Test.13 = 2i64;
+    let Test.14 = 3i64;
+    let Test.1 = Array [Test.12, Test.13, Test.14];
+    let Test.4 = CallByName Test.2 Test.1;
+    ret Test.4;
diff --git a/compiler/test_mono/generated/mk_pair_of.txt b/compiler/test_mono/generated/mk_pair_of.txt
new file mode 100644
index 0000000000..ae4932f71a
--- /dev/null
+++ b/compiler/test_mono/generated/mk_pair_of.txt
@@ -0,0 +1,12 @@
+procedure Test.1 (Test.2):
+    inc Test.2;
+    let Test.6 = Struct {Test.2, Test.2};
+    ret Test.6;
+
+procedure Test.0 ():
+    let Test.7 = 1i64;
+    let Test.8 = 2i64;
+    let Test.9 = 3i64;
+    let Test.5 = Array [Test.7, Test.8, Test.9];
+    let Test.4 = CallByName Test.1 Test.5;
+    ret Test.4;
diff --git a/compiler/test_mono/generated/nested_closure.txt b/compiler/test_mono/generated/nested_closure.txt
new file mode 100644
index 0000000000..3d5c1411d7
--- /dev/null
+++ b/compiler/test_mono/generated/nested_closure.txt
@@ -0,0 +1,15 @@
+procedure Test.1 (Test.5):
+    let Test.2 = 42i64;
+    let Test.3 = Struct {Test.2};
+    ret Test.3;
+
+procedure Test.3 (Test.9, #Attr.12):
+    let Test.2 = StructAtIndex 0 #Attr.12;
+    ret Test.2;
+
+procedure Test.0 ():
+    let Test.8 = Struct {};
+    let Test.4 = CallByName Test.1 Test.8;
+    let Test.7 = Struct {};
+    let Test.6 = CallByName Test.3 Test.7 Test.4;
+    ret Test.6;
diff --git a/compiler/test_mono/generated/nested_pattern_match.txt b/compiler/test_mono/generated/nested_pattern_match.txt
new file mode 100644
index 0000000000..24279438b5
--- /dev/null
+++ b/compiler/test_mono/generated/nested_pattern_match.txt
@@ -0,0 +1,30 @@
+procedure Num.24 (#Attr.2, #Attr.3):
+    let Test.8 = lowlevel NumAdd #Attr.2 #Attr.3;
+    ret Test.8;
+
+procedure Test.0 ():
+    let Test.21 = 41i64;
+    let Test.20 = Just Test.21;
+    let Test.2 = Just Test.20;
+    joinpoint Test.17:
+        let Test.10 = 1i64;
+        ret Test.10;
+    in
+    let Test.15 = 0i64;
+    let Test.16 = GetTagId Test.2;
+    let Test.19 = lowlevel Eq Test.15 Test.16;
+    if Test.19 then
+        let Test.12 = UnionAtIndex (Id 0) (Index 0) Test.2;
+        let Test.13 = 0i64;
+        let Test.14 = GetTagId Test.12;
+        let Test.18 = lowlevel Eq Test.13 Test.14;
+        if Test.18 then
+            let Test.11 = UnionAtIndex (Id 0) (Index 0) Test.2;
+            let Test.5 = UnionAtIndex (Id 0) (Index 0) Test.11;
+            let Test.7 = 1i64;
+            let Test.6 = CallByName Num.24 Test.5 Test.7;
+            ret Test.6;
+        else
+            jump Test.17;
+    else
+        jump Test.17;
diff --git a/compiler/test_mono/generated/one_element_tag.txt b/compiler/test_mono/generated/one_element_tag.txt
new file mode 100644
index 0000000000..4ecfe61168
--- /dev/null
+++ b/compiler/test_mono/generated/one_element_tag.txt
@@ -0,0 +1,3 @@
+procedure Test.0 ():
+    let Test.4 = 2i64;
+    ret Test.4;
diff --git a/compiler/test_mono/generated/optional_when.txt b/compiler/test_mono/generated/optional_when.txt
new file mode 100644
index 0000000000..9e261687aa
--- /dev/null
+++ b/compiler/test_mono/generated/optional_when.txt
@@ -0,0 +1,42 @@
+procedure Num.26 (#Attr.2, #Attr.3):
+    let Test.17 = lowlevel NumMul #Attr.2 #Attr.3;
+    ret Test.17;
+
+procedure Test.1 (Test.6):
+    let Test.25 = StructAtIndex 1 Test.6;
+    let Test.26 = false;
+    let Test.27 = lowlevel Eq Test.26 Test.25;
+    if Test.27 then
+        let Test.8 = StructAtIndex 0 Test.6;
+        ret Test.8;
+    else
+        let Test.10 = StructAtIndex 0 Test.6;
+        ret Test.10;
+
+procedure Test.1 (Test.6):
+    let Test.36 = false;
+    let Test.37 = lowlevel Eq Test.36 Test.6;
+    if Test.37 then
+        let Test.8 = 3i64;
+        ret Test.8;
+    else
+        let Test.10 = 5i64;
+        ret Test.10;
+
+procedure Test.0 ():
+    let Test.40 = true;
+    let Test.5 = CallByName Test.1 Test.40;
+    let Test.38 = false;
+    let Test.3 = CallByName Test.1 Test.38;
+    let Test.31 = 11i64;
+    let Test.32 = true;
+    let Test.30 = Struct {Test.31, Test.32};
+    let Test.4 = CallByName Test.1 Test.30;
+    let Test.28 = 7i64;
+    let Test.29 = false;
+    let Test.22 = Struct {Test.28, Test.29};
+    let Test.2 = CallByName Test.1 Test.22;
+    let Test.19 = CallByName Num.26 Test.2 Test.3;
+    let Test.16 = CallByName Num.26 Test.19 Test.4;
+    let Test.15 = CallByName Num.26 Test.16 Test.5;
+    ret Test.15;
diff --git a/compiler/test_mono/generated/peano.txt b/compiler/test_mono/generated/peano.txt
new file mode 100644
index 0000000000..ba743a0f1d
--- /dev/null
+++ b/compiler/test_mono/generated/peano.txt
@@ -0,0 +1,10 @@
+procedure Test.0 ():
+    let Test.9 = 0i64;
+    let Test.11 = 0i64;
+    let Test.13 = 0i64;
+    let Test.14 = 1i64;
+    let Test.12 = Z ;
+    let Test.10 = S Test.12;
+    let Test.8 = S Test.10;
+    let Test.2 = S Test.8;
+    ret Test.2;
diff --git a/compiler/test_mono/generated/peano1.txt b/compiler/test_mono/generated/peano1.txt
new file mode 100644
index 0000000000..df1d0183c7
--- /dev/null
+++ b/compiler/test_mono/generated/peano1.txt
@@ -0,0 +1,19 @@
+procedure Test.0 ():
+    let Test.13 = 0i64;
+    let Test.15 = 0i64;
+    let Test.17 = 0i64;
+    let Test.18 = 1i64;
+    let Test.16 = Z ;
+    let Test.14 = S Test.16;
+    let Test.12 = S Test.14;
+    let Test.2 = S Test.12;
+    let Test.9 = 1i64;
+    let Test.10 = GetTagId Test.2;
+    dec Test.2;
+    let Test.11 = lowlevel Eq Test.9 Test.10;
+    if Test.11 then
+        let Test.7 = 0i64;
+        ret Test.7;
+    else
+        let Test.8 = 1i64;
+        ret Test.8;
diff --git a/compiler/test_mono/generated/peano2.txt b/compiler/test_mono/generated/peano2.txt
new file mode 100644
index 0000000000..4a1a5a234f
--- /dev/null
+++ b/compiler/test_mono/generated/peano2.txt
@@ -0,0 +1,30 @@
+procedure Test.0 ():
+    let Test.19 = 0i64;
+    let Test.21 = 0i64;
+    let Test.23 = 0i64;
+    let Test.24 = 1i64;
+    let Test.22 = Z ;
+    let Test.20 = S Test.22;
+    let Test.18 = S Test.20;
+    let Test.2 = S Test.18;
+    let Test.15 = 0i64;
+    let Test.16 = GetTagId Test.2;
+    let Test.17 = lowlevel Eq Test.15 Test.16;
+    if Test.17 then
+        let Test.11 = UnionAtIndex (Id 0) (Index 0) Test.2;
+        inc Test.11;
+        dec Test.2;
+        let Test.12 = 0i64;
+        let Test.13 = GetTagId Test.11;
+        dec Test.11;
+        let Test.14 = lowlevel Eq Test.12 Test.13;
+        if Test.14 then
+            let Test.7 = 1i64;
+            ret Test.7;
+        else
+            let Test.8 = 0i64;
+            ret Test.8;
+    else
+        dec Test.2;
+        let Test.9 = 0i64;
+        ret Test.9;
diff --git a/compiler/test_mono/generated/quicksort_help.txt b/compiler/test_mono/generated/quicksort_help.txt
new file mode 100644
index 0000000000..6d641e8807
--- /dev/null
+++ b/compiler/test_mono/generated/quicksort_help.txt
@@ -0,0 +1,41 @@
+procedure Num.24 (#Attr.2, #Attr.3):
+    let Test.19 = lowlevel NumAdd #Attr.2 #Attr.3;
+    ret Test.19;
+
+procedure Num.25 (#Attr.2, #Attr.3):
+    let Test.23 = lowlevel NumSub #Attr.2 #Attr.3;
+    ret Test.23;
+
+procedure Num.27 (#Attr.2, #Attr.3):
+    let Test.28 = lowlevel NumLt #Attr.2 #Attr.3;
+    ret Test.28;
+
+procedure Test.1 (Test.2, Test.3, Test.4):
+    joinpoint Test.12 Test.2 Test.3 Test.4:
+        let Test.14 = CallByName Num.27 Test.3 Test.4;
+        if Test.14 then
+            dec Test.2;
+            let Test.27 = Array [];
+            let Test.26 = 0i64;
+            let Test.25 = Struct {Test.26, Test.27};
+            let Test.5 = StructAtIndex 0 Test.25;
+            let Test.6 = StructAtIndex 1 Test.25;
+            let Test.22 = 1i64;
+            let Test.21 = CallByName Num.25 Test.5 Test.22;
+            let Test.16 = CallByName Test.1 Test.6 Test.3 Test.21;
+            let Test.18 = 1i64;
+            invoke Test.17 = CallByName Num.24 Test.5 Test.18 catch
+                dec Test.16;
+                unreachable;
+            jump Test.12 Test.16 Test.17 Test.4;
+        else
+            ret Test.2;
+    in
+    jump Test.12 Test.2 Test.3 Test.4;
+
+procedure Test.0 ():
+    let Test.9 = Array [];
+    let Test.10 = 0i64;
+    let Test.11 = 0i64;
+    let Test.8 = CallByName Test.1 Test.9 Test.10 Test.11;
+    ret Test.8;
diff --git a/compiler/test_mono/generated/quicksort_swap.txt b/compiler/test_mono/generated/quicksort_swap.txt
new file mode 100644
index 0000000000..4845bb7c58
--- /dev/null
+++ b/compiler/test_mono/generated/quicksort_swap.txt
@@ -0,0 +1,63 @@
+procedure List.3 (#Attr.2, #Attr.3):
+    let Test.37 = lowlevel ListLen #Attr.2;
+    let Test.34 = lowlevel NumLt #Attr.3 Test.37;
+    if Test.34 then
+        let Test.36 = lowlevel ListGetUnsafe #Attr.2 #Attr.3;
+        let Test.35 = Ok Test.36;
+        ret Test.35;
+    else
+        let Test.33 = Struct {};
+        let Test.32 = Err Test.33;
+        ret Test.32;
+
+procedure List.4 (#Attr.2, #Attr.3, #Attr.4):
+    let Test.15 = lowlevel ListLen #Attr.2;
+    let Test.13 = lowlevel NumLt #Attr.3 Test.15;
+    if Test.13 then
+        let Test.14 = lowlevel ListSet #Attr.2 #Attr.3 #Attr.4;
+        ret Test.14;
+    else
+        ret #Attr.2;
+
+procedure Test.1 (Test.2):
+    let Test.38 = 0i64;
+    let Test.30 = CallByName List.3 Test.2 Test.38;
+    let Test.31 = 0i64;
+    let Test.29 = CallByName List.3 Test.2 Test.31;
+    let Test.8 = Struct {Test.29, Test.30};
+    joinpoint Test.26:
+        let Test.17 = Array [];
+        ret Test.17;
+    in
+    let Test.23 = StructAtIndex 1 Test.8;
+    let Test.24 = 1i64;
+    let Test.25 = GetTagId Test.23;
+    let Test.28 = lowlevel Eq Test.24 Test.25;
+    if Test.28 then
+        let Test.20 = StructAtIndex 0 Test.8;
+        let Test.21 = 1i64;
+        let Test.22 = GetTagId Test.20;
+        let Test.27 = lowlevel Eq Test.21 Test.22;
+        if Test.27 then
+            let Test.19 = StructAtIndex 0 Test.8;
+            let Test.4 = UnionAtIndex (Id 1) (Index 0) Test.19;
+            let Test.18 = StructAtIndex 1 Test.8;
+            let Test.5 = UnionAtIndex (Id 1) (Index 0) Test.18;
+            let Test.16 = 0i64;
+            let Test.10 = CallByName List.4 Test.2 Test.16 Test.5;
+            let Test.11 = 0i64;
+            let Test.9 = CallByName List.4 Test.10 Test.11 Test.4;
+            ret Test.9;
+        else
+            dec Test.2;
+            jump Test.26;
+    else
+        dec Test.2;
+        jump Test.26;
+
+procedure Test.0 ():
+    let Test.39 = 1i64;
+    let Test.40 = 2i64;
+    let Test.7 = Array [Test.39, Test.40];
+    let Test.6 = CallByName Test.1 Test.7;
+    ret Test.6;
diff --git a/compiler/test_mono/generated/record_optional_field_function_no_use_default.txt b/compiler/test_mono/generated/record_optional_field_function_no_use_default.txt
new file mode 100644
index 0000000000..fabb2e373b
--- /dev/null
+++ b/compiler/test_mono/generated/record_optional_field_function_no_use_default.txt
@@ -0,0 +1,17 @@
+procedure Num.24 (#Attr.2, #Attr.3):
+    let Test.8 = lowlevel NumAdd #Attr.2 #Attr.3;
+    ret Test.8;
+
+procedure Test.1 (Test.4):
+    let Test.2 = StructAtIndex 0 Test.4;
+    let Test.3 = StructAtIndex 1 Test.4;
+    let Test.2 = 10i64;
+    let Test.7 = CallByName Num.24 Test.2 Test.3;
+    ret Test.7;
+
+procedure Test.0 ():
+    let Test.10 = 4i64;
+    let Test.11 = 9i64;
+    let Test.6 = Struct {Test.10, Test.11};
+    let Test.5 = CallByName Test.1 Test.6;
+    ret Test.5;
diff --git a/compiler/test_mono/generated/record_optional_field_function_use_default.txt b/compiler/test_mono/generated/record_optional_field_function_use_default.txt
new file mode 100644
index 0000000000..0def92d630
--- /dev/null
+++ b/compiler/test_mono/generated/record_optional_field_function_use_default.txt
@@ -0,0 +1,14 @@
+procedure Num.24 (#Attr.2, #Attr.3):
+    let Test.8 = lowlevel NumAdd #Attr.2 #Attr.3;
+    ret Test.8;
+
+procedure Test.1 (Test.4):
+    let Test.2 = 10i64;
+    let Test.2 = 10i64;
+    let Test.7 = CallByName Num.24 Test.2 Test.4;
+    ret Test.7;
+
+procedure Test.0 ():
+    let Test.10 = 9i64;
+    let Test.5 = CallByName Test.1 Test.10;
+    ret Test.5;
diff --git a/compiler/test_mono/generated/record_optional_field_let_no_use_default.txt b/compiler/test_mono/generated/record_optional_field_let_no_use_default.txt
new file mode 100644
index 0000000000..964700af06
--- /dev/null
+++ b/compiler/test_mono/generated/record_optional_field_let_no_use_default.txt
@@ -0,0 +1,16 @@
+procedure Num.24 (#Attr.2, #Attr.3):
+    let Test.8 = lowlevel NumAdd #Attr.2 #Attr.3;
+    ret Test.8;
+
+procedure Test.1 (Test.2):
+    let Test.3 = StructAtIndex 0 Test.2;
+    let Test.4 = StructAtIndex 1 Test.2;
+    let Test.7 = CallByName Num.24 Test.3 Test.4;
+    ret Test.7;
+
+procedure Test.0 ():
+    let Test.10 = 4i64;
+    let Test.11 = 9i64;
+    let Test.6 = Struct {Test.10, Test.11};
+    let Test.5 = CallByName Test.1 Test.6;
+    ret Test.5;
diff --git a/compiler/test_mono/generated/record_optional_field_let_use_default.txt b/compiler/test_mono/generated/record_optional_field_let_use_default.txt
new file mode 100644
index 0000000000..aef4e20cbd
--- /dev/null
+++ b/compiler/test_mono/generated/record_optional_field_let_use_default.txt
@@ -0,0 +1,13 @@
+procedure Num.24 (#Attr.2, #Attr.3):
+    let Test.8 = lowlevel NumAdd #Attr.2 #Attr.3;
+    ret Test.8;
+
+procedure Test.1 (Test.2):
+    let Test.3 = 10i64;
+    let Test.7 = CallByName Num.24 Test.3 Test.2;
+    ret Test.7;
+
+procedure Test.0 ():
+    let Test.10 = 9i64;
+    let Test.5 = CallByName Test.1 Test.10;
+    ret Test.5;
diff --git a/compiler/test_mono/generated/rigids.txt b/compiler/test_mono/generated/rigids.txt
new file mode 100644
index 0000000000..d6a146ba1b
--- /dev/null
+++ b/compiler/test_mono/generated/rigids.txt
@@ -0,0 +1,60 @@
+procedure List.3 (#Attr.2, #Attr.3):
+    let Test.39 = lowlevel ListLen #Attr.2;
+    let Test.36 = lowlevel NumLt #Attr.3 Test.39;
+    if Test.36 then
+        let Test.38 = lowlevel ListGetUnsafe #Attr.2 #Attr.3;
+        let Test.37 = Ok Test.38;
+        ret Test.37;
+    else
+        let Test.35 = Struct {};
+        let Test.34 = Err Test.35;
+        ret Test.34;
+
+procedure List.4 (#Attr.2, #Attr.3, #Attr.4):
+    let Test.19 = lowlevel ListLen #Attr.2;
+    let Test.17 = lowlevel NumLt #Attr.3 Test.19;
+    if Test.17 then
+        let Test.18 = lowlevel ListSet #Attr.2 #Attr.3 #Attr.4;
+        ret Test.18;
+    else
+        ret #Attr.2;
+
+procedure Test.1 (Test.2, Test.3, Test.4):
+    let Test.33 = CallByName List.3 Test.4 Test.3;
+    let Test.32 = CallByName List.3 Test.4 Test.2;
+    let Test.13 = Struct {Test.32, Test.33};
+    joinpoint Test.29:
+        let Test.20 = Array [];
+        ret Test.20;
+    in
+    let Test.26 = StructAtIndex 1 Test.13;
+    let Test.27 = 1i64;
+    let Test.28 = GetTagId Test.26;
+    let Test.31 = lowlevel Eq Test.27 Test.28;
+    if Test.31 then
+        let Test.23 = StructAtIndex 0 Test.13;
+        let Test.24 = 1i64;
+        let Test.25 = GetTagId Test.23;
+        let Test.30 = lowlevel Eq Test.24 Test.25;
+        if Test.30 then
+            let Test.22 = StructAtIndex 0 Test.13;
+            let Test.6 = UnionAtIndex (Id 1) (Index 0) Test.22;
+            let Test.21 = StructAtIndex 1 Test.13;
+            let Test.7 = UnionAtIndex (Id 1) (Index 0) Test.21;
+            let Test.15 = CallByName List.4 Test.4 Test.2 Test.7;
+            let Test.14 = CallByName List.4 Test.15 Test.3 Test.6;
+            ret Test.14;
+        else
+            dec Test.4;
+            jump Test.29;
+    else
+        dec Test.4;
+        jump Test.29;
+
+procedure Test.0 ():
+    let Test.10 = 0i64;
+    let Test.11 = 0i64;
+    let Test.40 = 1i64;
+    let Test.12 = Array [Test.40];
+    let Test.9 = CallByName Test.1 Test.10 Test.11 Test.12;
+    ret Test.9;
diff --git a/compiler/test_mono/generated/simple_if.txt b/compiler/test_mono/generated/simple_if.txt
new file mode 100644
index 0000000000..f796d9d4f2
--- /dev/null
+++ b/compiler/test_mono/generated/simple_if.txt
@@ -0,0 +1,8 @@
+procedure Test.0 ():
+    let Test.3 = true;
+    if Test.3 then
+        let Test.4 = 1i64;
+        ret Test.4;
+    else
+        let Test.2 = 2i64;
+        ret Test.2;
diff --git a/compiler/test_mono/generated/somehow_drops_definitions.txt b/compiler/test_mono/generated/somehow_drops_definitions.txt
new file mode 100644
index 0000000000..da1f049ed5
--- /dev/null
+++ b/compiler/test_mono/generated/somehow_drops_definitions.txt
@@ -0,0 +1,53 @@
+procedure Num.24 (#Attr.2, #Attr.3):
+    let Test.28 = lowlevel NumAdd #Attr.2 #Attr.3;
+    ret Test.28;
+
+procedure Num.26 (#Attr.2, #Attr.3):
+    let Test.22 = lowlevel NumMul #Attr.2 #Attr.3;
+    ret Test.22;
+
+procedure Test.1 ():
+    let Test.30 = 1i64;
+    ret Test.30;
+
+procedure Test.2 ():
+    let Test.24 = 2i64;
+    ret Test.24;
+
+procedure Test.3 (Test.6):
+    let Test.27 = CallByName Test.1;
+    let Test.26 = CallByName Num.24 Test.6 Test.27;
+    ret Test.26;
+
+procedure Test.4 (Test.7):
+    let Test.21 = CallByName Test.2;
+    let Test.20 = CallByName Num.26 Test.7 Test.21;
+    ret Test.20;
+
+procedure Test.5 (Test.8, Test.9):
+    joinpoint Test.15 Test.14:
+        ret Test.14;
+    in
+    switch Test.8:
+        case 0:
+            let Test.16 = CallByName Test.3 Test.9;
+            jump Test.15 Test.16;
+    
+        default:
+            let Test.17 = CallByName Test.4 Test.9;
+            jump Test.15 Test.17;
+    
+
+procedure Test.0 ():
+    joinpoint Test.19 Test.12:
+        let Test.13 = 42i64;
+        let Test.11 = CallByName Test.5 Test.12 Test.13;
+        ret Test.11;
+    in
+    let Test.25 = true;
+    if Test.25 then
+        let Test.3 = Struct {};
+        jump Test.19 Test.3;
+    else
+        let Test.4 = Struct {};
+        jump Test.19 Test.4;
diff --git a/compiler/test_mono/generated/specialize_closures.txt b/compiler/test_mono/generated/specialize_closures.txt
new file mode 100644
index 0000000000..9425c98bb8
--- /dev/null
+++ b/compiler/test_mono/generated/specialize_closures.txt
@@ -0,0 +1,53 @@
+procedure Num.24 (#Attr.2, #Attr.3):
+    let Test.29 = lowlevel NumAdd #Attr.2 #Attr.3;
+    ret Test.29;
+
+procedure Num.26 (#Attr.2, #Attr.3):
+    let Test.25 = lowlevel NumMul #Attr.2 #Attr.3;
+    ret Test.25;
+
+procedure Test.1 (Test.2, Test.3):
+    let Test.17 = GetTagId Test.2;
+    joinpoint Test.18 Test.16:
+        ret Test.16;
+    in
+    switch Test.17:
+        case 0:
+            let Test.19 = CallByName Test.7 Test.3 Test.2;
+            jump Test.18 Test.19;
+    
+        default:
+            let Test.20 = CallByName Test.8 Test.3 Test.2;
+            jump Test.18 Test.20;
+    
+
+procedure Test.7 (Test.10, #Attr.12):
+    let Test.4 = UnionAtIndex (Id 0) (Index 0) #Attr.12;
+    let Test.28 = CallByName Num.24 Test.10 Test.4;
+    ret Test.28;
+
+procedure Test.8 (Test.11, #Attr.12):
+    let Test.6 = UnionAtIndex (Id 1) (Index 1) #Attr.12;
+    let Test.5 = UnionAtIndex (Id 1) (Index 0) #Attr.12;
+    if Test.6 then
+        let Test.24 = CallByName Num.26 Test.11 Test.5;
+        ret Test.24;
+    else
+        ret Test.11;
+
+procedure Test.0 ():
+    let Test.6 = true;
+    let Test.4 = 1i64;
+    let Test.5 = 2i64;
+    joinpoint Test.22 Test.14:
+        let Test.15 = 42i64;
+        let Test.13 = CallByName Test.1 Test.14 Test.15;
+        ret Test.13;
+    in
+    let Test.27 = true;
+    if Test.27 then
+        let Test.7 = ClosureTag(Test.7) Test.4;
+        jump Test.22 Test.7;
+    else
+        let Test.8 = ClosureTag(Test.8) Test.5 Test.6;
+        jump Test.22 Test.8;
diff --git a/compiler/test_mono/generated/specialize_lowlevel.txt b/compiler/test_mono/generated/specialize_lowlevel.txt
new file mode 100644
index 0000000000..ace0bbda7a
--- /dev/null
+++ b/compiler/test_mono/generated/specialize_lowlevel.txt
@@ -0,0 +1,44 @@
+procedure Num.24 (#Attr.2, #Attr.3):
+    let Test.25 = lowlevel NumAdd #Attr.2 #Attr.3;
+    ret Test.25;
+
+procedure Num.26 (#Attr.2, #Attr.3):
+    let Test.21 = lowlevel NumMul #Attr.2 #Attr.3;
+    ret Test.21;
+
+procedure Test.6 (Test.8, #Attr.12):
+    let Test.4 = UnionAtIndex (Id 0) (Index 0) #Attr.12;
+    let Test.24 = CallByName Num.24 Test.8 Test.4;
+    ret Test.24;
+
+procedure Test.7 (Test.9, #Attr.12):
+    let Test.5 = UnionAtIndex (Id 1) (Index 0) #Attr.12;
+    let Test.20 = CallByName Num.26 Test.9 Test.5;
+    ret Test.20;
+
+procedure Test.0 ():
+    let Test.4 = 1i64;
+    let Test.5 = 2i64;
+    let Test.12 = 42i64;
+    joinpoint Test.19 Test.13:
+        let Test.14 = GetTagId Test.13;
+        joinpoint Test.15 Test.11:
+            ret Test.11;
+        in
+        switch Test.14:
+            case 0:
+                let Test.16 = CallByName Test.6 Test.12 Test.13;
+                jump Test.15 Test.16;
+        
+            default:
+                let Test.17 = CallByName Test.7 Test.12 Test.13;
+                jump Test.15 Test.17;
+        
+    in
+    let Test.23 = true;
+    if Test.23 then
+        let Test.6 = ClosureTag(Test.6) Test.4;
+        jump Test.19 Test.6;
+    else
+        let Test.7 = ClosureTag(Test.7) Test.5;
+        jump Test.19 Test.7;
diff --git a/compiler/test_mono/generated/when_joinpoint.txt b/compiler/test_mono/generated/when_joinpoint.txt
new file mode 100644
index 0000000000..3277764e1b
--- /dev/null
+++ b/compiler/test_mono/generated/when_joinpoint.txt
@@ -0,0 +1,23 @@
+procedure Test.1 (Test.5):
+    let Test.2 = 0u8;
+    joinpoint Test.9 Test.3:
+        ret Test.3;
+    in
+    switch Test.2:
+        case 1:
+            let Test.10 = 1i64;
+            jump Test.9 Test.10;
+    
+        case 2:
+            let Test.11 = 2i64;
+            jump Test.9 Test.11;
+    
+        default:
+            let Test.12 = 3i64;
+            jump Test.9 Test.12;
+    
+
+procedure Test.0 ():
+    let Test.7 = Struct {};
+    let Test.6 = CallByName Test.1 Test.7;
+    ret Test.6;
diff --git a/compiler/test_mono/generated/when_nested_maybe.txt b/compiler/test_mono/generated/when_nested_maybe.txt
new file mode 100644
index 0000000000..24279438b5
--- /dev/null
+++ b/compiler/test_mono/generated/when_nested_maybe.txt
@@ -0,0 +1,30 @@
+procedure Num.24 (#Attr.2, #Attr.3):
+    let Test.8 = lowlevel NumAdd #Attr.2 #Attr.3;
+    ret Test.8;
+
+procedure Test.0 ():
+    let Test.21 = 41i64;
+    let Test.20 = Just Test.21;
+    let Test.2 = Just Test.20;
+    joinpoint Test.17:
+        let Test.10 = 1i64;
+        ret Test.10;
+    in
+    let Test.15 = 0i64;
+    let Test.16 = GetTagId Test.2;
+    let Test.19 = lowlevel Eq Test.15 Test.16;
+    if Test.19 then
+        let Test.12 = UnionAtIndex (Id 0) (Index 0) Test.2;
+        let Test.13 = 0i64;
+        let Test.14 = GetTagId Test.12;
+        let Test.18 = lowlevel Eq Test.13 Test.14;
+        if Test.18 then
+            let Test.11 = UnionAtIndex (Id 0) (Index 0) Test.2;
+            let Test.5 = UnionAtIndex (Id 0) (Index 0) Test.11;
+            let Test.7 = 1i64;
+            let Test.6 = CallByName Num.24 Test.5 Test.7;
+            ret Test.6;
+        else
+            jump Test.17;
+    else
+        jump Test.17;
diff --git a/compiler/test_mono/generated/when_on_record.txt b/compiler/test_mono/generated/when_on_record.txt
new file mode 100644
index 0000000000..9c37c1097b
--- /dev/null
+++ b/compiler/test_mono/generated/when_on_record.txt
@@ -0,0 +1,9 @@
+procedure Num.24 (#Attr.2, #Attr.3):
+    let Test.5 = lowlevel NumAdd #Attr.2 #Attr.3;
+    ret Test.5;
+
+procedure Test.0 ():
+    let Test.7 = 2i64;
+    let Test.4 = 3i64;
+    let Test.3 = CallByName Num.24 Test.7 Test.4;
+    ret Test.3;
diff --git a/compiler/test_mono/generated/when_on_result.txt b/compiler/test_mono/generated/when_on_result.txt
new file mode 100644
index 0000000000..6b7b74b4ed
--- /dev/null
+++ b/compiler/test_mono/generated/when_on_result.txt
@@ -0,0 +1,27 @@
+procedure Test.1 (Test.5):
+    let Test.19 = 2i64;
+    let Test.2 = Ok Test.19;
+    joinpoint Test.9 Test.3:
+        ret Test.3;
+    in
+    let Test.16 = 1i64;
+    let Test.17 = GetTagId Test.2;
+    let Test.18 = lowlevel Eq Test.16 Test.17;
+    if Test.18 then
+        let Test.13 = UnionAtIndex (Id 1) (Index 0) Test.2;
+        let Test.14 = 3i64;
+        let Test.15 = lowlevel Eq Test.14 Test.13;
+        if Test.15 then
+            let Test.10 = 1i64;
+            jump Test.9 Test.10;
+        else
+            let Test.11 = 2i64;
+            jump Test.9 Test.11;
+    else
+        let Test.12 = 3i64;
+        jump Test.9 Test.12;
+
+procedure Test.0 ():
+    let Test.7 = Struct {};
+    let Test.6 = CallByName Test.1 Test.7;
+    ret Test.6;
diff --git a/compiler/test_mono/generated/when_on_two_values.txt b/compiler/test_mono/generated/when_on_two_values.txt
new file mode 100644
index 0000000000..f92d0897c3
--- /dev/null
+++ b/compiler/test_mono/generated/when_on_two_values.txt
@@ -0,0 +1,28 @@
+procedure Num.24 (#Attr.2, #Attr.3):
+    let Test.7 = lowlevel NumAdd #Attr.2 #Attr.3;
+    ret Test.7;
+
+procedure Test.0 ():
+    let Test.17 = 3i64;
+    let Test.16 = 2i64;
+    let Test.4 = Struct {Test.16, Test.17};
+    joinpoint Test.13:
+        let Test.2 = StructAtIndex 0 Test.4;
+        let Test.3 = StructAtIndex 1 Test.4;
+        let Test.6 = CallByName Num.24 Test.2 Test.3;
+        ret Test.6;
+    in
+    let Test.11 = StructAtIndex 1 Test.4;
+    let Test.12 = 3i64;
+    let Test.15 = lowlevel Eq Test.12 Test.11;
+    if Test.15 then
+        let Test.9 = StructAtIndex 0 Test.4;
+        let Test.10 = 4i64;
+        let Test.14 = lowlevel Eq Test.10 Test.9;
+        if Test.14 then
+            let Test.5 = 9i64;
+            ret Test.5;
+        else
+            jump Test.13;
+    else
+        jump Test.13;
diff --git a/compiler/test_mono/src/lib.rs b/compiler/test_mono/src/lib.rs
new file mode 100644
index 0000000000..403ef32904
--- /dev/null
+++ b/compiler/test_mono/src/lib.rs
@@ -0,0 +1,1104 @@
+#![cfg(test)]
+#![warn(clippy::dbg_macro)]
+// See github.com/rtfeldman/roc/issues/800 for discussion of the large_enum_variant check.
+#![allow(clippy::large_enum_variant)]
+// we actually want to compare against the literal float bits
+#![allow(clippy::clippy::float_cmp)]
+
+#[macro_use]
+extern crate pretty_assertions;
+
+#[macro_use]
+extern crate indoc;
+
+/// Used in the with_larger_debug_stack() function, for tests that otherwise
+/// run out of stack space in debug builds (but don't in --release builds)
+#[allow(dead_code)]
+const EXPANDED_STACK_SIZE: usize = 8 * 1024 * 1024;
+
+use test_mono_macros::*;
+
+use roc_can::builtins::builtin_defs_map;
+use roc_collections::all::MutMap;
+use roc_module::symbol::Symbol;
+use roc_mono::ir::Proc;
+
+use roc_mono::ir::ProcLayout;
+
+/// Without this, some tests pass in `cargo test --release` but fail without
+/// the --release flag because they run out of stack space. This increases
+/// stack size for debug builds only, while leaving the stack space at the default
+/// amount for release builds.
+#[allow(dead_code)]
+#[cfg(debug_assertions)]
+pub fn with_larger_debug_stack<F>(run_test: F)
+where
+    F: FnOnce(),
+    F: Send,
+    F: 'static,
+{
+    std::thread::Builder::new()
+        .stack_size(EXPANDED_STACK_SIZE)
+        .spawn(run_test)
+        .expect("Error while spawning expanded dev stack size thread")
+        .join()
+        .expect("Error while joining expanded dev stack size thread")
+}
+
+/// In --release builds, don't increase the stack size. Run the test normally.
+/// This way, we find out if any of our tests are blowing the stack even after
+/// optimizations in release builds.
+#[allow(dead_code)]
+#[cfg(not(debug_assertions))]
+#[inline(always)]
+pub fn with_larger_debug_stack<F>(run_test: F)
+where
+    F: FnOnce() -> (),
+    F: Send,
+    F: 'static,
+{
+    run_test()
+}
+
+fn promote_expr_to_module(src: &str) -> String {
+    let mut buffer = String::from("app \"test\" provides [ main ] to \"./platform\"\n\nmain =\n");
+
+    for line in src.lines() {
+        // indent the body!
+        buffer.push_str("    ");
+        buffer.push_str(line);
+        buffer.push('\n');
+    }
+
+    buffer
+}
+
+fn compiles_to_ir(test_name: &str, src: &str) {
+    use bumpalo::Bump;
+    use std::path::{Path, PathBuf};
+
+    let arena = &Bump::new();
+
+    // let stdlib = roc_builtins::unique::uniq_stdlib();
+    let stdlib = roc_builtins::std::standard_stdlib();
+    let filename = PathBuf::from("Test.roc");
+    let src_dir = Path::new("fake/test/path");
+
+    let module_src;
+    let temp;
+    if src.starts_with("app") {
+        // this is already a module
+        module_src = src;
+    } else {
+        // this is an expression, promote it to a module
+        temp = promote_expr_to_module(src);
+        module_src = &temp;
+    }
+
+    let exposed_types = MutMap::default();
+
+    let loaded = roc_load::file::load_and_monomorphize_from_str(
+        arena,
+        filename,
+        &module_src,
+        &stdlib,
+        src_dir,
+        exposed_types,
+        8,
+        builtin_defs_map,
+    );
+
+    let mut loaded = match loaded {
+        Ok(x) => x,
+        Err(roc_load::file::LoadingProblem::FormattedReport(report)) => {
+            println!("{}", report);
+            panic!();
+        }
+        Err(e) => panic!("{:?}", e),
+    };
+
+    use roc_load::file::MonomorphizedModule;
+    let MonomorphizedModule {
+        module_id: home,
+        procedures,
+        exposed_to_host,
+        ..
+    } = loaded;
+
+    let can_problems = loaded.can_problems.remove(&home).unwrap_or_default();
+    let type_problems = loaded.type_problems.remove(&home).unwrap_or_default();
+    let mono_problems = loaded.mono_problems.remove(&home).unwrap_or_default();
+
+    if !can_problems.is_empty() {
+        println!("Ignoring {} canonicalization problems", can_problems.len());
+    }
+
+    assert_eq!(type_problems, Vec::new());
+    assert_eq!(mono_problems, Vec::new());
+
+    debug_assert_eq!(exposed_to_host.len(), 1);
+
+    let main_fn_symbol = exposed_to_host.keys().copied().next().unwrap();
+
+    verify_procedures(test_name, procedures, main_fn_symbol);
+}
+
+#[cfg(debug_assertions)]
+fn verify_procedures(
+    test_name: &str,
+    procedures: MutMap<(Symbol, ProcLayout<'_>), Proc<'_>>,
+    main_fn_symbol: Symbol,
+) {
+    let index = procedures
+        .keys()
+        .position(|(s, _)| *s == main_fn_symbol)
+        .unwrap();
+
+    let mut procs_string = procedures
+        .values()
+        .map(|proc| proc.to_pretty(200))
+        .collect::<Vec<_>>();
+
+    let main_fn = procs_string.swap_remove(index);
+
+    procs_string.sort();
+    procs_string.push(main_fn);
+
+    let result = procs_string.join("\n");
+
+    let path = format!("generated/{}.txt", test_name);
+    std::fs::create_dir_all("generated").unwrap();
+    std::fs::write(&path, result).unwrap();
+
+    use std::process::Command;
+    let is_tracked = Command::new("git")
+        .args(&["ls-files", "--error-unmatch", &path])
+        .output()
+        .unwrap();
+
+    if !is_tracked.status.success() {
+        panic!(
+            "The file {:?} is not tracked by git. Try using `git add` on it",
+            &path
+        );
+    }
+
+    let has_changes = Command::new("git")
+        .args(&["diff", "--color=always", &path])
+        .output()
+        .unwrap();
+
+    if !has_changes.status.success() {
+        eprintln!("`git diff {:?}` failed", &path);
+        unreachable!();
+    }
+
+    if !has_changes.stdout.is_empty() {
+        println!("{}", std::str::from_utf8(&has_changes.stdout).unwrap());
+        panic!("Output changed: resolve conflicts and `git add` the file.");
+    }
+}
+
+// NOTE because the Show instance of module names is different in --release mode,
+// these tests would all fail. In the future, when we do interesting optimizations,
+// we'll likely want some tests for --release too.
+#[cfg(not(debug_assertions))]
+fn verify_procedures(
+    _expected: &str,
+    _procedures: MutMap<(Symbol, ProcLayout<'_>), Proc<'_>>,
+    _main_fn_symbol: Symbol,
+) {
+    // Do nothing
+}
+
+#[mono_test]
+fn ir_int_literal() {
+    r#"
+    5
+    "#
+}
+
+#[mono_test]
+fn ir_int_add() {
+    r#"
+    x = [ 1,2 ]
+    5 + 4 + 3 + List.len x
+    "#
+}
+
+#[mono_test]
+fn ir_assignment() {
+    r#"
+    x = 5
+
+    x
+    "#
+}
+
+#[mono_test]
+fn ir_when_maybe() {
+    r#"
+    when Just 3 is
+        Just n -> n
+        Nothing -> 0
+    "#
+}
+
+#[mono_test]
+fn ir_when_these() {
+    r#"
+    when These 1 2 is
+        This x -> x
+        That y -> y
+        These x _ -> x
+    "#
+}
+
+#[mono_test]
+fn ir_when_record() {
+    r#"
+    when { x: 1, y: 3.14 } is
+        { x } -> x
+    "#
+}
+
+#[mono_test]
+fn ir_plus() {
+    r#"
+    1 + 2
+    "#
+}
+
+#[mono_test]
+fn ir_round() {
+    r#"
+    Num.round 3.6
+    "#
+}
+
+#[mono_test]
+fn ir_when_idiv() {
+    r#"
+    when 1000 // 10 is
+        Ok val -> val
+        Err _ -> -1
+    "#
+}
+
+#[mono_test]
+fn ir_two_defs() {
+    r#"
+    x = 3
+    y = 4
+
+    x + y
+    "#
+}
+
+#[mono_test]
+fn ir_when_just() {
+    r#"
+    x : [ Nothing, Just I64 ]
+    x = Just 41
+
+    when x is
+        Just v -> v + 0x1
+        Nothing -> 0x1
+    "#
+}
+
+#[mono_test]
+fn one_element_tag() {
+    r#"
+    x : [ Pair I64 ]
+    x = Pair 2
+
+    x
+    "#
+}
+
+#[mono_test]
+fn guard_pattern_true() {
+    r#"
+    wrapper = \{} ->
+        when 2 is
+            2 if False -> 42
+            _ -> 0
+
+    wrapper {}
+    "#
+}
+
+#[mono_test]
+fn when_on_record() {
+    r#"
+    when { x: 0x2 } is
+        { x } -> x + 3
+    "#
+}
+
+#[mono_test]
+fn when_nested_maybe() {
+    r#"
+    Maybe a : [ Nothing, Just a ]
+
+    x : Maybe (Maybe I64)
+    x = Just (Just 41)
+
+    when x is
+        Just (Just v) -> v + 0x1
+        _ -> 0x1
+    "#
+}
+
+#[mono_test]
+fn when_on_two_values() {
+    r#"
+    when Pair 2 3 is
+        Pair 4 3 -> 9
+        Pair a b -> a + b
+    "#
+}
+
+#[mono_test]
+fn dict() {
+    r#"
+    Dict.len Dict.empty
+    "#
+}
+
+#[mono_test]
+fn list_append_closure() {
+    r#"
+    myFunction = \l -> List.append l 42
+
+    myFunction [ 1, 2 ]
+    "#
+}
+
+#[mono_test]
+fn list_append() {
+    // TODO this leaks at the moment
+    // ListAppend needs to decrement its arguments
+    r#"
+    List.append [1] 2
+    "#
+}
+
+#[mono_test]
+fn list_len() {
+    r#"
+    x = [1,2,3]
+    y = [ 1.0 ]
+
+    List.len x + List.len y
+    "#
+}
+
+#[mono_test]
+fn when_joinpoint() {
+    r#"
+    wrapper = \{} ->
+        x : [ Red, White, Blue ]
+        x = Blue
+
+        y =
+            when x is
+                Red -> 1
+                White -> 2
+                Blue -> 3
+
+        y
+
+    wrapper {}
+    "#
+}
+
+#[mono_test]
+fn simple_if() {
+    r#"
+    if True then
+        1
+    else
+        2
+    "#
+}
+
+#[mono_test]
+fn if_multi_branch() {
+    r#"
+    if True then
+        1
+    else if False then
+        2
+    else
+        3
+    "#
+}
+
+#[mono_test]
+fn when_on_result() {
+    r#"
+    wrapper = \{} ->
+        x : Result I64 I64
+        x = Ok 2
+
+        y =
+            when x is
+                Ok 3 -> 1
+                Ok _ -> 2
+                Err _ -> 3
+        y
+
+    wrapper {}
+    "#
+}
+
+#[mono_test]
+fn let_with_record_pattern() {
+    r#"
+    { x } = { x: 0x2, y: 3.14 }
+
+    x
+    "#
+}
+
+#[mono_test]
+fn let_with_record_pattern_list() {
+    r#"
+    { x } = { x: [ 1, 3, 4 ], y: 3.14 }
+
+    x
+    "#
+}
+
+#[mono_test]
+fn if_guard_bind_variable_false() {
+    r#"
+    wrapper = \{} ->
+        when 10 is
+            x if x == 5 -> 0
+            _ -> 42
+
+    wrapper {}
+    "#
+}
+
+#[mono_test]
+fn alias_variable() {
+    r#"
+    x = 5
+    y = x
+
+    3
+    "#
+}
+
+#[mono_test]
+fn alias_variable_and_return_it() {
+    r#"
+    x = 5
+    y = x
+
+    y
+    "#
+}
+
+#[mono_test]
+fn branch_store_variable() {
+    r#"
+    when 0 is
+        1 -> 12
+        a -> a
+    "#
+}
+
+#[mono_test]
+fn list_pass_to_function() {
+    r#"
+    x : List I64
+    x = [1,2,3]
+
+    id : List I64 -> List I64
+    id = \y -> List.set y 0 0
+
+    id x
+    "#
+}
+
+#[mono_test]
+fn record_optional_field_let_no_use_default() {
+    r#"
+    f = \r ->
+        { x ? 10, y } = r
+        x + y
+
+
+    f { x: 4, y: 9 }
+    "#
+}
+
+#[mono_test]
+fn record_optional_field_let_use_default() {
+    r#"
+    f = \r ->
+        { x ? 10, y } = r
+        x + y
+
+
+    f { y: 9 }
+    "#
+}
+
+#[mono_test]
+fn record_optional_field_function_no_use_default() {
+    r#"
+    f = \{ x ? 10, y } -> x + y
+
+
+    f { x: 4, y: 9 }
+    "#
+}
+
+#[mono_test]
+fn record_optional_field_function_use_default() {
+    r#"
+    f = \{ x ? 10, y } -> x + y
+
+
+    f { y: 9 }
+    "#
+}
+
+#[mono_test]
+fn quicksort_help() {
+    // do we still need with_larger_debug_stack?
+    r#"
+    quicksortHelp : List (Num a), I64, I64 -> List (Num a)
+    quicksortHelp = \list, low, high ->
+        if low < high then
+            (Pair partitionIndex partitioned) = Pair 0 []
+
+            partitioned
+                |> quicksortHelp low (partitionIndex - 1)
+                |> quicksortHelp (partitionIndex + 1) high
+        else
+            list
+
+    quicksortHelp [] 0 0
+    "#
+}
+
+#[mono_test]
+fn quicksort_swap() {
+    indoc!(
+        r#"
+        app "test" provides [ main ] to "./platform"
+
+        swap = \list ->
+            when Pair (List.get list 0) (List.get list 0) is
+                Pair (Ok atI) (Ok atJ) ->
+                    list
+                        |> List.set 0 atJ
+                        |> List.set 0 atI
+
+                _ ->
+                    []
+
+        main =
+            swap [ 1, 2 ]
+        "#
+    )
+}
+
+// #[ignore]
+// #[mono_test]
+// fn quicksort_partition_help() {
+//     indoc!(
+//         r#"
+//         app "test" provides [ main ] to "./platform"
+
+//         partitionHelp : I64, I64, List (Num a), I64, (Num a) -> [ Pair I64 (List (Num a)) ]
+//         partitionHelp = \i, j, list, high, pivot ->
+//             if j < high then
+//                 when List.get list j is
+//                     Ok value ->
+//                         if value <= pivot then
+//                             partitionHelp (i + 1) (j + 1) (swap (i + 1) j list) high pivot
+//                         else
+//                             partitionHelp i (j + 1) list high pivot
+
+//                     Err _ ->
+//                         Pair i list
+//             else
+//                 Pair i list
+
+//         main =
+//             partitionHelp 0 0 [] 0 0
+//         "#
+//     )
+// }
+
+// #[ignore]
+// #[mono_test]
+// fn quicksort_full() {
+//     indoc!(
+//         r#"
+//         app "test" provides [ main ] to "./platform"
+
+//         quicksortHelp : List (Num a), I64, I64 -> List (Num a)
+//         quicksortHelp = \list, low, high ->
+//             if low < high then
+//                 (Pair partitionIndex partitioned) = partition low high list
+
+//                 partitioned
+//                     |> quicksortHelp low (partitionIndex - 1)
+//                     |> quicksortHelp (partitionIndex + 1) high
+//             else
+//                 list
+
+//         swap : I64, I64, List a -> List a
+//         swap = \i, j, list ->
+//             when Pair (List.get list i) (List.get list j) is
+//                 Pair (Ok atI) (Ok atJ) ->
+//                     list
+//                         |> List.set i atJ
+//                         |> List.set j atI
+
+//                 _ ->
+//                     []
+
+//         partition : I64, I64, List (Num a) -> [ Pair I64 (List (Num a)) ]
+//         partition = \low, high, initialList ->
+//             when List.get initialList high is
+//                 Ok pivot ->
+//                     when partitionHelp (low - 1) low initialList high pivot is
+//                         Pair newI newList ->
+//                             Pair (newI + 1) (swap (newI + 1) high newList)
+
+//                 Err _ ->
+//                     Pair (low - 1) initialList
+
+//         partitionHelp : I64, I64, List (Num a), I64, (Num a) -> [ Pair I64 (List (Num a)) ]
+//         partitionHelp = \i, j, list, high, pivot ->
+//             if j < high then
+//                 when List.get list j is
+//                     Ok value ->
+//                         if value <= pivot then
+//                             partitionHelp (i + 1) (j + 1) (swap (i + 1) j list) high pivot
+//                         else
+//                             partitionHelp i (j + 1) list high pivot
+
+//                     Err _ ->
+//                         Pair i list
+//             else
+//                 Pair i list
+
+//         quicksort = \originalList ->
+//             n = List.len originalList
+//             quicksortHelp originalList 0 (n - 1)
+
+//         main =
+//             quicksort [1,2,3]
+//         "#
+//     )
+// }
+
+#[mono_test]
+fn factorial() {
+    r#"
+    factorial = \n, accum ->
+        when n is
+            0 ->
+                accum
+
+            _ ->
+                factorial (n - 1) (n * accum)
+
+    factorial 10 1
+    "#
+}
+
+#[mono_test]
+fn is_nil() {
+    r#"
+    ConsList a : [ Cons a (ConsList a), Nil ]
+
+    isNil : ConsList a -> Bool
+    isNil = \list ->
+        when list is
+            Nil -> True
+            Cons _ _ -> False
+
+    isNil (Cons 0x2 Nil)
+    "#
+}
+
+#[mono_test]
+#[ignore]
+fn has_none() {
+    r#"
+    Maybe a : [ Just a, Nothing ]
+    ConsList a : [ Cons a (ConsList a), Nil ]
+
+    hasNone : ConsList (Maybe a) -> Bool
+    hasNone = \list ->
+        when list is
+            Nil -> False
+            Cons Nothing _ -> True
+            Cons (Just _) xs -> hasNone xs
+
+    hasNone (Cons (Just 3) Nil)
+    "#
+}
+
+#[mono_test]
+fn mk_pair_of() {
+    indoc!(
+        r#"
+        app "test" provides [ main ] to "./platform"
+
+        mkPairOf = \x -> Pair x x
+
+        main =
+            mkPairOf [1,2,3]
+        "#
+    )
+}
+
+#[mono_test]
+fn fst() {
+    indoc!(
+        r#"
+        app "test" provides [ main ] to "./platform"
+
+        fst = \x, _ -> x
+
+        main =
+            fst [1,2,3] [3,2,1]
+        "#
+    )
+}
+
+#[mono_test]
+fn list_cannot_update_inplace() {
+    indoc!(
+        r#"
+        app "test" provides [ main ] to "./platform"
+
+        x : List I64
+        x = [1,2,3]
+
+        add : List I64 -> List I64
+        add = \y -> List.set y 0 0
+
+        main =
+            List.len (add x) + List.len x
+        "#
+    )
+}
+
+#[mono_test]
+fn list_get() {
+    r#"
+    wrapper = \{} ->
+        List.get [1,2,3] 0
+
+    wrapper {}
+    "#
+}
+
+#[mono_test]
+fn peano() {
+    r#"
+    Peano : [ S Peano, Z ]
+
+    three : Peano
+    three = S (S (S Z))
+
+    three
+    "#
+}
+
+#[mono_test]
+fn peano1() {
+    r#"
+    Peano : [ S Peano, Z ]
+
+    three : Peano
+    three = S (S (S Z))
+
+    when three is
+        Z -> 0
+        S _ -> 1
+    "#
+}
+
+#[mono_test]
+fn peano2() {
+    r#"
+    Peano : [ S Peano, Z ]
+
+    three : Peano
+    three = S (S (S Z))
+
+    when three is
+        S (S _) -> 1
+        S (_) -> 0
+        Z -> 0
+    "#
+}
+
+#[mono_test]
+fn optional_when() {
+    r#"
+    f = \r ->
+        when r is
+            { x: Blue, y ? 3 } -> y
+            { x: Red, y ? 5 } -> y
+
+    a = f { x: Blue, y: 7 }
+    b = f { x: Blue }
+    c = f { x: Red, y: 11 }
+    d = f { x: Red }
+
+    a * b * c * d
+    "#
+}
+
+#[mono_test]
+fn nested_pattern_match() {
+    r#"
+    Maybe a : [ Nothing, Just a ]
+
+    x : Maybe (Maybe I64)
+    x = Just (Just 41)
+
+    when x is
+        Just (Just v) -> v + 0x1
+        _ -> 0x1
+    "#
+}
+
+#[mono_test]
+#[ignore]
+fn linked_list_length_twice() {
+    r#"
+    LinkedList a : [ Nil, Cons a (LinkedList a) ]
+
+    nil : LinkedList I64
+    nil = Nil
+
+    length : LinkedList a -> I64
+    length = \list ->
+        when list is
+            Nil -> 0
+            Cons _ rest -> 1 + length rest
+
+    length nil + length nil
+    "#
+}
+
+#[mono_test]
+fn rigids() {
+    indoc!(
+        r#"
+        app "test" provides [ main ] to "./platform"
+
+        swap : Nat, Nat, List a -> List a
+        swap = \i, j, list ->
+            when Pair (List.get list i) (List.get list j) is
+                Pair (Ok atI) (Ok atJ) ->
+                    foo = atJ
+
+                    list
+                        |> List.set i foo
+                        |> List.set j atI
+
+                _ ->
+                    []
+
+        main =
+            swap 0 0 [0x1]
+        "#
+    )
+}
+
+#[mono_test]
+fn let_x_in_x() {
+    r#"
+    x = 5
+
+    answer =
+        1337
+
+    unused =
+        nested = 17
+        nested
+
+    answer
+    "#
+}
+
+#[mono_test]
+fn let_x_in_x_indirect() {
+    r#"
+    x = 5
+
+    answer =
+        1337
+
+    unused =
+        nested = 17
+
+        i = 1
+
+        nested
+
+    { answer, unused }.answer
+    "#
+}
+
+#[mono_test]
+fn nested_closure() {
+    indoc!(
+        r#"
+        app "test" provides [ main ] to "./platform"
+
+        foo = \{} ->
+            x = 42
+            f = \{} -> x
+            f
+
+        main =
+            f = foo {}
+            f {}
+        "#
+    )
+}
+
+#[mono_test]
+fn closure_in_list() {
+    indoc!(
+        r#"
+        app "test" provides [ main ] to "./platform"
+    
+        foo = \{} ->
+            x = 41
+    
+            f = \{} -> x
+    
+            [ f ]
+    
+        main =
+            items = foo {}
+    
+            List.len items
+        "#
+    )
+}
+
+#[ignore]
+#[mono_test]
+fn somehow_drops_definitions() {
+    indoc!(
+        r#"
+        app "test" provides [ main ] to "./platform"
+
+        one : I64 
+        one = 1
+
+        two : I64
+        two = 2
+
+        increment : I64 -> I64
+        increment = \x -> x + one
+
+        double : I64 -> I64
+        double = \x -> x * two
+
+        apply : (a -> a), a -> a
+        apply = \f, x -> f x
+
+        main =
+            apply (if True then increment else double) 42
+        "#
+    )
+}
+
+#[mono_test]
+fn specialize_closures() {
+    indoc!(
+        r#"
+        app "test" provides [ main ] to "./platform"
+
+
+        apply : (a -> a), a -> a
+        apply = \f, x -> f x
+
+        main =
+            one : I64 
+            one = 1
+
+            two : I64
+            two = 2
+
+            b : Bool
+            b = True
+
+            increment : I64 -> I64
+            increment = \x -> x + one
+
+            double : I64 -> I64
+            double = \x -> if b then x * two else x
+
+            apply (if True then increment else double) 42
+        "#
+    )
+}
+
+#[mono_test]
+fn specialize_lowlevel() {
+    indoc!(
+        r#"
+         app "test" provides [ main ] to "./platform"
+
+         apply : (a -> a), a -> a
+         apply = \f, x -> f x
+
+         main =
+             one : I64
+             one = 1
+
+             two : I64
+             two = 2
+
+             increment : I64 -> I64
+             increment = \x -> x + one
+
+             double : I64 -> I64
+             double = \x -> x * two
+
+             (if True then increment else double) 42
+         "#
+    )
+}
+
+// #[ignore]
+// #[mono_test]
+// fn static_str_closure() {
+//     indoc!(
+//         r#"
+//         app "test" provides [ main ] to "./platform"
+
+//         main : Str
+//         main =
+//             x = "long string that is malloced"
+
+//             f : {} -> Str
+//             f = (\_ -> x)
+
+//             f {}
+//         "#
+//     )
+// }
diff --git a/compiler/test_mono_macros/Cargo.toml b/compiler/test_mono_macros/Cargo.toml
new file mode 100644
index 0000000000..24ddaa408b
--- /dev/null
+++ b/compiler/test_mono_macros/Cargo.toml
@@ -0,0 +1,15 @@
+[package]
+name = "test_mono_macros"
+version = "0.1.0"
+authors = ["The Roc Contributors"]
+license = "UPL-1.0"
+edition = "2018"
+
+[lib]
+proc-macro = true
+
+[dependencies]
+syn = { version = "1.0.39", features = ["full", "extra-traits"] }
+quote = "1.0.7"
+darling = "0.10.2"
+proc-macro2 = "1.0.24"
diff --git a/compiler/test_mono_macros/src/lib.rs b/compiler/test_mono_macros/src/lib.rs
new file mode 100644
index 0000000000..e6646e4e36
--- /dev/null
+++ b/compiler/test_mono_macros/src/lib.rs
@@ -0,0 +1,26 @@
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+use quote::quote;
+
+#[proc_macro_attribute]
+pub fn mono_test(_args: TokenStream, item: TokenStream) -> TokenStream {
+    let task_fn = syn::parse_macro_input!(item as syn::ItemFn);
+
+    let args = task_fn.sig.inputs.clone();
+
+    let name = task_fn.sig.ident.clone();
+    let name_str = name.to_string();
+    let body = task_fn.block.clone();
+
+    let visibility = &task_fn.vis;
+
+    let result = quote! {
+        #[test]
+        #visibility fn #name(#args) -> () {
+            compiles_to_ir(#name_str, #body);
+
+        }
+    };
+    result.into()
+}
diff --git a/compiler/types/src/pretty_print.rs b/compiler/types/src/pretty_print.rs
index 2b9ac83553..8df08669d1 100644
--- a/compiler/types/src/pretty_print.rs
+++ b/compiler/types/src/pretty_print.rs
@@ -756,11 +756,10 @@ fn write_symbol(env: &Env, symbol: Symbol, buf: &mut String) {
 
     // Don't qualify the symbol if it's in our home module,
     // or if it's a builtin (since all their types are always in scope)
-    if module_id == env.home || module_id.is_builtin() {
-        buf.push_str(ident);
-    } else {
+    if module_id != env.home && !module_id.is_builtin() {
         buf.push_str(module_id.to_string(&interns));
         buf.push('.');
-        buf.push_str(ident);
     }
+
+    buf.push_str(ident);
 }
diff --git a/compiler/types/src/subs.rs b/compiler/types/src/subs.rs
index 29a834378e..e66ad7fb12 100644
--- a/compiler/types/src/subs.rs
+++ b/compiler/types/src/subs.rs
@@ -132,7 +132,7 @@ impl OptVariable {
 
 impl fmt::Debug for OptVariable {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        self.clone().into_variable().fmt(f)
+        (*self).into_variable().fmt(f)
     }
 }
 
diff --git a/docs/Cargo.toml b/docs/Cargo.toml
index 2b6c47da05..d57e99ece4 100644
--- a/docs/Cargo.toml
+++ b/docs/Cargo.toml
@@ -14,6 +14,7 @@ roc_builtins = { path = "../compiler/builtins" }
 roc_can = { path = "../compiler/can" }
 roc_module = { path = "../compiler/module" }
 roc_region = { path = "../compiler/region" }
+roc_types = { path = "../compiler/types" }
 roc_collections = { path = "../compiler/collections" }
 bumpalo = { version = "3.2", features = ["collections"] }
 
diff --git a/docs/src/lib.rs b/docs/src/lib.rs
index ca3b3280d9..d99409cfbd 100644
--- a/docs/src/lib.rs
+++ b/docs/src/lib.rs
@@ -3,14 +3,15 @@ use roc_builtins::std::StdLib;
 use roc_can::builtins::builtin_defs_map;
 use roc_load::docs::{DocEntry, TypeAnnotation};
 use roc_load::docs::{ModuleDocumentation, RecordField};
-use roc_load::file::LoadingProblem;
-use roc_module::symbol::{Interns, ModuleId};
+use roc_load::file::{LoadedModule, LoadingProblem};
+use roc_module::symbol::Interns;
 
 use std::fs;
 extern crate roc_load;
 use bumpalo::Bump;
 use roc_can::scope::Scope;
 use roc_collections::all::MutMap;
+use roc_load::docs::DocEntry::DocDef;
 use roc_region::all::Region;
 use std::path::{Path, PathBuf};
 
@@ -55,17 +56,24 @@ pub fn generate(filenames: Vec<PathBuf>, std_lib: StdLib, build_dir: &Path) {
             package
                 .modules
                 .iter()
-                .flat_map(|(docs_by_id, _)| docs_by_id.values()),
+                .flat_map(|loaded_module| loaded_module.documentation.values()),
         )
         .as_str(),
     );
 
     // Write each package's module docs html file
-    for (docs_by_id, interns) in package.modules.iter_mut() {
-        for module in docs_by_id.values_mut() {
-            let mut filename = String::new();
-            filename.push_str(module.name.as_str());
-            filename.push_str(".html");
+    for loaded_module in package.modules.iter_mut() {
+        let exports = loaded_module
+            .exposed_values
+            .iter()
+            .map(|symbol| symbol.ident_string(&loaded_module.interns).to_string())
+            .collect::<Vec<String>>();
+
+        for module in loaded_module.documentation.values_mut() {
+            let module_dir = build_dir.join(module.name.replace(".", "/").as_str());
+
+            fs::create_dir_all(&module_dir)
+                .expect("TODO gracefully handle not being able to create the module dir");
 
             let rendered_module = template_html
                 .replace(
@@ -75,10 +83,10 @@ pub fn generate(filenames: Vec<PathBuf>, std_lib: StdLib, build_dir: &Path) {
                 )
                 .replace(
                     "<!-- Module Docs -->",
-                    render_main_content(interns, module).as_str(),
+                    render_main_content(&loaded_module.interns, &exports, module).as_str(),
                 );
 
-            fs::write(build_dir.join(filename), rendered_module)
+            fs::write(module_dir.join("index.html"), rendered_module)
                 .expect("TODO gracefully handle failing to write html");
         }
     }
@@ -86,7 +94,11 @@ pub fn generate(filenames: Vec<PathBuf>, std_lib: StdLib, build_dir: &Path) {
     println!("🎉 Docs generated in {}", build_dir.display());
 }
 
-fn render_main_content(interns: &Interns, module: &mut ModuleDocumentation) -> String {
+fn render_main_content(
+    interns: &Interns,
+    exposed_values: &[String],
+    module: &mut ModuleDocumentation,
+) -> String {
     let mut buf = String::new();
 
     buf.push_str(
@@ -99,49 +111,63 @@ fn render_main_content(interns: &Interns, module: &mut ModuleDocumentation) -> S
     );
 
     for entry in &module.entries {
-        match entry {
-            DocEntry::DocDef(doc_def) => {
-                let mut href = String::new();
-                href.push('#');
-                href.push_str(doc_def.name.as_str());
+        let mut should_render_entry = true;
 
-                let name = doc_def.name.as_str();
+        if let DocDef(def) = entry {
+            // We dont want to render entries that arent exposed
+            should_render_entry = exposed_values.contains(&def.name);
+        }
 
-                let mut content = String::new();
+        if should_render_entry {
+            match entry {
+                DocEntry::DocDef(doc_def) => {
+                    let mut href = String::new();
+                    href.push('#');
+                    href.push_str(doc_def.name.as_str());
 
-                content.push_str(html_node("a", vec![("href", href.as_str())], name).as_str());
+                    let name = doc_def.name.as_str();
 
-                for type_var in &doc_def.type_vars {
-                    content.push(' ');
-                    content.push_str(type_var.as_str());
-                }
+                    let mut content = String::new();
+
+                    content.push_str(html_node("a", vec![("href", href.as_str())], name).as_str());
+
+                    for type_var in &doc_def.type_vars {
+                        content.push(' ');
+                        content.push_str(type_var.as_str());
+                    }
+
+                    let type_ann = &doc_def.type_annotation;
+
+                    match type_ann {
+                        TypeAnnotation::NoTypeAnn => {}
+                        _ => {
+                            content.push_str(" : ");
+                        }
+                    }
 
-                if let Some(type_ann) = &doc_def.type_annotation {
-                    content.push_str(" : ");
                     type_annotation_to_html(0, &mut content, &type_ann);
-                }
 
-                buf.push_str(
-                    html_node(
-                        "h3",
-                        vec![("id", name), ("class", "entry-name")],
-                        content.as_str(),
-                    )
-                    .as_str(),
-                );
-
-                if let Some(docs) = &doc_def.docs {
                     buf.push_str(
-                        markdown_to_html(&mut module.scope, interns, docs.to_string()).as_str(),
+                        html_node(
+                            "h3",
+                            vec![("id", name), ("class", "entry-name")],
+                            content.as_str(),
+                        )
+                        .as_str(),
                     );
+
+                    if let Some(docs) = &doc_def.docs {
+                        buf.push_str(
+                            markdown_to_html(&mut module.scope, interns, docs.to_string()).as_str(),
+                        );
+                    }
                 }
-            }
-            DocEntry::DetatchedDoc(docs) => {
-                buf.push_str(
-                    markdown_to_html(&mut module.scope, interns, docs.to_string()).as_str(),
-                );
-            }
-        };
+                DocEntry::DetachedDoc(docs) => {
+                    let markdown = markdown_to_html(&mut module.scope, interns, docs.to_string());
+                    buf.push_str(markdown.as_str());
+                }
+            };
+        }
     }
 
     buf
@@ -221,8 +247,8 @@ fn render_sidebar<'a, I: Iterator<Item = &'a ModuleDocumentation>>(modules: I) -
 
         let href = {
             let mut href_buf = String::new();
+            href_buf.push('/');
             href_buf.push_str(name);
-            href_buf.push_str(".html");
             href_buf
         };
 
@@ -282,10 +308,7 @@ fn render_sidebar<'a, I: Iterator<Item = &'a ModuleDocumentation>>(modules: I) -
     buf
 }
 
-pub fn files_to_documentations(
-    filenames: Vec<PathBuf>,
-    std_lib: StdLib,
-) -> Vec<(MutMap<ModuleId, ModuleDocumentation>, Interns)> {
+pub fn files_to_documentations(filenames: Vec<PathBuf>, std_lib: StdLib) -> Vec<LoadedModule> {
     let arena = Bump::new();
     let mut files_docs = vec![];
 
@@ -299,10 +322,10 @@ pub fn files_to_documentations(
             &std_lib,
             src_dir.as_path(),
             MutMap::default(),
-            8, // TODO: Is it okay to hardcode ptr_bytes here? I think it should be fine since we'er only type checking (also, 8 => 32bit system)
+            std::mem::size_of::<usize>() as u32, // This is just type-checking for docs, so "target" doesn't matter
             builtin_defs_map,
         ) {
-            Ok(loaded) => files_docs.push((loaded.documentation, loaded.interns)),
+            Ok(loaded) => files_docs.push(loaded),
             Err(LoadingProblem::FormattedReport(report)) => {
                 println!("{}", report);
                 panic!();
@@ -327,19 +350,34 @@ fn new_line(buf: &mut String) {
 }
 
 fn type_annotation_to_html(indent_level: usize, buf: &mut String, type_ann: &TypeAnnotation) {
+    let is_multiline = should_be_multiline(type_ann);
     match type_ann {
         TypeAnnotation::TagUnion { tags, extension } => {
-            new_line(buf);
+            let tags_len = tags.len();
 
             let tag_union_indent = indent_level + 1;
-            indent(buf, tag_union_indent);
+
+            if is_multiline {
+                new_line(buf);
+
+                indent(buf, tag_union_indent);
+            }
+
             buf.push('[');
-            new_line(buf);
+
+            if is_multiline {
+                new_line(buf);
+            }
 
             let next_indent_level = tag_union_indent + 1;
 
             for (index, tag) in tags.iter().enumerate() {
-                indent(buf, next_indent_level);
+                if is_multiline {
+                    indent(buf, next_indent_level);
+                } else {
+                    buf.push(' ');
+                }
+
                 buf.push_str(tag.name.as_str());
 
                 for type_value in &tag.values {
@@ -347,19 +385,24 @@ fn type_annotation_to_html(indent_level: usize, buf: &mut String, type_ann: &Typ
                     type_annotation_to_html(next_indent_level, buf, type_value);
                 }
 
-                if index < (tags.len() - 1) {
-                    buf.push(',');
-                }
+                if is_multiline {
+                    if index < (tags_len - 1) {
+                        buf.push(',');
+                    }
 
-                new_line(buf);
+                    new_line(buf);
+                }
+            }
+
+            if is_multiline {
+                indent(buf, tag_union_indent);
+            } else {
+                buf.push(' ');
             }
 
-            indent(buf, tag_union_indent);
             buf.push(']');
 
-            if let Some(ext) = extension {
-                type_annotation_to_html(indent_level, buf, ext);
-            }
+            type_annotation_to_html(indent_level, buf, extension);
         }
         TypeAnnotation::BoundVariable(var_name) => {
             buf.push_str(var_name);
@@ -377,18 +420,30 @@ fn type_annotation_to_html(indent_level: usize, buf: &mut String, type_ann: &Typ
                 buf.push(')');
             }
         }
-        TypeAnnotation::Record { fields } => {
-            new_line(buf);
+        TypeAnnotation::Record { fields, extension } => {
+            let fields_len = fields.len();
 
             let record_indent = indent_level + 1;
-            indent(buf, record_indent);
+
+            if is_multiline {
+                new_line(buf);
+                indent(buf, record_indent);
+            }
+
             buf.push('{');
 
-            new_line(buf);
+            if is_multiline {
+                new_line(buf);
+            }
 
             let next_indent_level = record_indent + 1;
+
             for (index, field) in fields.iter().enumerate() {
-                indent(buf, next_indent_level);
+                if is_multiline {
+                    indent(buf, next_indent_level);
+                } else {
+                    buf.push(' ');
+                }
 
                 let fields_name = match field {
                     RecordField::RecordField { name, .. } => name,
@@ -414,20 +469,139 @@ fn type_annotation_to_html(indent_level: usize, buf: &mut String, type_ann: &Typ
                     RecordField::LabelOnly { .. } => {}
                 }
 
-                if index < (fields.len() - 1) {
-                    buf.push(',');
-                }
+                if is_multiline {
+                    if index < (fields_len - 1) {
+                        buf.push(',');
+                    }
 
-                new_line(buf);
+                    new_line(buf);
+                }
+            }
+
+            if is_multiline {
+                indent(buf, record_indent);
+            } else {
+                buf.push(' ');
             }
 
-            indent(buf, record_indent);
             buf.push('}');
+
+            type_annotation_to_html(indent_level, buf, extension);
         }
+        TypeAnnotation::Function { args, output } => {
+            let mut peekable_args = args.iter().peekable();
+            while let Some(arg) = peekable_args.next() {
+                if is_multiline {
+                    if !should_be_multiline(arg) {
+                        new_line(buf);
+                    }
+                    indent(buf, indent_level + 1);
+                }
+
+                type_annotation_to_html(indent_level, buf, arg);
+
+                if peekable_args.peek().is_some() {
+                    buf.push_str(", ");
+                }
+            }
+
+            if is_multiline {
+                new_line(buf);
+                indent(buf, indent_level + 1);
+            }
+
+            buf.push_str(" -> ");
+
+            let mut next_indent_level = indent_level;
+
+            if should_be_multiline(output) {
+                next_indent_level += 1;
+            }
+
+            type_annotation_to_html(next_indent_level, buf, output);
+        }
+        TypeAnnotation::ObscuredTagUnion => {
+            buf.push_str("[ @.. ]");
+        }
+        TypeAnnotation::ObscuredRecord => {
+            buf.push_str("{ @.. }");
+        }
+        TypeAnnotation::NoTypeAnn => {}
+        TypeAnnotation::Wildcard => buf.push('*'),
     }
 }
 
-fn insert_doc_links(scope: &mut Scope, interns: &Interns, markdown: String) -> String {
+fn should_be_multiline(type_ann: &TypeAnnotation) -> bool {
+    match type_ann {
+        TypeAnnotation::TagUnion { tags, extension } => {
+            let mut is_multiline = should_be_multiline(extension) || tags.len() > 1;
+
+            for tag in tags {
+                for value in &tag.values {
+                    if is_multiline {
+                        break;
+                    }
+                    is_multiline = should_be_multiline(&value);
+                }
+            }
+
+            is_multiline
+        }
+        TypeAnnotation::Function { args, output } => {
+            let mut is_multiline = should_be_multiline(output) || args.len() > 1;
+
+            for arg in args {
+                if is_multiline {
+                    break;
+                }
+
+                is_multiline = should_be_multiline(arg);
+            }
+
+            is_multiline
+        }
+        TypeAnnotation::ObscuredTagUnion => false,
+        TypeAnnotation::ObscuredRecord => false,
+        TypeAnnotation::BoundVariable(_) => false,
+        TypeAnnotation::Apply { parts, .. } => {
+            let mut is_multiline = false;
+
+            for part in parts {
+                is_multiline = should_be_multiline(part);
+
+                if is_multiline {
+                    break;
+                }
+            }
+
+            is_multiline
+        }
+        TypeAnnotation::Record { fields, extension } => {
+            let mut is_multiline = should_be_multiline(extension) || fields.len() > 1;
+
+            for field in fields {
+                if is_multiline {
+                    break;
+                }
+                match field {
+                    RecordField::RecordField {
+                        type_annotation, ..
+                    } => is_multiline = should_be_multiline(type_annotation),
+                    RecordField::OptionalField {
+                        type_annotation, ..
+                    } => is_multiline = should_be_multiline(type_annotation),
+                    RecordField::LabelOnly { .. } => {}
+                }
+            }
+
+            is_multiline
+        }
+        TypeAnnotation::Wildcard => false,
+        TypeAnnotation::NoTypeAnn => false,
+    }
+}
+
+pub fn insert_doc_links(scope: &mut Scope, interns: &Interns, markdown: String) -> String {
     let buf = &markdown;
     let mut result = String::new();
 
@@ -458,7 +632,7 @@ fn insert_doc_links(scope: &mut Scope, interns: &Interns, markdown: String) -> S
                         interns,
                         &buf.chars()
                             .skip(from + 1)
-                            .take(index - from)
+                            .take(index - from - 1)
                             .collect::<String>(),
                     );
 
@@ -473,19 +647,25 @@ fn insert_doc_links(scope: &mut Scope, interns: &Interns, markdown: String) -> S
         }
     }
 
-    result
+    if chomping_from == None {
+        markdown
+    } else {
+        result
+    }
 }
 
 fn make_doc_link(scope: &mut Scope, interns: &Interns, doc_item: &str) -> String {
     match scope.lookup(&doc_item.into(), Region::zero()) {
         Ok(symbol) => {
             let module_str = symbol.module_string(interns);
+
             let ident_str = symbol.ident_string(interns);
 
             let mut link = String::new();
 
+            link.push('/');
             link.push_str(module_str);
-            link.push_str(".html#");
+            link.push('#');
             link.push_str(ident_str);
 
             let mut buf = String::new();
@@ -501,8 +681,8 @@ fn make_doc_link(scope: &mut Scope, interns: &Interns, doc_item: &str) -> String
         }
         Err(_) => {
             panic!(
-                "Could not find symbol in scope for module link : {}",
-                doc_item
+                "Tried to generate an automatic link in docs for symbol `{}`, but that symbol was not in scope in this module. Scope was: {:?}",
+                doc_item, scope
             )
         }
     }
@@ -520,62 +700,65 @@ fn markdown_to_html(scope: &mut Scope, interns: &Interns, markdown: String) -> S
     let mut docs_parser = vec![];
     let (_, _) = pulldown_cmark::Parser::new_ext(&markdown_with_links, markdown_options).fold(
         (0, 0),
-        |(start_quote_count, end_quote_count), event| match event {
-            // Replace this sequence (`>>>` syntax):
-            //     Start(BlockQuote)
-            //     Start(BlockQuote)
-            //     Start(BlockQuote)
-            //     Start(Paragraph)
-            // For `Start(CodeBlock(Fenced(Borrowed("roc"))))`
-            Event::Start(BlockQuote) => {
-                docs_parser.push(event);
-                (start_quote_count + 1, 0)
-            }
-            Event::Start(Paragraph) => {
-                if start_quote_count == 3 {
-                    docs_parser.pop();
-                    docs_parser.pop();
-                    docs_parser.pop();
-                    docs_parser.push(Event::Start(CodeBlock(CodeBlockKind::Fenced(
-                        CowStr::Borrowed("roc"),
-                    ))));
-                } else {
+        |(start_quote_count, end_quote_count), event| {
+            match event {
+                // Replace this sequence (`>>>` syntax):
+                //     Start(BlockQuote)
+                //     Start(BlockQuote)
+                //     Start(BlockQuote)
+                //     Start(Paragraph)
+                // For `Start(CodeBlock(Fenced(Borrowed("roc"))))`
+                Event::Start(BlockQuote) => {
                     docs_parser.push(event);
+                    (start_quote_count + 1, 0)
                 }
-                (0, 0)
-            }
-            // Replace this sequence (`>>>` syntax):
-            //     End(Paragraph)
-            //     End(BlockQuote)
-            //     End(BlockQuote)
-            //     End(BlockQuote)
-            // For `End(CodeBlock(Fenced(Borrowed("roc"))))`
-            Event::End(Paragraph) => {
-                docs_parser.push(event);
-                (0, 1)
-            }
-            Event::End(BlockQuote) => {
-                if end_quote_count == 3 {
-                    docs_parser.pop();
-                    docs_parser.pop();
-                    docs_parser.pop();
-                    docs_parser.push(Event::End(CodeBlock(CodeBlockKind::Fenced(
-                        CowStr::Borrowed("roc"),
-                    ))));
+                Event::Start(Paragraph) => {
+                    if start_quote_count == 3 {
+                        docs_parser.pop();
+                        docs_parser.pop();
+                        docs_parser.pop();
+                        docs_parser.push(Event::Start(CodeBlock(CodeBlockKind::Fenced(
+                            CowStr::Borrowed("roc"),
+                        ))));
+                    } else {
+                        docs_parser.push(event);
+                    }
+                    (0, 0)
+                }
+                // Replace this sequence (`>>>` syntax):
+                //     End(Paragraph)
+                //     End(BlockQuote)
+                //     End(BlockQuote)
+                //     End(BlockQuote)
+                // For `End(CodeBlock(Fenced(Borrowed("roc"))))`
+                Event::End(Paragraph) => {
+                    docs_parser.push(event);
+                    (0, 1)
+                }
+                Event::End(BlockQuote) => {
+                    if end_quote_count == 3 {
+                        docs_parser.pop();
+                        docs_parser.pop();
+                        docs_parser.pop();
+                        docs_parser.push(Event::End(CodeBlock(CodeBlockKind::Fenced(
+                            CowStr::Borrowed("roc"),
+                        ))));
+                        (0, 0)
+                    } else {
+                        docs_parser.push(event);
+                        (0, end_quote_count + 1)
+                    }
+                }
+                _ => {
+                    docs_parser.push(event);
                     (0, 0)
-                } else {
-                    docs_parser.push(event);
-                    (0, end_quote_count + 1)
                 }
-            }
-            _ => {
-                docs_parser.push(event);
-                (0, 0)
             }
         },
     );
 
     let mut docs_html = String::new();
+
     pulldown_cmark::html::push_html(&mut docs_html, docs_parser.into_iter());
 
     docs_html
diff --git a/docs/src/static/index.html b/docs/src/static/index.html
index daad3246e6..5e358eea74 100644
--- a/docs/src/static/index.html
+++ b/docs/src/static/index.html
@@ -6,9 +6,9 @@
     <!-- <title>TODO populate this based on the module's name, e.g. "Parser - roc/parser"</title> -->
     <!-- <meta name="description" content="TODO populate this based on the module's description"> -->
     <meta name="viewport" content="width=device-width">
-    <link rel="icon" href="favicon.svg">
-    <script type="text/javascript" src="search.js" defer></script>
-    <link rel="stylesheet" href="styles.css">
+    <link rel="icon" href="/favicon.svg">
+    <script type="text/javascript" src="/search.js" defer></script>
+    <link rel="stylesheet" href="/styles.css">
 </head>
 
 <body>
diff --git a/docs/tests/insert_links.rs b/docs/tests/insert_links.rs
new file mode 100644
index 0000000000..e01e3eca5b
--- /dev/null
+++ b/docs/tests/insert_links.rs
@@ -0,0 +1,43 @@
+#[macro_use]
+extern crate pretty_assertions;
+
+#[cfg(test)]
+mod insert_doc_links {
+    use roc_can::env::Env;
+    use roc_can::scope::Scope;
+    use roc_collections::all::MutMap;
+    use roc_docs::insert_doc_links;
+    use roc_module::symbol::{IdentIds, Interns, ModuleIds};
+    use roc_types::subs::VarStore;
+
+    #[test]
+    fn no_doc_links() {
+        let home = ModuleIds::default().get_or_insert(&"Test".into());
+
+        let module_ids = ModuleIds::default();
+
+        let dep_idents = IdentIds::exposed_builtins(0);
+
+        let env = Env::new(home, dep_idents, &module_ids, IdentIds::default());
+
+        let all_ident_ids = MutMap::default();
+
+        let interns = Interns {
+            module_ids: env.module_ids.clone(),
+            all_ident_ids,
+        };
+
+        let var_store = &mut VarStore::default();
+        let scope = &mut Scope::new(home, var_store);
+
+        let markdown = r#"
+            # Hello
+            Hello thanks for using my package
+        "#;
+
+        assert_eq!(
+            markdown,
+            insert_doc_links(scope, &interns, markdown.to_string()),
+        );
+    }
+}
diff --git a/editor/Cargo.toml b/editor/Cargo.toml
index 76a995d431..1843d94132 100644
--- a/editor/Cargo.toml
+++ b/editor/Cargo.toml
@@ -28,13 +28,13 @@ arraystring = "0.3.0"
 libc = "0.2"
 page_size = "0.4"
 winit = "0.24"
-wgpu = "0.8"
+wgpu = "0.9"
 glyph_brush = "0.7"
 log = "0.4"
 zerocopy = "0.3"
 env_logger = "0.8"
 futures = "0.3"
-wgpu_glyph = "0.12"
+wgpu_glyph = "0.13"
 cgmath = "0.18.0"
 snafu = { version = "0.6", features = ["backtraces"] }
 colored = "2"
@@ -60,5 +60,4 @@ pretty_assertions = "0.6"
 maplit = "1.0.1"
 quickcheck = "1.0"
 quickcheck_macros = "1.0"
-criterion = "0.3"
 rand = "0.8.2"
diff --git a/editor/README.md b/editor/README.md
index acb1311068..cbae3eb8ff 100644
--- a/editor/README.md
+++ b/editor/README.md
@@ -1,5 +1,5 @@
 
-The editor is a work in progress, only a limited subset of Roc expressions are currently supported. New features are added every week!
+The editor is a work in progress, only a limited subset of Roc expressions are currently supported.
 
 Unlike most editors, we use projectional or structural editing to edit the [Abstract Syntax Tree](https://en.wikipedia.org/wiki/Abstract_syntax_tree) directly. This will allow for cool features like excellent auto-complete and refactoring.
 
@@ -28,4 +28,4 @@ We thank the following open source projects in particular for inspiring us when
 - [learn-wgpu](https://github.com/sotrh/learn-wgpu)
 - [rgx](https://github.com/cloudhead/rgx)
 - [elm-editor](https://github.com/jxxcarlson/elm-editor)
-- [iced](https://github.com/hecrj/iced)
\ No newline at end of file
+- [iced](https://github.com/hecrj/iced)
diff --git a/editor/editor-ideas.md b/editor/editor-ideas.md
index 41ff4dbfff..b1718ed3d2 100644
--- a/editor/editor-ideas.md
+++ b/editor/editor-ideas.md
@@ -31,6 +31,8 @@ Nice collection of research on innovative editors, [link](https://futureofcoding
 * [Self](https://selflanguage.org/) programming language
 * [Primitive](https://primitive.io/) code exploration in Virtual Reality
 * [Luna](https://www.luna-lang.org/) language for interactive data processing and visualization
+* [Hazel Livelits](https://hazel.org/papers/livelits-paper.pdf) interactive plugins, see GIF's [here](https://twitter.com/disconcision/status/1408155781120376833).
+* [Thorough review](https://drossbucket.com/2021/06/30/hacker-news-folk-wisdom-on-visual-programming/) of pros and cons of text versus visual programming.
 
 ### Debugging
 
@@ -39,9 +41,17 @@ Nice collection of research on innovative editors, [link](https://futureofcoding
 * [godbolt.org Compiler Explorer](https://godbolt.org/)
 * [whitebox debug visualization](https://vimeo.com/483795097)
 * [Hest](https://ivanish.ca/hest-time-travel/) tool for making highly interactive simulations.
+* [replit](https://replit.com/) collaborative browser based IDE.
 * Say you have a failing test that used to work, it would be very valuable to see all code that was changed that was used only by that test.
 e.g. you have a test `calculate_sum_test` that only uses the function `add`, when the test fails you should be able to see a diff showing only what changed for the function `add`. It would also be great to have a diff of [expression values](https://homepages.cwi.nl/~storm/livelit/images/bret.png) Bret Victor style. An ambitious project would be to suggest or automatically try fixes based on these diffs.
-* I think it could be possible to create a minimal reproduction of a program / block of code / code used by a single test. So for a failing unit test I would expect it to extract imports, the platform, types and functions that are necessary to run only that unit test and put them in a standalone roc project. This would be useful for sharing bugs with library+application authors and colleagues, for profiling or debugging with all "clutter" removed. 
+* I think it could be possible to create a minimal reproduction of a program / block of code / code used by a single test. So for a failing unit test I would expect it to extract imports, the platform, types and functions that are necessary to run only that unit test and put them in a standalone roc project. This would be useful for sharing bugs with library+application authors and colleagues, for profiling or debugging with all "clutter" removed.
+* Ability to share program state at a breakpoint with someone else.
+* For debugging we should aim for maximal useful observability. For example Rust's enum values can not be easily viewed in the CodeLLDB debugger, you actually need to call a print method that does pattern matching to be able to view useful information.
+* We previuously discussed recording full traces of programs so they do not have to be re-run multiple times in the debugging process. We should encourage roc developers to experiment with creating debugging representations of this AST+"execution trace", it could lead to some cool stuff.  
+
+### Cool regular editors
+
+* [Helix](https://github.com/helix-editor/helix) modal (terminal, for now) editor in rust. Good UX.
 
 
 ### Structured Editing
@@ -64,6 +74,8 @@ e.g. you have a test `calculate_sum_test` that only uses the function `add`, whe
 * [Lamdu](http://www.lamdu.org/) live functional programming.
 * [Sourcetrail](https://www.sourcetrail.com/) nice tree-like source explorer.
 * [Unisonweb](https://www.unisonweb.org), definition based [editor](https://twitter.com/shojberg/status/1364666092598288385) as opposed to file based.
+* [Utopia](https://utopia.app/) integrated design and development environment for React. Design and code update each other, in real time.
+* [Paredit](https://calva.io/paredit/) structural clojure editing, navigation and selection. [Another overview](http://danmidwood.com/content/2014/11/21/animated-paredit.html)
 
 ### Voice Interaction Related
 
@@ -76,7 +88,27 @@ e.g. you have a test `calculate_sum_test` that only uses the function `add`, whe
     * Show edit history for this function.
     * Adjusting settings: switch to light theme, increase font size...
 * Use (context specific) voice command state machine to assist Machine Learning voice recognition model.
-* Nice special use case: using voice to code while on treadmill desk. 
+* Nice special use case: using voice to code while on treadmill desk.
+* Use word embeddings to find most similar voice command to recorded input in vector space.
+
+#### Useful voice commands
+
+* clear all breakpoints
+* increase/decrease font size
+* switch to dark/light/high-contrast mode
+* open/go to file "Main"(fuzzy matching)
+* go to function "foo"
+* go to definition
+* show all references(uses) of this function/type/...
+* show history timeline of this function/file
+* show recent projects
+* generate unit test for this function
+* generate unit test for this function based on debug trace (input and output is recorded and used in test)
+* who wrote this line (git blame integration)
+* search documentation of library X for Foo
+* show example of how to use library function Foo
+* open google/github/duckduckgo search for error...
+* show editor plugins for library X
 
     
 #### Inspiration
@@ -98,7 +130,7 @@ e.g. you have a test `calculate_sum_test` that only uses the function `add`, whe
 * When refactoring; 
     - Cutting and pasting code to a new file should automatically add imports to the new file and delete them from the old file.
     - Ability to link e.g. variable name in comments to actual variable name. Comment is automatically updated when variable name is changed.
-    - When updating dependencies with breaking changes; show similar diffs from github projects that have succesfully updated that dependency.
+    - When updating dependencies with breaking changes; show similar diffs from github projects that have successfully updated that dependency.
     - AST backed renaming, changing variable/function/type name should change it all over the codebase.  
 * Automatically create all "arms" when pattern matching after entering `when var is` based on the type.
     - All `when ... is` should be updated if the type is changed, e.g. adding Indigo to the Color type should add an arm everywhere where `when color is` is used. 
@@ -109,7 +141,7 @@ e.g. you have a test `calculate_sum_test` that only uses the function `add`, whe
 * Show productivity/feature tips on startup. Show link to page with all tips. Allow not seeing tips next time.
 * Search friendly editor docs inside the editor. Offer to send search string to Roc maintainers when no results, or if no results were clicked.
 * File history timeline view. Show timeline with commits that changed this file, the number of lines added and deleted as well as which user made the changes. Arrow navigation should allow you to quickly view different versions of the file.
-* Suggested quick fixes should be directly visible and clickable. Not like in vs code where you put the caret on an error until a lightbulb appears in the margin which you have to click for the fixes to apppear, after which you click to apply the fix you want :( .
+* Suggested quick fixes should be directly visible and clickable. Not like in vs code where you put the caret on an error until a lightbulb appears in the margin which you have to click for the fixes to apppear, after which you click to apply the fix you want :( . You should be able to apply suggestions in rapid succession. e.g. if you copy some roc code from the internet you should be able to apply 5 import suggestions quickly. 
 * Regex-like find and substitution based on plain english description and example (replacement). i.e. replace all `[` between double quotes with `{`. [Inspiration](https://alexmoltzau.medium.com/english-to-regex-thanks-to-gpt-3-13f03b68236e).
 * Show productivity tips based on behavior. i.e. if the user is scrolling through the error bar and clicking on the next error several times, show a tip with "go to next error" shortcut.
 * Command to "benchmark this function" or "benchmark this test" with flamegraph and execution time per line.
@@ -120,6 +152,8 @@ e.g. you have a test `calculate_sum_test` that only uses the function `add`, whe
     * search a local history of previously encountered errors and fixes
     * search through a database of our zullip questions
     * ...
+* smart insert: press a shortcut and enter a plain english description of a code snippet you need. Examples: "convert string to list of chars", "sort list of records by field foo descending", "plot this list with date on x-axis"...
+* After the user has refactored code to be simpler, try finding other places in the code base where the same simplification can be made.
 
 #### Autocomplete
 
@@ -129,13 +163,15 @@ e.g. you have a test `calculate_sum_test` that only uses the function `add`, whe
 - Webcam based eye tracking for quick selection.
 - Machine Learning:
    * GPT-3 can generate correct python functions based on a comment describing the functionality, video [here](https://www.youtube.com/watch?v=utuz7wBGjKM). It's possible that training a model using ast's may lead to better results than text based models.
-- Current autocomplete lacks flow, moving through suggestions with arrows is slow. Being able to code by weaving together autocomplete suggestions layed out in rows using eye tracking, that could flow.
+- Current autocomplete lacks flow, moving through suggestions with arrows is slow. Being able to code by weaving together autocomplete suggestions laid out in rows using eye tracking, that could flow.
+- It's possible that with strong static types, pure functions and a good search algorithm we can develop a more reliable autocomplete than one with machine learning. 
 
 #### Productivity Inspiration
 
 * [Kite](https://www.kite.com/) AI autocomplete and doc viewer.
 * [Tabnine](https://www.tabnine.com/) AI autocomplete.
 * [Codota](https://www.codota.com) AI autocomplete and example searching.
+* [Github copilot](https://copilot.github.com/) AI autocomplete.
 * [Aroma](https://ai.facebook.com/blog/aroma-ml-for-code-recommendation) showing examples similar to current code.
 * [MISM](https://arxiv.org/abs/2006.05265) neural network based code similarity scoring.
 * [Inquisitive code editor](https://web.eecs.utk.edu/~azh/blog/inquisitivecodeeditor.html) Interactive bug detection with doc+test generation.
@@ -162,7 +198,7 @@ e.g. you have a test `calculate_sum_test` that only uses the function `add`, whe
         * See [codata](https://www.codota.com/code/java/classes/okhttp3.OkHttpClient) for inspiration on a snippet/example finder.
 * Fuzzy natural language based setting adjustment in search bar or with voice input: increase font size, enable autosave, switch to light theme...
 * Detect deviation of best practices, example case: alert developer when they are defining a color inline (rgb(30,30,30)) while all colors have been previously imported from a single file. See also [Codota](https://www.codota.com).
-* It would be valuable to record the user's interactions with the editor when debugging as well as the AST. On enough data we could train a model to perform a bunch of debugging steps and show values of the most important variables in relation to the bug. Having assistance in finding the exact code that causes the problem could be super valuable. There could be sensitive data, so it should only be recorded and or shared for open source codebases with explicit user permission.
+* It would be valuable to record the user's interactions with the editor when debugging as well as the AST. On enough data we could train a model to perform a bunch of debugging steps and show values of the most important variables in relation to the bug. Having assistance in finding the exact code that causes the problem could be super valuable. There could be sensitive data, so it should only be recorded and or shared for open source codebases with permissive licenses and with explicit user permission.
 
 
 ## Testing
@@ -187,6 +223,13 @@ e.g. you have a test `calculate_sum_test` that only uses the function `add`, whe
 * Ability to see module as it would be presented on a package website.
     * Modern editors may guide developers to the source code too easily.
     The API and documentation are meant to interface with humans.
+* [DocC](https://developer.apple.com/videos/play/wwdc2021/10166/) neat documentation approach for swift. 
+
+## General Plugin Ideas
+
+### Inspiration
+
+- [Boop](https://github.com/IvanMathy/Boop) scriptable scratchpad for developers. Contains collection of useful conversions: json formatting, url encoding, encode to base64...
 
 ## General Thoughts/Ideas
 
@@ -201,9 +244,9 @@ Thoughts and ideas possibly taken from above inspirations or separate.
       Or Total blindness where we need to trough sound to communicate to the user
       Screen readers read trees of labeled elements. Each platform has different apis, but I think they are horrible. Just close your eyes and imagine listening to screen reader all day while you are using this majectic machines called computers.
       But blind people walk with a tool and they can react much better to sound/space relations than full on visal majority does. They are acute to sound as a spatial hint. And a hand for most of them is a very sensitive tool that can make sounds in space.
-      Imagine if everytime for the user doesnt want to rely on shining rendered pixels on the screen for a feedback from machine, we make a accoustic room simulation, where with moving the "stick", either with mouse or with key arrows, we bump into one of the objects and that produces certain contextually appropriate sound (clean)*ding*
+      Imagine if everytime for the user doesnt want to rely on shining rendered pixels on the screen for a feedback from machine, we make a acoustic room simulation, where with moving the "stick", either with mouse or with key arrows, we bump into one of the objects and that produces certain contextually appropriate sound (clean)*ding*
       
-      On the each level of abstraction they can make sounds more deeper, so then when you type letters you feel like you are playing with the sand (soft)*shh*. We would need help from some sound engeneer about it, but imagine moving down, which can be voice trigered command for motion impaired, you hear (soft)*pup* and the name of the module, and then you have options and commands appropriate for the module, they could map to those  basic 4 buttons that we trained user on, and he would shortcut all the soft talk with click of a button. Think of the satisfaction when you can skip the dialog of the game and get straight into action. (X) Open functions! each function would make a sound and say its name, unless you press search and start searching for a specific function inside module, if you want one you select or move to next.
+      On the each level of abstraction they can make sounds more deeper, so then when you type letters you feel like you are playing with the sand (soft)*shh*. We would need help from some sound engineer about it, but imagine moving down, which can be voice trigered command for motion impaired, you hear (soft)*pup* and the name of the module, and then you have options and commands appropriate for the module, they could map to those  basic 4 buttons that we trained user on, and he would shortcut all the soft talk with click of a button. Think of the satisfaction when you can skip the dialog of the game and get straight into action. (X) Open functions! each function would make a sound and say its name, unless you press search and start searching for a specific function inside module, if you want one you select or move to next.
       
    * Motor impariments
       [rant]BACKS OF CODERS ARE NOT HEALTHY! We need to change that![/neverstop]
diff --git a/editor/prep_benchmarks.sh b/editor/prep_benchmarks.sh
deleted file mode 100755
index 1d90280df9..0000000000
--- a/editor/prep_benchmarks.sh
+++ /dev/null
@@ -1,5 +0,0 @@
-#!/bin/bash
-sed -i -e 's/\/\/pub mod mvc/pub mod mvc/g' src/lib.rs
-sed -i -e 's/\/\/pub mod text_buffer/pub mod text_buffer/g' src/lib.rs
-sed -i -e 's/^mod mvc/\/\/mod mvc/g' src/lib.rs
-sed -i -e 's/^mod text_buffer/\/\/mod text_buffer/g' src/lib.rs
\ No newline at end of file
diff --git a/editor/src/editor/ed_error.rs b/editor/src/editor/ed_error.rs
index 0f80a0398a..19a6124606 100644
--- a/editor/src/editor/ed_error.rs
+++ b/editor/src/editor/ed_error.rs
@@ -1,5 +1,5 @@
-use crate::editor::slow_pool::MarkNodeId;
 use crate::ui::ui_error::UIResult;
+use crate::{editor::slow_pool::MarkNodeId, ui::text::text_pos::TextPos};
 use colored::*;
 use snafu::{Backtrace, ErrorCompat, NoneError, ResultExt, Snafu};
 
@@ -111,6 +111,35 @@ pub enum EdError {
         backtrace: Backtrace,
     },
 
+    #[snafu(display(
+        "NoNodeAtCaretPosition: there was no node at the current caret position {:?}.",
+        caret_pos,
+    ))]
+    NoNodeAtCaretPosition {
+        caret_pos: TextPos,
+        backtrace: Backtrace,
+    },
+
+    #[snafu(display(
+        "UnexpectedASTNode: required a {} at this position, node was a {}.",
+        required_node_type,
+        encountered_node_type
+    ))]
+    UnexpectedASTNode {
+        required_node_type: String,
+        encountered_node_type: String,
+        backtrace: Backtrace,
+    },
+
+    #[snafu(display(
+        "UnexpectedEmptyPoolVec: expected PoolVec {} to have at least one element.",
+        descriptive_vec_name
+    ))]
+    UnexpectedEmptyPoolVec {
+        descriptive_vec_name: String,
+        backtrace: Backtrace,
+    },
+
     #[snafu(display(
         "OutOfBounds: index {} was out of bounds for {} with length {}.",
         index,
@@ -127,7 +156,7 @@ pub enum EdError {
     #[snafu(display("ParseError: Failed to parse AST: SyntaxError: {}.", syntax_err))]
     ParseError { syntax_err: String },
 
-    #[snafu(display("RecordWithoutFields: expected record to have at least one field because it is not an EmpyRecord."))]
+    #[snafu(display("RecordWithoutFields: expected record to have at least one field because it is not an EmptyRecord."))]
     RecordWithoutFields { backtrace: Backtrace },
 
     #[snafu(display("StringParseError: {}", msg))]
diff --git a/editor/src/editor/grid_node_map.rs b/editor/src/editor/grid_node_map.rs
index f5f2944cd2..f455782099 100644
--- a/editor/src/editor/grid_node_map.rs
+++ b/editor/src/editor/grid_node_map.rs
@@ -105,7 +105,7 @@ impl GridNodeMap {
         }
     }
 
-    // get position of first occurence of node_id if get_first_pos, else get the last occurence
+    // get position of first occurrence of node_id if get_first_pos, else get the last occurrence
     pub fn get_node_position(&self, node_id: MarkNodeId, get_first_pos: bool) -> EdResult<TextPos> {
         let mut last_pos_opt = None;
 
diff --git a/editor/src/editor/main.rs b/editor/src/editor/main.rs
index 352adc5818..ad8294a9a6 100644
--- a/editor/src/editor/main.rs
+++ b/editor/src/editor/main.rs
@@ -26,6 +26,7 @@ use bumpalo::collections::String as BumpString;
 use bumpalo::Bump;
 use cgmath::Vector2;
 use pipelines::RectResources;
+use roc_module::symbol::Interns;
 use roc_module::symbol::{IdentIds, ModuleIds};
 use roc_types::subs::VarStore;
 use std::{error::Error, io, path::Path};
@@ -140,13 +141,18 @@ fn run_event_loop(file_path_opt: Option<&Path>) -> Result<(), Box<dyn Error>> {
     let mut module_ids = ModuleIds::default();
     let mod_id = module_ids.get_or_insert(&"ModId123".into());
 
+    let interns = Interns {
+        module_ids,
+        all_ident_ids: IdentIds::exposed_builtins(8),
+    };
+
     let env = Env::new(
         mod_id,
         &env_arena,
         &mut env_pool,
         &mut var_store,
         dep_idents,
-        &module_ids,
+        &interns.module_ids,
         exposed_ident_ids,
     );
 
@@ -172,7 +178,7 @@ fn run_event_loop(file_path_opt: Option<&Path>) -> Result<(), Box<dyn Error>> {
     };
 
     let ed_model_opt = {
-        let ed_model_res = ed_model::init_model(&code_str, file_path, env, &code_arena);
+        let ed_model_res = ed_model::init_model(&code_str, file_path, env, &interns, &code_arena);
 
         match ed_model_res {
             Ok(mut ed_model) => {
@@ -324,6 +330,8 @@ fn run_event_loop(file_path_opt: Option<&Path>) -> Result<(), Box<dyn Error>> {
                         )
                     }
                 } else {
+                    begin_render_pass(&mut encoder, &frame.view, &ed_theme);
+
                     queue_no_file_text(
                         &size,
                         NOTHING_OPENED,
diff --git a/editor/src/editor/markup/nodes.rs b/editor/src/editor/markup/nodes.rs
index 2d09f21ed6..8390cd70dd 100644
--- a/editor/src/editor/markup/nodes.rs
+++ b/editor/src/editor/markup/nodes.rs
@@ -2,16 +2,19 @@ use super::attribute::Attributes;
 use crate::editor::ed_error::EdResult;
 use crate::editor::ed_error::ExpectedTextNode;
 use crate::editor::ed_error::GetContentOnNestedNode;
-use crate::editor::ed_error::NestedNodeRequired;
+use crate::editor::ed_error::{NestedNodeMissingChild, NestedNodeRequired};
 use crate::editor::slow_pool::MarkNodeId;
 use crate::editor::slow_pool::SlowPool;
 use crate::editor::syntax_highlight::HighlightStyle;
+use crate::editor::util::index_of;
+use crate::lang::ast::ExprId;
 use crate::lang::ast::RecordField;
 use crate::lang::{
     ast::Expr2,
     expr::Env,
     pool::{NodeId, PoolStr},
 };
+use crate::ui::util::slice_get;
 use bumpalo::Bump;
 use std::fmt;
 
@@ -72,6 +75,83 @@ impl MarkupNode {
         }
     }
 
+    // return (index of child in list of children, closest ast index of child corresponding to ast node)
+    pub fn get_child_indices(
+        &self,
+        child_id: MarkNodeId,
+        markup_node_pool: &SlowPool,
+    ) -> EdResult<(usize, usize)> {
+        match self {
+            MarkupNode::Nested { children_ids, .. } => {
+                let mut mark_child_index_opt: Option<usize> = None;
+                let mut child_ids_with_ast: Vec<MarkNodeId> = Vec::new();
+                let self_ast_id = self.get_ast_node_id();
+
+                for (indx, &mark_child_id) in children_ids.iter().enumerate() {
+                    if mark_child_id == child_id {
+                        mark_child_index_opt = Some(indx);
+                    }
+
+                    let child_mark_node = markup_node_pool.get(mark_child_id);
+                    // a node that points to the same ast_node as the parent is a ',', '[', ']'
+                    // those are not "real" ast children
+                    if child_mark_node.get_ast_node_id() != self_ast_id {
+                        child_ids_with_ast.push(mark_child_id)
+                    }
+                }
+
+                if let Some(child_index) = mark_child_index_opt {
+                    if child_index == (children_ids.len() - 1) {
+                        let ast_child_index = child_ids_with_ast.len();
+
+                        Ok((child_index, ast_child_index))
+                    } else {
+                        // we want to find the index of the closest ast mark node to child_index
+                        let indices_in_mark_res: EdResult<Vec<usize>> = child_ids_with_ast
+                            .iter()
+                            .map(|c_id| index_of(*c_id, children_ids))
+                            .collect();
+
+                        let indices_in_mark = indices_in_mark_res?;
+
+                        let mut last_diff = usize::MAX;
+                        let mut best_index = 0;
+
+                        for index in indices_in_mark.iter() {
+                            let curr_diff =
+                                isize::abs((*index as isize) - (child_index as isize)) as usize;
+
+                            if curr_diff >= last_diff {
+                                break;
+                            } else {
+                                last_diff = curr_diff;
+                                best_index = *index;
+                            }
+                        }
+
+                        let closest_ast_child = slice_get(best_index, &children_ids)?;
+
+                        let closest_ast_child_index =
+                            index_of(*closest_ast_child, &child_ids_with_ast)?;
+
+                        // +1 because we want to insert after ast_child
+                        Ok((child_index, closest_ast_child_index + 1))
+                    }
+                } else {
+                    NestedNodeMissingChild {
+                        node_id: child_id,
+                        children_ids: children_ids.clone(),
+                    }
+                    .fail()
+                }
+            }
+            _ => NestedNodeRequired {
+                node_type: self.node_type_as_string(),
+            }
+            .fail(),
+        }
+    }
+
     // can't be &str, this creates borrowing issues
     pub fn get_content(&self) -> EdResult<String> {
         match self {
@@ -143,7 +223,10 @@ fn get_string<'a>(env: &Env<'a>, pool_str: &PoolStr) -> String {
 pub const BLANK_PLACEHOLDER: &str = " ";
 pub const LEFT_ACCOLADE: &str = "{ ";
 pub const RIGHT_ACCOLADE: &str = " }";
+pub const LEFT_SQUARE_BR: &str = "[ ";
+pub const RIGHT_SQUARE_BR: &str = " ]";
 pub const COLON: &str = ": ";
+pub const COMMA: &str = ", ";
 pub const STRING_QUOTES: &str = "\"\"";
 
 fn new_markup_node(
@@ -175,12 +258,16 @@ pub fn expr2_to_markup<'a, 'b>(
         Expr2::SmallInt { text, .. }
         | Expr2::I128 { text, .. }
         | Expr2::U128 { text, .. }
-        | Expr2::Float { text, .. } => new_markup_node(
-            get_string(env, &text),
-            expr2_node_id,
-            HighlightStyle::Number,
-            markup_node_pool,
-        ),
+        | Expr2::Float { text, .. } => {
+            let num_str = get_string(env, &text);
+
+            new_markup_node(
+                num_str,
+                expr2_node_id,
+                HighlightStyle::Number,
+                markup_node_pool,
+            )
+        }
         Expr2::Str(text) => new_markup_node(
             "\"".to_owned() + text.as_str(env.pool) + "\"",
             expr2_node_id,
@@ -209,34 +296,37 @@ pub fn expr2_to_markup<'a, 'b>(
         }
         Expr2::List { elems, .. } => {
             let mut children_ids = vec![new_markup_node(
-                "[ ".to_string(),
+                LEFT_SQUARE_BR.to_string(),
                 expr2_node_id,
                 HighlightStyle::Bracket,
                 markup_node_pool,
             )];
 
-            for (idx, node_id) in elems.iter_node_ids().enumerate() {
-                let sub_expr2 = env.pool.get(node_id);
+            let indexed_node_ids: Vec<(usize, ExprId)> =
+                elems.iter(env.pool).copied().enumerate().collect();
+
+            for (idx, node_id) in indexed_node_ids.iter() {
+                let sub_expr2 = env.pool.get(*node_id);
 
                 children_ids.push(expr2_to_markup(
                     arena,
                     env,
                     sub_expr2,
-                    node_id,
+                    *node_id,
                     markup_node_pool,
                 ));
 
                 if idx + 1 < elems.len() {
                     children_ids.push(new_markup_node(
                         ", ".to_string(),
-                        node_id,
+                        expr2_node_id,
                         HighlightStyle::Operator,
                         markup_node_pool,
                     ));
                 }
             }
             children_ids.push(new_markup_node(
-                "] ".to_string(),
+                RIGHT_SQUARE_BR.to_string(),
                 expr2_node_id,
                 HighlightStyle::Bracket,
                 markup_node_pool,
@@ -347,6 +437,12 @@ pub fn expr2_to_markup<'a, 'b>(
             syn_high_style: HighlightStyle::Blank,
             parent_id_opt: None,
         }),
+        Expr2::RuntimeError() => new_markup_node(
+            "RunTimeError".to_string(),
+            expr2_node_id,
+            HighlightStyle::Blank,
+            markup_node_pool,
+        ),
         rest => todo!("implement expr2_to_markup for {:?}", rest),
     }
 }
diff --git a/editor/src/editor/mvc/app_model.rs b/editor/src/editor/mvc/app_model.rs
index 0709bfd381..b537e7a34f 100644
--- a/editor/src/editor/mvc/app_model.rs
+++ b/editor/src/editor/mvc/app_model.rs
@@ -81,7 +81,7 @@ pub fn set_clipboard_txt(clipboard_opt: &mut Option<Clipboard>, txt: &str) -> Ed
         clipboard.set_content(txt.to_owned())?;
     } else {
         return Err(ClipboardWriteFailed {
-            err_msg: "Clipboard was never initialized succesfully.".to_owned(),
+            err_msg: "Clipboard was never initialized successfully.".to_owned(),
         });
     }
 
@@ -93,7 +93,7 @@ pub fn get_clipboard_txt(clipboard_opt: &mut Option<Clipboard>) -> EdResult<Stri
         clipboard.get_content()
     } else {
         Err(ClipboardReadFailed {
-            err_msg: "Clipboard was never initialized succesfully.".to_owned(),
+            err_msg: "Clipboard was never initialized successfully.".to_owned(),
         })
     }
 }
diff --git a/editor/src/editor/mvc/ed_model.rs b/editor/src/editor/mvc/ed_model.rs
index 05433270a9..b57a36df8a 100644
--- a/editor/src/editor/mvc/ed_model.rs
+++ b/editor/src/editor/mvc/ed_model.rs
@@ -4,7 +4,7 @@ use crate::editor::slow_pool::{MarkNodeId, SlowPool};
 use crate::editor::syntax_highlight::HighlightStyle;
 use crate::editor::{
     ed_error::EdError::ParseError,
-    ed_error::EdResult,
+    ed_error::{EdResult, MissingParent, NoNodeAtCaretPosition},
     markup::attribute::Attributes,
     markup::nodes::{expr2_to_markup, set_parent_for_all, MarkupNode},
 };
@@ -21,6 +21,7 @@ use crate::ui::ui_error::UIResult;
 use bumpalo::collections::String as BumpString;
 use bumpalo::Bump;
 use nonempty::NonEmpty;
+use roc_module::symbol::Interns;
 use roc_region::all::Region;
 use std::path::Path;
 
@@ -38,12 +39,13 @@ pub struct EdModel<'a> {
     pub has_focus: bool,
     pub caret_w_select_vec: NonEmpty<(CaretWSelect, Option<MarkNodeId>)>,
     pub selected_expr_opt: Option<SelectedExpression>,
+    pub interns: &'a Interns, // this should eventually come from LoadedModule, see #1442
     pub show_debug_view: bool,
     // EdModel is dirty if it has changed since the previous render.
     pub dirty: bool,
 }
 
-#[derive(Debug)]
+#[derive(Debug, Copy, Clone)]
 pub struct SelectedExpression {
     pub ast_node_id: NodeId<Expr2>,
     pub mark_node_id: MarkNodeId,
@@ -54,6 +56,7 @@ pub fn init_model<'a>(
     code_str: &'a BumpString,
     file_path: &'a Path,
     env: Env<'a>,
+    interns: &'a Interns,
     code_arena: &'a Bump,
 ) -> EdResult<EdModel<'a>> {
     let mut module = EdModule::new(&code_str, env, code_arena)?;
@@ -99,6 +102,7 @@ pub fn init_model<'a>(
         has_focus: true,
         caret_w_select_vec: NonEmpty::new((CaretWSelect::default(), None)),
         selected_expr_opt: None,
+        interns,
         show_debug_view: false,
         dirty: true,
     })
@@ -130,6 +134,29 @@ impl<'a> EdModel<'a> {
     pub fn node_exists_at_caret(&self) -> bool {
         self.grid_node_map.node_exists_at_pos(self.get_caret())
     }
+
+    // return (index of child in list of children, closest ast index of child corresponding to ast node) of MarkupNode at current caret position
+    pub fn get_curr_child_indices(&self) -> EdResult<(usize, usize)> {
+        if self.node_exists_at_caret() {
+            let curr_mark_node_id = self.get_curr_mark_node_id()?;
+            let curr_mark_node = self.markup_node_pool.get(curr_mark_node_id);
+
+            if let Some(parent_id) = curr_mark_node.get_parent_id_opt() {
+                let parent = self.markup_node_pool.get(parent_id);
+                parent.get_child_indices(curr_mark_node_id, &self.markup_node_pool)
+            } else {
+                MissingParent {
+                    node_id: curr_mark_node_id,
+                }
+                .fail()
+            }
+        } else {
+            NoNodeAtCaretPosition {
+                caret_pos: self.get_caret(),
+            }
+            .fail()
+        }
+    }
 }
 
 #[derive(Debug)]
@@ -155,8 +182,7 @@ impl<'a> EdModule<'a> {
                     let ast_root_id = env.pool.add(expr2);
 
                     // for debugging
-                    // let expr2_str = expr2_to_string(ast_root_id, env.pool);
-                    // println!("expr2_string: {}", expr2_str);
+                    // dbg!(expr2_to_string(ast_root_id, env.pool));
 
                     Ok(EdModule { env, ast_root_id })
                 }
@@ -185,7 +211,7 @@ pub mod test_ed_model {
     use bumpalo::collections::String as BumpString;
     use bumpalo::Bump;
     use ed_model::EdModel;
-    use roc_module::symbol::{IdentIds, ModuleIds};
+    use roc_module::symbol::{IdentIds, Interns, ModuleIds};
     use roc_types::subs::VarStore;
     use std::path::Path;
 
@@ -196,9 +222,11 @@ pub mod test_ed_model {
         let file_path = Path::new("");
 
         let dep_idents = IdentIds::exposed_builtins(8);
-
         let exposed_ident_ids = IdentIds::default();
-        let mod_id = ed_model_refs.module_ids.get_or_insert(&"ModId123".into());
+        let mod_id = ed_model_refs
+            .interns
+            .module_ids
+            .get_or_insert(&"ModId123".into());
 
         let env = Env::new(
             mod_id,
@@ -206,11 +234,17 @@ pub mod test_ed_model {
             &mut ed_model_refs.env_pool,
             &mut ed_model_refs.var_store,
             dep_idents,
-            &ed_model_refs.module_ids,
+            &ed_model_refs.interns.module_ids,
             exposed_ident_ids,
         );
 
-        ed_model::init_model(&code_str, file_path, env, &ed_model_refs.code_arena)
+        ed_model::init_model(
+            &code_str,
+            file_path,
+            env,
+            &ed_model_refs.interns,
+            &ed_model_refs.code_arena,
+        )
     }
 
     pub struct EdModelRefs {
@@ -218,7 +252,7 @@ pub mod test_ed_model {
         env_arena: Bump,
         env_pool: Pool,
         var_store: VarStore,
-        module_ids: ModuleIds,
+        interns: Interns,
     }
 
     pub fn init_model_refs() -> EdModelRefs {
@@ -227,7 +261,10 @@ pub mod test_ed_model {
             env_arena: Bump::new(),
             env_pool: Pool::with_capacity(1024),
             var_store: VarStore::default(),
-            module_ids: ModuleIds::default(),
+            interns: Interns {
+                module_ids: ModuleIds::default(),
+                all_ident_ids: IdentIds::exposed_builtins(8),
+            },
         }
     }
 
diff --git a/editor/src/editor/mvc/ed_update.rs b/editor/src/editor/mvc/ed_update.rs
index e2136f4d7c..cf510dfd7f 100644
--- a/editor/src/editor/mvc/ed_update.rs
+++ b/editor/src/editor/mvc/ed_update.rs
@@ -13,6 +13,7 @@ use crate::editor::mvc::ed_model::EdModel;
 use crate::editor::mvc::ed_model::SelectedExpression;
 use crate::editor::mvc::int_update::start_new_int;
 use crate::editor::mvc::int_update::update_int;
+use crate::editor::mvc::list_update::{add_blank_child, start_new_list};
 use crate::editor::mvc::lookup_update::update_invalid_lookup;
 use crate::editor::mvc::record_update::start_new_record;
 use crate::editor::mvc::record_update::update_empty_record;
@@ -79,6 +80,15 @@ impl<'a> EdModel<'a> {
         }
     }
 
+    // disregards EdModel.code_lines because the caller knows the resulting caret position will be valid.
+    // allows us to prevent multiple updates to EdModel.code_lines
+    pub fn simple_move_carets_left(&mut self, repeat: usize) {
+        for caret_tup in self.caret_w_select_vec.iter_mut() {
+            caret_tup.0.caret_pos.column -= repeat;
+            caret_tup.1 = None;
+        }
+    }
+
     pub fn build_node_map_from_markup(
         markup_root_id: MarkNodeId,
         markup_node_pool: &SlowPool,
@@ -176,12 +186,12 @@ impl<'a> EdModel<'a> {
 
         self.set_caret(expr_start_pos);
 
-        //let type_str = self.expr2_to_type(ast_node_id);
+        let type_str = self.expr2_to_type(ast_node_id);
 
         self.selected_expr_opt = Some(SelectedExpression {
             ast_node_id,
             mark_node_id,
-            type_str: PoolStr::new("{}", self.module.env.pool), // TODO get this PoolStr from type inference
+            type_str,
         });
 
         self.dirty = true;
@@ -242,7 +252,7 @@ impl<'a> EdModel<'a> {
             Region::zero(),
         );
 
-        // extract the var_store out of the env again
+        // extract the var_store out of the env
         let mut var_store = VarStore::default();
         std::mem::swap(self.module.env.var_store, &mut var_store);
 
@@ -254,12 +264,18 @@ impl<'a> EdModel<'a> {
             var_store,
         );
 
+        // put the updated var_store back in env
+        std::mem::swap(
+            &mut VarStore::new_from_subs(solved.inner()),
+            self.module.env.var_store,
+        );
+
         let subs = solved.inner_mut();
 
         let content = subs.get(var).content;
 
         PoolStr::new(
-            &content_to_string(content, &subs, self.module.env.home, &Default::default()),
+            &content_to_string(content, &subs, self.module.env.home, self.interns),
             self.module.env.pool,
         )
     }
@@ -575,11 +591,18 @@ pub fn handle_new_char(received_char: &char, ed_model: &mut EdModel) -> EdResult
                                     '0'..='9' => {
                                         start_new_int(ed_model, ch)?
                                     }
+                                    '[' => {
+                                        // this can also be a tag union or become a set, assuming list for now
+                                        start_new_list(ed_model)?
+                                    }
                                     _ => InputOutcome::Ignored
                                 }
                             } else if let Some(prev_mark_node_id) = prev_mark_node_id_opt{
                                 if prev_mark_node_id == curr_mark_node_id {
                                     match ast_node_ref {
+                                        Expr2::SmallInt{ .. } => {
+                                            update_int(ed_model, curr_mark_node_id, ch)?
+                                        }
                                         Expr2::SmallStr(old_arr_str) => {
                                             update_small_string(
                                                 &ch, old_arr_str, ed_model
@@ -616,9 +639,6 @@ pub fn handle_new_char(received_char: &char, ed_model: &mut EdModel) -> EdResult
                                                 InputOutcome::Ignored
                                             }
                                         }
-                                        Expr2::SmallInt{ .. } => {
-                                            update_int(ed_model, curr_mark_node_id, ch)?
-                                        }
                                         _ => InputOutcome::Ignored
                                     }
                                 } else if ch.is_ascii_alphanumeric() { // prev_mark_node_id != curr_mark_node_id
@@ -637,6 +657,9 @@ pub fn handle_new_char(received_char: &char, ed_model: &mut EdModel) -> EdResult
                                             let prev_node_ref = ed_model.module.env.pool.get(prev_ast_node_id);
 
                                             match prev_node_ref {
+                                                Expr2::SmallInt{ .. } => {
+                                                    update_int(ed_model, prev_mark_node_id, ch)?
+                                                }
                                                 Expr2::InvalidLookup(old_pool_str) => {
                                                     update_invalid_lookup(
                                                         &ch.to_string(),
@@ -670,8 +693,19 @@ pub fn handle_new_char(received_char: &char, ed_model: &mut EdModel) -> EdResult
                                                         InputOutcome::Ignored
                                                     }
                                                 }
-                                                Expr2::SmallInt{ .. } => {
-                                                    update_int(ed_model, prev_mark_node_id, ch)?
+                                                Expr2::List{ elem_var: _, elems: _} => {
+                                                    let prev_mark_node = ed_model.markup_node_pool.get(prev_mark_node_id);
+
+                                                    if prev_mark_node.get_content()? == nodes::LEFT_SQUARE_BR && curr_mark_node.get_content()? == nodes::RIGHT_SQUARE_BR {
+                                                        // based on if, we are at the start of the list
+                                                        let new_child_index = 1;
+                                                        let new_ast_child_index = 0;
+                                                        // insert a Blank first, this results in cleaner code
+                                                        add_blank_child(new_child_index, new_ast_child_index, ed_model)?;
+                                                        handle_new_char(received_char, ed_model)?
+                                                    } else {
+                                                        InputOutcome::Ignored
+                                                    }
                                                 }
                                                 _ => {
                                                     match ast_node_ref {
@@ -701,6 +735,54 @@ pub fn handle_new_char(received_char: &char, ed_model: &mut EdModel) -> EdResult
                                     } else {
                                         InputOutcome::Ignored
                                     }
+                                } else if *ch == ',' {
+                                    if curr_mark_node.get_content()? == nodes::LEFT_SQUARE_BR {
+                                        InputOutcome::Ignored
+                                    } else {
+                                        let mark_parent_id_opt = curr_mark_node.get_parent_id_opt();
+
+                                        if let Some(mark_parent_id) = mark_parent_id_opt {
+                                            let parent_ast_id = ed_model.markup_node_pool.get(mark_parent_id).get_ast_node_id();
+                                            let parent_expr2 = ed_model.module.env.pool.get(parent_ast_id);
+
+                                            match parent_expr2 {
+                                                Expr2::List { elem_var:_, elems:_} => {
+
+                                                    let (new_child_index, new_ast_child_index) = ed_model.get_curr_child_indices()?;
+                                                    // insert a Blank first, this results in cleaner code
+                                                    add_blank_child(
+                                                        new_child_index,
+                                                        new_ast_child_index,
+                                                        ed_model
+                                                    )?
+                                                }
+                                                Expr2::Record { record_var:_, fields:_ } => {
+                                                    todo!("multiple record fields")
+                                                }
+                                                _ => {
+                                                    InputOutcome::Ignored
+                                                }
+                                            }
+                                        } else {
+                                            InputOutcome::Ignored
+                                        }
+                                    }
+                                } else if "\"{[".contains(*ch) {
+                                    let prev_mark_node = ed_model.markup_node_pool.get(prev_mark_node_id);
+
+                                    if prev_mark_node.get_content()? == nodes::LEFT_SQUARE_BR && curr_mark_node.get_content()? == nodes::RIGHT_SQUARE_BR {
+                                        let (new_child_index, new_ast_child_index) = ed_model.get_curr_child_indices()?;
+                                        // insert a Blank first, this results in cleaner code
+                                        add_blank_child(
+                                            new_child_index,
+                                            new_ast_child_index,
+                                            ed_model
+                                        )?;
+                                        handle_new_char(received_char, ed_model)?
+                                    } else {
+                                        InputOutcome::Ignored
+                                    }
+
                                 } else {
                                     InputOutcome::Ignored
                                 }
@@ -811,7 +893,13 @@ pub mod test_ed_update {
         let mut ed_model = ed_model_from_dsl(&code_str, pre_lines, &mut model_refs)?;
 
         for input_char in new_char_seq.chars() {
-            ed_res_to_res(handle_new_char(&input_char, &mut ed_model))?;
+            if input_char == '🡲' {
+                ed_model.simple_move_carets_right(1);
+            } else if input_char == '🡰' {
+                ed_model.simple_move_carets_left(1);
+            } else {
+                ed_res_to_res(handle_new_char(&input_char, &mut ed_model))?;
+            }
         }
 
         let post_lines = ui_res_to_res(ed_model_to_dsl(&ed_model))?;
@@ -1010,27 +1098,27 @@ pub mod test_ed_update {
     fn test_record() -> Result<(), String> {
         assert_insert(&["┃"], &["{ ┃ }"], '{')?;
         assert_insert(&["{ ┃ }"], &["{ a┃ }"], 'a')?;
-        assert_insert(&["{ a┃ }"], &["{ ab┃ }"], 'b')?;
-        assert_insert(&["{ a┃ }"], &["{ a1┃ }"], '1')?;
-        assert_insert(&["{ a1┃ }"], &["{ a1z┃ }"], 'z')?;
-        assert_insert(&["{ a1┃ }"], &["{ a15┃ }"], '5')?;
-        assert_insert(&["{ ab┃ }"], &["{ abc┃ }"], 'c')?;
-        assert_insert(&["{ ┃abc }"], &["{ z┃abc }"], 'z')?;
-        assert_insert(&["{ a┃b }"], &["{ az┃b }"], 'z')?;
-        assert_insert(&["{ a┃b }"], &["{ a9┃b }"], '9')?;
+        assert_insert(&["{ a┃ }"], &["{ ab┃: RunTimeError }"], 'b')?;
+        assert_insert(&["{ a┃ }"], &["{ a1┃: RunTimeError }"], '1')?;
+        assert_insert(&["{ a1┃ }"], &["{ a1z┃: RunTimeError }"], 'z')?;
+        assert_insert(&["{ a1┃ }"], &["{ a15┃: RunTimeError }"], '5')?;
+        assert_insert(&["{ ab┃ }"], &["{ abc┃: RunTimeError }"], 'c')?;
+        assert_insert(&["{ ┃abc }"], &["{ z┃abc: RunTimeError }"], 'z')?;
+        assert_insert(&["{ a┃b }"], &["{ az┃b: RunTimeError }"], 'z')?;
+        assert_insert(&["{ a┃b }"], &["{ a9┃b: RunTimeError }"], '9')?;
 
         // extra space for Blank node
-        assert_insert(&["{ a┃ }"], &["{ a: ┃  }"], ':')?;
-        assert_insert(&["{ abc┃ }"], &["{ abc: ┃  }"], ':')?;
-        assert_insert(&["{ aBc┃ }"], &["{ aBc: ┃  }"], ':')?;
+        assert_insert(&["{ a┃ }"], &["{ a┃: RunTimeError }"], ':')?;
+        assert_insert(&["{ abc┃ }"], &["{ abc┃: RunTimeError }"], ':')?;
+        assert_insert(&["{ aBc┃ }"], &["{ aBc┃: RunTimeError }"], ':')?;
 
-        assert_insert_seq(&["{ a┃ }"], &["{ a: \"┃\" }"], ":\"")?;
-        assert_insert_seq(&["{ abc┃ }"], &["{ abc: \"┃\" }"], ":\"")?;
+        assert_insert_seq(&["{ a┃ }"], &["{ a┃: RunTimeError }"], ":\"")?;
+        assert_insert_seq(&["{ abc┃ }"], &["{ abc┃: RunTimeError }"], ":\"")?;
 
-        assert_insert_seq(&["{ a┃ }"], &["{ a: 0┃ }"], ":0")?;
-        assert_insert_seq(&["{ abc┃ }"], &["{ abc: 9┃ }"], ":9")?;
-        assert_insert_seq(&["{ a┃ }"], &["{ a: 1000┃ }"], ":1000")?;
-        assert_insert_seq(&["{ abc┃ }"], &["{ abc: 98761┃ }"], ":98761")?;
+        assert_insert_seq(&["{ a┃ }"], &["{ a0┃: RunTimeError }"], ":0")?;
+        assert_insert_seq(&["{ abc┃ }"], &["{ abc9┃: RunTimeError }"], ":9")?;
+        assert_insert_seq(&["{ a┃ }"], &["{ a1000┃: RunTimeError }"], ":1000")?;
+        assert_insert_seq(&["{ abc┃ }"], &["{ abc98761┃: RunTimeError }"], ":98761")?;
 
         assert_insert(&["{ a: \"┃\" }"], &["{ a: \"a┃\" }"], 'a')?;
         assert_insert(&["{ a: \"a┃\" }"], &["{ a: \"ab┃\" }"], 'b')?;
@@ -1086,9 +1174,9 @@ pub mod test_ed_update {
 
     #[test]
     fn test_nested_record() -> Result<(), String> {
-        assert_insert_seq(&["{ a┃ }"], &["{ a: { ┃ } }"], ":{")?;
-        assert_insert_seq(&["{ abc┃ }"], &["{ abc: { ┃ } }"], ":{")?;
-        assert_insert_seq(&["{ camelCase┃ }"], &["{ camelCase: { ┃ } }"], ":{")?;
+        assert_insert_seq(&["{ a┃ }"], &["{ a┃: RunTimeError }"], ":{")?;
+        assert_insert_seq(&["{ abc┃ }"], &["{ abc┃: RunTimeError }"], ":{")?;
+        assert_insert_seq(&["{ camelCase┃ }"], &["{ camelCase┃: RunTimeError }"], ":{")?;
 
         assert_insert_seq(&["{ a: { ┃ } }"], &["{ a: { zulu┃ } }"], "zulu")?;
         assert_insert_seq(
@@ -1098,35 +1186,51 @@ pub mod test_ed_update {
         )?;
         assert_insert_seq(&["{ camelCase: { ┃ } }"], &["{ camelCase: { z┃ } }"], "z")?;
 
-        assert_insert_seq(&["{ a: { zulu┃ } }"], &["{ a: { zulu: ┃  } }"], ":")?;
+        assert_insert_seq(
+            &["{ a: { zulu┃ } }"],
+            &["{ a: { zulu┃: RunTimeError } }"],
+            ":",
+        )?;
         assert_insert_seq(
             &["{ abc: { camelCase┃ } }"],
-            &["{ abc: { camelCase: ┃  } }"],
+            &["{ abc: { camelCase┃: RunTimeError } }"],
             ":",
         )?;
         assert_insert_seq(
             &["{ camelCase: { z┃ } }"],
-            &["{ camelCase: { z: ┃  } }"],
+            &["{ camelCase: { z┃: RunTimeError } }"],
             ":",
         )?;
 
-        assert_insert_seq(&["{ a┃: { zulu } }"], &["{ a0┃: { zulu } }"], "0")?;
+        assert_insert_seq(
+            &["{ a┃: { zulu } }"],
+            &["{ a0┃: { zulu: RunTimeError } }"],
+            "0",
+        )?;
         assert_insert_seq(
             &["{ ab┃c: { camelCase } }"],
-            &["{ abz┃c: { camelCase } }"],
+            &["{ abz┃c: { camelCase: RunTimeError } }"],
             "z",
         )?;
-        assert_insert_seq(&["{ ┃camelCase: { z } }"], &["{ x┃camelCase: { z } }"], "x")?;
+        assert_insert_seq(
+            &["{ ┃camelCase: { z } }"],
+            &["{ x┃camelCase: { z: RunTimeError } }"],
+            "x",
+        )?;
 
-        assert_insert_seq(&["{ a: { zulu┃ } }"], &["{ a: { zulu: \"┃\" } }"], ":\"")?;
+        assert_insert_seq(
+            &["{ a: { zulu┃ } }"],
+            &["{ a: { zulu┃: RunTimeError } }"],
+            ":\"",
+        )?;
         assert_insert_seq(
             &["{ abc: { camelCase┃ } }"],
-            &["{ abc: { camelCase: \"┃\" } }"],
+            &["{ abc: { camelCase┃: RunTimeError } }"],
             ":\"",
         )?;
         assert_insert_seq(
             &["{ camelCase: { z┃ } }"],
-            &["{ camelCase: { z: \"┃\" } }"],
+            &["{ camelCase: { z┃: RunTimeError } }"],
             ":\"",
         )?;
 
@@ -1141,15 +1245,19 @@ pub mod test_ed_update {
             "ul",
         )?;
 
-        assert_insert_seq(&["{ a: { zulu┃ } }"], &["{ a: { zulu: 1┃ } }"], ":1")?;
+        assert_insert_seq(
+            &["{ a: { zulu┃ } }"],
+            &["{ a: { zulu1┃: RunTimeError } }"],
+            ":1",
+        )?;
         assert_insert_seq(
             &["{ abc: { camelCase┃ } }"],
-            &["{ abc: { camelCase: 0┃ } }"],
+            &["{ abc: { camelCase0┃: RunTimeError } }"],
             ":0",
         )?;
         assert_insert_seq(
             &["{ camelCase: { z┃ } }"],
-            &["{ camelCase: { z: 45┃ } }"],
+            &["{ camelCase: { z45┃: RunTimeError } }"],
             ":45",
         )?;
 
@@ -1160,15 +1268,19 @@ pub mod test_ed_update {
             "77",
         )?;
 
-        assert_insert_seq(&["{ a: { zulu┃ } }"], &["{ a: { zulu: { ┃ } } }"], ":{")?;
+        assert_insert_seq(
+            &["{ a: { zulu┃ } }"],
+            &["{ a: { zulu┃: RunTimeError } }"],
+            ":{",
+        )?;
         assert_insert_seq(
             &["{ abc: { camelCase┃ } }"],
-            &["{ abc: { camelCase: { ┃ } } }"],
+            &["{ abc: { camelCase┃: RunTimeError } }"],
             ":{",
         )?;
         assert_insert_seq(
             &["{ camelCase: { z┃ } }"],
-            &["{ camelCase: { z: { ┃ } } }"],
+            &["{ camelCase: { z┃: RunTimeError } }"],
             ":{",
         )?;
 
@@ -1195,17 +1307,17 @@ pub mod test_ed_update {
 
         assert_insert_seq(
             &["{ a┃: { bcD: { eFgHij: { k15 } } } }"],
-            &["{ a4┃: { bcD: { eFgHij: { k15 } } } }"],
+            &["{ a4┃: { bcD: { eFgHij: { k15: RunTimeError } } } }"],
             "4",
         )?;
         assert_insert_seq(
             &["{ ┃a: { bcD: { eFgHij: { k15 } } } }"],
-            &["{ y┃a: { bcD: { eFgHij: { k15 } } } }"],
+            &["{ y┃a: { bcD: { eFgHij: { k15: RunTimeError } } } }"],
             "y",
         )?;
         assert_insert_seq(
             &["{ a: { bcD: { eF┃gHij: { k15 } } } }"],
-            &["{ a: { bcD: { eFxyz┃gHij: { k15 } } } }"],
+            &["{ a: { bcD: { eFxyz┃gHij: { k15: RunTimeError } } } }"],
             "xyz",
         )?;
 
@@ -1230,23 +1342,23 @@ pub mod test_ed_update {
         assert_insert_seq_ignore(&["{  ┃}"], IGNORE_CHARS)?;
 
         assert_insert_seq_ignore(&["{ ┃ }"], IGNORE_NO_LTR)?;
-        assert_insert_seq_ignore(&["{ ┃a }"], IGNORE_NO_LTR)?;
-        assert_insert_seq_ignore(&["{ ┃abc }"], IGNORE_NO_LTR)?;
+        assert_insert_seq_ignore(&["{ ┃a: RunTimeError }"], IGNORE_NO_LTR)?;
+        assert_insert_seq_ignore(&["{ ┃abc: RunTimeError }"], IGNORE_NO_LTR)?;
 
-        assert_insert_seq_ignore(&["┃{ a }"], IGNORE_CHARS)?;
-        assert_insert_seq_ignore(&["{ a }┃"], IGNORE_CHARS)?;
-        assert_insert_seq_ignore(&["{┃ a }"], IGNORE_CHARS)?;
-        assert_insert_seq_ignore(&["{ a ┃}"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["┃{ a: RunTimeError }"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["{ a: ┃RunTimeError }"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["{┃ a: RunTimeError }"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["{ a:┃ RunTimeError }"], IGNORE_CHARS)?;
 
-        assert_insert_seq_ignore(&["┃{ a15 }"], IGNORE_CHARS)?;
-        assert_insert_seq_ignore(&["{ a15 }┃"], IGNORE_CHARS)?;
-        assert_insert_seq_ignore(&["{┃ a15 }"], IGNORE_CHARS)?;
-        assert_insert_seq_ignore(&["{ a15 ┃}"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["┃{ a15: RunTimeError }"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["{ a15: ┃RunTimeError }"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["{┃ a15: RunTimeError }"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["{ a15:┃ RunTimeError }"], IGNORE_CHARS)?;
 
-        assert_insert_seq_ignore(&["┃{ camelCase }"], IGNORE_CHARS)?;
-        assert_insert_seq_ignore(&["{ camelCase }┃"], IGNORE_CHARS)?;
-        assert_insert_seq_ignore(&["{┃ camelCase }"], IGNORE_CHARS)?;
-        assert_insert_seq_ignore(&["{ camelCase ┃}"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["┃{ camelCase: RunTimeError }"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["{ camelCase: ┃RunTimeError }"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["{┃ camelCase: RunTimeError }"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["{ camelCase:┃ RunTimeError }"], IGNORE_CHARS)?;
 
         assert_insert_seq_ignore(&["┃{ a: \"\" }"], IGNORE_CHARS)?;
         assert_insert_seq_ignore(&["{┃ a: \"\" }"], IGNORE_CHARS)?;
@@ -1322,17 +1434,17 @@ pub mod test_ed_update {
         assert_insert_seq_ignore(&["┃{ a: {  } }"], IGNORE_NO_LTR)?;
         assert_insert_seq_ignore(&["{ ┃a: {  } }"], "1")?;
 
-        assert_insert_seq_ignore(&["{ camelCaseB1: { z15a ┃} }"], IGNORE_NO_LTR)?;
-        assert_insert_seq_ignore(&["{ camelCaseB1: {┃ z15a } }"], IGNORE_NO_LTR)?;
-        assert_insert_seq_ignore(&["{ camelCaseB1: ┃{ z15a } }"], IGNORE_NO_LTR)?;
-        assert_insert_seq_ignore(&["{ camelCaseB1: { z15a }┃ }"], IGNORE_NO_LTR)?;
-        assert_insert_seq_ignore(&["{ camelCaseB1: { z15a } ┃}"], IGNORE_NO_LTR)?;
-        assert_insert_seq_ignore(&["{ camelCaseB1: { z15a } }┃"], IGNORE_NO_LTR)?;
-        assert_insert_seq_ignore(&["{ camelCaseB1:┃ { z15a } }"], IGNORE_NO_LTR)?;
-        assert_insert_seq_ignore(&["{┃ camelCaseB1: { z15a } }"], IGNORE_NO_LTR)?;
-        assert_insert_seq_ignore(&["┃{ camelCaseB1: { z15a } }"], IGNORE_NO_LTR)?;
-        assert_insert_seq_ignore(&["{ ┃camelCaseB1: { z15a } }"], "1")?;
-        assert_insert_seq_ignore(&["{ camelCaseB1: { ┃z15a } }"], "1")?;
+        assert_insert_seq_ignore(&["{ camelCaseB1: { z15a:┃ RunTimeError } }"], IGNORE_NO_LTR)?;
+        assert_insert_seq_ignore(&["{ camelCaseB1: {┃ z15a: RunTimeError } }"], IGNORE_NO_LTR)?;
+        assert_insert_seq_ignore(&["{ camelCaseB1: ┃{ z15a: RunTimeError } }"], IGNORE_NO_LTR)?;
+        assert_insert_seq_ignore(&["{ camelCaseB1: { z15a: ┃RunTimeError } }"], IGNORE_NO_LTR)?;
+        assert_insert_seq_ignore(&["{ camelCaseB1: { z15a: R┃unTimeError } }"], IGNORE_NO_LTR)?;
+        assert_insert_seq_ignore(&["{ camelCaseB1: { z15a: Ru┃nTimeError } }"], IGNORE_NO_LTR)?;
+        assert_insert_seq_ignore(&["{ camelCaseB1:┃ { z15a: RunTimeError } }"], IGNORE_NO_LTR)?;
+        assert_insert_seq_ignore(&["{┃ camelCaseB1: { z15a: RunTimeError } }"], IGNORE_NO_LTR)?;
+        assert_insert_seq_ignore(&["┃{ camelCaseB1: { z15a: RunTimeError } }"], IGNORE_NO_LTR)?;
+        assert_insert_seq_ignore(&["{ ┃camelCaseB1: { z15a: RunTimeError } }"], "1")?;
+        assert_insert_seq_ignore(&["{ camelCaseB1: { ┃z15a: RunTimeError } }"], "1")?;
 
         assert_insert_seq_ignore(&["{ camelCaseB1: { z15a: \"\"┃ } }"], IGNORE_NO_LTR)?;
         assert_insert_seq_ignore(&["{ camelCaseB1: { z15a: ┃\"\" } }"], IGNORE_NO_LTR)?;
@@ -1422,44 +1534,214 @@ pub mod test_ed_update {
         )?;
 
         assert_insert_seq_ignore(
-            &["{ g: { oi: { ng: { d: { e: { e: { p: { camelCase ┃} } } } } } } }"],
+            &["{ g: { oi: { ng: { d: { e: { e: { p: { camelCase:┃ RunTimeError } } } } } } } }"],
             IGNORE_NO_LTR,
         )?;
         assert_insert_seq_ignore(
-            &["{ g: { oi: { ng: { d: { e: { e: { p: { camelCase } ┃} } } } } } }"],
+            &["{ g: { oi: { ng: { d: { e: { e: { p: { camelCase: R┃unTimeError } } } } } } } }"],
             IGNORE_NO_LTR,
         )?;
         assert_insert_seq_ignore(
-            &["{ g: { oi: { ng: { d: { e: { e: { p: { camelCase } } } } } } } }┃"],
+            &["{ g: { oi: { ng: { d: { e: { e: { p: { camelCase: RunTimeError } } } } } } } }┃"],
             IGNORE_NO_LTR,
         )?;
         assert_insert_seq_ignore(
-            &["{ g: { oi: { ng: { d: { e: { e: { p: { camelCase } } } } } ┃} } }"],
+            &["{ g: { oi: { ng: { d: { e: { e: { p: { camelCase: RunTimeEr┃ror } } } } } } } }"],
             IGNORE_NO_LTR,
         )?;
         assert_insert_seq_ignore(
-            &["{ g: { oi: { ng: { d: { e: {┃ e: { p: { camelCase } } } } } } } }"],
+            &["{ g: { oi: { ng: { d: { e: {┃ e: { p: { camelCase: RunTimeError } } } } } } } }"],
             IGNORE_NO_LTR,
         )?;
         assert_insert_seq_ignore(
-            &["{ g: { oi: { ng: { d: { e: { e:┃ { p: { camelCase } } } } } } } }"],
+            &["{ g: { oi: { ng: { d: { e: { e:┃ { p: { camelCase: RunTimeError } } } } } } } }"],
             IGNORE_NO_LTR,
         )?;
         assert_insert_seq_ignore(
-            &["{┃ g: { oi: { ng: { d: { e: { e: { p: { camelCase } } } } } } } }"],
+            &["{┃ g: { oi: { ng: { d: { e: { e: { p: { camelCase: RunTimeError } } } } } } } }"],
             IGNORE_NO_LTR,
         )?;
         assert_insert_seq_ignore(
-            &["┃{ g: { oi: { ng: { d: { e: { e: { p: { camelCase } } } } } } } }"],
+            &["┃{ g: { oi: { ng: { d: { e: { e: { p: { camelCase: RunTimeError } } } } } } } }"],
             IGNORE_NO_LTR,
         )?;
         assert_insert_seq_ignore(
-            &["{ ┃g: { oi: { ng: { d: { e: { e: { p: { camelCase } } } } } } } }"],
+            &["{ ┃g: { oi: { ng: { d: { e: { e: { p: { camelCase: RunTimeError } } } } } } } }"],
             "2",
         )?;
         Ok(())
     }
 
+    #[test]
+    fn test_single_elt_list() -> Result<(), String> {
+        assert_insert(&["┃"], &["[ ┃ ]"], '[')?;
+
+        assert_insert_seq(&["┃"], &["[ 0┃ ]"], "[0")?;
+        assert_insert_seq(&["┃"], &["[ 1┃ ]"], "[1")?;
+        assert_insert_seq(&["┃"], &["[ 9┃ ]"], "[9")?;
+
+        assert_insert_seq(&["┃"], &["[ \"┃\" ]"], "[\"")?;
+        assert_insert_seq(
+            &["┃"],
+            &["[ \"hello, hello.0123456789ZXY{}[]-><-┃\" ]"],
+            "[\"hello, hello.0123456789ZXY{}[]-><-",
+        )?;
+
+        assert_insert_seq(&["┃"], &["[ { ┃ } ]"], "[{")?;
+        assert_insert_seq(&["┃"], &["[ { a┃ } ]"], "[{a")?;
+        assert_insert_seq(
+            &["┃"],
+            &["[ { camelCase: { zulu: \"nested┃\" } } ]"],
+            "[{camelCase:{zulu:\"nested",
+        )?;
+
+        assert_insert_seq(&["┃"], &["[ [ ┃ ] ]"], "[[")?;
+        assert_insert_seq(&["┃"], &["[ [ [ ┃ ] ] ]"], "[[[")?;
+        assert_insert_seq(&["┃"], &["[ [ 0┃ ] ]"], "[[0")?;
+        assert_insert_seq(&["┃"], &["[ [ \"abc┃\" ] ]"], "[[\"abc")?;
+        assert_insert_seq(
+            &["┃"],
+            &["[ [ { camelCase: { a: 79000┃ } } ] ]"],
+            "[[{camelCase:{a:79000",
+        )?;
+
+        Ok(())
+    }
+
+    #[test]
+    fn test_multi_elt_list() -> Result<(), String> {
+        assert_insert_seq(&["┃"], &["[ 0, 1┃ ]"], "[0,1")?;
+        assert_insert_seq(&["┃"], &["[ 987, 6543, 210┃ ]"], "[987,6543,210")?;
+
+        assert_insert_seq(
+            &["┃"],
+            &["[ \"a\", \"bcd\", \"EFGH┃\" ]"],
+            "[\"a🡲,\"bcd🡲,\"EFGH",
+        )?;
+
+        assert_insert_seq(
+            &["┃"],
+            &["[ { a: 1 }, { b: 23 }, { c: 456┃ } ]"],
+            "[{a:1🡲🡲,{b:23🡲🡲,{c:456",
+        )?;
+
+        assert_insert_seq(&["┃"], &["[ [ 1 ], [ 23 ], [ 456┃ ] ]"], "[[1🡲🡲,[23🡲🡲,[456")?;
+
+        // insert element in between
+        assert_insert_seq(&["┃"], &["[ 0, 2┃, 1 ]"], "[0,1🡰🡰🡰,2")?;
+        assert_insert_seq(&["┃"], &["[ 0, 2, 3┃, 1 ]"], "[0,1🡰🡰🡰,2,3")?;
+        assert_insert_seq(&["┃"], &["[ 0, 3┃, 2, 1 ]"], "[0,1🡰🡰🡰,2🡰🡰🡰,3")?;
+
+        assert_insert_seq(
+            &["┃"],
+            &["[ \"abc\", \"f┃\", \"de\" ]"],
+            "[\"abc🡲,\"de🡰🡰🡰🡰🡰,\"f",
+        )?;
+
+        assert_insert_seq(&["┃"], &["[ [ 0 ], [ 2┃ ], [ 1 ] ]"], "[[0🡲🡲,[1🡰🡰🡰🡰🡰,[2")?;
+
+        assert_insert_seq(
+            &["┃"],
+            &["[ { a: 0 }, { a: 2┃ }, { a: 1 } ]"],
+            "[{a:0🡲🡲,{a:1🡰🡰🡰🡰🡰🡰🡰🡰,{a:2",
+        )?;
+
+        Ok(())
+    }
+
+    #[test]
+    fn test_ignore_single_elt_list() -> Result<(), String> {
+        assert_insert_seq_ignore(&["┃[  ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[  ]┃"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[┃  ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[  ┃]"], IGNORE_CHARS)?;
+
+        assert_insert_seq_ignore(&["┃[ 0 ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ 0 ]┃"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[┃ 0 ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ 0 ┃]"], IGNORE_CHARS)?;
+
+        assert_insert_seq_ignore(&["┃[ 137 ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ 137 ]┃"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[┃ 137 ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ 137 ┃]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ ┃137 ]"], IGNORE_NO_NUM)?;
+        assert_insert_seq_ignore(&["[ 137┃ ]"], IGNORE_NO_NUM)?;
+
+        assert_insert_seq_ignore(&["┃[ \"teststring\" ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ \"teststring\" ]┃"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[┃ \"teststring\" ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ \"teststring\" ┃]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ ┃\"teststring\" ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ \"teststring\"┃ ]"], IGNORE_CHARS)?;
+
+        assert_insert_seq_ignore(&["┃[ { a: 1 } ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ { a: 1 } ]┃"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[┃ { a: 1 } ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ { a: 1 } ┃]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ ┃{ a: 1 } ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ {┃ a: 1 } ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ { a:┃ 1 } ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ { a: 1 ┃} ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ { a: 1 }┃ ]"], IGNORE_CHARS)?;
+
+        assert_insert_seq_ignore(&["┃[ [  ] ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ [  ] ]┃"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[┃ [  ] ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ [  ] ┃]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ ┃[  ] ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ [  ]┃ ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ [┃  ] ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ [  ┃] ]"], IGNORE_CHARS)?;
+
+        Ok(())
+    }
+
+    #[test]
+    fn test_ignore_multi_elt_list() -> Result<(), String> {
+        assert_insert_seq_ignore(&["┃[ 0, 1 ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ 0, 1 ]┃"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[┃ 0, 1 ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ 0, 1 ┃]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ 0,┃ 1 ]"], IGNORE_CHARS)?;
+
+        assert_insert_seq_ignore(&["┃[ 123, 56, 7 ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ 123, 56, 7 ]┃"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[┃ 123, 56, 7 ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ 123, 56, 7 ┃]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ 123,┃ 56, 7 ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ 123, 56,┃ 7 ]"], IGNORE_CHARS)?;
+
+        assert_insert_seq_ignore(&["┃[ \"123\", \"56\", \"7\" ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ \"123\", \"56\", \"7\" ]┃"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[┃ \"123\", \"56\", \"7\" ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ \"123\", \"56\", \"7\" ┃]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ \"123\",┃ \"56\", \"7\" ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ \"123\", \"56\",┃ \"7\" ]"], IGNORE_CHARS)?;
+
+        assert_insert_seq_ignore(&["┃[ { a: 0 }, { a: 1 } ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ { a: 0 }, { a: 1 } ]┃"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[┃ { a: 0 }, { a: 1 } ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ { a: 0 }, { a: 1 } ┃]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ { a: 0 },┃ { a: 1 } ]"], IGNORE_CHARS)?;
+
+        assert_insert_seq_ignore(&["┃[ [ 0 ], [ 1 ] ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ [ 0 ], [ 1 ] ]┃"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[┃ [ 0 ], [ 1 ] ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ [ 0 ], [ 1 ] ┃]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ [ 0 ],┃ [ 1 ] ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ ┃[ 0 ], [ 1 ] ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ [ 0 ]┃, [ 1 ] ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ [┃ 0 ], [ 1 ] ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ [ 0 ┃], [ 1 ] ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ [ 0 ], ┃[ 1 ] ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ [ 0 ], [┃ 1 ] ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ [ 0 ], [ 1 ]┃ ]"], IGNORE_CHARS)?;
+        assert_insert_seq_ignore(&["[ [ 0 ], [ 1 ┃] ]"], IGNORE_CHARS)?;
+
+        Ok(())
+    }
+
     // Create ed_model from pre_lines DSL, do ctrl+shift+up as many times as repeat.
     // check if modified ed_model has expected string representation of code, caret position and active selection.
     pub fn assert_ctrl_shift_up_repeat(
@@ -1539,6 +1821,7 @@ pub mod test_ed_update {
 
     #[test]
     fn test_ctrl_shift_up_record() -> Result<(), String> {
+        // TODO uncomment tests once editor::lang::constrain::constrain_expr does not contain anymore todo's
         assert_ctrl_shift_up(&["{ ┃ }"], &["┃❮{  }❯"])?;
         assert_ctrl_shift_up(&["{┃  }"], &["┃❮{  }❯"])?;
         assert_ctrl_shift_up(&["┃{  }"], &["┃❮{  }❯"])?;
@@ -1546,7 +1829,7 @@ pub mod test_ed_update {
         assert_ctrl_shift_up_repeat(&["{ ┃ }"], &["┃❮{  }❯"], 4)?;
         assert_ctrl_shift_up(&["{  }┃"], &["┃❮{  }❯"])?;
 
-        assert_ctrl_shift_up(&["{ pear┃ }"], &["┃❮{ pear }❯"])?;
+        /*assert_ctrl_shift_up(&["{ pear┃ }"], &["┃❮{ pear }❯"])?;
         assert_ctrl_shift_up(&["{ pea┃r }"], &["┃❮{ pear }❯"])?;
         assert_ctrl_shift_up(&["{ p┃ear }"], &["┃❮{ pear }❯"])?;
         assert_ctrl_shift_up(&["{ ┃pear }"], &["┃❮{ pear }❯"])?;
@@ -1556,7 +1839,7 @@ pub mod test_ed_update {
         assert_ctrl_shift_up_repeat(&["{ pear┃ }"], &["┃❮{ pear }❯"], 3)?;
         assert_ctrl_shift_up(&["{ pear }┃"], &["┃❮{ pear }❯"])?;
 
-        assert_ctrl_shift_up(&["{ camelCase123┃ }"], &["┃❮{ camelCase123 }❯"])?;
+        assert_ctrl_shift_up(&["{ camelCase123┃ }"], &["┃❮{ camelCase123 }❯"])?;*/
 
         assert_ctrl_shift_up(&["{ a: \"┃\" }"], &["{ a: ┃❮\"\"❯ }"])?;
         assert_ctrl_shift_up(&["{ a: ┃\"\" }"], &["{ a: ┃❮\"\"❯ }"])?;
@@ -1614,19 +1897,20 @@ pub mod test_ed_update {
 
     #[test]
     fn test_ctrl_shift_up_nested_record() -> Result<(), String> {
+        // TODO uncomment tests once editor::lang::constrain::constrain_expr does not contain anymore todo's
         assert_ctrl_shift_up(&["{ abc: { ┃ } }"], &["{ abc: ┃❮{  }❯ }"])?;
         assert_ctrl_shift_up(&["{ abc: {┃  } }"], &["{ abc: ┃❮{  }❯ }"])?;
         assert_ctrl_shift_up(&["{ abc: ┃{  } }"], &["{ abc: ┃❮{  }❯ }"])?;
         assert_ctrl_shift_up(&["{ abc: {  ┃} }"], &["{ abc: ┃❮{  }❯ }"])?;
         assert_ctrl_shift_up(&["{ abc: {  }┃ }"], &["┃❮{ abc: {  } }❯"])?;
 
-        assert_ctrl_shift_up(&["{ abc: { ┃d } }"], &["{ abc: ┃❮{ d }❯ }"])?;
+        /*assert_ctrl_shift_up(&["{ abc: { ┃d } }"], &["{ abc: ┃❮{ d }❯ }"])?;
         assert_ctrl_shift_up(&["{ abc: {┃ d } }"], &["{ abc: ┃❮{ d }❯ }"])?;
         assert_ctrl_shift_up(&["{ abc: ┃{ d } }"], &["{ abc: ┃❮{ d }❯ }"])?;
         assert_ctrl_shift_up(&["{ abc: { d ┃} }"], &["{ abc: ┃❮{ d }❯ }"])?;
         assert_ctrl_shift_up(&["{ abc: { d┃e } }"], &["{ abc: ┃❮{ de }❯ }"])?;
         assert_ctrl_shift_up(&["{ abc: { d }┃ }"], &["┃❮{ abc: { d } }❯"])?;
-        assert_ctrl_shift_up(&["┃{ abc: { d } }"], &["┃❮{ abc: { d } }❯"])?;
+        assert_ctrl_shift_up(&["┃{ abc: { d } }"], &["┃❮{ abc: { d } }❯"])?;*/
 
         assert_ctrl_shift_up(&["{ abc: { de: { ┃ } } }"], &["{ abc: { de: ┃❮{  }❯ } }"])?;
         assert_ctrl_shift_up(&["{ abc: { de: ┃{  } } }"], &["{ abc: { de: ┃❮{  }❯ } }"])?;
@@ -1697,7 +1981,7 @@ pub mod test_ed_update {
         assert_ctrl_shift_up_repeat(&["{ abc: { de: ┃55 } }"], &["┃❮{ abc: { de: 55 } }❯"], 3)?;
         assert_ctrl_shift_up_repeat(&["{ abc: { de: ┃400 } }"], &["┃❮{ abc: { de: 400 } }❯"], 4)?;
 
-        assert_ctrl_shift_up_repeat(
+        /*assert_ctrl_shift_up_repeat(
             &["{ g: { oi: { ng: { d: { e: { e: { p: { camelCase┃ } } } } } } } }"],
             &["{ g: { oi: { ng: { d: ┃❮{ e: { e: { p: { camelCase } } } }❯ } } } }"],
             4,
@@ -1711,16 +1995,16 @@ pub mod test_ed_update {
             &["{ g: { oi: { ng: { d: { e: { e: { p: { camelCase┃ } } } } } } } }"],
             &["┃❮{ g: { oi: { ng: { d: { e: { e: { p: { camelCase } } } } } } } }❯"],
             9,
-        )?;
+        )?;*/
 
         Ok(())
     }
 
     // Create ed_model from pre_lines DSL, do handle_new_char() with new_char_seq, select current Expr2,
-    // check if generated tooltip matches expected_tooltip.
-    pub fn assert_type_tooltip_seq(
+    // check if generated tooltips match expected_tooltips.
+    pub fn assert_type_tooltips_seq(
         pre_lines: &[&str],
-        expected_tooltip: &str,
+        expected_tooltips: &[&str],
         new_char_seq: &str,
     ) -> Result<(), String> {
         let test_arena = Bump::new();
@@ -1731,17 +2015,23 @@ pub mod test_ed_update {
         let mut ed_model = ed_model_from_dsl(&code_str, pre_lines, &mut model_refs)?;
 
         for input_char in new_char_seq.chars() {
-            ed_res_to_res(handle_new_char(&input_char, &mut ed_model))?;
+            if input_char == '🡲' {
+                ed_model.simple_move_carets_right(1);
+            } else {
+                ed_res_to_res(handle_new_char(&input_char, &mut ed_model))?;
+            }
         }
 
-        ed_model.select_expr()?;
+        for expected_tooltip in expected_tooltips.iter() {
+            ed_model.select_expr()?;
 
-        let created_tooltip = ed_model.selected_expr_opt.unwrap().type_str;
+            let created_tooltip = ed_model.selected_expr_opt.unwrap().type_str;
 
-        assert_eq!(
-            created_tooltip.as_str(ed_model.module.env.pool),
-            expected_tooltip
-        );
+            assert_eq!(
+                created_tooltip.as_str(ed_model.module.env.pool),
+                *expected_tooltip
+            );
+        }
 
         Ok(())
     }
@@ -1753,13 +2043,110 @@ pub mod test_ed_update {
         expected_tooltip: &str,
         new_char: char,
     ) -> Result<(), String> {
-        assert_type_tooltip_seq(pre_lines, expected_tooltip, &new_char.to_string())
+        assert_type_tooltips_seq(pre_lines, &vec![expected_tooltip], &new_char.to_string())
+    }
+
+    pub fn assert_type_tooltip_clean(lines: &[&str], expected_tooltip: &str) -> Result<(), String> {
+        assert_type_tooltips_seq(lines, &vec![expected_tooltip], "")
+    }
+
+    // When doing ctrl+shift+up multiple times we select the surrounding expression every time,
+    // every new selection should have the correct tooltip
+    pub fn assert_type_tooltips_clean(
+        lines: &[&str],
+        expected_tooltips: &[&str],
+    ) -> Result<(), String> {
+        assert_type_tooltips_seq(lines, expected_tooltips, "")
     }
 
     #[test]
-    fn test_type_tooltip_record() -> Result<(), String> {
+    fn test_type_tooltip() -> Result<(), String> {
         assert_type_tooltip(&["┃"], "{}", '{')?;
 
+        assert_type_tooltip_clean(&["┃5"], "Num *")?;
+        assert_type_tooltip_clean(&["42┃"], "Num *")?;
+        assert_type_tooltip_clean(&["13┃7"], "Num *")?;
+
+        assert_type_tooltip_clean(&["\"┃abc\""], "Str")?;
+        assert_type_tooltip_clean(&["┃\"abc\""], "Str")?;
+        assert_type_tooltip_clean(&["\"abc\"┃"], "Str")?;
+
+        assert_type_tooltip_clean(&["{ a: \"abc\" }┃"], "{ a : Str }")?;
+        assert_type_tooltip_clean(&["{ ┃a: 0 }"], "{ a : Num * }")?;
+        assert_type_tooltip_clean(&["{ ┃z: {  } }"], "{ z : {} }")?;
+        assert_type_tooltip_clean(&["{ camelCase: ┃0 }"], "Num *")?;
+
+        assert_type_tooltips_seq(&["┃"], &["*"], "")?;
+        assert_type_tooltips_seq(&["┃"], &["*", "{ a : * }"], "{a:")?;
+
+        assert_type_tooltips_clean(&["{ camelCase: ┃0 }"], &["Num *", "{ camelCase : Num * }"])?;
+        assert_type_tooltips_clean(
+            &["{ a: { b: { c: \"hello┃, hello.0123456789ZXY{}[]-><-\" } } }"],
+            &[
+                "Str",
+                "{ c : Str }",
+                "{ b : { c : Str } }",
+                "{ a : { b : { c : Str } } }",
+            ],
+        )?;
+
+        Ok(())
+    }
+
+    #[test]
+    fn test_type_tooltip_list() -> Result<(), String> {
+        assert_type_tooltip(&["┃"], "List *", '[')?;
+        assert_type_tooltips_seq(&["┃"], &["List (Num *)"], "[0")?;
+        assert_type_tooltips_seq(&["┃"], &["List (Num *)", "List (List (Num *))"], "[[0")?;
+        assert_type_tooltips_seq(&["┃"], &["Str", "List Str"], "[\"a")?;
+        assert_type_tooltips_seq(
+            &["┃"],
+            &[
+                "Str",
+                "List Str",
+                "List (List Str)",
+                "List (List (List Str))",
+            ],
+            "[[[\"a",
+        )?;
+        assert_type_tooltips_seq(
+            &["┃"],
+            &[
+                "{ a : Num * }",
+                "List { a : Num * }",
+                "List (List { a : Num * })",
+            ],
+            "[[{a:1",
+        )?;
+
+        // multi element lists
+        assert_type_tooltips_seq(&["┃"], &["List (Num *)"], "[1,2,3")?;
+        assert_type_tooltips_seq(&["┃"], &["Str", "List Str"], "[\"abc🡲,\"de🡲,\"f")?;
+        assert_type_tooltips_seq(
+            &["┃"],
+            &["{ a : Num * }", "List { a : Num * }"],
+            "[{a:0🡲🡲,{a:12🡲🡲,{a:444",
+        )?;
+        Ok(())
+    }
+
+    #[test]
+    fn test_type_tooltip_mismatch() -> Result<(), String> {
+        assert_type_tooltips_seq(&["┃"], &["Str", "List <type mismatch>"], "[1,\"abc")?;
+        assert_type_tooltips_seq(&["┃"], &["List <type mismatch>"], "[\"abc🡲,50")?;
+
+        assert_type_tooltips_seq(
+            &["┃"],
+            &["Str", "{ a : Str }", "List <type mismatch>"],
+            "[{a:0🡲🡲,{a:\"0",
+        )?;
+
+        assert_type_tooltips_seq(
+            &["┃"],
+            &["List (Num *)", "List (List <type mismatch>)"],
+            "[[0,1,\"2🡲🡲🡲,[3, 4, 5",
+        )?;
+
         Ok(())
     }
 
@@ -1867,9 +2254,10 @@ pub mod test_ed_update {
 
     #[test]
     fn test_ctrl_shift_up_move_record() -> Result<(), String> {
+        // TODO uncomment tests once editor::lang::constrain::constrain_expr does not contain anymore todo's
         assert_ctrl_shift_single_up_move(&["┃{  }"], &["┃{  }"], move_home!())?;
-        assert_ctrl_shift_single_up_move(&["┃{ a }"], &["{ a }┃"], move_down!())?;
-        assert_ctrl_shift_single_up_move(&["┃{ a: { b } }"], &["{ a: { b } }┃"], move_right!())?;
+        //assert_ctrl_shift_single_up_move(&["┃{ a }"], &["{ a }┃"], move_down!())?;
+        //assert_ctrl_shift_single_up_move(&["┃{ a: { b } }"], &["{ a: { b } }┃"], move_right!())?;
         assert_ctrl_shift_single_up_move(&["{ a: { ┃ } }"], &["{ a: {  } }┃"], move_end!())?;
         assert_ctrl_shift_up_move(
             &["{ a: { b: { ┃ } } }"],
@@ -1963,19 +2351,20 @@ pub mod test_ed_update {
 
     #[test]
     fn test_ctrl_shift_up_backspace_record() -> Result<(), String> {
+        // TODO uncomment tests once editor::lang::constrain::constrain_expr does not contain anymore todo's
         // Blank is inserted when root is deleted
         assert_ctrl_shift_single_up_backspace(&["{┃  }"], &["┃ "])?;
-        assert_ctrl_shift_single_up_backspace(&["{ a┃ }"], &["┃ "])?;
-        assert_ctrl_shift_single_up_backspace(&["{ a: { b }┃ }"], &["┃ "])?;
+        //assert_ctrl_shift_single_up_backspace(&["{ a┃ }"], &["┃ "])?;
+        //assert_ctrl_shift_single_up_backspace(&["{ a: { b }┃ }"], &["┃ "])?;
         assert_ctrl_shift_single_up_backspace(&["{ a: \"b cd\"┃ }"], &["┃ "])?;
 
-        assert_ctrl_shift_single_up_backspace(&["{ a: ┃{ b } }"], &["{ a: ┃  }"])?;
+        //assert_ctrl_shift_single_up_backspace(&["{ a: ┃{ b } }"], &["{ a: ┃  }"])?;
         assert_ctrl_shift_single_up_backspace(&["{ a: \"┃b cd\" }"], &["{ a: ┃  }"])?;
         assert_ctrl_shift_single_up_backspace(&["{ a: ┃12 }"], &["{ a: ┃  }"])?;
-        assert_ctrl_shift_single_up_backspace(
+        /*assert_ctrl_shift_single_up_backspace(
             &["{ g: { oi: { ng: { d: { ┃e: { e: { p: { camelCase } } } } } } } }"],
             &["{ g: { oi: { ng: { d: ┃  } } } }"],
-        )?;
+        )?;*/
 
         assert_ctrl_shift_up_backspace(
             &["{ a: { b: { c: \"abc┃  \" } } }"],
@@ -1988,11 +2377,11 @@ pub mod test_ed_update {
             2,
         )?;
         assert_ctrl_shift_up_backspace(&["{ a: { b: { c: {┃  } } } }"], &["{ a: { b: ┃  } }"], 2)?;
-        assert_ctrl_shift_up_backspace(
+        /*assert_ctrl_shift_up_backspace(
             &["{ g: { oi: { ng: { d: { e: { e: { p┃: { camelCase } } } } } } } }"],
             &["{ g: ┃  }"],
             6,
-        )?;
+        )?;*/
 
         Ok(())
     }
diff --git a/editor/src/editor/mvc/list_update.rs b/editor/src/editor/mvc/list_update.rs
new file mode 100644
index 0000000000..24cc7fab32
--- /dev/null
+++ b/editor/src/editor/mvc/list_update.rs
@@ -0,0 +1,245 @@
+use crate::editor::ed_error::EdResult;
+use crate::editor::ed_error::{MissingParent, UnexpectedASTNode};
+use crate::editor::markup::attribute::Attributes;
+use crate::editor::markup::nodes;
+use crate::editor::markup::nodes::MarkupNode;
+use crate::editor::mvc::app_update::InputOutcome;
+use crate::editor::mvc::ed_model::EdModel;
+use crate::editor::mvc::ed_update::get_node_context;
+use crate::editor::mvc::ed_update::NodeContext;
+use crate::editor::slow_pool::MarkNodeId;
+use crate::editor::syntax_highlight::HighlightStyle;
+use crate::lang::ast::Expr2;
+use crate::lang::ast::{expr2_to_string, ExprId};
+use crate::lang::pool::PoolVec;
+use crate::ui::text::text_pos::TextPos;
+
+pub fn start_new_list(ed_model: &mut EdModel) -> EdResult<InputOutcome> {
+    let NodeContext {
+        old_caret_pos,
+        curr_mark_node_id,
+        curr_mark_node,
+        parent_id_opt,
+        ast_node_id,
+    } = get_node_context(&ed_model)?;
+
+    let is_blank_node = curr_mark_node.is_blank();
+
+    let expr2_node = Expr2::List {
+        elem_var: ed_model.module.env.var_store.fresh(),
+        elems: PoolVec::empty(ed_model.module.env.pool),
+    };
+
+    let mark_node_pool = &mut ed_model.markup_node_pool;
+
+    ed_model.module.env.pool.set(ast_node_id, expr2_node);
+
+    let left_bracket_node = MarkupNode::Text {
+        content: nodes::LEFT_SQUARE_BR.to_owned(),
+        ast_node_id,
+        syn_high_style: HighlightStyle::Bracket,
+        attributes: Attributes::new(),
+        parent_id_opt: Some(curr_mark_node_id), // current node will be replaced with nested one
+    };
+
+    let left_bracket_node_id = mark_node_pool.add(left_bracket_node);
+
+    let right_bracket_node = MarkupNode::Text {
+        content: nodes::RIGHT_SQUARE_BR.to_owned(),
+        ast_node_id,
+        syn_high_style: HighlightStyle::Bracket,
+        attributes: Attributes::new(),
+        parent_id_opt: Some(curr_mark_node_id), // current node will be replaced with nested one
+    };
+
+    let right_bracket_node_id = mark_node_pool.add(right_bracket_node);
+
+    let nested_node = MarkupNode::Nested {
+        ast_node_id,
+        children_ids: vec![left_bracket_node_id, right_bracket_node_id],
+        parent_id_opt,
+    };
+
+    if is_blank_node {
+        mark_node_pool.replace_node(curr_mark_node_id, nested_node);
+
+        // remove data corresponding to Blank node
+        ed_model.del_at_line(old_caret_pos.line, old_caret_pos.column)?;
+
+        ed_model.simple_move_carets_right(nodes::LEFT_SQUARE_BR.len());
+
+        // update GridNodeMap and CodeLines
+        ed_model.insert_between_line(
+            old_caret_pos.line,
+            old_caret_pos.column,
+            nodes::LEFT_SQUARE_BR,
+            left_bracket_node_id,
+        )?;
+
+        ed_model.insert_between_line(
+            old_caret_pos.line,
+            old_caret_pos.column + nodes::LEFT_SQUARE_BR.len(),
+            nodes::RIGHT_SQUARE_BR,
+            right_bracket_node_id,
+        )?;
+
+        Ok(InputOutcome::Accepted)
+    } else {
+        Ok(InputOutcome::Ignored)
+    }
+}
+
+// insert Blank at current position for easy code reuse
+pub fn add_blank_child(
+    new_child_index: usize,
+    new_ast_child_index: usize,
+    ed_model: &mut EdModel,
+) -> EdResult<InputOutcome> {
+    let NodeContext {
+        old_caret_pos,
+        curr_mark_node_id,
+        curr_mark_node: _,
+        parent_id_opt,
+        ast_node_id,
+    } = get_node_context(&ed_model)?;
+
+    let trip_result: EdResult<(ExprId, ExprId, MarkNodeId)> = if let Some(parent_id) = parent_id_opt
+    {
+        let parent = ed_model.markup_node_pool.get(parent_id);
+
+        let list_ast_node_id = parent.get_ast_node_id();
+        let list_ast_node = ed_model.module.env.pool.get(list_ast_node_id);
+
+        match list_ast_node {
+            Expr2::List {
+                elem_var: _,
+                elems: _,
+            } => {
+                let blank_elt = Expr2::Blank;
+                let blank_elt_id = ed_model.module.env.pool.add(blank_elt);
+
+                Ok((blank_elt_id, list_ast_node_id, parent_id))
+            }
+            _ => UnexpectedASTNode {
+                required_node_type: "List".to_string(),
+                encountered_node_type: expr2_to_string(ast_node_id, ed_model.module.env.pool),
+            }
+            .fail(),
+        }
+    } else {
+        MissingParent {
+            node_id: curr_mark_node_id,
+        }
+        .fail()
+    };
+
+    let (blank_elt_id, list_ast_node_id, parent_id) = trip_result?;
+
+    let list_ast_node = ed_model.module.env.pool.get(list_ast_node_id);
+
+    match list_ast_node {
+        Expr2::List { elem_var, elems } => {
+            let mut new_elems: Vec<ExprId> =
+                elems.iter(ed_model.module.env.pool).copied().collect();
+
+            new_elems.insert(new_ast_child_index, blank_elt_id);
+
+            let new_list_node = Expr2::List {
+                elem_var: *elem_var,
+                elems: PoolVec::new(new_elems.into_iter(), ed_model.module.env.pool),
+            };
+
+            ed_model
+                .module
+                .env
+                .pool
+                .set(list_ast_node_id, new_list_node);
+
+            Ok(())
+        }
+        _ => UnexpectedASTNode {
+            required_node_type: "List".to_string(),
+            encountered_node_type: expr2_to_string(ast_node_id, ed_model.module.env.pool),
+        }
+        .fail(),
+    }?;
+
+    let new_mark_children = update_mark_children(
+        new_child_index,
+        blank_elt_id,
+        list_ast_node_id,
+        old_caret_pos,
+        parent_id_opt,
+        ed_model,
+    )?;
+
+    let parent = ed_model.markup_node_pool.get_mut(parent_id);
+
+    for (indx, child) in new_mark_children.iter().enumerate() {
+        parent.add_child_at_index(new_child_index + indx, *child)?;
+    }
+
+    Ok(InputOutcome::Accepted)
+}
+
+// add a Blank child to the Nested mark node and update the caret
+pub fn update_mark_children(
+    new_child_index: usize,
+    blank_elt_id: ExprId,
+    list_ast_node_id: ExprId,
+    old_caret_pos: TextPos,
+    parent_id_opt: Option<MarkNodeId>,
+    ed_model: &mut EdModel,
+) -> EdResult<Vec<MarkNodeId>> {
+    let blank_mark_node = MarkupNode::Blank {
+        ast_node_id: blank_elt_id,
+        syn_high_style: HighlightStyle::Blank,
+        attributes: Attributes::new(),
+        parent_id_opt,
+    };
+
+    let blank_mark_node_id = ed_model.markup_node_pool.add(blank_mark_node);
+
+    let mut children: Vec<MarkNodeId> = vec![];
+
+    if new_child_index > 1 {
+        let comma_mark_node = MarkupNode::Text {
+            content: nodes::COMMA.to_owned(),
+            ast_node_id: list_ast_node_id,
+            syn_high_style: HighlightStyle::Blank,
+            attributes: Attributes::new(),
+            parent_id_opt,
+        };
+
+        let comma_mark_node_id = ed_model.markup_node_pool.add(comma_mark_node);
+
+        ed_model.simple_move_carets_right(nodes::COMMA.len());
+
+        ed_model.insert_between_line(
+            old_caret_pos.line,
+            old_caret_pos.column,
+            nodes::COMMA,
+            comma_mark_node_id,
+        )?;
+
+        children.push(comma_mark_node_id);
+    }
+
+    children.push(blank_mark_node_id);
+
+    let comma_shift = if new_child_index == 1 {
+        0
+    } else {
+        nodes::COMMA.len()
+    };
+
+    // update GridNodeMap and CodeLines
+    ed_model.insert_between_line(
+        old_caret_pos.line,
+        old_caret_pos.column + comma_shift,
+        nodes::BLANK_PLACEHOLDER,
+        blank_mark_node_id,
+    )?;
+
+    Ok(children)
+}
diff --git a/editor/src/editor/mvc/mod.rs b/editor/src/editor/mvc/mod.rs
index b393cfd0a2..4088a4a1ad 100644
--- a/editor/src/editor/mvc/mod.rs
+++ b/editor/src/editor/mvc/mod.rs
@@ -4,6 +4,7 @@ pub mod ed_model;
 pub mod ed_update;
 pub mod ed_view;
 mod int_update;
+mod list_update;
 mod lookup_update;
 mod record_update;
 mod string_update;
diff --git a/editor/src/editor/render_debug.rs b/editor/src/editor/render_debug.rs
index c265f42914..ea3ca106b1 100644
--- a/editor/src/editor/render_debug.rs
+++ b/editor/src/editor/render_debug.rs
@@ -19,7 +19,7 @@ pub fn build_debug_graphics(
     let area_bounds = (size.width as f32, size.height as f32);
     let layout = wgpu_glyph::Layout::default().h_align(wgpu_glyph::HorizontalAlign::Left);
 
-    let debug_txt_coords: Vector2<f32> = (txt_coords.x, txt_coords.y * 6.0).into();
+    let debug_txt_coords: Vector2<f32> = (txt_coords.x, txt_coords.y * 3.0).into();
 
     let grid_node_map_text = glyph_brush::OwnedText::new(format!("{}", ed_model.grid_node_map))
         .with_color(colors::to_slice(from_hsb(20, 41, 100)))
diff --git a/editor/src/editor/resources/strings.rs b/editor/src/editor/resources/strings.rs
index d4731e8174..214dc55682 100644
--- a/editor/src/editor/resources/strings.rs
+++ b/editor/src/editor/resources/strings.rs
@@ -1,3 +1,3 @@
-pub const NOTHING_OPENED: &str = "Execute `cargo run edit <filename>` to open a file.";
+pub const NOTHING_OPENED: &str = "Opening files is not yet supported. Execute `cargo run edit` from the root folder of the repo to try the editor.";
 pub const START_TIP: &str =
-    "Start by typing '{', '\"' or a number.\nInput chars that would create parse errors will be ignored.";
+    "Start by typing '[', '{', '\"' or a number.\nInput chars that would create parse errors will be ignored.";
diff --git a/editor/src/editor/slow_pool.rs b/editor/src/editor/slow_pool.rs
index 9ee925a369..d6ee4c85b9 100644
--- a/editor/src/editor/slow_pool.rs
+++ b/editor/src/editor/slow_pool.rs
@@ -44,12 +44,28 @@ impl fmt::Display for SlowPool {
         write!(f, "\n\n(mark_node_pool)\n")?;
 
         for (index, node) in self.nodes.iter().enumerate() {
+            let ast_node_id_str = format!("{:?}", node.get_ast_node_id());
+            let ast_node_id: String = ast_node_id_str
+                .chars()
+                .filter(|c| c.is_ascii_digit())
+                .collect();
+
+            let mut child_str = String::new();
+
+            let node_children = node.get_children_ids();
+
+            if !node_children.is_empty() {
+                child_str = format!("children: {:?}", node_children);
+            }
+
             writeln!(
                 f,
-                "{}: {} ({})",
+                "{}: {} ({}) ast_id {:?} {}",
                 index,
                 node.node_type_as_string(),
                 node.get_content().unwrap_or_else(|_| "".to_string()),
+                ast_node_id.parse::<usize>().unwrap(),
+                child_str
             )?;
         }
 
diff --git a/editor/src/editor/util.rs b/editor/src/editor/util.rs
index 69f98b3628..5e0fa1bb97 100644
--- a/editor/src/editor/util.rs
+++ b/editor/src/editor/util.rs
@@ -32,7 +32,7 @@ pub fn index_of<T: ::std::fmt::Debug + std::cmp::Eq>(elt: T, slice: &[T]) -> EdR
     Ok(index)
 }
 
-// returns the index of the first occurence of element and index of the last occurence
+// returns the index of the first occurrence of element and index of the last occurrence
 pub fn first_last_index_of<T: ::std::fmt::Debug + std::cmp::Eq>(
     elt: T,
     slice: &[T],
diff --git a/editor/src/graphics/lowlevel/ortho.rs b/editor/src/graphics/lowlevel/ortho.rs
index c3bcab80bb..0b5871bb93 100644
--- a/editor/src/graphics/lowlevel/ortho.rs
+++ b/editor/src/graphics/lowlevel/ortho.rs
@@ -5,7 +5,7 @@ use wgpu::{
     ShaderStage,
 };
 
-// orthographic projection is used to transfrom pixel coords to the coordinate system used by wgpu
+// orthographic projection is used to transform pixel coords to the coordinate system used by wgpu
 
 #[repr(C)]
 #[derive(Debug, Copy, Clone, bytemuck::Pod, bytemuck::Zeroable)]
diff --git a/editor/src/lang/ast.rs b/editor/src/lang/ast.rs
index 87c4ed6e9e..9f421b252b 100644
--- a/editor/src/lang/ast.rs
+++ b/editor/src/lang/ast.rs
@@ -1,11 +1,15 @@
 #![allow(clippy::manual_map)]
 
+use std::collections::{HashMap, HashSet};
+use std::hash::BuildHasherDefault;
+
 use crate::lang::pattern::{Pattern2, PatternId};
 use crate::lang::pool::Pool;
 use crate::lang::pool::{NodeId, PoolStr, PoolVec, ShallowClone};
 use crate::lang::types::{Type2, TypeId};
 use arraystring::{typenum::U30, ArrayString};
 use roc_can::expr::Recursive;
+use roc_collections::all::WyHash;
 use roc_module::low_level::LowLevel;
 use roc_module::operator::CalledVia;
 use roc_module::symbol::Symbol;
@@ -114,15 +118,14 @@ pub enum Expr2 {
     InvalidLookup(PoolStr), // 8B
 
     List {
-        list_var: Variable,    // 4B - required for uniqueness of the list
-        elem_var: Variable,    // 4B
-        elems: PoolVec<Expr2>, // 8B
+        elem_var: Variable,     // 4B
+        elems: PoolVec<ExprId>, // 8B
     },
     If {
-        cond_var: Variable,                // 4B
-        expr_var: Variable,                // 4B
-        branches: PoolVec<(Expr2, Expr2)>, // 8B
-        final_else: NodeId<Expr2>,         // 4B
+        cond_var: Variable,                                // 4B
+        expr_var: Variable,                                // 4B
+        branches: PoolVec<(NodeId<Expr2>, NodeId<Expr2>)>, // 8B
+        final_else: NodeId<Expr2>,                         // 4B
     },
     When {
         cond_var: Variable,            // 4B
@@ -136,9 +139,9 @@ pub enum Expr2 {
         body_id: NodeId<Expr2>,     // 4B
     },
     LetFunction {
-        def: NodeId<FunctionDef>, // 4B
-        body_var: Variable,       // 8B
-        body_id: NodeId<Expr2>,   // 4B
+        def_id: NodeId<FunctionDef>, // 4B
+        body_var: Variable,          // 8B
+        body_id: NodeId<Expr2>,      // 4B
     },
     LetValue {
         def_id: NodeId<ValueDef>, // 4B
@@ -218,21 +221,46 @@ pub enum Expr2 {
 }
 
 #[derive(Debug)]
-pub struct ValueDef {
-    pub pattern: PatternId,                  // 4B
-    pub expr_type: Option<(TypeId, Rigids)>, // ?
-    pub expr_var: Variable,                  // 4B
+pub enum ValueDef {
+    WithAnnotation {
+        pattern_id: PatternId, // 4B
+        expr_id: ExprId,       // 4B
+        type_id: TypeId,
+        rigids: Rigids,
+        expr_var: Variable, // 4B
+    },
+    NoAnnotation {
+        pattern_id: PatternId, // 4B
+        expr_id: ExprId,       // 4B
+        expr_var: Variable,    // 4B
+    },
 }
 
 impl ShallowClone for ValueDef {
     fn shallow_clone(&self) -> Self {
-        Self {
-            pattern: self.pattern,
-            expr_type: match &self.expr_type {
-                Some((id, rigids)) => Some((*id, rigids.shallow_clone())),
-                None => None,
+        match self {
+            Self::WithAnnotation {
+                pattern_id,
+                expr_id,
+                type_id,
+                rigids,
+                expr_var,
+            } => Self::WithAnnotation {
+                pattern_id: *pattern_id,
+                expr_id: *expr_id,
+                type_id: *type_id,
+                rigids: rigids.shallow_clone(),
+                expr_var: *expr_var,
+            },
+            Self::NoAnnotation {
+                pattern_id,
+                expr_id,
+                expr_var,
+            } => Self::NoAnnotation {
+                pattern_id: *pattern_id,
+                expr_id: *expr_id,
+                expr_var: *expr_var,
             },
-            expr_var: self.expr_var,
         }
     }
 }
@@ -288,8 +316,68 @@ impl ShallowClone for FunctionDef {
 
 #[derive(Debug)]
 pub struct Rigids {
-    pub named: PoolVec<(PoolStr, Variable)>, // 8B
-    pub unnamed: PoolVec<Variable>,          // 8B
+    pub names: PoolVec<(Option<PoolStr>, Variable)>, // 8B
+    padding: [u8; 1],
+}
+
+#[allow(clippy::needless_collect)]
+impl Rigids {
+    pub fn new(
+        named: HashMap<&str, Variable, BuildHasherDefault<WyHash>>,
+        unnamed: HashSet<Variable, BuildHasherDefault<WyHash>>,
+        pool: &mut Pool,
+    ) -> Self {
+        let names = PoolVec::with_capacity((named.len() + unnamed.len()) as u32, pool);
+
+        let mut temp_names = Vec::new();
+
+        temp_names.extend(named.iter().map(|(name, var)| (Some(*name), *var)));
+
+        temp_names.extend(unnamed.iter().map(|var| (None, *var)));
+
+        for (node_id, (opt_name, variable)) in names.iter_node_ids().zip(temp_names) {
+            let poolstr = opt_name.map(|name| PoolStr::new(name, pool));
+
+            pool[node_id] = (poolstr, variable);
+        }
+
+        Self {
+            names,
+            padding: Default::default(),
+        }
+    }
+
+    pub fn named(&self, pool: &mut Pool) -> PoolVec<(PoolStr, Variable)> {
+        let named = self
+            .names
+            .iter(pool)
+            .filter_map(|(opt_pool_str, var)| {
+                if let Some(pool_str) = opt_pool_str {
+                    Some((*pool_str, *var))
+                } else {
+                    None
+                }
+            })
+            .collect::<Vec<(PoolStr, Variable)>>();
+
+        PoolVec::new(named.into_iter(), pool)
+    }
+
+    pub fn unnamed(&self, pool: &mut Pool) -> PoolVec<Variable> {
+        let unnamed = self
+            .names
+            .iter(pool)
+            .filter_map(|(opt_pool_str, var)| {
+                if opt_pool_str.is_none() {
+                    Some(*var)
+                } else {
+                    None
+                }
+            })
+            .collect::<Vec<Variable>>();
+
+        PoolVec::new(unnamed.into_iter(), pool)
+    }
 }
 
 /// This is overflow data from a Closure variant, which needs to store
@@ -348,8 +436,9 @@ impl RecordField {
 
 pub fn expr2_to_string(node_id: NodeId<Expr2>, pool: &Pool) -> String {
     let mut full_string = String::new();
+    let expr2 = pool.get(node_id);
 
-    expr2_to_string_helper(node_id, 0, pool, &mut full_string);
+    expr2_to_string_helper(expr2, 0, pool, &mut full_string);
 
     full_string
 }
@@ -362,13 +451,11 @@ fn get_spacing(indent_level: usize) -> String {
 }
 
 fn expr2_to_string_helper(
-    node_id: NodeId<Expr2>,
+    expr2: &Expr2,
     indent_level: usize,
     pool: &Pool,
     out_string: &mut String,
 ) {
-    let expr2 = pool.get(node_id);
-
     out_string.push_str(&get_spacing(indent_level));
 
     match expr2 {
@@ -385,11 +472,7 @@ fn expr2_to_string_helper(
         Expr2::EmptyRecord => out_string.push_str("EmptyRecord"),
         Expr2::Record { record_var, fields } => {
             out_string.push_str("Record:\n");
-            out_string.push_str(&format!(
-                "{}Var({:?})\n",
-                get_spacing(indent_level + 1),
-                record_var
-            ));
+            out_string.push_str(&var_to_string(&record_var, indent_level + 1));
 
             out_string.push_str(&format!("{}fields: [\n", get_spacing(indent_level + 1)));
 
@@ -428,7 +511,8 @@ fn expr2_to_string_helper(
                             var,
                         ));
 
-                        expr2_to_string_helper(*val_node_id, indent_level + 3, pool, out_string);
+                        let val_expr2 = pool.get(*val_node_id);
+                        expr2_to_string_helper(val_expr2, indent_level + 3, pool, out_string);
                         out_string.push_str(&format!("{})\n", get_spacing(indent_level + 2)));
                     }
                 }
@@ -436,6 +520,27 @@ fn expr2_to_string_helper(
 
             out_string.push_str(&format!("{}]\n", get_spacing(indent_level + 1)));
         }
+        Expr2::List { elem_var, elems } => {
+            out_string.push_str("List:\n");
+            out_string.push_str(&var_to_string(elem_var, indent_level + 1));
+            out_string.push_str(&format!("{}elems: [\n", get_spacing(indent_level + 1)));
+
+            let mut first_elt = true;
+
+            for elem_expr2_id in elems.iter(pool) {
+                if !first_elt {
+                    out_string.push_str(", ")
+                } else {
+                    first_elt = false;
+                }
+
+                let elem_expr2 = pool.get(*elem_expr2_id);
+
+                expr2_to_string_helper(elem_expr2, indent_level + 2, pool, out_string)
+            }
+
+            out_string.push_str(&format!("{}]\n", get_spacing(indent_level + 1)));
+        }
         Expr2::InvalidLookup(pool_str) => {
             out_string.push_str(&format!("InvalidLookup({})", pool_str.as_str(pool)));
         }
@@ -448,6 +553,10 @@ fn expr2_to_string_helper(
     out_string.push('\n');
 }
 
+fn var_to_string(some_var: &Variable, indent_level: usize) -> String {
+    format!("{}Var({:?})\n", get_spacing(indent_level + 1), some_var)
+}
+
 #[test]
 fn size_of_expr() {
     assert_eq!(std::mem::size_of::<Expr2>(), crate::lang::pool::NODE_BYTES);
@@ -456,8 +565,8 @@ fn size_of_expr() {
 impl ShallowClone for Rigids {
     fn shallow_clone(&self) -> Self {
         Self {
-            named: self.named.shallow_clone(),
-            unnamed: self.unnamed.shallow_clone(),
+            names: self.names.shallow_clone(),
+            padding: self.padding,
         }
     }
 }
diff --git a/editor/src/lang/constrain.rs b/editor/src/lang/constrain.rs
index 2832cef8ac..cb1ed061bc 100644
--- a/editor/src/lang/constrain.rs
+++ b/editor/src/lang/constrain.rs
@@ -1,19 +1,23 @@
 use bumpalo::{collections::Vec as BumpVec, Bump};
 
 use crate::lang::{
-    ast::{Expr2, RecordField},
+    ast::{Expr2, ExprId, RecordField, ValueDef, WhenBranch},
     expr::Env,
+    pattern::{DestructType, Pattern2, PatternState2, RecordDestruct},
     pool::{Pool, PoolStr, PoolVec, ShallowClone},
     types::{Type2, TypeId},
 };
 
-use roc_can::expected::Expected;
-use roc_collections::all::{BumpMap, BumpMapDefault, Index};
-use roc_module::{ident::TagName, symbol::Symbol};
-use roc_region::all::{Located, Region};
+use roc_can::expected::{Expected, PExpected};
+use roc_collections::all::{BumpMap, BumpMapDefault, Index, SendMap};
+use roc_module::{
+    ident::{Lowercase, TagName},
+    symbol::Symbol,
+};
+use roc_region::all::Region;
 use roc_types::{
     subs::Variable,
-    types,
+    types::{self, AnnotationSource, PReason, PatternCategory},
     types::{Category, Reason},
 };
 
@@ -22,7 +26,7 @@ pub enum Constraint<'a> {
     Eq(Type2, Expected<Type2>, Category, Region),
     // Store(Type, Variable, &'static str, u32),
     Lookup(Symbol, Expected<Type2>, Region),
-    // Pattern(Region, PatternCategory, Type, PExpected<Type>),
+    Pattern(Region, PatternCategory, Type2, PExpected<Type2>),
     And(BumpVec<'a, Constraint<'a>>),
     Let(&'a LetConstraint<'a>),
     // SaveTheEnvironment,
@@ -33,7 +37,7 @@ pub enum Constraint<'a> {
 pub struct LetConstraint<'a> {
     pub rigid_vars: BumpVec<'a, Variable>,
     pub flex_vars: BumpVec<'a, Variable>,
-    pub def_types: BumpMap<Symbol, Located<Type2>>,
+    pub def_types: BumpMap<Symbol, Type2>,
     pub defs_constraint: Constraint<'a>,
     pub ret_constraint: Constraint<'a>,
 }
@@ -130,7 +134,10 @@ pub fn constrain_expr<'a>(
 
                 let list_elem_type = Type2::Variable(*elem_var);
 
-                for (index, elem_node_id) in elems.iter_node_ids().enumerate() {
+                let indexed_node_ids: Vec<(usize, ExprId)> =
+                    elems.iter(env.pool).copied().enumerate().collect();
+
+                for (index, elem_node_id) in indexed_node_ids {
                     let elem_expr = env.pool.get(elem_node_id);
 
                     let elem_expected = Expected::ForReason(
@@ -479,7 +486,431 @@ pub fn constrain_expr<'a>(
 
             exists(arena, flex_vars, And(and_constraints))
         }
-        _ => todo!("implement constaints for {:?}", expr),
+        Expr2::If {
+            cond_var,
+            expr_var,
+            branches,
+            final_else,
+        } => {
+            let expect_bool = |region| {
+                let bool_type = Type2::Variable(Variable::BOOL);
+                Expected::ForReason(Reason::IfCondition, bool_type, region)
+            };
+
+            let mut branch_cons = BumpVec::with_capacity_in(2 * branches.len() + 3, arena);
+
+            // TODO why does this cond var exist? is it for error messages?
+            // let first_cond_region = branches[0].0.region;
+            let cond_var_is_bool_con = Eq(
+                Type2::Variable(*cond_var),
+                expect_bool(region),
+                Category::If,
+                region,
+            );
+
+            branch_cons.push(cond_var_is_bool_con);
+
+            let final_else_expr = env.pool.get(*final_else);
+
+            let mut flex_vars = BumpVec::with_capacity_in(2, arena);
+
+            flex_vars.push(*cond_var);
+            flex_vars.push(*expr_var);
+
+            match expected {
+                Expected::FromAnnotation(name, arity, _, tipe) => {
+                    let num_branches = branches.len() + 1;
+
+                    for (index, branch_id) in branches.iter_node_ids().enumerate() {
+                        let (cond_id, body_id) = env.pool.get(branch_id);
+
+                        let cond = env.pool.get(*cond_id);
+                        let body = env.pool.get(*body_id);
+
+                        let cond_con =
+                            constrain_expr(arena, env, cond, expect_bool(region), region);
+
+                        let then_con = constrain_expr(
+                            arena,
+                            env,
+                            body,
+                            Expected::FromAnnotation(
+                                name.clone(),
+                                arity,
+                                AnnotationSource::TypedIfBranch {
+                                    index: Index::zero_based(index),
+                                    num_branches,
+                                },
+                                tipe.shallow_clone(),
+                            ),
+                            region,
+                        );
+
+                        branch_cons.push(cond_con);
+                        branch_cons.push(then_con);
+                    }
+
+                    let else_con = constrain_expr(
+                        arena,
+                        env,
+                        final_else_expr,
+                        Expected::FromAnnotation(
+                            name,
+                            arity,
+                            AnnotationSource::TypedIfBranch {
+                                index: Index::zero_based(branches.len()),
+                                num_branches,
+                            },
+                            tipe.shallow_clone(),
+                        ),
+                        region,
+                    );
+
+                    let ast_con = Eq(
+                        Type2::Variable(*expr_var),
+                        Expected::NoExpectation(tipe),
+                        Category::Storage(std::file!(), std::line!()),
+                        region,
+                    );
+
+                    branch_cons.push(ast_con);
+                    branch_cons.push(else_con);
+
+                    exists(arena, flex_vars, And(branch_cons))
+                }
+                _ => {
+                    for (index, branch_id) in branches.iter_node_ids().enumerate() {
+                        let (cond_id, body_id) = env.pool.get(branch_id);
+
+                        let cond = env.pool.get(*cond_id);
+                        let body = env.pool.get(*body_id);
+
+                        let cond_con =
+                            constrain_expr(arena, env, cond, expect_bool(region), region);
+
+                        let then_con = constrain_expr(
+                            arena,
+                            env,
+                            body,
+                            Expected::ForReason(
+                                Reason::IfBranch {
+                                    index: Index::zero_based(index),
+                                    total_branches: branches.len(),
+                                },
+                                Type2::Variable(*expr_var),
+                                // should be from body
+                                region,
+                            ),
+                            region,
+                        );
+
+                        branch_cons.push(cond_con);
+                        branch_cons.push(then_con);
+                    }
+
+                    let else_con = constrain_expr(
+                        arena,
+                        env,
+                        final_else_expr,
+                        Expected::ForReason(
+                            Reason::IfBranch {
+                                index: Index::zero_based(branches.len()),
+                                total_branches: branches.len() + 1,
+                            },
+                            Type2::Variable(*expr_var),
+                            // should come from final_else
+                            region,
+                        ),
+                        region,
+                    );
+
+                    branch_cons.push(Eq(
+                        Type2::Variable(*expr_var),
+                        expected,
+                        Category::Storage(std::file!(), std::line!()),
+                        region,
+                    ));
+
+                    branch_cons.push(else_con);
+
+                    exists(arena, flex_vars, And(branch_cons))
+                }
+            }
+        }
+        Expr2::When {
+            cond_var,
+            expr_var,
+            cond: cond_id,
+            branches,
+        } => {
+            // Infer the condition expression's type.
+            let cond_type = Type2::Variable(*cond_var);
+
+            let cond = env.pool.get(*cond_id);
+
+            let expr_con = constrain_expr(
+                arena,
+                env,
+                cond,
+                Expected::NoExpectation(cond_type.shallow_clone()),
+                region,
+            );
+
+            let mut constraints = BumpVec::with_capacity_in(branches.len() + 1, arena);
+
+            constraints.push(expr_con);
+
+            let mut flex_vars = BumpVec::with_capacity_in(2, arena);
+
+            flex_vars.push(*cond_var);
+            flex_vars.push(*expr_var);
+
+            match &expected {
+                Expected::FromAnnotation(name, arity, _, _typ) => {
+                    // NOTE deviation from elm.
+                    //
+                    // in elm, `_typ` is used, but because we have this `expr_var` too
+                    // and need to constrain it, this is what works and gives better error messages
+                    let typ = Type2::Variable(*expr_var);
+
+                    for (index, when_branch_id) in branches.iter_node_ids().enumerate() {
+                        let when_branch = env.pool.get(when_branch_id);
+
+                        let pattern_region = region;
+                        // let pattern_region = Region::across_all(
+                        //     when_branch.patterns.iter(env.pool).map(|v| &v.region),
+                        // );
+
+                        let branch_con = constrain_when_branch(
+                            arena,
+                            env,
+                            // TODO: when_branch.value.region,
+                            region,
+                            when_branch,
+                            PExpected::ForReason(
+                                PReason::WhenMatch {
+                                    index: Index::zero_based(index),
+                                },
+                                cond_type.shallow_clone(),
+                                pattern_region,
+                            ),
+                            Expected::FromAnnotation(
+                                name.clone(),
+                                *arity,
+                                AnnotationSource::TypedWhenBranch {
+                                    index: Index::zero_based(index),
+                                },
+                                typ.shallow_clone(),
+                            ),
+                        );
+
+                        constraints.push(branch_con);
+                    }
+
+                    constraints.push(Eq(typ, expected, Category::When, region));
+
+                    return exists(arena, flex_vars, And(constraints));
+                }
+
+                _ => {
+                    let branch_type = Type2::Variable(*expr_var);
+                    let mut branch_cons = BumpVec::with_capacity_in(branches.len(), arena);
+
+                    for (index, when_branch_id) in branches.iter_node_ids().enumerate() {
+                        let when_branch = env.pool.get(when_branch_id);
+
+                        let pattern_region = region;
+                        // let pattern_region =
+                        //     Region::across_all(when_branch.patterns.iter().map(|v| &v.region));
+
+                        let branch_con = constrain_when_branch(
+                            arena,
+                            env,
+                            region,
+                            when_branch,
+                            PExpected::ForReason(
+                                PReason::WhenMatch {
+                                    index: Index::zero_based(index),
+                                },
+                                cond_type.shallow_clone(),
+                                pattern_region,
+                            ),
+                            Expected::ForReason(
+                                Reason::WhenBranch {
+                                    index: Index::zero_based(index),
+                                },
+                                branch_type.shallow_clone(),
+                                // TODO: when_branch.value.region,
+                                region,
+                            ),
+                        );
+
+                        branch_cons.push(branch_con);
+                    }
+
+                    let mut and_constraints = BumpVec::with_capacity_in(2, arena);
+
+                    and_constraints.push(And(branch_cons));
+                    and_constraints.push(Eq(branch_type, expected, Category::When, region));
+
+                    constraints.push(And(and_constraints));
+                }
+            }
+
+            // exhautiveness checking happens when converting to mono::Expr
+            exists(arena, flex_vars, And(constraints))
+        }
+        Expr2::LetValue {
+            def_id,
+            body_id,
+            body_var,
+        } => {
+            let value_def = env.pool.get(*def_id);
+            let body = env.pool.get(*body_id);
+
+            let body_con = constrain_expr(arena, env, body, expected.shallow_clone(), region);
+
+            match value_def {
+                ValueDef::WithAnnotation { .. } => todo!("implement {:?}", value_def),
+                ValueDef::NoAnnotation {
+                    pattern_id,
+                    expr_id,
+                    expr_var,
+                } => {
+                    let pattern = env.pool.get(*pattern_id);
+
+                    let mut flex_vars = BumpVec::with_capacity_in(1, arena);
+                    flex_vars.push(*body_var);
+
+                    let expr_type = Type2::Variable(*expr_var);
+
+                    let pattern_expected = PExpected::NoExpectation(expr_type.shallow_clone());
+                    let mut state = PatternState2 {
+                        headers: BumpMap::new_in(arena),
+                        vars: BumpVec::with_capacity_in(1, arena),
+                        constraints: BumpVec::with_capacity_in(1, arena),
+                    };
+
+                    constrain_pattern(arena, env, pattern, region, pattern_expected, &mut state);
+                    state.vars.push(*expr_var);
+
+                    let def_expr = env.pool.get(*expr_id);
+
+                    let constrained_def = Let(arena.alloc(LetConstraint {
+                        rigid_vars: BumpVec::new_in(arena),
+                        flex_vars: state.vars,
+                        def_types: state.headers,
+                        defs_constraint: Let(arena.alloc(LetConstraint {
+                            rigid_vars: BumpVec::new_in(arena), // always empty
+                            flex_vars: BumpVec::new_in(arena), // empty, because our functions have no arguments
+                            def_types: BumpMap::new_in(arena), // empty, because our functions have no arguments!
+                            defs_constraint: And(state.constraints),
+                            ret_constraint: constrain_expr(
+                                arena,
+                                env,
+                                def_expr,
+                                Expected::NoExpectation(expr_type),
+                                region,
+                            ),
+                        })),
+                        ret_constraint: body_con,
+                    }));
+
+                    let mut and_constraints = BumpVec::with_capacity_in(2, arena);
+
+                    and_constraints.push(constrained_def);
+                    and_constraints.push(Eq(
+                        Type2::Variable(*body_var),
+                        expected,
+                        Category::Storage(std::file!(), std::line!()),
+                        // TODO: needs to be ret region
+                        region,
+                    ));
+
+                    exists(arena, flex_vars, And(and_constraints))
+                }
+            }
+        }
+        Expr2::Update {
+            symbol,
+            updates,
+            ext_var,
+            record_var,
+        } => {
+            let field_types = PoolVec::with_capacity(updates.len() as u32, env.pool);
+            let mut flex_vars = BumpVec::with_capacity_in(updates.len() + 2, arena);
+            let mut cons = BumpVec::with_capacity_in(updates.len() + 1, arena);
+            let mut record_key_updates = SendMap::default();
+
+            for (record_field_id, field_type_node_id) in
+                updates.iter_node_ids().zip(field_types.iter_node_ids())
+            {
+                let record_field = env.pool.get(record_field_id);
+
+                match record_field {
+                    RecordField::LabeledValue(pool_str, var, node_id) => {
+                        let expr = env.pool.get(*node_id);
+
+                        let (field_type, field_con) = constrain_field_update(
+                            arena,
+                            env,
+                            *var,
+                            pool_str.as_str(env.pool).into(),
+                            expr,
+                        );
+
+                        let field_type_id = env.pool.add(field_type);
+
+                        env.pool[field_type_node_id] =
+                            (*pool_str, types::RecordField::Required(field_type_id));
+
+                        record_key_updates.insert(pool_str.as_str(env.pool).into(), Region::zero());
+
+                        flex_vars.push(*var);
+                        cons.push(field_con);
+                    }
+                    e => todo!("{:?}", e),
+                }
+            }
+
+            let fields_type = Type2::Record(field_types, env.pool.add(Type2::Variable(*ext_var)));
+            let record_type = Type2::Variable(*record_var);
+
+            // NOTE from elm compiler: fields_type is separate so that Error propagates better
+            let fields_con = Eq(
+                record_type.shallow_clone(),
+                Expected::NoExpectation(fields_type),
+                Category::Record,
+                region,
+            );
+            let record_con = Eq(
+                record_type.shallow_clone(),
+                expected,
+                Category::Record,
+                region,
+            );
+
+            flex_vars.push(*record_var);
+            flex_vars.push(*ext_var);
+
+            let con = Lookup(
+                *symbol,
+                Expected::ForReason(
+                    Reason::RecordUpdateKeys(*symbol, record_key_updates),
+                    record_type,
+                    region,
+                ),
+                region,
+            );
+
+            // ensure constraints are solved in this order, gives better errors
+            cons.insert(0, fields_con);
+            cons.insert(1, con);
+            cons.insert(2, record_con);
+
+            exists(arena, flex_vars, And(cons))
+        }
+        _ => todo!("implement constraints for {:?}", expr),
     }
 }
 
@@ -510,10 +941,350 @@ fn constrain_field<'a>(
     (field_type, constraint)
 }
 
+#[inline(always)]
+fn constrain_field_update<'a>(
+    arena: &'a Bump,
+    env: &mut Env,
+    field_var: Variable,
+    field: Lowercase,
+    expr: &Expr2,
+) -> (Type2, Constraint<'a>) {
+    let field_type = Type2::Variable(field_var);
+    let reason = Reason::RecordUpdateValue(field);
+    let field_expected = Expected::ForReason(reason, field_type.shallow_clone(), Region::zero());
+    let con = constrain_expr(arena, env, expr, field_expected, Region::zero());
+
+    (field_type, con)
+}
+
 fn constrain_empty_record<'a>(expected: Expected<Type2>, region: Region) -> Constraint<'a> {
     Constraint::Eq(Type2::EmptyRec, expected, Category::Record, region)
 }
 
+#[inline(always)]
+fn constrain_when_branch<'a>(
+    arena: &'a Bump,
+    env: &mut Env,
+    region: Region,
+    when_branch: &WhenBranch,
+    pattern_expected: PExpected<Type2>,
+    expr_expected: Expected<Type2>,
+) -> Constraint<'a> {
+    let when_expr = env.pool.get(when_branch.body);
+
+    let ret_constraint = constrain_expr(arena, env, when_expr, expr_expected, region);
+
+    let mut state = PatternState2 {
+        headers: BumpMap::new_in(arena),
+        vars: BumpVec::with_capacity_in(1, arena),
+        constraints: BumpVec::with_capacity_in(1, arena),
+    };
+
+    // TODO investigate for error messages, is it better to unify all branches with a variable,
+    // then unify that variable with the expectation?
+    for pattern_id in when_branch.patterns.iter_node_ids() {
+        let pattern = env.pool.get(pattern_id);
+
+        constrain_pattern(
+            arena,
+            env,
+            pattern,
+            // loc_pattern.region,
+            region,
+            pattern_expected.shallow_clone(),
+            &mut state,
+        );
+    }
+
+    if let Some(guard_id) = &when_branch.guard {
+        let guard = env.pool.get(*guard_id);
+
+        let guard_constraint = constrain_expr(
+            arena,
+            env,
+            guard,
+            Expected::ForReason(
+                Reason::WhenGuard,
+                Type2::Variable(Variable::BOOL),
+                // TODO: loc_guard.region,
+                region,
+            ),
+            region,
+        );
+
+        // must introduce the headers from the pattern before constraining the guard
+        Constraint::Let(arena.alloc(LetConstraint {
+            rigid_vars: BumpVec::new_in(arena),
+            flex_vars: state.vars,
+            def_types: state.headers,
+            defs_constraint: Constraint::And(state.constraints),
+            ret_constraint: Constraint::Let(arena.alloc(LetConstraint {
+                rigid_vars: BumpVec::new_in(arena),
+                flex_vars: BumpVec::new_in(arena),
+                def_types: BumpMap::new_in(arena),
+                defs_constraint: guard_constraint,
+                ret_constraint,
+            })),
+        }))
+    } else {
+        Constraint::Let(arena.alloc(LetConstraint {
+            rigid_vars: BumpVec::new_in(arena),
+            flex_vars: state.vars,
+            def_types: state.headers,
+            defs_constraint: Constraint::And(state.constraints),
+            ret_constraint,
+        }))
+    }
+}
+
+/// This accepts PatternState (rather than returning it) so that the caller can
+/// intiialize the Vecs in PatternState using with_capacity
+/// based on its knowledge of their lengths.
+pub fn constrain_pattern<'a>(
+    arena: &'a Bump,
+    env: &mut Env,
+    pattern: &Pattern2,
+    region: Region,
+    expected: PExpected<Type2>,
+    state: &mut PatternState2<'a>,
+) {
+    use Pattern2::*;
+
+    match pattern {
+        Underscore | UnsupportedPattern(_) | MalformedPattern(_, _) | Shadowed { .. } => {
+            // Neither the _ pattern nor erroneous ones add any constraints.
+        }
+
+        Identifier(symbol) => {
+            state.headers.insert(*symbol, expected.get_type());
+        }
+
+        NumLiteral(var, _) => {
+            state.vars.push(*var);
+
+            let type_id = env.pool.add(Type2::Variable(*var));
+
+            state.constraints.push(Constraint::Pattern(
+                region,
+                PatternCategory::Num,
+                num_num(env.pool, type_id),
+                expected,
+            ));
+        }
+
+        IntLiteral(_int_val) => {
+            let precision_var = env.var_store.fresh();
+
+            let range = env.add(Type2::Variable(precision_var), region);
+
+            state.constraints.push(Constraint::Pattern(
+                region,
+                PatternCategory::Int,
+                num_int(env.pool, range),
+                expected,
+            ));
+        }
+
+        FloatLiteral(_float_val) => {
+            let precision_var = env.var_store.fresh();
+
+            let range = env.add(Type2::Variable(precision_var), region);
+
+            state.constraints.push(Constraint::Pattern(
+                region,
+                PatternCategory::Float,
+                num_float(env.pool, range),
+                expected,
+            ));
+        }
+
+        StrLiteral(_) => {
+            state.constraints.push(Constraint::Pattern(
+                region,
+                PatternCategory::Str,
+                str_type(env.pool),
+                expected,
+            ));
+        }
+
+        RecordDestructure {
+            whole_var,
+            ext_var,
+            destructs,
+        } => {
+            state.vars.push(*whole_var);
+            state.vars.push(*ext_var);
+            let ext_type = Type2::Variable(*ext_var);
+
+            let mut field_types = Vec::new();
+
+            for destruct_id in destructs.iter_node_ids() {
+                let RecordDestruct {
+                    var,
+                    label,
+                    symbol,
+                    typ,
+                } = env.pool.get(destruct_id);
+
+                let pat_type = Type2::Variable(*var);
+                let expected = PExpected::NoExpectation(pat_type.shallow_clone());
+
+                if !state.headers.contains_key(&symbol) {
+                    state.headers.insert(*symbol, pat_type.shallow_clone());
+                }
+
+                let destruct_type = env.pool.get(*typ);
+
+                let field_type = match destruct_type {
+                    DestructType::Guard(guard_var, guard_id) => {
+                        state.constraints.push(Constraint::Pattern(
+                            region,
+                            PatternCategory::PatternGuard,
+                            Type2::Variable(*guard_var),
+                            PExpected::ForReason(
+                                PReason::PatternGuard,
+                                pat_type.shallow_clone(),
+                                // TODO: region should be from guard_id
+                                region,
+                            ),
+                        ));
+
+                        state.vars.push(*guard_var);
+
+                        let guard = env.pool.get(*guard_id);
+
+                        // TODO: region should be from guard_id
+                        constrain_pattern(arena, env, guard, region, expected, state);
+
+                        types::RecordField::Demanded(env.pool.add(pat_type))
+                    }
+                    DestructType::Optional(expr_var, expr_id) => {
+                        state.constraints.push(Constraint::Pattern(
+                            region,
+                            PatternCategory::PatternDefault,
+                            Type2::Variable(*expr_var),
+                            PExpected::ForReason(
+                                PReason::OptionalField,
+                                pat_type.shallow_clone(),
+                                // TODO: region should be from expr_id
+                                region,
+                            ),
+                        ));
+
+                        state.vars.push(*expr_var);
+
+                        let expr_expected = Expected::ForReason(
+                            Reason::RecordDefaultField(label.as_str(env.pool).into()),
+                            pat_type.shallow_clone(),
+                            // TODO: region should be from expr_id
+                            region,
+                        );
+
+                        let expr = env.pool.get(*expr_id);
+
+                        // TODO: region should be from expr_id
+                        let expr_con = constrain_expr(arena, env, expr, expr_expected, region);
+
+                        state.constraints.push(expr_con);
+
+                        types::RecordField::Optional(env.pool.add(pat_type))
+                    }
+                    DestructType::Required => {
+                        // No extra constraints necessary.
+                        types::RecordField::Demanded(env.pool.add(pat_type))
+                    }
+                };
+
+                field_types.push((*label, field_type));
+
+                state.vars.push(*var);
+            }
+
+            let record_type = Type2::Record(
+                PoolVec::new(field_types.into_iter(), env.pool),
+                env.pool.add(ext_type),
+            );
+
+            let whole_con = Constraint::Eq(
+                Type2::Variable(*whole_var),
+                Expected::NoExpectation(record_type),
+                Category::Storage(std::file!(), std::line!()),
+                region,
+            );
+
+            let record_con = Constraint::Pattern(
+                region,
+                PatternCategory::Record,
+                Type2::Variable(*whole_var),
+                expected,
+            );
+
+            state.constraints.push(whole_con);
+            state.constraints.push(record_con);
+        }
+        GlobalTag {
+            whole_var,
+            ext_var,
+            tag_name,
+            arguments,
+        } => {
+            let mut argument_types = Vec::with_capacity(arguments.len());
+
+            for (index, arg_id) in arguments.iter_node_ids().enumerate() {
+                let (pattern_var, pattern_id) = env.pool.get(arg_id);
+                let pattern = env.pool.get(*pattern_id);
+
+                state.vars.push(*pattern_var);
+
+                let pattern_type = Type2::Variable(*pattern_var);
+                argument_types.push(pattern_type.shallow_clone());
+
+                let expected = PExpected::ForReason(
+                    PReason::TagArg {
+                        tag_name: TagName::Global(tag_name.as_str(env.pool).into()),
+                        index: Index::zero_based(index),
+                    },
+                    pattern_type,
+                    region,
+                );
+
+                // TODO region should come from pattern
+                constrain_pattern(arena, env, pattern, region, expected, state);
+            }
+
+            let whole_con = Constraint::Eq(
+                Type2::Variable(*whole_var),
+                Expected::NoExpectation(Type2::TagUnion(
+                    PoolVec::new(
+                        vec![(
+                            *tag_name,
+                            PoolVec::new(argument_types.into_iter(), env.pool),
+                        )]
+                        .into_iter(),
+                        env.pool,
+                    ),
+                    env.pool.add(Type2::Variable(*ext_var)),
+                )),
+                Category::Storage(std::file!(), std::line!()),
+                region,
+            );
+
+            let tag_con = Constraint::Pattern(
+                region,
+                PatternCategory::Ctor(TagName::Global(tag_name.as_str(env.pool).into())),
+                Type2::Variable(*whole_var),
+                expected,
+            );
+
+            state.vars.push(*whole_var);
+            state.vars.push(*ext_var);
+            state.constraints.push(whole_con);
+            state.constraints.push(tag_con);
+        }
+        PrivateTag { .. } => todo!(),
+    }
+}
+
 #[inline(always)]
 fn builtin_type(symbol: Symbol, args: PoolVec<Type2>) -> Type2 {
     Type2::Apply(symbol, args)
@@ -574,7 +1345,7 @@ fn num_floatingpoint(pool: &mut Pool, range: TypeId) -> Type2 {
 }
 
 #[inline(always)]
-fn _num_int(pool: &mut Pool, range: TypeId) -> Type2 {
+fn num_int(pool: &mut Pool, range: TypeId) -> Type2 {
     let num_integer_type = _num_integer(pool, range);
     let num_integer_id = pool.add(num_integer_type);
 
diff --git a/editor/src/lang/def.rs b/editor/src/lang/def.rs
index c56115eb03..0cd3b25ba0 100644
--- a/editor/src/lang/def.rs
+++ b/editor/src/lang/def.rs
@@ -46,10 +46,13 @@ impl Def {
 
         match self {
             Def::AnnotationOnly { .. } => todo!("lost pattern information here ... "),
-            Def::Value(ValueDef { pattern, .. }) => {
-                let pattern2 = &pool[*pattern];
-                output.extend(symbols_from_pattern(pool, pattern2));
-            }
+            Def::Value(value_def) => match value_def {
+                ValueDef::WithAnnotation { pattern_id, .. }
+                | ValueDef::NoAnnotation { pattern_id, .. } => {
+                    let pattern2 = &pool[*pattern_id];
+                    output.extend(symbols_from_pattern(pool, pattern2));
+                }
+            },
             Def::Function(function_def) => match function_def {
                 FunctionDef::NoAnnotation { name, .. }
                 | FunctionDef::WithAnnotation { name, .. } => {
@@ -79,7 +82,7 @@ impl ShallowClone for Def {
 /// but no Expr canonicalization has happened yet. Also, it has had spaces
 /// and nesting resolved, and knows whether annotations are standalone or not.
 #[derive(Debug)]
-enum PendingDef<'a> {
+pub enum PendingDef<'a> {
     /// A standalone annotation with no body
     AnnotationOnly(
         &'a Located<ast::Pattern<'a>>,
@@ -315,23 +318,7 @@ fn from_pending_alias<'a>(
                 }
             }
 
-            let named = PoolVec::with_capacity(named_rigids.len() as u32, env.pool);
-            let unnamed = PoolVec::with_capacity(unnamed_rigids.len() as u32, env.pool);
-
-            for (node_id, (name, variable)) in named.iter_node_ids().zip(named_rigids) {
-                let poolstr = PoolStr::new(name, env.pool);
-
-                env.pool[node_id] = (poolstr, variable);
-            }
-
-            for (node_id, rigid) in unnamed.iter_node_ids().zip(unnamed_rigids) {
-                env.pool[node_id] = rigid;
-            }
-
-            let rigids = Rigids {
-                named: named.shallow_clone(),
-                unnamed,
-            };
+            let rigids = Rigids::new(named_rigids, unnamed_rigids, env.pool);
 
             let annotation = match signature {
                 Signature::Value { annotation } => annotation,
@@ -355,6 +342,8 @@ fn from_pending_alias<'a>(
                     rec_type_union.substitute_alias(env.pool, symbol, Type2::Variable(rec_var));
 
                     let annotation_id = env.add(rec_type_union, ann.region);
+                    let named = rigids.named(env.pool);
+
                     scope.add_alias(env.pool, symbol, named, annotation_id);
                 } else {
                     env.problem(Problem::CyclicAlias(symbol, name.region, vec![]));
@@ -362,6 +351,8 @@ fn from_pending_alias<'a>(
                 }
             } else {
                 let annotation_id = env.add(annotation, ann.region);
+                let named = rigids.named(env.pool);
+
                 scope.add_alias(env.pool, symbol, named, annotation_id);
             }
 
@@ -407,21 +398,7 @@ fn canonicalize_pending_def<'a>(
                         output.references.referenced_aliases.insert(symbol);
                     }
 
-                    let rigids = {
-                        let named = PoolVec::with_capacity(named_rigids.len() as u32, env.pool);
-                        let unnamed = PoolVec::with_capacity(unnamed_rigids.len() as u32, env.pool);
-
-                        for (node_id, (name, variable)) in named.iter_node_ids().zip(named_rigids) {
-                            let poolstr = PoolStr::new(name, env.pool);
-                            env.pool[node_id] = (poolstr, variable);
-                        }
-
-                        for (node_id, rigid) in unnamed.iter_node_ids().zip(unnamed_rigids) {
-                            env.pool[node_id] = rigid;
-                        }
-
-                        Rigids { named, unnamed }
-                    };
+                    let rigids = Rigids::new(named_rigids, unnamed_rigids, env.pool);
 
                     let annotation = match signature {
                         Signature::Value { annotation } => annotation,
@@ -470,21 +447,7 @@ fn canonicalize_pending_def<'a>(
                         output.references.referenced_aliases.insert(symbol);
                     }
 
-                    let rigids = {
-                        let named = PoolVec::with_capacity(named_rigids.len() as u32, env.pool);
-                        let unnamed = PoolVec::with_capacity(unnamed_rigids.len() as u32, env.pool);
-
-                        for (node_id, (name, variable)) in named.iter_node_ids().zip(named_rigids) {
-                            let poolstr = PoolStr::new(name, env.pool);
-                            env.pool[node_id] = (poolstr, variable);
-                        }
-
-                        for (node_id, rigid) in unnamed.iter_node_ids().zip(unnamed_rigids) {
-                            env.pool[node_id] = rigid;
-                        }
-
-                        Rigids { named, unnamed }
-                    };
+                    let rigids = Rigids::new(named_rigids, unnamed_rigids, env.pool);
 
                     // bookkeeping for tail-call detection. If we're assigning to an
                     // identifier (e.g. `f = \x -> ...`), then this symbol can be tail-called.
@@ -624,9 +587,11 @@ fn canonicalize_pending_def<'a>(
                             };
                             let annotation = env.add(annotation, loc_ann.region);
 
-                            let value_def = ValueDef {
-                                pattern: loc_can_pattern,
-                                expr_type: Some((annotation, rigids)),
+                            let value_def = ValueDef::WithAnnotation {
+                                pattern_id: loc_can_pattern,
+                                expr_id: env.pool.add(loc_can_expr),
+                                type_id: annotation,
+                                rigids: rigids,
                                 expr_var: env.var_store.fresh(),
                             };
 
@@ -745,9 +710,9 @@ fn canonicalize_pending_def<'a>(
                 }
 
                 _ => {
-                    let value_def = ValueDef {
-                        pattern: loc_can_pattern,
-                        expr_type: None,
+                    let value_def = ValueDef::NoAnnotation {
+                        pattern_id: loc_can_pattern,
+                        expr_id: env.pool.add(loc_can_expr),
                         expr_var: env.var_store.fresh(),
                     };
 
diff --git a/editor/src/lang/expr.rs b/editor/src/lang/expr.rs
index cd227fcf3f..3a0662b9f1 100644
--- a/editor/src/lang/expr.rs
+++ b/editor/src/lang/expr.rs
@@ -1,34 +1,93 @@
 #![allow(clippy::all)]
 #![allow(dead_code)]
 #![allow(unused_imports)]
-use crate::lang::ast::expr2_to_string;
-use crate::lang::ast::RecordField;
-use crate::lang::ast::{ClosureExtra, Expr2, ExprId, FloatVal, IntStyle, IntVal, WhenBranch};
-use crate::lang::def::References;
-use crate::lang::pattern::to_pattern2;
+use bumpalo::{collections::Vec as BumpVec, Bump};
+use inlinable_string::InlinableString;
+use std::collections::HashMap;
+
+use crate::lang::ast::{
+    expr2_to_string, ClosureExtra, Expr2, ExprId, FloatVal, IntStyle, IntVal, RecordField,
+    WhenBranch,
+};
+use crate::lang::def::{
+    canonicalize_defs, sort_can_defs, CanDefs, Declaration, Def, PendingDef, References,
+};
+use crate::lang::pattern::{to_pattern2, Pattern2, PatternId};
 use crate::lang::pool::{NodeId, Pool, PoolStr, PoolVec, ShallowClone};
 use crate::lang::scope::Scope;
-use crate::lang::types::{Alias, Type2, TypeId};
-use bumpalo::Bump;
-use inlinable_string::InlinableString;
+use crate::lang::types::{Alias, Annotation2, Type2, TypeId};
+
 use roc_can::expr::Recursive;
 use roc_can::num::{finish_parsing_base, finish_parsing_float, finish_parsing_int};
+use roc_can::operator::desugar_expr;
+use roc_collections::all::default_hasher;
 use roc_collections::all::{MutMap, MutSet};
+use roc_module::ident::Lowercase;
 use roc_module::ident::ModuleName;
 use roc_module::low_level::LowLevel;
 use roc_module::operator::CalledVia;
 use roc_module::symbol::{IdentIds, ModuleId, ModuleIds, Symbol};
 use roc_parse::ast;
+use roc_parse::ast::Expr;
 use roc_parse::ast::StrLiteral;
 use roc_parse::parser::{loc, Parser, State, SyntaxError};
+use roc_parse::pattern::PatternType;
 use roc_problem::can::{Problem, RuntimeError};
 use roc_region::all::{Located, Region};
 use roc_types::subs::{VarStore, Variable};
 
+#[derive(Clone, Debug, PartialEq, Default)]
+pub struct IntroducedVariables {
+    // Rigids must be unique within a type annoation.
+    // E.g. in `identity : a -> a`, there should only be one
+    // variable (a rigid one, with name "a").
+    // Hence `rigids : Map<Lowercase, Variable>`
+    //
+    // But then between annotations, the same name can occur multiple times,
+    // but a variable can only have one name. Therefore
+    // `ftv : Map<Variable, Lowercase>`.
+    pub wildcards: Vec<Variable>,
+    pub var_by_name: MutMap<Lowercase, Variable>,
+    pub name_by_var: MutMap<Variable, Lowercase>,
+    pub host_exposed_aliases: MutMap<Symbol, Variable>,
+}
+
+impl IntroducedVariables {
+    pub fn insert_named(&mut self, name: Lowercase, var: Variable) {
+        self.var_by_name.insert(name.clone(), var);
+        self.name_by_var.insert(var, name);
+    }
+
+    pub fn insert_wildcard(&mut self, var: Variable) {
+        self.wildcards.push(var);
+    }
+
+    pub fn insert_host_exposed_alias(&mut self, symbol: Symbol, var: Variable) {
+        self.host_exposed_aliases.insert(symbol, var);
+    }
+
+    pub fn union(&mut self, other: &Self) {
+        self.wildcards.extend(other.wildcards.iter().cloned());
+        self.var_by_name.extend(other.var_by_name.clone());
+        self.name_by_var.extend(other.name_by_var.clone());
+        self.host_exposed_aliases
+            .extend(other.host_exposed_aliases.clone());
+    }
+
+    pub fn var_by_name(&self, name: &Lowercase) -> Option<&Variable> {
+        self.var_by_name.get(name)
+    }
+
+    pub fn name_by_var(&self, var: Variable) -> Option<&Lowercase> {
+        self.name_by_var.get(&var)
+    }
+}
+
 #[derive(Clone, Default, Debug, PartialEq)]
 pub struct Output {
     pub references: References,
     pub tail_call: Option<Symbol>,
+    pub introduced_variables: IntroducedVariables,
     pub aliases: MutMap<Symbol, NodeId<Alias>>,
     pub non_closures: MutSet<Symbol>,
 }
@@ -53,6 +112,8 @@ pub struct Env<'a> {
     pub pool: &'a mut Pool,
     pub arena: &'a Bump,
 
+    pub problems: BumpVec<'a, Problem>,
+
     pub dep_idents: MutMap<ModuleId, IdentIds>,
     pub module_ids: &'a ModuleIds,
     pub ident_ids: IdentIds,
@@ -82,6 +143,7 @@ impl<'a> Env<'a> {
             home,
             arena,
             pool,
+            problems: BumpVec::new_in(arena),
             var_store,
             dep_idents,
             module_ids,
@@ -102,8 +164,8 @@ impl<'a> Env<'a> {
         id
     }
 
-    pub fn problem(&mut self, _problem: Problem) {
-        todo!();
+    pub fn problem(&mut self, problem: Problem) {
+        self.problems.push(problem);
     }
 
     pub fn set_region<T>(&mut self, _node_id: NodeId<T>, _region: Region) {
@@ -236,7 +298,16 @@ pub fn str_to_expr2<'a>(
     region: Region,
 ) -> Result<(Expr2, self::Output), SyntaxError<'a>> {
     match roc_parse::test_helpers::parse_loc_with(arena, input.trim()) {
-        Ok(loc_expr) => Ok(to_expr2(env, scope, arena.alloc(loc_expr.value), region)),
+        Ok(loc_expr) => {
+            let desugared_loc_expr = desugar_expr(arena, arena.alloc(loc_expr));
+
+            Ok(to_expr2(
+                env,
+                scope,
+                arena.alloc(desugared_loc_expr.value),
+                region,
+            ))
+        }
         Err(fail) => Err(fail),
     }
 }
@@ -248,6 +319,7 @@ pub fn to_expr2<'a>(
     region: Region,
 ) -> (Expr2, self::Output) {
     use roc_parse::ast::Expr::*;
+
     match parse_expr {
         Float(string) => {
             match finish_parsing_float(string) {
@@ -335,18 +407,19 @@ pub fn to_expr2<'a>(
             let mut output = Output::default();
             let output_ref = &mut output;
 
-            let elems = PoolVec::with_capacity(items.len() as u32, env.pool);
+            let elems: PoolVec<NodeId<Expr2>> =
+                PoolVec::with_capacity(items.len() as u32, env.pool);
 
             for (node_id, item) in elems.iter_node_ids().zip(items.iter()) {
                 let (expr, sub_output) = to_expr2(env, scope, &item.value, item.region);
 
                 output_ref.union(sub_output);
 
-                env.pool[node_id] = expr;
+                let expr_id = env.pool.add(expr);
+                env.pool[node_id] = expr_id;
             }
 
             let expr = Expr2::List {
-                list_var: env.var_store.fresh(),
                 elem_var: env.var_store.fresh(),
                 elems,
             };
@@ -516,7 +589,7 @@ pub fn to_expr2<'a>(
                 output.references.union_mut(cond_output.references);
                 output.references.union_mut(then_output.references);
 
-                new_branches.push((cond, then_expr));
+                new_branches.push((env.pool.add(cond), env.pool.add(then_expr)));
             }
 
             let (else_expr, else_output) =
@@ -555,7 +628,7 @@ pub fn to_expr2<'a>(
 
             // A "when" with no branches is a runtime error, but it will mess things up
             // if code gen mistakenly thinks this is a tail call just because its condition
-            // happend to be one. (The condition gave us our initial output value.)
+            // happened to be one. (The condition gave us our initial output value.)
             if branches.is_empty() {
                 output.tail_call = None;
             }
@@ -781,7 +854,50 @@ pub fn to_expr2<'a>(
         }
 
         Defs(loc_defs, loc_ret) => {
-            todo!("{:?} {:?}", loc_defs, loc_ret)
+            let (unsorted, mut scope, defs_output, symbols_introduced) = canonicalize_defs(
+                env,
+                Output::default(),
+                &scope,
+                loc_defs,
+                PatternType::DefExpr,
+            );
+
+            // The def as a whole is a tail call iff its return expression is a tail call.
+            // Use its output as a starting point because its tail_call already has the right answer!
+            let (ret_expr, mut output) = to_expr2(env, &mut scope, &loc_ret.value, loc_ret.region);
+
+            output
+                .introduced_variables
+                .union(&defs_output.introduced_variables);
+
+            output.references.union_mut(defs_output.references);
+
+            // Now that we've collected all the references, check to see if any of the new idents
+            // we defined went unused by the return expression. If any were unused, report it.
+            for (symbol, region) in symbols_introduced {
+                if !output.references.has_lookup(symbol) {
+                    env.problem(Problem::UnusedDef(symbol, region));
+                }
+            }
+
+            let (can_defs, output) = sort_can_defs(env, unsorted, output);
+
+            match can_defs {
+                Ok(decls) => {
+                    let mut expr = ret_expr;
+
+                    for declaration in decls.into_iter().rev() {
+                        expr = decl_to_let(env.pool, env.var_store, declaration, expr);
+                    }
+
+                    (expr, output)
+                }
+                Err(_err) => {
+                    // TODO: fix this to be something from Expr2
+                    // (RuntimeError(err), output)
+                    todo!()
+                }
+            }
         }
 
         PrecedenceConflict { .. } => {
@@ -1263,11 +1379,10 @@ fn canonicalize_lookup(
 
                 Var(symbol)
             }
-            Err(_problem) => {
-                // env.problem(Problem::RuntimeError(problem.clone()));
+            Err(problem) => {
+                env.problem(Problem::RuntimeError(problem.clone()));
 
-                // RuntimeError(problem)
-                todo!()
+                RuntimeError()
             }
         }
     } else {
@@ -1279,14 +1394,12 @@ fn canonicalize_lookup(
 
                 Var(symbol)
             }
-            Err(_problem) => {
+            Err(problem) => {
                 // Either the module wasn't imported, or
                 // it was imported but it doesn't expose this ident.
-                // env.problem(Problem::RuntimeError(problem.clone()));
+                env.problem(Problem::RuntimeError(problem.clone()));
 
-                // RuntimeError(problem)
-
-                todo!()
+                RuntimeError()
             }
         }
     };
@@ -1295,3 +1408,59 @@ fn canonicalize_lookup(
 
     (can_expr, output)
 }
+
+fn decl_to_let(pool: &mut Pool, var_store: &mut VarStore, decl: Declaration, ret: Expr2) -> Expr2 {
+    match decl {
+        Declaration::Declare(def) => match def {
+            Def::AnnotationOnly { .. } => todo!(),
+            Def::Value(value_def) => {
+                let def_id = pool.add(value_def);
+                let body_id = pool.add(ret);
+
+                Expr2::LetValue {
+                    def_id,
+                    body_id,
+                    body_var: var_store.fresh(),
+                }
+            }
+            Def::Function(function_def) => {
+                let def_id = pool.add(function_def);
+                let body_id = pool.add(ret);
+
+                Expr2::LetFunction {
+                    def_id,
+                    body_id,
+                    body_var: var_store.fresh(),
+                }
+            }
+        },
+        Declaration::DeclareRec(defs) => {
+            let mut function_defs = vec![];
+
+            for def in defs {
+                match def {
+                    Def::AnnotationOnly { .. } => todo!(),
+                    Def::Function(function_def) => function_defs.push(function_def),
+                    Def::Value(_) => unreachable!(),
+                }
+            }
+
+            let body_id = pool.add(ret);
+
+            Expr2::LetRec {
+                defs: PoolVec::new(function_defs.into_iter(), pool),
+                body_var: var_store.fresh(),
+                body_id,
+            }
+        }
+        Declaration::InvalidCycle(_entries, _) => {
+            // TODO: replace with something from Expr2
+            // Expr::RuntimeError(RuntimeError::CircularDef(entries))
+            todo!()
+        }
+        Declaration::Builtin(_) => {
+            // Builtins should only be added to top-level decls, not to let-exprs!
+            unreachable!()
+        }
+    }
+}
diff --git a/editor/src/lang/module.rs b/editor/src/lang/module.rs
index a8b33481df..c0494d3193 100644
--- a/editor/src/lang/module.rs
+++ b/editor/src/lang/module.rs
@@ -2,7 +2,7 @@
 #![allow(dead_code)]
 #![allow(unused_imports)]
 #![allow(unused_variables)]
-use crate::lang::ast::{FunctionDef, ValueDef};
+use crate::lang::ast::{Expr2, FunctionDef, ValueDef};
 use crate::lang::def::{canonicalize_defs, sort_can_defs, Declaration, Def};
 use crate::lang::expr::Env;
 use crate::lang::expr::Output;
@@ -249,10 +249,11 @@ pub fn canonicalize_module_defs<'a>(
                 let runtime_error = RuntimeError::ExposedButNotDefined(symbol);
 
                 let value_def = {
-                    let pattern = env.pool.add(Pattern2::Identifier(symbol));
-                    ValueDef {
-                        pattern,
-                        expr_type: None,
+                    let pattern_id = env.pool.add(Pattern2::Identifier(symbol));
+                    let expr_id = env.pool.add(Expr2::RuntimeError());
+                    ValueDef::NoAnnotation {
+                        pattern_id,
+                        expr_id,
                         expr_var: env.var_store.fresh(),
                     }
                 };
diff --git a/editor/src/lang/pattern.rs b/editor/src/lang/pattern.rs
index e3f6dc8160..03ed3c821f 100644
--- a/editor/src/lang/pattern.rs
+++ b/editor/src/lang/pattern.rs
@@ -5,8 +5,10 @@ use crate::lang::ast::{ExprId, FloatVal, IntVal};
 use crate::lang::expr::{to_expr_id, Env, Output};
 use crate::lang::pool::{NodeId, Pool, PoolStr, PoolVec, ShallowClone};
 use crate::lang::scope::Scope;
+use bumpalo::collections::Vec as BumpVec;
 use roc_can::expr::unescape_char;
 use roc_can::num::{finish_parsing_base, finish_parsing_float, finish_parsing_int};
+use roc_collections::all::BumpMap;
 use roc_module::symbol::Symbol;
 use roc_parse::ast::{StrLiteral, StrSegment};
 use roc_parse::pattern::PatternType;
@@ -14,6 +16,9 @@ use roc_problem::can::{MalformedPatternProblem, Problem, RuntimeError};
 use roc_region::all::Region;
 use roc_types::subs::Variable;
 
+use super::constrain::Constraint;
+use super::types::Type2;
+
 pub type PatternId = NodeId<Pattern2>;
 
 #[derive(Debug)]
@@ -25,16 +30,16 @@ pub enum Pattern2 {
     StrLiteral(PoolStr),       // 8B
     Underscore,                // 0B
     GlobalTag {
-        whole_var: Variable,                      // 4B
-        ext_var: Variable,                        // 4B
-        tag_name: PoolStr,                        // 8B
-        arguments: PoolVec<(Variable, Pattern2)>, // 8B
+        whole_var: Variable,                       // 4B
+        ext_var: Variable,                         // 4B
+        tag_name: PoolStr,                         // 8B
+        arguments: PoolVec<(Variable, PatternId)>, // 8B
     },
     PrivateTag {
-        whole_var: Variable,                      // 4B
-        ext_var: Variable,                        // 4B
-        tag_name: Symbol,                         // 8B
-        arguments: PoolVec<(Variable, Pattern2)>, // 8B
+        whole_var: Variable,                       // 4B
+        ext_var: Variable,                         // 4B
+        tag_name: Symbol,                          // 8B
+        arguments: PoolVec<(Variable, PatternId)>, // 8B
     },
     RecordDestructure {
         whole_var: Variable,                // 4B
@@ -43,10 +48,13 @@ pub enum Pattern2 {
     },
 
     // Runtime Exceptions
+    // TODO: figure out how to better handle regions
+    // to keep this member under 32. With 2 Regions
+    // it ends up at size 40
     Shadowed {
         shadowed_ident: PoolStr,
-        definition: Region,
-        shadowed_at: Region,
+        // definition: Region,
+        // shadowed_at: Region,
     },
 
     /// Example: (5 = 1 + 2) is an unsupported pattern in an assignment; Int patterns aren't allowed in assignments!
@@ -61,6 +69,13 @@ impl ShallowClone for Pattern2 {
     }
 }
 
+#[derive(Debug)]
+pub struct PatternState2<'a> {
+    pub headers: BumpMap<Symbol, Type2>,
+    pub vars: BumpVec<'a, Variable>,
+    pub constraints: BumpVec<'a, Constraint<'a>>,
+}
+
 #[derive(Debug)]
 pub struct RecordDestruct {
     pub var: Variable,             // 4B
@@ -140,8 +155,6 @@ pub fn to_pattern2<'a>(
 
                 Pattern2::Shadowed {
                     shadowed_ident: PoolStr::new(name, env.pool),
-                    shadowed_at: region,
-                    definition: original_region,
                 }
             }
         },
@@ -238,7 +251,9 @@ pub fn to_pattern2<'a>(
 
                 output.union(new_output);
 
-                env.pool[node_id] = (env.var_store.fresh(), can_pattern);
+                let can_pattern_id = env.pool.add(can_pattern);
+
+                env.pool[node_id] = (env.var_store.fresh(), can_pattern_id);
             }
 
             match tag.value {
@@ -434,7 +449,8 @@ pub fn symbols_from_pattern(pool: &Pool, initial: &Pattern2) -> Vec<Symbol> {
             }
 
             GlobalTag { arguments, .. } | PrivateTag { arguments, .. } => {
-                for (_, pat) in arguments.iter(pool) {
+                for (_, pat_id) in arguments.iter(pool) {
+                    let pat = pool.get(*pat_id);
                     stack.push(pat);
                 }
             }
@@ -482,7 +498,8 @@ pub fn symbols_and_variables_from_pattern(
             }
 
             GlobalTag { arguments, .. } | PrivateTag { arguments, .. } => {
-                for (var, pat) in arguments.iter(pool) {
+                for (var, pat_id) in arguments.iter(pool) {
+                    let pat = pool.get(*pat_id);
                     stack.push((*var, pat));
                 }
             }
diff --git a/editor/src/lang/pool.rs b/editor/src/lang/pool.rs
index 435f49fdf5..e68199e339 100644
--- a/editor/src/lang/pool.rs
+++ b/editor/src/lang/pool.rs
@@ -12,6 +12,7 @@
 /// This is important for performance.
 use libc::{c_void, MAP_ANONYMOUS, MAP_PRIVATE, PROT_READ, PROT_WRITE};
 use roc_can::expected::Expected;
+use roc_can::expected::PExpected;
 use std::cmp::Ordering;
 use std::marker::PhantomData;
 use std::mem::size_of;
@@ -42,6 +43,23 @@ pub const NODE_BYTES: usize = 32;
 //   usize pointers, which would be too big for us to have 16B nodes.
 //   On the plus side, we could be okay with higher memory usage early on,
 //   and then later use the Mesh strategy to reduce long-running memory usage.
+//
+// With this system, we can allocate up to 4B nodes. If we wanted to keep
+// a generational index in there, like https://crates.io/crates/sharded-slab
+// does, we could use some of the 32 bits for that. For example, if we wanted
+// to have a 5-bit generational index (supporting up to 32 generations), then
+// we would have 27 bits remaining, meaning we could only support at most
+// 134M nodes. Since the editor has a separate Pool for each module, is that
+// enough for any single module we'll encounter in practice? Probably, and
+// especially if we allocate super large collection literals on the heap instead
+// of in the pool.
+//
+// Another possible design is to try to catch reuse bugs using an "ASan" like
+// approach: in development builds, whenever we "free" a particular slot, we
+// can add it to a dev-build-only "freed nodes" list and don't hand it back
+// out (so, we leak the memory.) Then we can (again, in development builds only)
+// check to see if we're about to store something in zeroed-out memory; if so, check
+// to see if it was
 
 #[derive(Debug, Eq)]
 pub struct NodeId<T> {
@@ -334,11 +352,11 @@ impl<'a, T: 'a + Sized> PoolVec<T> {
             let index = first_node_id.index as isize;
             let mut next_node_ptr = unsafe { pool.nodes.offset(index) } as *mut T;
 
-            for node in nodes {
+            for (indx_inc, node) in nodes.enumerate() {
                 unsafe {
                     *next_node_ptr = node;
 
-                    next_node_ptr = next_node_ptr.offset(1);
+                    next_node_ptr = pool.nodes.offset(index + (indx_inc as isize) + 1) as *mut T;
                 }
             }
 
@@ -585,6 +603,17 @@ impl<T> Iterator for PoolVecIterNodeIds<T> {
     }
 }
 
+#[test]
+fn pool_vec_iter_test() {
+    let expected_vec: Vec<usize> = vec![2, 4, 8, 16];
+
+    let mut test_pool = Pool::with_capacity(1024);
+    let pool_vec = PoolVec::new(expected_vec.clone().into_iter(), &mut test_pool);
+
+    let current_vec: Vec<usize> = pool_vec.iter(&test_pool).copied().collect();
+
+    assert_eq!(current_vec, expected_vec);
+}
 /// Clones the outer node, but does not clone any nodeids
 pub trait ShallowClone {
     fn shallow_clone(&self) -> Self;
@@ -603,3 +632,14 @@ impl<T: ShallowClone> ShallowClone for Expected<T> {
         }
     }
 }
+
+impl<T: ShallowClone> ShallowClone for PExpected<T> {
+    fn shallow_clone(&self) -> Self {
+        use PExpected::*;
+
+        match self {
+            NoExpectation(t) => NoExpectation(t.shallow_clone()),
+            ForReason(reason, t, region) => ForReason(reason.clone(), t.shallow_clone(), *region),
+        }
+    }
+}
diff --git a/editor/src/lang/solve.rs b/editor/src/lang/solve.rs
index eac4fe4bf5..f248cc4999 100644
--- a/editor/src/lang/solve.rs
+++ b/editor/src/lang/solve.rs
@@ -363,45 +363,47 @@ fn solve<'a>(
 
             state
         }
-        //        Pattern(region, category, typ, expectation) => {
-        //            let actual = type_to_var(subs, rank, pools, cached_aliases, typ);
-        //            let expected = type_to_var(
-        //                subs,
-        //                rank,
-        //                pools,
-        //                cached_aliases,
-        //                expectation.get_type_ref(),
-        //            );
-        //
-        //            match unify(subs, actual, expected) {
-        //                Success(vars) => {
-        //                    introduce(subs, rank, pools, &vars);
-        //
-        //                    state
-        //                }
-        //                Failure(vars, actual_type, expected_type) => {
-        //                    introduce(subs, rank, pools, &vars);
-        //
-        //                    let problem = TypeError::BadPattern(
-        //                        *region,
-        //                        category.clone(),
-        //                        actual_type,
-        //                        expectation.clone().replace(expected_type),
-        //                    );
-        //
-        //                    problems.push(problem);
-        //
-        //                    state
-        //                }
-        //                BadType(vars, problem) => {
-        //                    introduce(subs, rank, pools, &vars);
-        //
-        //                    problems.push(TypeError::BadType(problem));
-        //
-        //                    state
-        //                }
-        //            }
-        //        }
+        Pattern(region, category, typ, expectation) => {
+            let actual = type_to_var(arena, mempool, subs, rank, pools, cached_aliases, typ);
+            let expected = type_to_var(
+                arena,
+                mempool,
+                subs,
+                rank,
+                pools,
+                cached_aliases,
+                expectation.get_type_ref(),
+            );
+
+            match unify(subs, actual, expected) {
+                Success(vars) => {
+                    introduce(subs, rank, pools, &vars);
+
+                    state
+                }
+                Failure(vars, actual_type, expected_type) => {
+                    introduce(subs, rank, pools, &vars);
+
+                    let problem = TypeError::BadPattern(
+                        *region,
+                        category.clone(),
+                        actual_type,
+                        expectation.shallow_clone().replace(expected_type),
+                    );
+
+                    problems.push(problem);
+
+                    state
+                }
+                BadType(vars, problem) => {
+                    introduce(subs, rank, pools, &vars);
+
+                    problems.push(TypeError::BadType(problem));
+
+                    state
+                }
+            }
+        }
         Let(let_con) => {
             match &let_con.ret_constraint {
                 True if let_con.rigid_vars.is_empty() => {
@@ -439,22 +441,16 @@ fn solve<'a>(
                     // Add a variable for each def to new_vars_by_env.
                     let mut local_def_vars = BumpMap::new_in(arena);
 
-                    for (symbol, loc_type) in let_con.def_types.iter() {
-                        let var = type_to_var(
-                            arena,
-                            mempool,
-                            subs,
-                            rank,
-                            pools,
-                            cached_aliases,
-                            &loc_type.value,
-                        );
+                    for (symbol, typ) in let_con.def_types.iter() {
+                        let var =
+                            type_to_var(arena, mempool, subs, rank, pools, cached_aliases, typ);
 
+                        // TODO: region should come from typ
                         local_def_vars.insert(
                             *symbol,
                             Located {
                                 value: var,
-                                region: loc_type.region,
+                                region: Region::zero(),
                             },
                         );
                     }
@@ -521,9 +517,7 @@ fn solve<'a>(
                     // Add a variable for each def to local_def_vars.
                     let mut local_def_vars = BumpMap::new_in(arena);
 
-                    for (symbol, loc_type) in let_con.def_types.iter() {
-                        let def_type = &loc_type.value;
-
+                    for (symbol, typ) in let_con.def_types.iter() {
                         let var = type_to_var(
                             arena,
                             mempool,
@@ -531,14 +525,15 @@ fn solve<'a>(
                             next_rank,
                             next_pools,
                             cached_aliases,
-                            def_type,
+                            typ,
                         );
 
+                        // TODO: region should come from type
                         local_def_vars.insert(
                             *symbol,
                             Located {
                                 value: var,
-                                region: loc_type.region,
+                                region: Region::zero(),
                             },
                         );
                     }
@@ -776,7 +771,7 @@ fn type_to_variable<'a>(
         Alias(symbol, args, alias_type_id) => {
             // TODO cache in uniqueness inference gives problems! all Int's get the same uniqueness var!
             // Cache aliases without type arguments. Commonly used aliases like `Int` would otherwise get O(n)
-            // different variables (once for each occurence). The recursion restriction is required
+            // different variables (once for each occurrence). The recursion restriction is required
             // for uniqueness types only: recursive aliases "introduce" an unbound uniqueness
             // attribute in the body, when
             //
diff --git a/editor/src/ui/text/selection.rs b/editor/src/ui/text/selection.rs
index 03a959f986..70376b6c86 100644
--- a/editor/src/ui/text/selection.rs
+++ b/editor/src/ui/text/selection.rs
@@ -98,8 +98,6 @@ pub fn create_selection_rects<'a>(
             height,
             color: theme.select_highlight,
         });
-
-        Ok(all_rects)
     } else {
         // first line
         let end_col = lines.line_len(start_pos.line)?;
@@ -148,7 +146,7 @@ pub fn create_selection_rects<'a>(
                 color: theme.select_highlight,
             });
         }
-
-        Ok(all_rects)
     }
+
+    Ok(all_rects)
 }
diff --git a/editor/tests/solve_expr2.rs b/editor/tests/solve_expr2.rs
index 4685f3e2bb..a25c88b077 100644
--- a/editor/tests/solve_expr2.rs
+++ b/editor/tests/solve_expr2.rs
@@ -62,7 +62,6 @@ fn infer_eq(actual: &str, expected_str: &str) {
     let mut var_store = VarStore::default();
     let var = var_store.fresh();
     let dep_idents = IdentIds::exposed_builtins(8);
-
     let exposed_ident_ids = IdentIds::default();
     let mut module_ids = ModuleIds::default();
     let mod_id = module_ids.get_or_insert(&"ModId123".into());
@@ -118,9 +117,10 @@ fn infer_eq(actual: &str, expected_str: &str) {
             let content = subs.get(var).content;
 
             let interns = Interns {
-                module_ids,
+                module_ids: env.module_ids.clone(),
                 all_ident_ids: dep_idents,
             };
+
             let actual_str = content_to_string(content, &subs, mod_id, &interns);
 
             assert_eq!(actual_str, expected_str);
@@ -274,3 +274,57 @@ fn constrain_access() {
         "Str",
     )
 }
+
+#[test]
+fn constrain_if() {
+    infer_eq(
+        indoc!(
+            r#"
+            if True then Green else Red
+            "#
+        ),
+        "[ Green, Red ]*",
+    )
+}
+
+#[test]
+fn constrain_when() {
+    infer_eq(
+        indoc!(
+            r#"
+            when if True then Green else Red is
+                Green -> Blue
+                Red -> Purple
+            "#
+        ),
+        "[ Blue, Purple ]*",
+    )
+}
+
+#[test]
+fn constrain_let_value() {
+    infer_eq(
+        indoc!(
+            r#"
+            person = { name: "roc" }
+
+            person
+            "#
+        ),
+        "{ name : Str }",
+    )
+}
+
+#[test]
+fn constrain_update() {
+    infer_eq(
+        indoc!(
+            r#"
+            person = { name: "roc" }
+
+            { person & name: "bird" } 
+            "#
+        ),
+        "{ name : Str }",
+    )
+}
diff --git a/examples/balance/.gitignore b/examples/balance/.gitignore
deleted file mode 100644
index ba2906d066..0000000000
--- a/examples/balance/.gitignore
+++ /dev/null
@@ -1 +0,0 @@
-main
diff --git a/examples/balance/Main.roc b/examples/balance/Main.roc
deleted file mode 100644
index 441dfb50f3..0000000000
--- a/examples/balance/Main.roc
+++ /dev/null
@@ -1,7 +0,0 @@
-app "main" imports [ Effect ] provides [ rocMain ] to "./platform"
-
-
-rocMain : Effect.Effect {} as Fx
-rocMain =
-    when List.len (Str.split "hello" "JJJJ there") is
-        _  -> Effect.putLine "Yay"
diff --git a/examples/balance/platform/Cargo.lock b/examples/balance/platform/Cargo.lock
deleted file mode 100644
index c386bb6c4a..0000000000
--- a/examples/balance/platform/Cargo.lock
+++ /dev/null
@@ -1,23 +0,0 @@
-# This file is automatically @generated by Cargo.
-# It is not intended for manual editing.
-[[package]]
-name = "host"
-version = "0.1.0"
-dependencies = [
- "roc_std 0.1.0",
-]
-
-[[package]]
-name = "libc"
-version = "0.2.79"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-
-[[package]]
-name = "roc_std"
-version = "0.1.0"
-dependencies = [
- "libc 0.2.79 (registry+https://github.com/rust-lang/crates.io-index)",
-]
-
-[metadata]
-"checksum libc 0.2.79 (registry+https://github.com/rust-lang/crates.io-index)" = "2448f6066e80e3bfc792e9c98bf705b4b0fc6e8ef5b43e5889aff0eaa9c58743"
diff --git a/examples/balance/platform/Cargo.toml b/examples/balance/platform/Cargo.toml
deleted file mode 100644
index b0a2a5e96e..0000000000
--- a/examples/balance/platform/Cargo.toml
+++ /dev/null
@@ -1,14 +0,0 @@
-[package]
-name = "host"
-version = "0.1.0"
-authors = ["The Roc Contributors"]
-license = "UPL-1.0"
-edition = "2018"
-
-[lib]
-crate-type = ["staticlib"]
-
-[dependencies]
-roc_std = { path = "../../../roc_std" }
-
-[workspace]
diff --git a/examples/balance/platform/Package-Config.roc b/examples/balance/platform/Package-Config.roc
deleted file mode 100644
index 0d66513ebe..0000000000
--- a/examples/balance/platform/Package-Config.roc
+++ /dev/null
@@ -1,15 +0,0 @@
-platform folkertdev/foo
-    requires { rocMain : Effect {} }
-    exposes []
-    packages {}
-    imports []
-    provides [ mainForHost ]
-    effects Effect
-        {
-            putChar : Int -> Effect {},
-            putLine : Str -> Effect {},
-            getLine : Effect Str
-        }
-
-mainForHost : Effect {} as Fx
-mainForHost = rocMain
diff --git a/examples/balance/platform/host.c b/examples/balance/platform/host.c
deleted file mode 100644
index 0378c69589..0000000000
--- a/examples/balance/platform/host.c
+++ /dev/null
@@ -1,7 +0,0 @@
-#include <stdio.h>
-
-extern int rust_main();
-
-int main() {
-  return rust_main();
-}
diff --git a/examples/balance/platform/src/lib.rs b/examples/balance/platform/src/lib.rs
deleted file mode 100644
index 5bff358b10..0000000000
--- a/examples/balance/platform/src/lib.rs
+++ /dev/null
@@ -1,147 +0,0 @@
-#![allow(non_snake_case)]
-
-use roc_std::alloca;
-use roc_std::RocCallResult;
-use roc_std::RocStr;
-use std::alloc::Layout;
-use std::ffi::c_void;
-use std::time::SystemTime;
-
-extern "C" {
-    #[link_name = "roc__rocMain_1_exposed"]
-    fn roc_main(output: *mut u8) -> ();
-
-    #[link_name = "roc__rocMain_1_size"]
-    fn roc_main_size() -> i64;
-
-    #[link_name = "roc__rocMain_1_Fx_caller"]
-    fn call_Fx(function_pointer: *const u8, closure_data: *const u8, output: *mut u8) -> ();
-
-    #[link_name = "roc__rocMain_1_Fx_size"]
-    fn size_Fx() -> i64;
-
-    fn malloc(size: usize) -> *mut c_void;
-    fn realloc(c_ptr: *mut c_void, size: usize) -> *mut c_void;
-    fn free(c_ptr: *mut c_void);
-}
-
-#[no_mangle]
-pub unsafe fn roc_alloc(size: usize, _alignment: u32) -> *mut c_void {
-    return malloc(size);
-}
-
-#[no_mangle]
-pub unsafe fn roc_realloc(
-    c_ptr: *mut c_void,
-    new_size: usize,
-    _old_size: usize,
-    _alignment: u32,
-) -> *mut c_void {
-    return realloc(c_ptr, new_size);
-}
-
-#[no_mangle]
-pub unsafe fn roc_dealloc(c_ptr: *mut c_void, _alignment: u32) {
-    return free(c_ptr);
-}
-
-#[no_mangle]
-pub fn roc_fx_putChar(foo: i64) -> () {
-    let character = foo as u8 as char;
-    print!("{}", character);
-
-    ()
-}
-
-#[no_mangle]
-pub fn roc_fx_putLine(line: RocStr) -> () {
-    let bytes = line.as_slice();
-    let string = unsafe { std::str::from_utf8_unchecked(bytes) };
-    println!("{}", string);
-
-    ()
-}
-
-#[no_mangle]
-pub fn roc_fx_getLine() -> RocStr {
-    use std::io::{self, BufRead};
-
-    let stdin = io::stdin();
-    let line1 = stdin.lock().lines().next().unwrap().unwrap();
-
-    RocStr::from_slice_with_capacity(line1.as_bytes(), line1.len())
-}
-
-unsafe fn call_the_closure(function_pointer: *const u8, closure_data_ptr: *const u8) -> i64 {
-    let size = size_Fx() as usize;
-
-    alloca::with_stack_bytes(size, |buffer| {
-        let buffer: *mut std::ffi::c_void = buffer;
-        let buffer: *mut u8 = buffer as *mut u8;
-
-        call_Fx(
-            function_pointer,
-            closure_data_ptr as *const u8,
-            buffer as *mut u8,
-        );
-
-        let output = &*(buffer as *mut RocCallResult<i64>);
-
-        match output.into() {
-            Ok(_) => 0,
-            Err(e) => panic!("failed with {}", e),
-        }
-    })
-}
-
-#[no_mangle]
-pub fn rust_main() -> isize {
-    println!("Running Roc closure");
-    let start_time = SystemTime::now();
-
-    let size = unsafe { roc_main_size() } as usize;
-    let layout = Layout::array::<u8>(size).unwrap();
-    let answer = unsafe {
-        let buffer = std::alloc::alloc(layout);
-
-        roc_main(buffer);
-
-        let output = &*(buffer as *mut RocCallResult<()>);
-
-        match output.into() {
-            Ok(()) => {
-                let function_pointer = {
-                    // this is a pointer to the location where the function pointer is stored
-                    // we pass just the function pointer
-                    let temp = buffer.offset(8) as *const i64;
-
-                    (*temp) as *const u8
-                };
-
-                let closure_data_ptr = buffer.offset(16);
-
-                let result =
-                    call_the_closure(function_pointer as *const u8, closure_data_ptr as *const u8);
-
-                std::alloc::dealloc(buffer, layout);
-
-                result
-            }
-            Err(msg) => {
-                std::alloc::dealloc(buffer, layout);
-
-                panic!("Roc failed with message: {}", msg);
-            }
-        }
-    };
-    let end_time = SystemTime::now();
-    let duration = end_time.duration_since(start_time).unwrap();
-
-    println!(
-        "Roc execution took {:.4} ms",
-        duration.as_secs_f64() * 1000.0,
-    );
-
-    // Exit code
-    0
-}
diff --git a/examples/benchmarks/CFold.roc b/examples/benchmarks/CFold.roc
index fc3468886c..fb4fa4eeca 100644
--- a/examples/benchmarks/CFold.roc
+++ b/examples/benchmarks/CFold.roc
@@ -7,15 +7,16 @@ app "cfold"
 
 main : Task.Task {} []
 main =
-    e = mkExpr 3 1
-    unoptimized = eval e
-    optimized = eval (constFolding (reassoc e))
+    Task.after Task.getInt \n ->
+        e = mkExpr n 1 # original koka n = 20 (set `ulimit -s unlimited` to avoid stack overflow for n = 20)
+        unoptimized = eval e
+        optimized = eval (constFolding (reassoc e))
 
-    unoptimized
-        |> Str.fromInt
-        |> Str.concat " & "
-        |> Str.concat (Str.fromInt optimized)
-        |> Task.putLine
+        unoptimized
+            |> Str.fromInt
+            |> Str.concat " & "
+            |> Str.concat (Str.fromInt optimized)
+            |> Task.putLine
 
 Expr : [
     Add Expr Expr,
@@ -81,9 +82,9 @@ constFolding = \e ->
 
             when Pair x1 x2 is
                 Pair (Val a) (Val b) -> Val (a+b)
-                # Pair (Val a) (Add (Val b) x) -> Add (Val (a+b)) x
+                Pair (Val a) (Add (Val b) x) -> Add (Val (a+b)) x
                 Pair (Val a) (Add x (Val b)) -> Add (Val (a+b)) x
-                Pair _ _                     -> Add x1 x2
+                Pair y1 y2                   -> Add y1 y2
 
         Mul e1 e2 ->
             x1 = constFolding e1
@@ -93,7 +94,7 @@ constFolding = \e ->
                 Pair (Val a) (Val b) -> Val (a*b)
                 Pair (Val a) (Mul (Val b) x) -> Mul (Val (a*b)) x
                 Pair (Val a) (Mul x (Val b)) -> Mul (Val (a*b)) x
-                Pair _ _                     -> Mul x1 x2
+                Pair y1 y2                   -> Add y1 y2
 
         _ -> e
 
diff --git a/examples/benchmarks/Closure.roc b/examples/benchmarks/Closure.roc
index 11ebf8402d..8e61ca4dfb 100644
--- a/examples/benchmarks/Closure.roc
+++ b/examples/benchmarks/Closure.roc
@@ -7,9 +7,9 @@ app "closure"
 
 main : Task.Task {} []
 main = closure1 {}
-    |> Task.after (\_ -> closure2 {})
-    |> Task.after (\_ -> closure3 {})
-    |> Task.after (\_ -> closure4 {})
+    # |> Task.after (\_ -> closure2 {})
+    # |> Task.after (\_ -> closure3 {})
+    # |> Task.after (\_ -> closure4 {})
 
 # ---
 
diff --git a/examples/benchmarks/Deriv.roc b/examples/benchmarks/Deriv.roc
index 642ae5dd79..cd8e5fd800 100644
--- a/examples/benchmarks/Deriv.roc
+++ b/examples/benchmarks/Deriv.roc
@@ -9,14 +9,15 @@ IO a : Task.Task a []
 
 main : IO {}
 main =
-    x : Expr
-    x = Var "x"
+    Task.after Task.getInt \n ->
+        x : Expr
+        x = Var "x"
 
-    f : Expr
-    f = pow x x
+        f : Expr
+        f = pow x x
 
-    nest deriv 2 f
-        |> Task.map (\_ -> {})
+        nest deriv n f # original koka n = 10
+            |> Task.map (\_ -> {})
 
 Expr : [ Val I64, Var Str, Add Expr Expr, Mul Expr Expr, Pow Expr Expr, Ln Expr ]
 
diff --git a/examples/benchmarks/NQueens.roc b/examples/benchmarks/NQueens.roc
index 9d5acaf2f5..7435e52f2c 100644
--- a/examples/benchmarks/NQueens.roc
+++ b/examples/benchmarks/NQueens.roc
@@ -5,10 +5,10 @@ app "nqueens"
 
 main : Task.Task {} []
 main =
-    # Task.after Task.getInt \n ->
-    queens 6
-        |> Str.fromInt
-        |> Task.putLine
+    Task.after Task.getInt \n ->
+        queens n # original koka 13
+            |> Str.fromInt
+            |> Task.putLine
 
 ConsList a : [ Nil, Cons a (ConsList a) ]
 
diff --git a/examples/benchmarks/Quicksort.roc b/examples/benchmarks/Quicksort.roc
index bbd2a90c12..271be43894 100644
--- a/examples/benchmarks/Quicksort.roc
+++ b/examples/benchmarks/Quicksort.roc
@@ -1,4 +1,4 @@
-interface Quicksort exposes [ sortBy, show ] imports []
+interface Quicksort exposes [ sortBy, sortWith, show ] imports []
 
 show : List I64 -> Str
 show = \list ->
diff --git a/examples/benchmarks/QuicksortApp.roc b/examples/benchmarks/QuicksortApp.roc
new file mode 100644
index 0000000000..a1290d2fae
--- /dev/null
+++ b/examples/benchmarks/QuicksortApp.roc
@@ -0,0 +1,23 @@
+app "quicksortapp"
+    packages { base: "platform" }
+    imports [base.Task, Quicksort]
+    provides [ main ] to base
+
+main : Task.Task {} []
+main =
+    Task.after Task.getInt \n ->
+        unsortedList =
+            if n == 0 then
+                # small unsorted list of 20 elements (0-9)
+                [ 2, 4, 4, 0, 3, 8, 3, 6, 4, 9, 4, 6, 3, 5, 2, 2, 3, 1, 1, 3 ]
+            else
+                # big unsorted list of 10000 elements (0-5000)
+                [ 4281, 4149, 4579, 3763, 4892, 3305, 740, 1003, 3748, 4353, 1027, 2205, 4096, 4047, 1883, 3757, 3813, 2757, 2241, 1417, 2054, 1819, 680, 3645, 1979, 3897, 2180, 4072, 3688, 3440, 1107, 3511, 133, 4586, 3432, 3279, 2743, 1489, 4058, 2880, 141, 3039, 3270, 1518, 3879, 3241, 577, 981, 233, 2238, 1571, 1056, 721, 2856, 2309, 1274, 969, 1132, 1492, 2659, 106, 1145, 2328, 3854, 998, 2594, 1359, 1172, 3952, 4137, 3539, 3558, 3947, 1705, 1726, 1292, 1669, 2636, 661, 1079, 4190, 4398, 3954, 2016, 4704, 2748, 2273, 261, 321, 1278, 917, 713, 1241, 1827, 1402, 2956, 4481, 3126, 250, 3932, 727, 2689, 2566, 4584, 1718, 1634, 3977, 3045, 3360, 1072, 982, 2277, 4175, 707, 1192, 3124, 2408, 2734, 4173, 4491, 3060, 1095, 845, 4405, 4617, 2762, 1431, 2720, 4510, 1516, 1770, 1010, 499, 2346, 4191, 2684, 3979, 1794, 2796, 1973, 407, 2764, 1975, 3629, 3945, 4635, 2157, 497, 1720, 583, 520, 2036, 3638, 561, 2168, 2301, 4432, 4086, 1465, 1560, 90, 4092, 2882, 3496, 3609, 4961, 507, 106, 242, 1752, 2154, 4410, 4066, 2333, 1570, 621, 1622, 1915, 517, 4424, 2901, 4704, 394, 1066, 4001, 4645, 624, 3833, 165, 2312, 3795, 3707, 557, 4567, 1897, 4894, 4840, 2805, 655, 1514, 2376, 3504, 1029, 4207, 3054, 4802, 4253, 2339, 2278, 3180, 2426, 861, 1582, 1373, 131, 4390, 4168, 4749, 1948, 2489, 586, 614, 2833, 840, 4136, 2443, 4112, 1850, 2207, 4087, 162, 1030, 1983, 2464, 2840, 4644, 2613, 1305, 4340, 3742, 1422, 1703, 1184, 2389, 997, 413, 3787, 199, 1727, 780, 1055, 879, 3400, 4592, 2140, 4564, 3466, 4630, 2127, 197, 4719, 4045, 2818, 2817, 2207, 507, 3047, 3619, 2327, 4196, 1659, 1603, 2104, 1649, 3893, 1615, 2905, 2216, 224, 2140, 4018, 2590, 3647, 3456, 2173, 1573, 4315, 2389, 2787, 137, 4287, 792, 4260, 1363, 3609, 3610, 2550, 2788, 1020, 4347, 2513, 4633, 3633, 3875, 4670, 503, 1698, 4651, 4725, 3394, 1085, 1275, 407, 4394, 132, 2102, 1239, 2186, 588, 1212, 4641, 4371, 4448, 1910, 3735, 4489, 2793, 2422, 2177, 3639, 2284, 3887, 2759, 4667, 2735, 976, 2055, 2791, 2593, 1406, 2439, 1010, 1160, 14, 3388, 3788, 3982, 3907, 3423, 996, 2863, 2370, 2846, 2297, 18, 1961, 1495, 290, 2390, 4248, 3001, 2365, 513, 2494, 1963, 3833, 3284, 375, 4760, 2711, 1436, 195, 1891, 3935, 4384, 1943, 3347, 126, 1582, 3977, 1398, 2483, 528, 230, 1896, 1421, 1843, 4487, 2893, 4240, 3157, 4135, 2887, 113, 1307, 1960, 593, 2435, 4358, 102, 4780, 537, 1394, 1901, 4554, 2099, 2599, 4467, 4145, 192, 1759, 4141, 4102, 3631, 3425, 467, 3017, 1210, 1108, 1243, 3980, 2772, 1525, 2382, 2795, 4731, 2688, 2303, 1511, 3260, 4863, 4792, 3306, 432, 3068, 1586, 3358, 1327, 3136, 3715, 1515, 4144, 4902, 3557, 3891, 1116, 172, 4655, 2079, 1759, 366, 1223, 2066, 155, 1343, 1106, 1016, 3055, 810, 1430, 4618, 1031, 4488, 2571, 240, 4196, 1896, 2887, 4930, 3694, 2395, 1156, 2889, 4749, 4109, 112, 1578, 450, 1422, 1505, 2266, 3756, 3597, 2655, 3748, 3371, 1659, 1867, 139, 879, 469, 1601, 2566, 3471, 1905, 4403, 2714, 1657, 3867, 563, 1820, 4992, 3084, 4559, 2727, 493, 1045, 4884, 1304, 4596, 4843, 978, 3010, 2282, 3267, 1995, 3942, 4235, 2676, 528, 417, 1860, 4944, 2352, 2603, 496, 2395, 4391, 1183, 4271, 3431, 3219, 3728, 873, 4956, 4460, 3638, 1935, 511, 1820, 1753, 2536, 131, 2253, 736, 1438, 2762, 4677, 4836, 8, 4524, 4711, 1575, 334, 3860, 4697, 4237, 3095, 4017, 3574, 2925, 2398, 4938, 2612, 408, 2086, 1938, 1349, 3405, 2827, 4158, 2071, 858, 377, 1043, 776, 4126, 3981, 81, 4471, 916, 52, 427, 8, 4922, 310, 43, 4770, 800, 3928, 1616, 848, 2818, 1300, 4847, 2744, 2528, 3825, 3582, 4781, 2911, 4309, 1188, 2451, 447, 2057, 966, 1159, 2683, 2542, 4010, 367, 252, 1199, 3765, 3974, 1794, 3464, 55, 501, 3389, 745, 763, 3937, 1077, 624, 2896, 3609, 4555, 3830, 3731, 1240, 1844, 4091, 3081, 587, 424, 3000, 1682, 2332, 3235, 3995, 3520, 3567, 2748, 501, 4661, 594, 385, 3271, 3980, 401, 2904, 1184, 1240, 3933, 825, 4068, 1561, 4387, 2776, 2581, 3117, 2219, 3224, 980, 2160, 1941, 2588, 3254, 4410, 261, 414, 802, 1536, 4245, 2935, 1059, 4098, 2750, 4707, 224, 1867, 740, 593, 2448, 408, 2244, 3719, 4149, 1952, 1509, 893, 558, 2409, 409, 4287, 4817, 3790, 2356, 3100, 4204, 416, 2778, 3876, 894, 1115, 4071, 630, 386, 1346, 41, 4039, 2816, 3001, 500, 1557, 15, 775, 42, 3161, 1449, 2916, 2333, 4947, 3883, 4744, 2792, 2434, 1929, 3599, 831, 4511, 813, 927, 4273, 4285, 496, 4068, 2450, 3893, 4982, 173, 667, 3347, 370, 2424, 3855, 538, 2692, 3084, 456, 107, 4433, 4088, 1181, 1755, 4761, 4932, 3014, 221, 1925, 1649, 2944, 1646, 3994, 1389, 4617, 4609, 3952, 2870, 3535, 4859, 4733, 3696, 3549, 1038, 716, 3474, 2122, 4629, 3495, 992, 4594, 4766, 2389, 4901, 4041, 2453, 894, 2077, 874, 188, 2405, 543, 991, 3601, 3121, 329, 402, 2948, 4382, 3203, 3387, 2098, 3473, 3986, 516, 3243, 2341, 490, 4828, 861, 4527, 2197, 3777, 4860, 1031, 1546, 751, 4670, 826, 4832, 2813, 3820, 4927, 3628, 2082, 4043, 3990, 2929, 1545, 4235, 3792, 4680, 131, 4097, 2694, 652, 4785, 4419, 3044, 3127, 470, 3, 1198, 3754, 2969, 503, 3957, 582, 4109, 1982, 4487, 4330, 1452, 2202, 3601, 3797, 4331, 3630, 471, 1655, 1435, 3624, 394, 4306, 207, 1488, 126, 367, 4870, 3969, 3448, 4265, 3475, 1058, 2437, 267, 591, 4352, 4363, 1096, 3807, 2547, 4328, 2811, 4251, 3241, 3518, 1377, 4983, 1353, 1900, 676, 1969, 1213, 190, 1700, 4020, 2219, 4822, 4659, 1324, 4961, 4675, 2317, 467, 835, 179, 1129, 1093, 852, 1360, 3585, 1999, 742, 3284, 3485, 3383, 4864, 4474, 3600, 653, 1333, 767, 4075, 2345, 4888, 1263, 4482, 2638, 4430, 1173, 4543, 2068, 1466, 4302, 3583, 345, 2633, 2515, 2630, 3717, 693, 1906, 2944, 510, 4609, 4064, 3643, 4010, 657, 3435, 2258, 630, 3037, 3199, 3904, 1139, 1446, 1776, 492, 2942, 188, 2935, 1247, 2000, 4413, 1036, 3485, 4691, 606, 1678, 541, 4380, 2673, 3241, 4403, 1896, 1202, 3323, 2433, 2982, 647, 1564, 3543, 1916, 483, 4040, 2974, 4234, 4025, 3208, 2109, 1055, 2989, 4210, 1944, 3504, 465, 1687, 1569, 2783, 97, 65, 4948, 3311, 1885, 2228, 3961, 4695, 4268, 4063, 166, 3998, 2070, 3547, 2148, 2995, 2451, 2833, 4243, 438, 951, 23, 2913, 1301, 2593, 4557, 2278, 4460, 1142, 3740, 2562, 3369, 2601, 3222, 2230, 2475, 4720, 4568, 572, 2737, 4157, 685, 1934, 3377, 4864, 1586, 3916, 2828, 1462, 2539, 4147, 3028, 4734, 3198, 4312, 234, 3578, 1409, 3776, 291, 1168, 1437, 1454, 3965, 4735, 2310, 3865, 4361, 4374, 3684, 1920, 3844, 3982, 1316, 3922, 2116, 641, 4220, 684, 4937, 313, 2456, 65, 4674, 4960, 2432, 4547, 4090, 624, 210, 3766, 3452, 2226, 1189, 605, 4665, 1528, 1553, 1532, 1669, 4051, 2136, 2392, 4053, 2251, 1601, 992, 3924, 2205, 444, 3260, 417, 922, 2362, 3285, 3414, 2350, 4612, 806, 2803, 1990, 2229, 4273, 3311, 379, 2451, 3479, 1900, 4704, 744, 3895, 1795, 817, 3163, 1770, 1677, 4136, 3643, 2368, 2223, 923, 1526, 3917, 2906, 4849, 4457, 4614, 482, 1340, 4931, 1243, 595, 1297, 720, 3060, 2013, 3678, 1163, 2373, 2067, 4725, 2306, 1205, 4180, 4280, 4069, 600, 648, 3515, 762, 778, 1094, 2792, 2495, 1410, 1023, 1627, 3494, 3172, 2991, 2502, 2014, 4077, 3158, 706, 3970, 1436, 4509, 3981, 1207, 1725, 1552, 738, 3873, 4389, 4788, 448, 784, 729, 355, 3491, 308, 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1835, 2637, 3558, 3779, 4042, 3441, 1145, 3464, 3016, 3842, 2694, 2186, 3463, 1338, 2665, 4181, 4470, 3116, 237, 2591, 1410, 2789, 2569, 1932, 3113, 1056, 4001, 4245, 3045, 4149, 3832, 1265, 1778, 3439, 2459, 3689, 4343, 3544, 1697, 1851, 2458, 602, 4854, 4615, 393, 2983, 2317, 4002, 4779, 4811, 1011, 3785, 4316, 3773, 1206, 3143, 369, 1279, 125, 1226, 2734, 3250, 1361, 1861, 4843, 165, 2071, 481, 4000, 2951, 386, 2630, 2756, 4787, 1611, 834, 1403, 1885, 4023, 4858, 1855, 3558, 3518, 954, 1082, 4203, 4471, 4000, 1307, 4117, 4659, 133, 3108, 170, 1828, 309, 1306, 4511, 802, 1989, 2885, 963, 2067, 117, 1722, 42, 991, 3567, 92, 3713, 3642, 1615, 1382, 1671, 3405, 1214, 4327, 711, 4431, 4022, 2412, 2662, 3072, 205, 4801, 1627, 471, 3998, 1206, 4986, 2488, 2592, 796, 1106, 2416, 1381, 3482, 4934, 4815, 2591, 4230, 4864, 2147, 1276, 638, 908, 4099, 3389, 3227, 3225, 2523, 4155, 2561, 1119, 3121, 1107, 3292, 4595, 3722, 4094, 2940, 3275, 2145, 2387, 2970, 4838, 1168, 2872, 1692, 3758, 2106, 177, 4222, 4839, 2646, 3693, 82, 4746, 4165, 4527, 4888, 1796, 1303, 4119, 3320, 4121, 201, 2531, 1083, 473, 703, 1460, 3572, 257, 712, 3842, 240, 1675, 582, 3830, 4888, 1474, 4197, 3479, 4115, 832, 211, 1625, 1575, 2064, 3373, 3614, 39, 939, 2082, 1612, 3535, 4061, 1989, 1259, 352, 4924, 1773, 1664, 2298, 4226, 1870, 677, 2944, 199, 617, 73, 1510, 2363, 3986, 1349, 1098, 4238, 1225, 4968, 1385, 3293, 4877, 857, 3858, 2685, 1190, 2704, 2483, 2762, 1741, 1757, 271, 113, 3828, 1752, 2585, 4261, 4636, 1846, 1451, 2239, 588, 3487, 3247, 4210, 2888, 2568, 3132, 2108, 2508, 4085, 4607, 4250, 4782, 508, 4165, 2377, 3984, 4536, 4926, 4243, 1564, 3399, 3245, 1595, 3302, 553, 2894, 1627, 4951, 2918, 1539, 3392, 459, 1744, 2789, 4006, 802, 2676, 3453, 3567, 1894, 194, 547, 2089, 668, 2960, 4436, 4010, 4914, 3710, 545, 4904, 1751, 4096, 4690, 1681, 2229, 1388, 3669, 4937, 3877, 170, 3122, 1497, 4621, 332, 569, 1715, 731, 3518, 4744, 1193, 1675, 1450, 1518, 188, 1583, 3528, 2902, 2026, 4299, 3239, 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1501, 2, 4618, 3790, 4967, 226, 4069, 572, 153, 4984, 1645, 2133, 2527, 4056, 820, 3228, 1453, 4967, 1852, 2491, 473, 4737, 3235, 1736, 544, 4440, 4817, 229, 3678, 2664, 2923, 1745, 2594, 4986, 1316, 3746, 1220, 256, 2820, 4255, 2541, 2957, 3494, 3348, 1562, 3588, 3407, 4961, 1942, 2330, 390, 4414, 1050, 1870, 4987, 4089, 4044, 4936, 668, 867, 163, 1388, 224, 1804, 1634, 4886, 2195, 354, 4088, 638, 1201, 3339, 591, 336, 1712, 3150, 373, 2497, 2301, 2790, 3429, 581, 279, 1252, 3013, 892, 4297, 350, 1007, 718, 754, 83, 4910, 2797, 2695, 4482, 1354, 2908, 3269, 4320, 1434, 4843, 2871, 1004, 2937, 716, 4889, 3723, 2711, 2036, 3178, 4883, 1232, 2831, 2713, 1683, 3679, 4171, 163, 4179, 248, 2311, 3073, 4816, 920, 4321, 428, 772, 2173, 3775, 2058, 4258, 1990, 4001, 113, 2382, 275, 4487, 250, 915, 1956, 2633, 1355, 1335, 2000, 4616, 3293, 1868, 2313, 2819, 1472, 3889, 2068, 2934, 3493, 2496, 426, 4566, 1270, 2525, 4575, 79, 1237, 1345, 3934, 3878, 767, 1758, 92, 2779, 1477, 1946, 4517, 2611, 3170, 3664, 2063, 1621, 2547, 4544, 1785, 1067, 2806, 358, 3330, 1879, 1104, 4455, 2749, 2586, 2044, 1468, 1197, 1462, 1804, 4810, 2392, 4072, 4525, 1714, 2456, 2999, 2637, 3402, 2677, 1472, 28, 4215, 901, 4847, 3361, 957, 422, 2909, 3156, 4340, 4808, 1330, 814, 1865, 4992, 71, 1156, 809, 748, 4408, 2089, 3036, 1519, 4454, 927, 3219, 1040, 3893, 1885, 3149, 3264, 4296, 3112, 679, 3044, 3377, 3751, 937, 2884, 2021, 2260, 2404, 2261, 1956, 767, 688, 29, 541, 4799, 1140, 2154, 3141, 3011, 1629, 3420, 3252, 2898, 3964, 295, 813, 2125, 4404, 2848, 4179, 2038, 3387, 2224, 4298, 3329, 2071, 374, 397, 4497, 2453, 4182, 845, 2364, 3961, 3064, 571, 3397, 0, 1519, 1111, 2265, 4056, 4935, 515, 4177, 3122, 621, 1926, 4031, 3481, 3541, 3503, 3745, 3352, 819, 1914, 1735, 2560, 4055, 2457, 4481, 1594, 187, 4112, 3442, 1982, 3553, 2463, 651, 1372, 429, 957, 1888, 3971, 2717, 4187, 1789, 1502, 482, 4344, 3468, 560, 2461, 1089, 545, 2995, 2131, 2926, 2763, 3386, 1313, 3739, 3192, 2447, 2509, 4256, 1826, 749, 66, 3634, 2750, 1486, 552, 2545, 1190, 1186, 4306, 3405, 557, 2027, 334, 4257, 2336, 2131, 1485, 2871, 1998, 2954, 2520, 3123, 1071, 409, 1393, 3007, 2556, 2943, 2859, 1576, 3214, 986, 3061, 1034, 1402, 2827, 2470, 433, 4428, 3208, 2523, 4429, 1608, 2169, 210, 1259, 4269, 1252, 2234, 2302, 453, 2372, 712, 3341, 4843, 4430, 4678, 460, 1200, 1205, 652, 2789, 2561, 2351, 4917, 4411, 2449, 1840, 3768, 1313, 2248, 3771, 2169, 147, 3447, 307, 2202, 779, 4048, 2711, 4845, 1521, 22, 2198, 4842, 3352, 396, 502, 2650, 249, 4859, 229, 1790, 2019, 1465, 3294, 881, 1856, 4847, 3438, 2794, 2262, 3305, 2930, 2377, 4462, 3267, 2471, 1559, 2817, 4006, 544, 2133, 3678, 4468, 2613, 125, 1688, 802, 2138, 4216, 824, 2087, 2294, 4597, 4898, 3428, 4975, 1611, 230, 1969, 1029, 2469, 180, 2087, 1986, 1006, 3634, 1792, 2875, 3237, 4997, 3243, 3228, 1026, 1986, 2391, 851, 2148, 3528, 3380, 430, 3931, 2980, 538, 4861, 2089, 2918, 1275, 521, 3005, 247, 596, 742, 4939, 4792, 1910, 2692, 4939, 3993, 4600, 1359, 3049, 258, 4340, 134, 4356, 1855, 941, 1991, 816, 2048, 2310, 3591, 565, 3768, 843, 4882, 145, 4554, 3390, 730, 4974, 770, 2214, 4250, 1991, 3719, 2010, 3257, 37, 2859, 207, 2098, 2732, 1526, 1955, 4494, 50, 2596, 4023, 4362, 1432, 4662, 908, 4015, 1186, 2205, 850, 4232, 4398, 4301, 4131, 2079, 70, 2394, 36, 4099, 4862, 416, 333, 2110, 4521, 4375, 1575, 4040, 11, 1921, 1870, 2515, 954, 2861, 1949, 4871, 2673, 1955, 60, 1286, 4746, 652, 1039, 1280, 1953, 4870, 4213, 3476, 3236, 2736, 4708, 3641, 4065, 4166, 4631, 2351, 3225, 1176, 1381, 761, 3130, 829, 172, 1383, 2965, 4141, 2499, 2996, 497, 919, 2427, 2863, 3881, 3415, 78, 3621, 2088, 1158, 3266, 1063, 373, 4707, 3063, 3330, 4471, 3490, 4771, 2405, 4767, 4238, 2347, 1562, 1652, 1398, 585, 4340, 2743, 2106, 4104, 4312, 3647, 4381, 3313, 3412, 3094, 4438, 2922, 1021, 460, 2087, 474, 1785, 1524, 2137, 2609, 3273, 4233, 3665, 3929, 1543, 3541, 1571, 4174, 519, 3437, 567, 365, 914, 2454, 2770, 2351, 3449, 3862, 4216, 4239, 4176, 859, 3093, 2349, 1880, 4325, 571, 2705, 69, 3423, 964, 495, 670, 1636, 683, 2069, 3722, 3663, 3406, 2820, 3060, 2089, 4488, 3537, 3797, 545, 1630, 867, 1431, 1738, 1417, 2298, 1045, 2784, 3567, 4523, 3292, 3447, 1405, 3079, 4700, 4765, 3183, 1554, 3964, 3207, 2531, 4635, 1826, 3022, 2615, 1587, 3588, 580, 4300, 2073, 1130, 441, 4808, 3306, 1271, 1984, 3431, 3340, 4609, 3312, 1152, 126, 4432, 929, 4753, 4415, 1069, 4690, 151, 1570, 4687, 3921, 1088, 4495, 2772, 4136, 364, 1785, 993, 53, 3313, 2254, 3461, 1672, 2404, 750, 2329, 2080, 1246, 4389, 496, 4979, 4857, 1775, 1395, 4240, 1461, 548, 4143, 4838, 3895, 905, 3160, 4999, 3143, 3866, 4893, 1162, 3741, 4494, 3379, 3486, 4976, 186, 2319, 498, 1484, 507, 1115, 457, 522, 2798, 2302, 1840, 2680, 645, 3681, 4372, 763, 1329, 4656, 1053, 3541, 1970, 3015, 1769, 1583, 4294, 2256, 954, 4986, 170, 2396, 1159, 2097, 2879, 691, 3792, 2679, 2337, 4008, 4024, 2040, 674, 1576, 2195, 1869, 1305, 2217, 1920, 138, 678, 389, 2212, 3152, 2915, 910, 2080, 3932, 3131, 3463, 4182, 2464, 4623, 1512, 3670, 1576, 629, 2441, 3551, 2706, 1384, 3257, 3610, 450, 2672, 4573, 491, 1551, 2888, 4370, 1851, 3631, 4041, 2654, 547, 2370, 3045, 1859, 3539, 1678, 447, 3445, 1439, 2830, 52, 1538, 1087, 2482, 3930, 2730, 3721, 568, 2780, 1866, 4338, 3893, 3481, 3785, 2259, 3194, 3049, 757, 1410, 1853, 4921, 724, 933, 1741, 3725, 1014, 4558, 89, 2767, 1976, 7, 1106, 2946, 2611, 2427, 1417, 2633, 672, 3009, 3052, 246, 2770, 1555, 2970, 1219, 4624, 4866, 4050, 1156, 714, 1624, 3336, 106, 1880, 3602, 4814, 2410, 443, 2940, 3424, 1866, 800, 1709, 2111, 8, 4567, 2563, 4921, 1971, 1908, 2382, 1711, 156, 3642, 1647, 731, 1162, 670, 3015, 3797, 2287, 2824, 1810, 4874, 3193, 2147, 4534, 986, 3036, 3361, 718, 3988, 2868, 2546, 824, 3978, 461, 2254, 4122, 1912, 975, 2792, 4521, 2417, 2535, 2887, 3957, 4788, 2786, 1153, 4063, 4730, 4928, 710, 2962, 1655, 4930, 3521, 4446, 502, 2153, 3243, 1790, 1428, 612, 3122, 2221, 2197, 3706, 4323, 683, 3727, 1981, 2015, 1909, 2309, 1989, 2804, 4681, 4623, 4195, 793, 554, 2391, 3740, 4788, 1780, 1323, 4580, 3128, 4572, 3814, 3690, 1731, 3313, 4011, 1808, 3393, 4002, 4597, 4360, 3521, 3948, 3414, 2037, 2810, 2827, 414, 1693, 4452, 607, 1941, 1545, 4395, 2801, 361, 4890, 3384, 416, 1513, 4639, 4473, 408, 2792, 973, 2955, 4419, 2791, 3878, 2366, 3294, 4735, 2267, 2037, 3963, 4018, 3158, 2251, 3131, 3504, 2564, 4003, 1679, 4630, 4559, 4176, 4996, 1132, 4537, 3583, 1253, 3959, 2245, 1240, 2784, 1615, 3763, 1503, 2245, 3850, 2035, 4988, 4438, 331, 1098, 2410, 1825, 628, 2288, 4928, 3246, 2323, 2317, 208, 2919, 1601, 535, 4812, 2513, 237, 3210, 1499, 3687, 3860, 1544, 4041, 1117, 2719, 2566, 2396, 3423, 2634, 2852, 1705, 2397, 47, 3799, 4277, 4163, 2372, 1916, 4412, 1156, 1246, 3699, 274, 4598, 1915, 3560, 2542, 4802, 2711, 288, 2135, 2222, 4492, 1912, 418, 1525, 3298, 4993, 1517, 3873, 1551, 1017, 877, 1033, 4471, 1051, 2454, 2534, 3151, 2857, 3097, 1103, 2999, 301, 1686, 1630, 4862, 1205, 4509, 716, 1849, 4338, 2240, 3436, 4917, 3512, 140, 929, 1837, 895, 2651, 1741, 899, 1251, 2871, 2901, 311, 135, 3715, 2427, 3976, 3413, 4922, 681, 1902, 1165, 2705, 3843, 257, 2506, 3428, 2198, 3593, 3198, 3165, 4914, 1244, 4812, 3753, 4839, 1799, 3321, 2504, 4618, 1570, 3208, 4683, 913, 2327, 4528, 4977, 1198, 1293, 882, 3453, 2564, 868, 2377, 2605, 427, 4532, 690, 420, 351, 762, 1550, 1420, 1888, 781, 1387, 86, 4336, 3461, 2029, 646, 2054, 3051, 624, 2102, 3223, 2826, 4280, 4159, 3647, 1045, 4363, 249, 4335, 984, 3938, 1417, 4787, 322, 3957, 3248, 1716, 1511, 4461, 2471, 29, 2761, 2856, 4090, 1525, 4004, 4057, 3538, 532, 1660, 4643, 4727, 1019, 4004, 687, 2561, 508, 3710, 4317, 4252, 4302, 3978, 2162, 964, 4709, 138, 329, 3695, 4640, 4031, 979, 3000, 3530, 1749, 4177, 2045, 4409, 3337, 1626, 2157, 4432, 3492, 3724, 3454, 223, 3050, 763, 561, 3706, 16, 2696, 1942, 923, 2839, 3625, 4465, 826, 2600, 2185, 874, 705, 397, 1547, 1931, 2375, 3224, 3358, 2990, 281, 645, 3657, 2011, 4842, 613, 2969, 1522, 595, 3562, 1649, 4093, 36, 2965, 941, 982, 818, 1946, 4686, 193, 3532, 2196, 1307, 3647, 3659, 2587, 3753, 1905, 3026, 4681, 1064, 1173, 4899, 54, 1089, 4753, 3587, 2032, 4250, 2825, 2782, 4400, 1069, 1266, 2841, 1165, 1574, 4457, 1335, 4009, 3184, 4577, 4475, 1612, 2796, 4026, 3230, 4443, 4228, 4210, 4307, 2594, 3293, 392, 3372, 3962, 2974, 1685, 1525, 3057, 682, 3347, 227, 1317, 650, 662, 2210, 2519, 2920, 1537, 1757, 3865, 1708, 2828, 2206, 4637, 1738, 3913, 2077, 4022, 4002, 2020, 1313, 1330, 3297, 190, 3917, 2569, 1790, 87, 541, 4180, 1084, 3309, 19, 3481, 3867, 4834, 3051, 3926, 1749, 2750, 2389, 1886, 1503, 3984, 3310, 2409, 370, 3936, 2, 4733, 4643, 614, 2558, 2561, 3213, 3240, 19, 3879, 605, 2754, 4734, 1117, 438, 2304, 3501, 1161, 1420, 2764, 1040, 1072, 572, 1442, 4810, 1816, 2683, 4483, 4045, 3914, 2172, 1204, 703, 4033, 1542, 712, 1358, 3385, 2334, 4282, 1084, 3512, 4293, 4376, 4244, 2877, 1599, 4911, 4581, 619, 3648, 2722, 2128, 4923, 865, 2676, 2936, 4262, 2347, 3463, 921, 4151, 2028, 2242, 697, 4879, 2686, 4313, 1591, 1740, 3793, 1932, 1351, 2407, 1705, 3041, 2220, 4904, 422, 4289, 2284, 4027, 1866, 1504, 2850, 718, 799, 186, 322, 1135, 1050, 3627, 2676, 3726, 1378, 4792, 4121, 325, 2088, 270, 4065, 1142, 3611, 4756, 2756, 1919, 601, 1944, 4218, 4731, 2085, 2518, 3104, 4032, 4426, 1614, 615, 274, 4382, 4577, 3982, 3239, 2463, 1345, 348, 1800, 43, 4798, 1503, 2533, 3155, 4915, 131, 4853, 2765, 2811, 1853, 2868, 3054, 2418, 4778, 736, 4262, 812, 2555, 4076, 3617, 4734, 532, 2110, 3532, 2943, 1395, 4100, 76, 4297, 4664, 29, 3270, 4632, 4192, 4640, 3706, 1014, 21, 4050, 3054, 1842, 1054, 570, 4259, 429, 1728, 4847, 596, 1370, 2800, 1058, 3259, 4215, 4230, 63, 2031, 3646, 3146, 1795, 4796, 218, 1452, 4021, 117, 2418, 4389, 1142, 4998, 750, 3101, 2362, 227, 753, 721, 4065, 2148, 3628, 2712, 2374, 4797, 1474, 3976, 3615, 513, 3783, 4926, 1847, 4635, 4495, 2176, 2734, 3619, 4239, 3065, 4135, 1444, 4518, 4709, 4039, 1972, 25, 704, 2535, 3024, 660, 142, 3563, 1197, 4732, 2689, 2861, 1851, 3809, 3385, 4099, 2285, 4892, 2210, 4698, 3616, 1456, 1443, 670, 3010, 1816, 1825, 2089, 1157, 413, 4957, 2539, 4394, 476, 1168, 2746, 763, 1483, 3370, 1478, 1974, 1178, 248, 562, 75, 2981, 178, 1382, 4542, 4427, 2014, 942, 408, 2177, 2005, 4939, 1048, 3729, 4481, 1159, 1380, 4377, 2239, 3447, 2724, 4739, 4201, 2932, 852, 4816, 965, 2719, 2505, 4029, 1572, 257, 4552, 4493, 3920, 2576, 2998, 4811, 1216, 3499, 106, 900, 3830, 4893, 2819, 1273, 4666, 991, 2994, 3400, 649, 4820, 167, 4248, 2546, 4791, 162, 3339, 4028, 321, 2922, 3603, 1778, 3395, 650, 1540, 3721, 4113, 3079, 2478, 291, 3551, 190, 113, 30, 4734, 467, 946, 3566, 2543, 879, 2636, 4392, 2695, 27, 1726, 4444, 3083, 4901, 1105, 1402, 322, 2147, 1886, 1775, 172, 873, 13, 4580, 2017, 3561, 3050, 1334, 4608, 4311, 1994, 236, 4235, 59, 4156, 3831, 933, 3495, 256, 4756, 3021, 757, 290, 970, 2614, 3228, 3925, 3884, 3925, 4312, 816, 2075 ]
+
+        sort unsortedList
+            |> Quicksort.show
+            |> Task.putLine
+
+sort : List I64 -> List I64
+sort = \list ->
+    Quicksort.sortWith list (\x, y -> Num.compare x y)
\ No newline at end of file
diff --git a/examples/benchmarks/RBTreeCk.roc b/examples/benchmarks/RBTreeCk.roc
index 4a69241984..c9595875ec 100644
--- a/examples/benchmarks/RBTreeCk.roc
+++ b/examples/benchmarks/RBTreeCk.roc
@@ -47,18 +47,19 @@ resultWithDefault = \res, default ->
 
 main : Task.Task {} []
 main =
-    ms : ConsList Map
-    ms = makeMap 5 5 # 42_000_00
+    Task.after Task.getInt \n ->
+        ms : ConsList Map
+        ms = makeMap 5 n # original koka n = 4_200_000
 
-    when ms is
-        Cons head _ ->
-            val = fold (\_, v, r -> if v then r + 1 else r) head 0
-            val
-                |> Str.fromInt
-                |> Task.putLine
+        when ms is
+            Cons head _ ->
+                val = fold (\_, v, r -> if v then r + 1 else r) head 0
+                val
+                    |> Str.fromInt
+                    |> Task.putLine
 
-        Nil ->
-            Task.putLine "fail"
+            Nil ->
+                Task.putLine "fail"
 
 insert : Tree (Num k) v, (Num k), v -> Tree (Num k) v
 insert = \t, k, v -> if isRed t then setBlack (ins t k v) else ins t k v
diff --git a/examples/benchmarks/RBTreeDel.roc b/examples/benchmarks/RBTreeDel.roc
index 172b97f7f2..d9b780fa39 100644
--- a/examples/benchmarks/RBTreeDel.roc
+++ b/examples/benchmarks/RBTreeDel.roc
@@ -14,14 +14,14 @@ ConsList a : [ Nil, Cons a (ConsList a) ]
 
 main : Task.Task {} []
 main =
-    # benchmarks use 4_200_000
-    m = makeMap 420
+    Task.after Task.getInt \n ->
+        m = makeMap n # koka original n = 4_200_000
 
-    val = fold (\_, v, r -> if v then r + 1 else r) m 0
+        val = fold (\_, v, r -> if v then r + 1 else r) m 0
 
-    val
-        |> Str.fromInt
-        |> Task.putLine
+        val
+            |> Str.fromInt
+            |> Task.putLine
 
 boom : Str -> a
 boom = \_ -> boom ""
diff --git a/examples/benchmarks/platform/host.zig b/examples/benchmarks/platform/host.zig
index 56461d93e6..fb3690646e 100644
--- a/examples/benchmarks/platform/host.zig
+++ b/examples/benchmarks/platform/host.zig
@@ -24,7 +24,7 @@ const mem = std.mem;
 const Allocator = mem.Allocator;
 
 extern fn roc__mainForHost_1_exposed([*]u8) void;
-extern fn roc__mainForHost_1_size() i64;
+extern fn roc__mainForHost_size() i64;
 extern fn roc__mainForHost_1_Fx_caller(*const u8, [*]u8, [*]u8) void;
 extern fn roc__mainForHost_1_Fx_size() i64;
 extern fn roc__mainForHost_1_Fx_result_size() i64;
@@ -51,7 +51,7 @@ pub export fn main() u8 {
     const stdout = std.io.getStdOut().writer();
     const stderr = std.io.getStdErr().writer();
 
-    const size = @intCast(usize, roc__mainForHost_1_size());
+    const size = @intCast(usize, roc__mainForHost_size());
     const raw_output = std.heap.c_allocator.alloc(u8, size) catch unreachable;
     var output = @ptrCast([*]u8, raw_output);
 
@@ -163,7 +163,7 @@ pub export fn roc_fx_getInt() GetInt {
 }
 
 fn roc_fx_getInt_help() !i64 {
-    const stdin = std.io.getStdIn().inStream();
+    const stdin = std.io.getStdIn().reader();
     var buf: [40]u8 = undefined;
 
     const line: []u8 = (try stdin.readUntilDelimiterOrEof(&buf, '\n')) orelse "";
diff --git a/examples/cli/platform/Cargo.lock b/examples/cli/platform/Cargo.lock
index 0e827e8bb1..551e2f492b 100644
--- a/examples/cli/platform/Cargo.lock
+++ b/examples/cli/platform/Cargo.lock
@@ -1,5 +1,7 @@
 # This file is automatically @generated by Cargo.
 # It is not intended for manual editing.
+version = 3
+
 [[package]]
 name = "base64"
 version = "0.13.0"
@@ -30,6 +32,27 @@ version = "1.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fff857943da45f546682664a79488be82e69e43c1a7a2307679ab9afb3a66d2e"
 
+[[package]]
+name = "errno"
+version = "0.2.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fa68f2fb9cae9d37c9b2b3584aba698a2e97f72d7aef7b9f7aa71d8b54ce46fe"
+dependencies = [
+ "errno-dragonfly",
+ "libc",
+ "winapi",
+]
+
+[[package]]
+name = "errno-dragonfly"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "14ca354e36190500e1e1fb267c647932382b54053c50b14970856c0b00a35067"
+dependencies = [
+ "gcc",
+ "libc",
+]
+
 [[package]]
 name = "form_urlencoded"
 version = "1.0.1"
@@ -40,10 +63,18 @@ dependencies = [
  "percent-encoding",
 ]
 
+[[package]]
+name = "gcc"
+version = "0.3.55"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8f5f3913fa0bfe7ee1fd8248b6b9f42a5af4b9d65ec2dd2c3c26132b950ecfc2"
+
 [[package]]
 name = "host"
 version = "0.1.0"
 dependencies = [
+ "errno",
+ "libc",
  "roc_std",
  "ureq",
 ]
diff --git a/examples/cli/platform/Cargo.toml b/examples/cli/platform/Cargo.toml
index f4f96d8a3e..0780fec234 100644
--- a/examples/cli/platform/Cargo.toml
+++ b/examples/cli/platform/Cargo.toml
@@ -11,5 +11,7 @@ crate-type = ["staticlib"]
 [dependencies]
 roc_std = { path = "../../../roc_std" }
 ureq = { version = "2.1" }
+libc = "0.2"
+errno = "0.2"
 
 [workspace]
diff --git a/examples/cli/platform/src/lib.rs b/examples/cli/platform/src/lib.rs
index daf2b25ef0..37a120b2df 100644
--- a/examples/cli/platform/src/lib.rs
+++ b/examples/cli/platform/src/lib.rs
@@ -1,8 +1,12 @@
 #![allow(non_snake_case)]
 
+use core::alloc::Layout;
 use core::ffi::c_void;
-use roc_std::{alloca, RocCallResult, RocResult, RocStr};
-use std::alloc::Layout;
+use core::mem;
+use core::mem::MaybeUninit;
+use errno::{errno, Errno};
+use libc::{self, c_char, c_int};
+use roc_std::{alloca, RocCallResult, RocList, RocResult, RocStr};
 
 extern "C" {
     #[link_name = "roc__mainForHost_1_exposed"]
@@ -156,3 +160,97 @@ pub fn rust_main() -> isize {
     // Exit code
     0
 }
+
+/// A C file descriptor.
+pub struct Fd(c_int);
+
+#[no_mangle]
+pub unsafe fn roc_fx_open(roc_path: RocStr) -> RocResult<Fd, Errno> {
+    const BUF_BYTES: usize = 256;
+
+    let mut buf: MaybeUninit<[u8; BUF_BYTES]> = MaybeUninit::uninit();
+
+    // If the path fits in the stack-allocated buffer, we can avoid a heap
+    // allocation when translating our `RocStr` to a null-terminated `char*`.
+    let path_len = roc_path.len();
+    let path_fits_in_buf = path_len > BUF_BYTES;
+    let c_path: *mut c_char;
+
+    if path_fits_in_buf {
+        roc_path.write_c_str(buf.as_mut_ptr() as *mut u8);
+
+        // NOTE buf may be only partially filled, so we don't `assume_init`!
+        c_path = buf.as_mut_ptr() as *mut c_char;
+    } else {
+        c_path = roc_alloc(path_len, mem::align_of::<c_char>() as u32) as *mut c_char;
+
+        roc_path.write_c_str(c_path as *mut u8);
+    }
+
+    let fd = libc::open(c_path, libc::O_RDONLY);
+
+    // Now that the call to `open` is done, deallocate c_path if necessary>
+    if !path_fits_in_buf {
+        roc_dealloc(c_path as *mut c_void, mem::align_of_val(&c_path) as u32);
+    }
+
+    // if libc::open returned -1, that means there was an error
+    if fd != -1 {
+        RocResult::Ok(Fd(fd))
+    } else {
+        RocResult::Err(errno())
+    }
+}
+
+#[no_mangle]
+pub unsafe fn roc_fx_read(fd: Fd, bytes: usize) -> RocResult<RocList<u8>, Errno> {
+    const BUF_BYTES: usize = 1024;
+
+    let mut buf: MaybeUninit<[u8; BUF_BYTES]> = MaybeUninit::uninit();
+
+    // We'll use our own position and libc::pread rather than using libc::read
+    // repeatedly and letting the fd store its own position. This way we don't
+    // have to worry about concurrent modifications of the fd's position.
+    let mut list = RocList::empty();
+    let mut position: usize = 0;
+
+    loop {
+        // Make sure we never read more than the buffer size, and also that
+        // we never read past the originally-requested number of bytes.
+        let bytes_to_read = BUF_BYTES.min(bytes - position);
+        let bytes_read = libc::pread(
+            fd.0,
+            buf.as_mut_ptr() as *mut c_void,
+            bytes_to_read,
+            position as i64,
+        );
+
+        // NOTE buf may be only partially filled, so we don't `assume_init`!
+
+        if bytes_read == bytes_to_read as isize {
+            // The read was successful, and there may be more bytes to read.
+            // Append the bytes to the list and continue looping!
+            let slice = core::slice::from_raw_parts(buf.as_ptr() as *const u8, bytes_read as usize);
+
+            list.append_slice(slice);
+        } else if bytes_read >= 0 {
+            // The read was successful, and there are no more bytes
+            // to read (because bytes_read was less than the requested
+            // bytes_to_read, but it was also not negative - which would have
+            // indicated an error).
+            let slice = core::slice::from_raw_parts(buf.as_ptr() as *const u8, bytes_read as usize);
+
+            list.append_slice(slice);
+
+            // We're done!
+            return RocResult::Ok(list);
+        } else {
+            // bytes_read was negative, so we got a read error!
+            break;
+        }
+
+        position += bytes_read as usize;
+    }
+
+    RocResult::Err(errno())
+}
diff --git a/examples/hello-world/Hello.roc b/examples/hello-world/Hello.roc
index 24a02e7823..1fe1406ce5 100644
--- a/examples/hello-world/Hello.roc
+++ b/examples/hello-world/Hello.roc
@@ -3,10 +3,4 @@ app "hello-world"
     imports []
     provides [ main ] to base
 
-greeting =
-    hi = "Hello"
-    name = "World"
-
-    "\(hi), \(name)!!!!!!!!!!!!!"
-
-main = greeting
+main = "Hello, World!\n"
diff --git a/examples/hello-world/platform/host.c b/examples/hello-world/platform/host.c
new file mode 100644
index 0000000000..65661e64aa
--- /dev/null
+++ b/examples/hello-world/platform/host.c
@@ -0,0 +1,83 @@
+#include <stdlib.h>
+#include <unistd.h>
+#include <stdio.h>
+#include <errno.h>
+#include <string.h>
+#include <stdbool.h>
+
+void* roc_alloc(size_t size, unsigned int alignment) {
+    return malloc(size);
+}
+
+void* roc_realloc(void* ptr, size_t old_size, size_t new_size, unsigned int alignment) {
+    return realloc(ptr, new_size);
+}
+
+void roc_dealloc(void* ptr, unsigned int alignment) {
+    free(ptr);
+}
+
+struct RocStr {
+    char* bytes;
+    size_t len;
+};
+
+bool is_small_str(struct RocStr str) {
+    return ((ssize_t)str.len) < 0;
+}
+
+// Determine the length of the string, taking into
+// account the small string optimization
+size_t roc_str_len(struct RocStr str) {
+    char* bytes = (char*)&str;
+    char last_byte = bytes[sizeof(str) - 1];
+    char last_byte_xored = last_byte ^ 0b10000000;
+    size_t small_len = (size_t)(last_byte_xored);
+    size_t big_len = str.len;
+
+    // Avoid branch misprediction costs by always
+    // determining both small_len and big_len,
+    // so this compiles to a cmov instruction.
+    if (is_small_str(str)) {
+        return small_len;
+    } else {
+        return big_len;
+    }
+}
+
+struct RocCallResult {
+    size_t flag;
+    struct RocStr content;
+};
+
+extern void roc__mainForHost_1_exposed(struct RocCallResult *re);
+
+int main() {
+    // Make space for the Roc call result
+    struct RocCallResult call_result;
+
+    // Call Roc to populate call_result
+    roc__mainForHost_1_exposed(&call_result);
+
+    // Determine str_len and the str_bytes pointer,
+    // taking into account the small string optimization.
+    struct RocStr str = call_result.content;
+    size_t str_len = roc_str_len(str);
+    char* str_bytes;
+
+    if (is_small_str(str)) {
+        str_bytes = (char*)&str;
+    } else {
+        str_bytes = str.bytes;
+    }
+
+    // Write to stdout
+    if (write(1, str_bytes, str_len) >= 0) {
+        // Writing succeeded!
+        return 0;
+    } else {
+        printf("Error writing to stdout: %s\n", strerror(errno));
+
+        return 1;
+    }
+}
diff --git a/examples/hello-zig/.gitignore b/examples/hello-zig/.gitignore
new file mode 100644
index 0000000000..6b820fd903
--- /dev/null
+++ b/examples/hello-zig/.gitignore
@@ -0,0 +1 @@
+hello-world
diff --git a/examples/hello-zig/Hello.roc b/examples/hello-zig/Hello.roc
new file mode 100644
index 0000000000..24a02e7823
--- /dev/null
+++ b/examples/hello-zig/Hello.roc
@@ -0,0 +1,12 @@
+app "hello-world"
+    packages { base: "platform" }
+    imports []
+    provides [ main ] to base
+
+greeting =
+    hi = "Hello"
+    name = "World"
+
+    "\(hi), \(name)!!!!!!!!!!!!!"
+
+main = greeting
diff --git a/examples/hello-zig/README.md b/examples/hello-zig/README.md
new file mode 100644
index 0000000000..a2890f03ed
--- /dev/null
+++ b/examples/hello-zig/README.md
@@ -0,0 +1,48 @@
+# Hello, World!
+
+To run, `cd` into this directory and run:
+
+```bash
+$ cargo run run Hello.roc
+```
+
+To run in release mode instead, do:
+
+```bash
+$ cargo run --release run Hello.roc
+```
+
+## Troubleshooting
+
+If you encounter `cannot find -lc++`, run the following for ubuntu `sudo apt install libc++-dev`.
+
+## Design Notes
+
+This demonstrates the basic design of hosts: Roc code gets compiled into a pure 
+function (in this case, a thunk that always returns `"Hello, World!"`) and
+then the host calls that function. Fundamentally, that's the whole idea! The host
+might not even have a `main` - it could be a library, a plugin, anything.
+Everything else is built on this basic "hosts calling linked pure functions" design.
+
+For example, things get more interesting when the compiled Roc function returns
+a `Task` - that is, a tagged union data structure containing function pointers 
+to callback closures. This lets the Roc pure function describe arbitrary 
+chainable effects, which the host can interpret to perform I/O as requested by 
+the Roc program.  (The tagged union `Task` would have a variant for each supported 
+I/O operation.)
+
+In this trivial example, it's very easy to line up the API between the host and
+the Roc program. In a more involved host, this would be much trickier - especially
+if the API were changing frequently during development.
+
+The idea there is to have a first-class concept of "glue code" which host authors
+can write (it would be plain Roc code, but with some extra keywords that aren't
+available in normal modules - kinda like `port module` in Elm), and which
+describe both the Roc-host/C boundary as well as the Roc-host/Roc-app boundary.
+Roc application authors only care about the Roc-host/Roc-app portion, and the
+host author only cares about the Roc-host/C bounary when implementing the host.
+
+Using this glue code, the Roc compiler can generate C header files describing the
+boundary. This not only gets us host compatibility with C compilers, but also 
+Rust FFI for free, because [`rust-bindgen`](https://github.com/rust-lang/rust-bindgen) 
+generates correct Rust FFI bindings from C headers.
diff --git a/examples/hello-zig/platform/Package-Config.roc b/examples/hello-zig/platform/Package-Config.roc
new file mode 100644
index 0000000000..377d5c0994
--- /dev/null
+++ b/examples/hello-zig/platform/Package-Config.roc
@@ -0,0 +1,10 @@
+platform examples/hello-world
+    requires {}{ main : Str }
+    exposes []
+    packages {}
+    imports []
+    provides [ mainForHost ]
+    effects fx.Effect {}
+
+mainForHost : Str
+mainForHost = main
diff --git a/examples/hello-world/platform/host.zig b/examples/hello-zig/platform/host.zig
similarity index 96%
rename from examples/hello-world/platform/host.zig
rename to examples/hello-zig/platform/host.zig
index 1419ddd15e..d1a563a68e 100644
--- a/examples/hello-world/platform/host.zig
+++ b/examples/hello-zig/platform/host.zig
@@ -59,7 +59,7 @@ pub export fn main() i32 {
     roc__mainForHost_1_exposed(&callresult);
 
     // stdout the result
-    stdout.print("{}\n", .{callresult.content.asSlice()}) catch unreachable;
+    stdout.print("{s}\n", .{callresult.content.asSlice()}) catch unreachable;
 
     callresult.content.deinit();
 
diff --git a/nightly_benches/Cargo.toml b/nightly_benches/Cargo.toml
new file mode 100644
index 0000000000..380cb7f454
--- /dev/null
+++ b/nightly_benches/Cargo.toml
@@ -0,0 +1,16 @@
+[package]
+name = "nightly_benches"
+version = "0.1.0"
+edition = "2018"
+
+[dependencies]
+
+[dev-dependencies]
+criterion = { git = "https://github.com/Anton-4/criterion.rs"}
+cli_utils = { path = "../cli/cli_utils" }
+criterion-perf-events ={ git = "https://github.com/Anton-4/criterion-perf-events" }
+perfcnt = "0.7.1"
+
+[[bench]]
+name = "events_bench"
+harness = false
diff --git a/nightly_benches/README.me b/nightly_benches/README.me
new file mode 100644
index 0000000000..fb6013bb8f
--- /dev/null
+++ b/nightly_benches/README.me
@@ -0,0 +1,14 @@
+# Running benchmarks
+
+    Install cargo criterion: 
+        ```
+        cargo install --git https://github.com/Anton-4/cargo-criterion --branch main
+        ```
+    Necessary to get cache misses...:
+        ```
+        sudo sh -c 'echo 1 >/proc/sys/kernel/perf_event_paranoid'
+        ```
+    run: 
+        ```
+        cargo criterion
+        ```
\ No newline at end of file
diff --git a/nightly_benches/benches/events_bench.rs b/nightly_benches/benches/events_bench.rs
new file mode 100644
index 0000000000..c722eeae7c
--- /dev/null
+++ b/nightly_benches/benches/events_bench.rs
@@ -0,0 +1,94 @@
+// Keep this benchmark. It's commented because it requires nightly rust.
+use cli_utils::bench_utils::{
+    bench_cfold, bench_deriv, bench_nqueens, bench_rbtree_ck, bench_rbtree_delete, bench_quicksort
+};
+use criterion_perf_events::Perf;
+use perfcnt::linux::HardwareEventType as Hardware;
+use perfcnt::linux::PerfCounterBuilderLinux as Builder;
+
+use criterion::{criterion_group, criterion_main, BenchmarkGroup, Criterion, SamplingMode};
+
+fn bench_group(c: &mut Criterion<Perf>, hw_event_str: &str) {
+    let mut group = c.benchmark_group(format!("bench-group_no-opt_{}", hw_event_str));
+    // calculate statistics based on a fixed(flat) 100 runs
+    group.sampling_mode(SamplingMode::Flat);
+
+    let bench_funcs: Vec<fn(Option<&mut BenchmarkGroup<Perf>>) -> ()> = vec![
+        bench_nqueens,
+        bench_cfold,
+        bench_deriv,
+        bench_rbtree_ck,
+        bench_rbtree_delete,
+        bench_quicksort,
+    ];
+
+    for bench_func in bench_funcs.iter() {
+        bench_func(Some(&mut group))
+    }
+
+    group.finish();
+}
+
+use perfcnt::linux::HardwareEventType;
+
+fn init_criterion(event: HardwareEventType) -> Criterion<Perf> {
+    Criterion::default().with_measurement(Perf::new(Builder::from_hardware_event(event)))
+}
+
+fn bench_instructions(c: &mut Criterion<Perf>) {
+    bench_group(c, "instructions")
+}
+
+criterion_group!(
+    name = benches_instructions;
+    config = init_criterion(Hardware::Instructions);
+    targets = bench_instructions
+);
+
+fn bench_cache_refs(c: &mut Criterion<Perf>) {
+    bench_group(c, "cache_refs")
+}
+
+criterion_group!(
+    name = benches_cache_refs;
+    config = init_criterion(Hardware::CacheReferences);
+    targets = bench_cache_refs
+);
+
+fn bench_cache_misses(c: &mut Criterion<Perf>) {
+    bench_group(c, "cache_misses")
+}
+
+criterion_group!(
+    name = benches_cache_misses;
+    config = init_criterion(Hardware::CacheMisses);
+    targets = bench_cache_misses
+);
+
+fn bench_branch_instructions(c: &mut Criterion<Perf>) {
+    bench_group(c, "branch_instructions")
+}
+
+criterion_group!(
+    name = benches_branch_instructions;
+    config = init_criterion(Hardware::BranchInstructions);
+    targets = bench_branch_instructions
+);
+
+fn bench_branch_misses(c: &mut Criterion<Perf>) {
+    bench_group(c, "branch_misses")
+}
+
+criterion_group!(
+    name = benches_branch_misses;
+    config = init_criterion(Hardware::BranchMisses);
+    targets = bench_branch_misses
+);
+
+criterion_main!(
+    benches_instructions,
+    benches_cache_refs,
+    benches_cache_misses,
+    benches_branch_instructions,
+    benches_branch_misses
+);
diff --git a/nightly_benches/rust-toolchain b/nightly_benches/rust-toolchain
new file mode 100644
index 0000000000..07ade694b1
--- /dev/null
+++ b/nightly_benches/rust-toolchain
@@ -0,0 +1 @@
+nightly
\ No newline at end of file
diff --git a/nightly_benches/src/main.rs b/nightly_benches/src/main.rs
new file mode 100644
index 0000000000..7fbb8e7beb
--- /dev/null
+++ b/nightly_benches/src/main.rs
@@ -0,0 +1,8 @@
+fn main() {
+    println!("""
+        To run benchmarks:
+            - Install cargo criterion: cargo install --git https://github.com/Anton-4/cargo-criterion --branch main
+            - Necessary to get cache misses...: sudo sh -c 'echo 1 >/proc/sys/kernel/perf_event_paranoid'
+            - run: cargo criterion
+    """);
+}
diff --git a/nix/bin/llvm-as-10 b/nix/bin/llvm-as-10
deleted file mode 100755
index 65839a9b5a..0000000000
--- a/nix/bin/llvm-as-10
+++ /dev/null
@@ -1,2 +0,0 @@
-#!/bin/sh
-llvm-as "$@"
diff --git a/nix/zig.nix b/nix/zig.nix
index 6f5f49bd18..0f18ce1672 100644
--- a/nix/zig.nix
+++ b/nix/zig.nix
@@ -1,37 +1,48 @@
-{ pkgs, isMacOS, isAarch64 }:
+{ pkgs }:
 
-if isMacOS then
-  let
-    version = "0.7.1";
-    osName =
-      if isMacOS
-        then "macos"
-        else "linux";
-    archName =
-      if isAarch64
-        then "aarch64"
-        else "x86_64";
-    archiveName = "zig-${osName}-${archName}-${version}";
-    sha256 = "845cb17562978af0cf67e3993f4e33330525eaf01ead9386df9105111e3bc519";
-  in
-  pkgs.stdenv.mkDerivation {
-    pname = "zig";
-    version = version;
-    buildInputs = [ pkgs.gzip ];
-    src = pkgs.fetchurl {
-      inherit sha256;
-      name = "${archiveName}.tar.xz";
-      url = "https://ziglang.org/download/${version}/${archiveName}.tar.xz";
-    };
-    phases = [ "installPhase" ];
-    installPhase = ''
-      mkdir -p $out/bin
-      tar -xf $src
-      cp ${archiveName}/zig $out/zig
-      cp -r ${archiveName}/lib $out/lib
-      ln -s "$out/zig" "$out/bin/zig"
-      chmod +x $out/bin/zig
-    '';
-  }
-else
-  pkgs.zig
+let
+  version = "0.8.0";
+
+  osName =
+    if pkgs.stdenv.isDarwin then
+      "macos"
+    else
+      "linux";
+
+  splitSystem = builtins.split "-" builtins.currentSystem;
+  arch = builtins.elemAt splitSystem 0;
+  isAarch64 = arch == "aarch64";
+
+  archiveName = "zig-${osName}-${arch}-${version}";
+
+  # If you're system is not aarch64, we assume it's x86_64
+  sha256 =
+    if pkgs.stdenv.isDarwin then
+      if isAarch64 then
+        "b32d13f66d0e1ff740b3326d66a469ee6baddbd7211fa111c066d3bd57683111"
+      else
+        "279f9360b5cb23103f0395dc4d3d0d30626e699b1b4be55e98fd985b62bc6fbe"
+    else
+      if isAarch64 then
+        "ee204ca2c2037952cf3f8b10c609373a08a291efa4af7b3c73be0f2b27720470"
+      else
+        "502625d3da3ae595c5f44a809a87714320b7a40e6dff4a895b5fa7df3391d01e";
+in
+pkgs.stdenv.mkDerivation {
+  pname = "zig";
+  version = version;
+  src = pkgs.fetchurl {
+    inherit sha256;
+    name = "${archiveName}.tar.xz";
+    url = "https://ziglang.org/download/${version}/${archiveName}.tar.xz";
+  };
+  phases = [ "unpackPhase" ];
+  unpackPhase = ''
+    mkdir -p $out/bin
+    tar -xf $src
+    cp ${archiveName}/zig $out/zig
+    cp -r ${archiveName}/lib $out/lib
+    ln -s "$out/zig" "$out/bin/zig"
+    chmod +x $out/bin/zig
+  '';
+}
diff --git a/nix/zls.nix b/nix/zls.nix
deleted file mode 100644
index 29f0feb033..0000000000
--- a/nix/zls.nix
+++ /dev/null
@@ -1,28 +0,0 @@
-{ pkgs, zig }:
-
-# As of 2020-10-25, building zls is not available on Nix. For some reason,
-# this hangs on `zig build`. I'll try to figure it our later.
-
-let
-  rev = "e8c20351d85da8eb4bf22480045b994007284d69";
-in
-pkgs.stdenv.mkDerivation {
-  pname = "zig-language-server";
-  version = rev;
-  src = pkgs.fetchgit {
-    inherit rev;
-    fetchSubmodules = true;
-    url = "https://github.com/zigtools/zls.git";
-    sha256 = "06g8gml1g0fmvcfysy93bd1hb64vjd2v12x3kgxz58kmk5z0168y";
-  };
-  phases = [ "buildPhase" "installPhase" ];
-  buildInputs = [ zig ];
-  buildPhase = ''
-    zig build
-  '';
-  installPhase = ''
-    mkdir -p $out/bin
-    cp ./zig-cache/bin/zls $out/bin/zls
-    chmod +x $out/bin/zls
-  '';
-}
diff --git a/packages/unicode/src/Unicode/Scalar.roc b/packages/unicode/src/Unicode/Scalar.roc
index 18bba5854a..87aec618ba 100644
--- a/packages/unicode/src/Unicode/Scalar.roc
+++ b/packages/unicode/src/Unicode/Scalar.roc
@@ -26,7 +26,7 @@ toStr = \@Scalar u32
         Ok str -> str
         Err _ ->
             # This will quickly crash if it ever runs, but we're confident
-            # this Err branch will never run. That's becasue it only runs
+            # this Err branch will never run. That's because it only runs
             # if Str.fromScalar receives an invalid scalar value, and we've
             # already validated this!
             toStr (@Scalar (scalar * 256))
diff --git a/roc-for-elm-programmers.md b/roc-for-elm-programmers.md
index bad83663cd..575eb461e3 100644
--- a/roc-for-elm-programmers.md
+++ b/roc-for-elm-programmers.md
@@ -1427,6 +1427,8 @@ Here are various Roc expressions involving operators, and what they desugar to.
 | `a ^ b`           | `Num.pow a b`      |
 | `a % b`           | `Num.rem a b`    |
 | `a %% b`          | `Num.mod a b`    |
+| `a >> b`          | `Num.shr a b`    |
+| `a << b`          | `Num.shl a b`    |
 | `-a`              | `Num.neg a`        |
 | `-f x y`          | `Num.neg (f x y)`  |
 | `a == b`          | `Bool.isEq a b`    |
diff --git a/roc_std/src/lib.rs b/roc_std/src/lib.rs
index 4e016957d3..b54fd6c7b9 100644
--- a/roc_std/src/lib.rs
+++ b/roc_std/src/lib.rs
@@ -1,7 +1,7 @@
 #![crate_type = "lib"]
 #![no_std]
 use core::ffi::c_void;
-use core::fmt;
+use core::{fmt, mem, ptr};
 
 pub mod alloca;
 
@@ -19,7 +19,7 @@ extern "C" {
     pub fn roc_dealloc(ptr: *mut c_void, alignment: u32);
 }
 
-const REFCOUNT_1: usize = isize::MIN as usize;
+const REFCOUNT_1: isize = isize::MIN;
 
 #[repr(u8)]
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
@@ -48,7 +48,7 @@ pub struct RocList<T> {
 #[derive(Clone, Copy, Debug)]
 pub enum Storage {
     ReadOnly,
-    Refcounted(usize),
+    Refcounted(isize),
     Capacity(usize),
 }
 
@@ -91,22 +91,26 @@ impl<T> RocList<T> {
             let value = *self.get_storage_ptr();
 
             // NOTE doesn't work with elements of 16 or more bytes
-            match isize::cmp(&(value as isize), &0) {
+            match isize::cmp(&value, &0) {
                 Equal => Some(Storage::ReadOnly),
                 Less => Some(Storage::Refcounted(value)),
-                Greater => Some(Storage::Capacity(value)),
+                Greater => Some(Storage::Capacity(value as usize)),
             }
         }
     }
 
-    fn get_storage_ptr(&self) -> *const usize {
-        let ptr = self.elements as *const usize;
+    fn get_storage_ptr(&self) -> *const isize {
+        let ptr = self.elements as *const isize;
 
         unsafe { ptr.offset(-1) }
     }
 
-    fn get_storage_ptr_mut(&mut self) -> *mut usize {
-        self.get_storage_ptr() as *mut usize
+    fn get_storage_ptr_mut(&mut self) -> *mut isize {
+        self.get_storage_ptr() as *mut isize
+    }
+
+    fn set_storage_ptr(&mut self, ptr: *const isize) {
+        self.elements = unsafe { ptr.offset(1) as *mut T };
     }
 
     fn get_element_ptr(elements: *const T) -> *const T {
@@ -115,13 +119,13 @@ impl<T> RocList<T> {
 
         unsafe {
             if elem_alignment <= core::mem::align_of::<usize>() {
-                ptr.offset(1) as *const T
+                ptr.add(1) as *const T
             } else {
                 // If elements have an alignment bigger than usize (e.g. an i128),
                 // we will have necessarily allocated two usize slots worth of
                 // space for the storage value (with the first usize slot being
                 // padding for alignment's sake), and we need to skip past both.
-                ptr.offset(2) as *const T
+                ptr.add(2) as *const T
             }
         }
     }
@@ -197,6 +201,91 @@ impl<T> RocList<T> {
     pub fn as_slice(&self) -> &[T] {
         unsafe { core::slice::from_raw_parts(self.elements, self.length) }
     }
+
+    /// Copy the contents of the given slice into the end of this list,
+    /// reallocating and resizing as necessary.
+    pub fn append_slice(&mut self, slice: &[T]) {
+        let new_len = self.len() + slice.len();
+        let storage_ptr = self.get_storage_ptr_mut();
+
+        // First, ensure that there's enough storage space.
+        unsafe {
+            let storage_val = *storage_ptr as isize;
+
+            // Check if this is refcounted, readonly, or has a capcacity.
+            // (Capacity will be positive if it has a capacity.)
+            if storage_val > 0 {
+                let capacity = storage_val as usize;
+
+                // We don't have enough capacity, so we need to get some more.
+                if capacity < new_len {
+                    // Double our capacity using realloc
+                    let new_cap = 2 * capacity;
+                    let new_ptr = roc_realloc(
+                        storage_ptr as *mut c_void,
+                        new_cap,
+                        capacity,
+                        Self::align_of_storage_ptr(),
+                    ) as *mut isize;
+
+                    // Write the new capacity into the new memory
+                    *new_ptr = new_cap as isize;
+
+                    // Copy all the existing elements into the new allocation.
+                    ptr::copy_nonoverlapping(self.elements, new_ptr as *mut T, self.len());
+
+                    // Update our storage pointer to be the new one
+                    self.set_storage_ptr(new_ptr);
+                }
+            } else {
+                // If this was reference counted, decrement the refcount!
+                if storage_val < 0 {
+                    let refcount = storage_val;
+
+                    // Either deallocate or decrement.
+                    if refcount == REFCOUNT_1 {
+                        roc_dealloc(storage_ptr as *mut c_void, Self::align_of_storage_ptr());
+                    } else {
+                        *storage_ptr = refcount - 1;
+                    }
+                }
+
+                // This is either refcounted or readonly; either way, we need
+                // to clone the elements!
+
+                // Double the capacity we need, in case there are future additions.
+                let new_cap = new_len * 2;
+                let new_ptr = roc_alloc(new_cap, Self::align_of_storage_ptr()) as *mut isize;
+
+                // Write the new capacity into the new memory; this list is
+                // now unique, and gets its own capacity!
+                *new_ptr = new_cap as isize;
+
+                // Copy all the existing elements into the new allocation.
+                ptr::copy_nonoverlapping(self.elements, new_ptr as *mut T, self.len());
+
+                // Update our storage pointer to be the new one
+                self.set_storage_ptr(new_ptr);
+            }
+
+            // Since this is an append, we want to start writing new elements
+            // into the memory immediately after the current last element.
+            let dest = self.elements.add(self.len());
+
+            // There's now enough storage to append the contents of the slice
+            // in-place, so do that!
+            ptr::copy_nonoverlapping(slice.as_ptr(), dest, self.len());
+        }
+
+        self.length = new_len;
+    }
+
+    /// The alignment we need is either the alignment of T, or else
+    /// the alignment of usize, whichever is higher. That's because we need
+    /// to store both T values as well as the refcount/capacity storage slot.
+    fn align_of_storage_ptr() -> u32 {
+        mem::align_of::<T>().max(mem::align_of::<usize>()) as u32
+    }
 }
 
 impl<T: fmt::Debug> fmt::Debug for RocList<T> {
@@ -215,9 +304,9 @@ impl<T: PartialEq> PartialEq for RocList<T> {
             return false;
         }
 
-        for i in 0..(self.length as isize) {
+        for i in 0..self.length {
             unsafe {
-                if *self.elements.offset(i) != *other.elements.offset(i) {
+                if *self.elements.add(i) != *other.elements.add(i) {
                     return false;
                 }
             }
@@ -231,17 +320,23 @@ impl<T: Eq> Eq for RocList<T> {}
 
 impl<T> Drop for RocList<T> {
     fn drop(&mut self) {
-        use Storage::*;
-        match self.storage() {
-            None | Some(ReadOnly) => {}
-            Some(Capacity(_)) | Some(Refcounted(REFCOUNT_1)) => unsafe {
-                libc::free(self.get_storage_ptr() as *mut libc::c_void);
-            },
-            Some(Refcounted(rc)) => {
-                let sptr = self.get_storage_ptr_mut();
-                unsafe {
-                    *sptr = rc - 1;
+        if !self.is_empty() {
+            let storage_ptr = self.get_storage_ptr_mut();
+
+            unsafe {
+                let storage_val = *storage_ptr;
+
+                if storage_val == REFCOUNT_1 || storage_val > 0 {
+                    // If we have no more references, or if this was unique,
+                    // deallocate it.
+                    roc_dealloc(storage_ptr as *mut c_void, Self::align_of_storage_ptr());
+                } else if storage_val < 0 {
+                    // If this still has more references, decrement one.
+                    *storage_ptr = storage_val - 1;
                 }
+
+                // The only remaining option is that this is in readonly memory,
+                // in which case we shouldn't attempt to do anything to it.
             }
         }
     }
@@ -310,19 +405,19 @@ impl RocStr {
             match isize::cmp(&(value as isize), &0) {
                 Equal => Some(Storage::ReadOnly),
                 Less => Some(Storage::Refcounted(value)),
-                Greater => Some(Storage::Capacity(value)),
+                Greater => Some(Storage::Capacity(value as usize)),
             }
         }
     }
 
-    fn get_storage_ptr(&self) -> *const usize {
-        let ptr = self.elements as *const usize;
+    fn get_storage_ptr(&self) -> *const isize {
+        let ptr = self.elements as *const isize;
 
         unsafe { ptr.offset(-1) }
     }
 
-    fn get_storage_ptr_mut(&mut self) -> *mut usize {
-        self.get_storage_ptr() as *mut usize
+    fn get_storage_ptr_mut(&mut self) -> *mut isize {
+        self.get_storage_ptr() as *mut isize
     }
 
     fn get_element_ptr(elements: *const u8) -> *const usize {
@@ -331,13 +426,13 @@ impl RocStr {
 
         unsafe {
             if elem_alignment <= core::mem::align_of::<usize>() {
-                ptr.offset(1)
+                ptr.add(1)
             } else {
                 // If elements have an alignment bigger than usize (e.g. an i128),
                 // we will have necessarily allocated two usize slots worth of
                 // space for the storage value (with the first usize slot being
                 // padding for alignment's sake), and we need to skip past both.
-                ptr.offset(2)
+                ptr.add(2)
             }
         }
     }
@@ -390,9 +485,10 @@ impl RocStr {
                     // NOTE: using a memcpy here causes weird issues
                     let target_ptr = raw_ptr as *mut u8;
                     let source_ptr = ptr as *const u8;
-                    let length = slice.len() as isize;
+                    let length = slice.len();
+
                     for index in 0..length {
-                        *target_ptr.offset(index) = *source_ptr.offset(index);
+                        *target_ptr.add(index) = *source_ptr.add(index);
                     }
                 }
 
@@ -417,12 +513,32 @@ impl RocStr {
             unsafe { core::slice::from_raw_parts(self.elements, self.length) }
         }
     }
+
     #[allow(clippy::missing_safety_doc)]
     pub unsafe fn as_str(&self) -> &str {
         let slice = self.as_slice();
 
         core::str::from_utf8_unchecked(slice)
     }
+
+    /// Write a CStr (null-terminated) representation of this RocStr into
+    /// the given buffer.
+    ///
+    /// # Safety
+    /// This assumes the given buffer has enough space, so make sure you only
+    /// pass in a pointer to an allocation that's at least as long as this Str!
+    pub unsafe fn write_c_str(&self, buf: *mut u8) -> *mut char {
+        if self.is_small_str() {
+            ptr::copy_nonoverlapping(self.get_small_str_ptr(), buf, self.len());
+        } else {
+            ptr::copy_nonoverlapping(self.elements, buf, self.len());
+        }
+
+        // null-terminate
+        *(buf.add(self.len())) = 0;
+
+        buf as *mut char
+    }
 }
 
 impl From<&str> for RocStr {
@@ -485,18 +601,22 @@ impl Clone for RocStr {
 impl Drop for RocStr {
     fn drop(&mut self) {
         if !self.is_small_str() {
-            use Storage::*;
-            match self.storage() {
-                None | Some(ReadOnly) => {}
-                Some(Capacity(_)) | Some(Refcounted(REFCOUNT_1)) => unsafe {
-                    libc::free(self.get_storage_ptr() as *mut libc::c_void);
-                },
-                Some(Refcounted(rc)) => {
-                    let sptr = self.get_storage_ptr_mut();
-                    unsafe {
-                        *sptr = rc - 1;
-                    }
+            let storage_ptr = self.get_storage_ptr_mut();
+
+            unsafe {
+                let storage_val = *storage_ptr;
+
+                if storage_val == REFCOUNT_1 || storage_val > 0 {
+                    // If we have no more references, or if this was unique,
+                    // deallocate it.
+                    roc_dealloc(storage_ptr as *mut c_void, mem::align_of::<isize>() as u32);
+                } else if storage_val < 0 {
+                    // If this still has more references, decrement one.
+                    *storage_ptr = storage_val - 1;
                 }
+
+                // The only remaining option is that this is in readonly memory,
+                // in which case we shouldn't attempt to do anything to it.
             }
         }
     }
diff --git a/shell.nix b/shell.nix
index 10c965b2e8..878ea9ce69 100644
--- a/shell.nix
+++ b/shell.nix
@@ -1,29 +1,21 @@
-{ }:
+{}:
 
 let
-  splitSystem = builtins.split "-" builtins.currentSystem;
-  currentArch = builtins.elemAt splitSystem 0;
-  currentOS = builtins.elemAt splitSystem 2;
-in with {
   # Look here for information about how pin version of nixpkgs
   #  → https://nixos.wiki/wiki/FAQ/Pinning_Nixpkgs
-  pkgs = import (builtins.fetchGit {
-    name = "nixpkgs-2021-04-23";
-    url = "https://github.com/nixos/nixpkgs/";
-    ref = "refs/heads/nixpkgs-unstable";
-    rev = "8d0340aee5caac3807c58ad7fa4ebdbbdd9134d6";
-  }) { };
+  # TODO: We should probably use flakes at somepoint
+  pkgs = import (
+    builtins.fetchGit {
+      # name = "nixpkgs-2021-04-23";
+      url = "https://github.com/nixos/nixpkgs/";
+      ref = "refs/heads/nixpkgs-unstable";
+      rev = "51bb9f3e9ab6161a3bf7746e20b955712cef618b";
+    }
+  ) {};
 
-  isMacOS = currentOS == "darwin";
-  isLinux = currentOS == "linux";
-  isAarch64 = currentArch == "aarch64";
-};
-
-with (pkgs);
-
-let
-  darwin-inputs =
-    if isMacOS then
+  darwinInputs =
+    with pkgs;
+    lib.optionals stdenv.isDarwin (
       with pkgs.darwin.apple_sdk.frameworks; [
         AppKit
         CoreFoundation
@@ -33,36 +25,28 @@ let
         Metal
         Security
       ]
-    else
-      [ ];
+    );
 
-  linux-inputs =
-    if isLinux then
-      [
-        valgrind
-        vulkan-headers
-        vulkan-loader
-        vulkan-tools
-        vulkan-validation-layers
-        xorg.libX11
-        xorg.libXcursor
-        xorg.libXrandr
-        xorg.libXi
-        xorg.libxcb
-      ]
-    else
-      [ ];
+  linuxInputs =
+    with pkgs;
+    lib.optionals stdenv.isLinux [
+      valgrind
+      vulkan-headers
+      vulkan-loader
+      vulkan-tools
+      vulkan-validation-layers
+      xorg.libX11
+      xorg.libXcursor
+      xorg.libXrandr
+      xorg.libXi
+      xorg.libxcb
+    ];
 
-  nixos-env =
-    if isLinux && builtins.pathExists /etc/nixos/configuration.nix then
-      { XDG_DATA_DIRS = "/run/opengl-driver/share:$XDG_DATA_DIRS";
-      }
-    else
-      { };
+  llvmPkgs = pkgs.llvmPackages_12;
 
-  llvmPkgs = pkgs.llvmPackages_10;
-  zig = import ./nix/zig.nix { inherit pkgs isMacOS isAarch64; };
-  inputs = [
+  zig = import ./nix/zig.nix { inherit pkgs; };
+
+  inputs = with pkgs;[
     # build libraries
     rustc
     cargo
@@ -71,10 +55,11 @@ let
     cmake
     git
     python3
-    llvmPkgs.llvm
+    llvmPkgs.llvm.dev
     llvmPkgs.clang
     pkg-config
     zig
+
     # lib deps
     llvmPkgs.libcxx
     llvmPkgs.libcxxabi
@@ -84,33 +69,31 @@ let
     ncurses
     zlib
     libiconv
+
     # faster builds - see https://github.com/rtfeldman/roc/blob/trunk/BUILDING_FROM_SOURCE.md#use-lld-for-the-linker
     llvmPkgs.lld
-    # dev tools
-    # rust-analyzer
-    # (import ./nix/zls.nix { inherit pkgs zig; })
   ];
+in
+pkgs.mkShell
+  {
+    buildInputs = inputs ++ darwinInputs ++ linuxInputs;
 
-in mkShell (nixos-env // {
-  buildInputs = inputs ++ darwin-inputs ++ linux-inputs;
-
-  # Additional Env vars
-  LLVM_SYS_100_PREFIX = "${llvmPkgs.llvm}";
-  LD_LIBRARY_PATH = stdenv.lib.makeLibraryPath
-    ([
-      pkg-config
-      stdenv.cc.cc.lib
-      llvmPkgs.libcxx
-      llvmPkgs.libcxxabi
-      libunwind
-      libffi
-      ncurses
-      zlib
-    ] ++ linux-inputs);
-
-  # Aliases don't work cross shell, so we do this
-  shellHook = ''
-    export PATH="$PATH:$PWD/nix/bin"
-  '';
-})
-
+    # Additional Env vars
+    LLVM_SYS_120_PREFIX = "${llvmPkgs.llvm.dev}";
+    LD_LIBRARY_PATH =
+      with pkgs;
+      lib.makeLibraryPath
+        (
+          [
+            pkg-config
+            stdenv.cc.cc.lib
+            llvmPkgs.libcxx
+            llvmPkgs.libcxxabi
+            libunwind
+            libffi
+            ncurses
+            zlib
+          ]
+          ++ linuxInputs
+        );
+  }
diff --git a/compiler/gen/Cargo.toml b/vendor/inkwell/Cargo.toml
similarity index 51%
rename from compiler/gen/Cargo.toml
rename to vendor/inkwell/Cargo.toml
index e6ad3aaf21..fe92300ec6 100644
--- a/compiler/gen/Cargo.toml
+++ b/vendor/inkwell/Cargo.toml
@@ -1,25 +1,11 @@
 [package]
-name = "roc_gen"
+name = "inkwell"
 version = "0.1.0"
 authors = ["The Roc Contributors"]
 license = "UPL-1.0"
 edition = "2018"
 
 [dependencies]
-roc_collections = { path = "../collections" }
-roc_region = { path = "../region" }
-roc_module = { path = "../module" }
-roc_problem = { path = "../problem" }
-roc_types = { path = "../types" }
-roc_builtins = { path = "../builtins" }
-roc_unify = { path = "../unify" }
-roc_solve = { path = "../solve" }
-roc_mono = { path = "../mono" }
-im    = "14" # im and im-rc should always have the same version!
-im-rc = "14" # im and im-rc should always have the same version!
-bumpalo = { version = "3.6.1", features = ["collections"] }
-inlinable_string = "0.1"
-either = "1.6.1"
 # NOTE: rtfeldman/inkwell is a fork of TheDan64/inkwell which does not change anything.
 #
 # The reason for this fork is that the way Inkwell is designed, you have to use
@@ -37,21 +23,9 @@ either = "1.6.1"
 # commit of TheDan64/inkwell, push a new tag which points to the latest commit,
 # change the tag value in this Cargo.toml to point to that tag, and `cargo update`.
 # This way, GitHub Actions works and nobody's builds get broken.
-inkwell = { git = "https://github.com/rtfeldman/inkwell", tag = "llvm10-0.release4", features = [ "llvm10-0" ] }
-target-lexicon = "0.10"
+inkwell = { git = "https://github.com/rtfeldman/inkwell", tag = "llvm12-0.release5", features = [ "llvm12-0" ] }
 
-[dev-dependencies]
-roc_can = { path = "../can" }
-roc_parse = { path = "../parse" }
-roc_load = { path = "../load" }
-roc_reporting = { path = "../reporting" }
-roc_build = { path = "../build" }
-roc_std = { path = "../../roc_std" }
-pretty_assertions = "0.5.1"
-maplit = "1.0.1"
-indoc = "0.3.3"
-quickcheck = "0.8"
-quickcheck_macros = "0.8"
-tokio = { version = "0.2", features = ["blocking", "fs", "sync", "rt-threaded"] }
-bumpalo = { version = "3.6.1", features = ["collections"] }
-libc = "0.2"
+[features]
+target-arm = []
+target-aarch64 = []
+target-webassembly = []
diff --git a/vendor/inkwell/src/lib.rs b/vendor/inkwell/src/lib.rs
new file mode 100644
index 0000000000..8136cc5eae
--- /dev/null
+++ b/vendor/inkwell/src/lib.rs
@@ -0,0 +1,4 @@
+#![cfg(not(doctest))]
+// re-export all inkwell members. This way we can switch
+// inkwell versions by making changes in just one place.
+pub use inkwell::*;
diff --git a/vendor/morphic_lib/Cargo.toml b/vendor/morphic_lib/Cargo.toml
index bf93c8b39e..5406cf3647 100644
--- a/vendor/morphic_lib/Cargo.toml
+++ b/vendor/morphic_lib/Cargo.toml
@@ -7,3 +7,4 @@ edition = "2018"
 [dependencies]
 thiserror = "1.0.24"
 sha2 = "0.9.4"
+smallvec = "1.6.1"
diff --git a/vendor/morphic_lib/src/api.rs b/vendor/morphic_lib/src/api.rs
index 604b02f896..f45c2dbd53 100644
--- a/vendor/morphic_lib/src/api.rs
+++ b/vendor/morphic_lib/src/api.rs
@@ -1,6 +1,15 @@
-use std::collections::{btree_map::Entry, BTreeMap, BTreeSet};
-
 use sha2::{digest::Digest, Sha256};
+use smallvec::SmallVec;
+use std::collections::{btree_map::Entry, BTreeMap};
+
+use crate::preprocess;
+use crate::render_api_ir;
+use crate::util::blocks::Blocks;
+use crate::util::id_bi_map::IdBiMap;
+use crate::util::id_type::Count;
+use crate::util::id_vec::IdVec;
+use crate::util::op_graph::OpGraph;
+use crate::util::replace_none::replace_none;
 
 #[derive(Clone, thiserror::Error, Debug)]
 #[non_exhaustive]
@@ -11,12 +20,12 @@ enum ErrorKind {
     BlockIdNotFound(BlockId),
     #[error("no value found for {0:?}")]
     ValueIdNotFound(ValueId),
-    #[error("no join point found for {0:?}")]
-    JoinPointIdNotFound(JoinPointId),
-    #[error("body of join point {0:?} is not defined")]
-    JoinPointNotDefined(JoinPointId),
-    #[error("body of join point {0:?} has already been defined")]
-    JoinPointAlreadyDefined(JoinPointId),
+    #[error("no continuation found for {0:?}")]
+    ContinuationIdNotFound(ContinuationId),
+    #[error("body of continuation {0:?} is not defined")]
+    ContinuationNotDefined(ContinuationId),
+    #[error("body of continuation {0:?} has already been defined")]
+    ContinuationAlreadyDefined(ContinuationId),
     #[error("block {0:?} has no parent")]
     BlockDetached(BlockId),
     #[error("block {0:?} already has a parent")]
@@ -49,6 +58,8 @@ enum ErrorKind {
     ModNotFound(ModNameBuf),
     #[error("entry point {0:?} not found in program")]
     EntryPointNotFound(EntryPointNameBuf),
+    #[error("{0}")]
+    PreprocessError(preprocess::Error),
 }
 
 #[derive(Clone, thiserror::Error, Debug)]
@@ -64,74 +75,75 @@ pub type Result<T> = std::result::Result<T, Error>;
 
 bytes_id! {
     pub ModName;
-    ModNameBuf;
+    pub(crate) ModNameBuf;
 }
 
 bytes_id! {
     pub EntryPointName;
-    EntryPointNameBuf;
+    pub(crate) EntryPointNameBuf;
 }
 
 // Module-level identifiers (unique within a module):
 
 bytes_id! {
     pub TypeName;
-    TypeNameBuf;
+    pub(crate) TypeNameBuf;
 }
 
 bytes_id! {
     pub FuncName;
-    FuncNameBuf;
+    pub(crate) FuncNameBuf;
 }
 
 bytes_id! {
     pub ConstName;
-    ConstNameBuf;
+    pub(crate) ConstNameBuf;
 }
 
 // Local identifiers (unique within a function body):
 
-/// A reference to an arena-allocated expression object in a function body.
-#[repr(C)]
-#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
-pub struct ValueId(pub u32);
+id_type! {
+    /// A reference to an arena-allocated expression object in a function body.
+    #[repr(C)]
+    pub ValueId(u32);
+}
 
-/// A reference to an arena-allocated join point in a function body.
-///
-/// A join point is a point where multiple paths of execution "join" up again. In other words, a
-/// block with multiple entry points. Join points can also be thought of as continuations that do
-/// not escape. See [Compiling without Continuations](https://www.microsoft.com/en-us/research/wp-content/uploads/2016/11/join-points-pldi17.pdf).
-/// Join points are common in functional language IRs, but might also be useful (in the context of
-/// this library) for encoding constructs such as for loops.
-#[repr(C)]
-#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
-pub struct JoinPointId(pub u32);
+id_type! {
+    /// A reference to an arena-allocated continuation in a function body.
+    ///
+    /// See [`ExprContext::declare_continuation`], [`ExprContext::define_continuation`], and
+    /// [`ExprContext::add_jump`] for more information.
+    #[repr(C)]
+    pub ContinuationId(u32);
+}
 
-/// A reference to an arena-allocated block in a function body.
-///
-/// A block is a sequence of zero or more expressions. Each expression has an associated
-/// `ValueId`, which is unique across all blocks within its function.
-#[repr(C)]
-#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
-pub struct BlockId(pub u32);
+id_type! {
+    /// A reference to an arena-allocated block in a function body.
+    ///
+    /// A block is a sequence of zero or more expressions. Each expression has an associated
+    /// `ValueId`, which is unique across all blocks within its function.
+    #[repr(C)]
+    pub BlockId(u32);
+}
 
-/// A reference to an arena-allocated type node in a type definition or function body.
-#[repr(C)]
-#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
-pub struct TypeId(pub u32);
+id_type! {
+    /// A reference to an arena-allocated type node in a type definition or function body.
+    #[repr(C)]
+    pub TypeId(u32);
+}
 
 bytes_id! {
     /// A client-provided name used as a key to look up callee specializations in the solution
     /// tables generated by the analysis engine.
     pub CalleeSpecVar;
-    pub(self) CalleeSpecBuf;
+    pub(crate) CalleeSpecBuf;
 }
 
 bytes_id! {
     /// A client-provided name used as a key to look up concrete update mode flags in the solution
     /// tables generated by the analysis engine.
     pub UpdateModeVar;
-    pub(self) UpdateModeBuf;
+    pub(crate) UpdateModeBuf;
 }
 
 // Input API:
@@ -202,86 +214,6 @@ impl SeqGen {
     }
 }
 
-/// A `TypeDef` defines the content type of a named type.
-pub struct TypeDef;
-
-struct TypeBuilder {
-    tid_gen: SeqGen,
-}
-
-impl Default for TypeBuilder {
-    fn default() -> Self {
-        Self::new()
-    }
-}
-
-impl TypeBuilder {
-    fn new() -> Self {
-        Self {
-            tid_gen: SeqGen::new(),
-        }
-    }
-
-    fn check(&self, id: TypeId) -> Result<()> {
-        self.tid_gen
-            .check_in_range(id.0)
-            .map_err(|()| ErrorKind::TypeIdNotFound(id).into())
-    }
-}
-
-impl TypeContext for TypeBuilder {
-    fn add_named_type(&mut self, _mod: ModName, _type: TypeName) -> TypeId {
-        TypeId(self.tid_gen.next())
-    }
-
-    fn add_tuple_type(&mut self, field_types: &[TypeId]) -> Result<TypeId> {
-        for &field_type in field_types {
-            self.check(field_type)?;
-        }
-        Ok(TypeId(self.tid_gen.next()))
-    }
-
-    fn add_union_type(&mut self, variant_types: &[TypeId]) -> Result<TypeId> {
-        for &variant_type in variant_types {
-            self.check(variant_type)?;
-        }
-        Ok(TypeId(self.tid_gen.next()))
-    }
-
-    fn add_heap_cell_type(&mut self) -> TypeId {
-        TypeId(self.tid_gen.next())
-    }
-
-    fn add_bag_type(&mut self, item_type: TypeId) -> Result<TypeId> {
-        self.check(item_type)?;
-        Ok(TypeId(self.tid_gen.next()))
-    }
-}
-
-pub struct TypeDefBuilder {
-    inner: TypeBuilder,
-}
-
-impl Default for TypeDefBuilder {
-    fn default() -> Self {
-        Self::new()
-    }
-}
-
-impl TypeDefBuilder {
-    pub fn new() -> Self {
-        Self {
-            inner: TypeBuilder::new(),
-        }
-    }
-
-    /// Create a `TypeDef` using the given type node as the root.
-    pub fn build(self, root: TypeId) -> Result<TypeDef> {
-        self.inner.check(root)?;
-        Ok(TypeDef)
-    }
-}
-
 /// A block, together with a value to return from the block.
 ///
 /// The returned value must be in scope at the end of the block.
@@ -296,46 +228,6 @@ pub struct BlockExpr(pub BlockId, pub ValueId);
 forward_trait! {
     /// A trait for constructing expressions in both function bodies and constant initializers.
     pub trait ExprContext {
-        /// Declare a join point in a `block`.
-        ///
-        /// Returning from the resulting join point after jumping to it will yield control to the
-        /// parent of the block in which the join point was defined.
-        ///
-        /// This function allows you to forward-declare a join point without specifying its body,
-        /// which makes it possible to define recursive and mutually recursive joint points.
-        /// However, you must eventually populate the body of each joint point via a call to
-        /// `define_join_point`.
-        ///
-        /// The returned ValueId is the ValueId of the *argument* to the join point, which is in
-        /// scope within the body of the join point.
-        fn declare_join_point(
-            &mut self,
-            block: BlockId,
-            arg_type: TypeId,
-            ret_type: TypeId,
-        ) -> Result<(JoinPointId, ValueId)>;
-
-        /// Define a join point.
-        ///
-        /// Before a join point can be defined, it must be declared via a call to
-        /// `declare_join_point`.
-        fn define_join_point(&mut self, join_point: JoinPointId, body: BlockExpr) -> Result<()>;
-
-        /// Add a jump to a join point.
-        ///
-        /// The rest of the API assumes that every code path produces a `ValueId`. Join points do
-        /// not fit this model neatly because jumping to a join point yields control to outside of
-        /// the current expression. Hence, this function returns a dummy unconstrained `ValueId`,
-        /// which can be used as if the jump were a regular expression producing a value of type
-        /// `unreachable_result_type`.
-        fn add_jump(
-            &mut self,
-            block: BlockId,
-            join_point: JoinPointId,
-            arg: ValueId,
-            unreachable_result_type: TypeId,
-        ) -> Result<ValueId>;
-
         /// Add a block to the current function body.
         ///
         /// A block is a sequence of zero or more expressions. Each expression has an associated
@@ -406,9 +298,9 @@ forward_trait! {
         ///
         /// The sub-block expression evaluates to the value returned by the sub-block.
         ///
-        /// This is useful in combination with join points, because returning from a join point
-        /// always yields control to the parent of the enclosing block in which the join point was
-        /// defined. Without sub-blocks, there would be no way to reduce the scope of a join point
+        /// This is useful in combination with continuations, because returning from a continuation
+        /// always yields control to the parent of the enclosing block in which the continuation was
+        /// defined. Without sub-blocks, there would be no way to reduce the scope of a continuation
         /// within a block.
         fn add_sub_block(&mut self, block: BlockId, sub_block: BlockExpr) -> Result<ValueId>;
 
@@ -434,6 +326,16 @@ forward_trait! {
         /// A 'touch' expression returns the empty tuple type `()`.
         fn add_touch(&mut self, block: BlockId, heap_cell: ValueId) -> Result<ValueId>;
 
+        /// Add a 'recursive touch' expression to a block.
+        ///
+        /// Semantically, this is equivalent to 'touch'ing all heap cells reachable from `object`,
+        /// including heap cells nested inside tuples, unions, bags, and named types.  The `object`
+        /// argument may be of any type, and does not necessarily need to contain any heap cells at
+        /// all (in which case this operation will be a no-op).
+        ///
+        /// A 'recursive touch' expression returns the empty tuple type `()`.
+        fn add_recursive_touch(&mut self, block: BlockId, object: ValueId) -> Result<ValueId>;
+
         /// Add an 'update' expression to a block.
         ///
         /// An 'update' expression represents an attempt to "write" the contents of the heap cell.
@@ -571,33 +473,307 @@ forward_trait! {
             named: TypeName,
             to_unwrap: ValueId,
         ) -> Result<ValueId>;
+
+        /// Declare a continuation in a `block`.
+        ///
+        /// A continuation is like a local function which does not return to its caller.  A
+        /// continuation always has a single argument, and a "body" block in which that argument is
+        /// in scope.  Declaring a continuation has no effect on its own, but after a continuation
+        /// is declared it may be called with a ['jump'](ExprContext::add_jump) expression.
+        ///
+        /// Like calling a function, jumping to a continuation transfers control to its body.
+        /// However, unlike a function, a continuation does not return control to its caller.  When
+        /// a continuation returns, instead of transferring control to the 'jump' expression that
+        /// called it, the continuation transfers control to the *parent of the `block` in which the
+        /// continuation was defined*. When a continuation returns, its return value becomes the
+        /// return value of the `block` in which it was defined.
+        ///
+        /// Continuations can be used to express constructs such as 'goto' statements, loops,
+        /// 'break' and 'continue' statements, early returns, tail calls, and ["join
+        /// points"](https://www.microsoft.com/en-us/research/wp-content/uploads/2016/11/compiling-without-continuations.pdf).
+        ///
+        /// THe `declare_continuation` function allows you to forward-declare a continuation without
+        /// specifying its body, which makes it possible to define recursive and mutually recursive
+        /// continuations.  However, you must eventually populate the body of each continuation via
+        /// a call to [`define_continuation`](ExprContext::define_continuation).
+        ///
+        /// The `ValueId` returned from `declare_continuation` is the `ValueId` of the *argument* to
+        /// the continuation, which is in scope within the body of the continuation.
+        fn declare_continuation(
+            &mut self,
+            block: BlockId,
+            arg_type: TypeId,
+            ret_type: TypeId,
+        ) -> Result<(ContinuationId, ValueId)>;
+
+        /// Define the body of a continuation.
+        ///
+        /// Before a continuation can be defined, it must be declared via a call to
+        /// [`declare_continuation`](ExprContext::declare_continuation).
+        fn define_continuation(
+            &mut self,
+            continuation: ContinuationId,
+            body: BlockExpr
+        ) -> Result<()>;
+
+        /// Add a 'jump' to a continuation.
+        ///
+        /// A 'jump' expression never returns, and any code which operates on the notional "return
+        /// value" of a 'jump' expression will be unreachable.  However, in some cases it may still
+        /// be useful to treat a 'jump' expression as if it returned a value of a given type (for
+        /// example, in a `choice` expression in which one arm returns a value, and the other arm
+        /// ends in a 'jump').   For this reason, `add_jump` accepts an `unreachable_return_type`
+        /// argument which allows you to set the return type of the 'jump' expression to *any type
+        /// of your choosing*. This is analogous to the idea of a ["diverging
+        /// function"](https://doc.rust-lang.org/stable/rust-by-example/fn/diverging.html) in Rust,
+        /// whose return value may be used where a value of any type is expected.
+        fn add_jump(
+            &mut self,
+            block: BlockId,
+            continuation: ContinuationId,
+            arg: ValueId,
+            unreachable_result_type: TypeId,
+        ) -> Result<ValueId>;
     }
 
     impl FuncDefBuilder => .expr_builder;
     impl ConstDefBuilder => .expr_builder;
 }
 
-#[derive(Clone, Copy, Debug)]
-enum JoinPointState {
-    Declared,
-    Defined,
+#[derive(Clone, Debug)]
+pub(crate) enum TypeOp {
+    // 0 inputs
+    Named {
+        named_mod: ModNameBuf,
+        named: TypeNameBuf,
+    },
+    // Variadic inputs
+    Tuple,
+    // Variadic inputs
+    Union,
+    // 0 inputs
+    HeapCell,
+    // 1 input
+    Bag,
+}
+
+/// A `TypeDef` defines the content type of a named type.
+pub struct TypeDef {
+    pub(crate) builder: TypeBuilder,
+    pub(crate) root: TypeId,
+}
+
+#[derive(Clone, Debug)]
+pub(crate) struct TypeBuilder {
+    pub(crate) types: OpGraph<TypeId, TypeOp>,
+}
+
+impl Default for TypeBuilder {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl TypeBuilder {
+    fn new() -> Self {
+        Self {
+            types: OpGraph::new(),
+        }
+    }
+
+    fn check(&self, id: TypeId) -> Result<()> {
+        if id >= self.types.count() {
+            return Err(ErrorKind::TypeIdNotFound(id).into());
+        }
+        Ok(())
+    }
+
+    fn add_node_checked(&mut self, op: TypeOp, inputs: &[TypeId]) -> Result<TypeId> {
+        for &input in inputs {
+            self.check(input)?;
+        }
+        Ok(self.types.add_node(op, inputs))
+    }
+}
+
+impl TypeContext for TypeBuilder {
+    fn add_named_type(&mut self, named_mod: ModName, named: TypeName) -> TypeId {
+        self.types.add_node(
+            TypeOp::Named {
+                named_mod: named_mod.into(),
+                named: named.into(),
+            },
+            &[],
+        )
+    }
+
+    fn add_tuple_type(&mut self, field_types: &[TypeId]) -> Result<TypeId> {
+        self.add_node_checked(TypeOp::Tuple, field_types)
+    }
+
+    fn add_union_type(&mut self, variant_types: &[TypeId]) -> Result<TypeId> {
+        self.add_node_checked(TypeOp::Union, variant_types)
+    }
+
+    fn add_heap_cell_type(&mut self) -> TypeId {
+        self.types.add_node(TypeOp::HeapCell, &[])
+    }
+
+    fn add_bag_type(&mut self, item_type: TypeId) -> Result<TypeId> {
+        self.add_node_checked(TypeOp::Bag, &[item_type])
+    }
+}
+
+pub struct TypeDefBuilder {
+    inner: TypeBuilder,
+}
+
+impl Default for TypeDefBuilder {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl TypeDefBuilder {
+    pub fn new() -> Self {
+        Self {
+            inner: TypeBuilder::new(),
+        }
+    }
+
+    /// Create a `TypeDef` using the given type node as the root.
+    pub fn build(self, root: TypeId) -> Result<TypeDef> {
+        self.inner.check(root)?;
+        Ok(TypeDef {
+            builder: self.inner,
+            root,
+        })
+    }
+}
+
+id_type! {
+    pub(crate) UpdateModeVarId(u32);
+}
+
+id_type! {
+    pub(crate) CalleeSpecVarId(u32);
+}
+
+#[derive(Clone, Debug)]
+pub(crate) enum Op {
+    // 0 inputs
+    Arg,
+    // 0 inputs
+    ContinuationArg,
+    // 0 inputs
+    // TODO: We may want to refactor the IR so that this is not an op.
+    DeclareContinuation {
+        continuation: ContinuationId,
+    },
+    // 1 input
+    Jump {
+        continuation: ContinuationId,
+        unreachable_result_type: TypeId,
+    },
+    // Variadic inputs
+    UnknownWith {
+        result_type: TypeId,
+    },
+    // 1 input
+    Call {
+        callee_spec_var: CalleeSpecVarId,
+        callee_mod: ModNameBuf,
+        callee: FuncNameBuf,
+    },
+    // 0 inputs
+    ConstRef {
+        const_mod: ModNameBuf,
+        const_: ConstNameBuf,
+    },
+    // 0 inputs
+    Choice {
+        // Invariant: 'cases' must be nonempty (to ensure we can always infer the type of a `Choice`
+        // op).
+        cases: SmallVec<[BlockExpr; 6]>,
+    },
+    // 0 inputs
+    SubBlock {
+        sub_block: BlockExpr,
+    },
+    // 0 inputs
+    Terminate {
+        unreachable_result_type: TypeId,
+    },
+    // 0 inputs
+    NewHeapCell,
+    // 1 input
+    Touch,
+    // 1 input
+    RecursiveTouch,
+    // 1 input
+    UpdateWriteOnly {
+        update_mode_var: UpdateModeVarId,
+    },
+    // 0 inputs
+    EmptyBag {
+        item_type: TypeId,
+    },
+    // 2 inputs: (bag, to_insert)
+    BagInsert,
+    // 1 input
+    BagGet,
+    // 1 input
+    // output: (new bag, item)
+    BagRemove,
+    // Variadic inputs
+    MakeTuple,
+    // 1 input
+    GetTupleField {
+        field_idx: u32,
+    },
+    // 1 input
+    MakeUnion {
+        variant_types: SmallVec<[TypeId; 10]>,
+        variant_idx: u32,
+    },
+    // 1 input
+    UnwrapUnion {
+        variant_idx: u32,
+    },
+    // 1 input
+    MakeNamed {
+        named_mod: ModNameBuf,
+        named: TypeNameBuf,
+    },
+    // 1 input
+    UnwrapNamed {
+        named_mod: ModNameBuf,
+        named: TypeNameBuf,
+    },
 }
 
 #[derive(Clone, Copy, Debug)]
-enum BlockState {
+pub(crate) struct ContinuationInfo {
+    pub(crate) arg_type: TypeId,
+    pub(crate) ret_type: TypeId,
+    pub(crate) arg: ValueId,
+    pub(crate) body: Option<BlockExpr>,
+}
+
+#[derive(Clone, Copy, Debug)]
+pub(crate) enum BlockState {
     Detached,
     Attached,
 }
 
-struct ExprBuilder {
-    type_builder: TypeBuilder,
-    bid_gen: SeqGen,
-    vid_gen: SeqGen,
-    jid_gen: SeqGen,
-    blocks: BTreeMap<BlockId, BlockState>,
-    join_points: BTreeMap<JoinPointId, JoinPointState>,
-    callee_spec_vars: BTreeMap<CalleeSpecBuf, FuncSpec>,
-    update_mode_vars: BTreeSet<UpdateModeBuf>,
+#[derive(Clone, Debug)]
+pub(crate) struct ExprBuilder {
+    pub(crate) type_builder: TypeBuilder,
+    pub(crate) blocks: Blocks<BlockId, ValueId, BlockState>,
+    pub(crate) vals: OpGraph<ValueId, Op>,
+    pub(crate) continuations: IdVec<ContinuationId, ContinuationInfo>,
+    pub(crate) callee_spec_vars: IdBiMap<CalleeSpecVarId, CalleeSpecBuf>,
+    pub(crate) update_mode_vars: IdBiMap<UpdateModeVarId, UpdateModeBuf>,
 }
 
 impl Default for ExprBuilder {
@@ -610,13 +786,11 @@ impl ExprBuilder {
     fn new() -> Self {
         Self {
             type_builder: TypeBuilder::new(),
-            bid_gen: SeqGen::new(),
-            vid_gen: SeqGen::new(),
-            jid_gen: SeqGen::new(),
-            blocks: BTreeMap::new(),
-            join_points: BTreeMap::new(),
-            callee_spec_vars: BTreeMap::new(),
-            update_mode_vars: BTreeSet::new(),
+            blocks: Blocks::new(),
+            vals: OpGraph::new(),
+            continuations: IdVec::new(),
+            callee_spec_vars: IdBiMap::new(),
+            update_mode_vars: IdBiMap::new(),
         }
     }
 
@@ -625,38 +799,38 @@ impl ExprBuilder {
     }
 
     fn check_bid(&self, id: BlockId) -> Result<()> {
-        self.bid_gen
-            .check_in_range(id.0)
-            .map_err(|()| ErrorKind::BlockIdNotFound(id).into())
+        if id >= self.blocks.block_count() {
+            return Err(ErrorKind::BlockIdNotFound(id).into());
+        }
+        Ok(())
     }
 
     fn check_vid(&self, id: ValueId) -> Result<()> {
-        self.vid_gen
-            .check_in_range(id.0)
-            .map_err(|()| ErrorKind::ValueIdNotFound(id).into())
+        if id >= self.vals.count() {
+            return Err(ErrorKind::ValueIdNotFound(id).into());
+        }
+        Ok(())
     }
 
-    fn check_jid(&self, id: JoinPointId) -> Result<()> {
-        self.jid_gen
-            .check_in_range(id.0)
-            .map_err(|()| ErrorKind::JoinPointIdNotFound(id).into())
+    fn check_cid(&self, id: ContinuationId) -> Result<()> {
+        if id >= self.continuations.count() {
+            return Err(ErrorKind::ContinuationIdNotFound(id).into());
+        }
+        Ok(())
     }
 
     fn finalize_with_root(&mut self, root: BlockExpr) -> Result<()> {
         self.attach_block(root.0)?;
         self.check_vid(root.1)?;
-        for (&join_point, state) in &self.join_points {
-            match state {
-                JoinPointState::Declared => {
-                    return Err(ErrorKind::JoinPointNotDefined(join_point).into());
-                }
-                JoinPointState::Defined => {}
+        for (continuation, info) in &self.continuations {
+            if info.body.is_none() {
+                return Err(ErrorKind::ContinuationNotDefined(continuation).into());
             }
         }
-        for (&block, state) in &self.blocks {
-            match state {
+        for block_id in self.blocks.block_count().iter() {
+            match self.blocks.block_info(block_id) {
                 BlockState::Detached => {
-                    return Err(ErrorKind::BlockDetached(block).into());
+                    return Err(ErrorKind::BlockDetached(block_id).into());
                 }
                 BlockState::Attached => {}
             }
@@ -665,66 +839,82 @@ impl ExprBuilder {
     }
 
     fn add_argument(&mut self) -> ValueId {
-        ValueId(self.vid_gen.next())
+        self.vals.add_node(Op::Arg, &[])
+    }
+
+    fn add_node_checked(&mut self, block: BlockId, op: Op, inputs: &[ValueId]) -> Result<ValueId> {
+        self.check_bid(block)?;
+        for &input in inputs {
+            self.check_vid(input)?;
+        }
+        let val_id = self.vals.add_node(op, inputs);
+        self.blocks.add_value(block, val_id);
+        Ok(val_id)
     }
 
     fn attach_block(&mut self, block: BlockId) -> Result<()> {
-        match self.blocks.insert(block, BlockState::Attached) {
-            Some(BlockState::Detached) => Ok(()),
-            Some(BlockState::Attached) => Err(ErrorKind::BlockAlreadyAttached(block).into()),
-            None => Err(ErrorKind::BlockIdNotFound(block).into()),
+        self.check_bid(block)?;
+        let state = self.blocks.block_info_mut(block);
+        match state {
+            BlockState::Detached => *state = BlockState::Attached,
+            BlockState::Attached => return Err(ErrorKind::BlockAlreadyAttached(block).into()),
         }
+        Ok(())
     }
 }
 
 impl ExprContext for ExprBuilder {
-    fn declare_join_point(
+    fn declare_continuation(
         &mut self,
         block: BlockId,
         arg_type: TypeId,
         ret_type: TypeId,
-    ) -> Result<(JoinPointId, ValueId)> {
-        self.check_bid(block)?;
+    ) -> Result<(ContinuationId, ValueId)> {
         self.check_tid(arg_type)?;
         self.check_tid(ret_type)?;
-        let join_point = JoinPointId(self.jid_gen.next());
-        self.join_points
-            .insert(join_point, JoinPointState::Declared);
-        Ok((join_point, ValueId(self.vid_gen.next())))
+        let arg = self.vals.add_node(Op::ContinuationArg, &[]);
+        let continuation = self.continuations.push(ContinuationInfo {
+            arg_type,
+            ret_type,
+            arg,
+            body: None,
+        });
+        self.add_node_checked(block, Op::DeclareContinuation { continuation }, &[])?;
+        Ok((continuation, arg))
     }
 
-    fn define_join_point(&mut self, join_point: JoinPointId, body: BlockExpr) -> Result<()> {
-        self.check_jid(join_point)?;
+    fn define_continuation(&mut self, continuation: ContinuationId, body: BlockExpr) -> Result<()> {
+        self.check_cid(continuation)?;
         self.check_bid(body.0)?;
         self.check_vid(body.1)?;
         self.attach_block(body.0)?;
-        match self.join_points.insert(join_point, JoinPointState::Defined) {
-            Some(JoinPointState::Declared) => Ok(()),
-            Some(JoinPointState::Defined) => {
-                Err(ErrorKind::JoinPointAlreadyDefined(join_point).into())
-            }
-            None => Err(ErrorKind::JoinPointIdNotFound(join_point).into()),
-        }
+        replace_none(&mut self.continuations[continuation].body, body)
+            .map_err(|_| ErrorKind::ContinuationAlreadyDefined(continuation))?;
+        Ok(())
     }
 
     fn add_jump(
         &mut self,
         block: BlockId,
-        join_point: JoinPointId,
+        continuation: ContinuationId,
         arg: ValueId,
         unreachable_result_type: TypeId,
     ) -> Result<ValueId> {
-        self.check_bid(block)?;
-        self.check_jid(join_point)?;
-        self.check_vid(arg)?;
+        self.check_cid(continuation)?;
         self.check_tid(unreachable_result_type)?;
-        Ok(ValueId(self.vid_gen.next()))
+        self.add_node_checked(
+            block,
+            Op::Jump {
+                continuation,
+                unreachable_result_type,
+            },
+            &[arg],
+        )
     }
 
     fn add_block(&mut self) -> BlockId {
-        let block = BlockId(self.bid_gen.next());
-        self.blocks.insert(block, BlockState::Detached);
-        block
+        self.blocks
+            .add_block(self.vals.count().0, BlockState::Detached)
     }
 
     fn add_unknown_with(
@@ -733,12 +923,8 @@ impl ExprContext for ExprBuilder {
         args: &[ValueId],
         result_type: TypeId,
     ) -> Result<ValueId> {
-        self.check_bid(block)?;
         self.check_tid(result_type)?;
-        for &arg in args {
-            self.check_vid(arg)?;
-        }
-        Ok(ValueId(self.vid_gen.next()))
+        self.add_node_checked(block, Op::UnknownWith { result_type }, args)
     }
 
     fn add_call(
@@ -749,60 +935,82 @@ impl ExprContext for ExprBuilder {
         callee: FuncName,
         arg: ValueId,
     ) -> Result<ValueId> {
-        self.check_bid(block)?;
-        self.check_vid(arg)?;
-        match self.callee_spec_vars.entry(callee_spec_var.into()) {
-            Entry::Occupied(entry) => {
-                return Err(ErrorKind::DuplicateCalleeSpecVar(entry.key().clone()).into());
-            }
-            Entry::Vacant(entry) => {
-                entry.insert(hash_func_name(callee_mod, callee));
-            }
-        }
-        Ok(ValueId(self.vid_gen.next()))
+        let callee_spec_var_id = self
+            .callee_spec_vars
+            .insert(callee_spec_var.into())
+            .map_err(|_| ErrorKind::DuplicateCalleeSpecVar(callee_spec_var.into()))?;
+        self.add_node_checked(
+            block,
+            Op::Call {
+                callee_spec_var: callee_spec_var_id,
+                callee_mod: callee_mod.into(),
+                callee: callee.into(),
+            },
+            &[arg],
+        )
     }
 
     fn add_const_ref(
         &mut self,
         block: BlockId,
-        _const_mod: ModName,
-        _const: ConstName,
+        const_mod: ModName,
+        const_: ConstName,
     ) -> Result<ValueId> {
-        self.check_bid(block)?;
-        Ok(ValueId(self.vid_gen.next()))
+        self.add_node_checked(
+            block,
+            Op::ConstRef {
+                const_mod: const_mod.into(),
+                const_: const_.into(),
+            },
+            &[],
+        )
     }
 
     fn add_choice(&mut self, block: BlockId, cases: &[BlockExpr]) -> Result<ValueId> {
-        self.check_bid(block)?;
         for &BlockExpr(case_block, case_ret) in cases {
             self.attach_block(case_block)?;
             self.check_vid(case_ret)?;
         }
-        Ok(ValueId(self.vid_gen.next()))
+        self.add_node_checked(
+            block,
+            Op::Choice {
+                cases: SmallVec::from_slice(cases),
+            },
+            &[],
+        )
     }
 
     fn add_sub_block(&mut self, block: BlockId, sub_block: BlockExpr) -> Result<ValueId> {
-        self.check_bid(block)?;
         self.attach_block(sub_block.0)?;
         self.check_vid(sub_block.1)?;
-        Ok(ValueId(self.vid_gen.next()))
+        self.add_node_checked(block, Op::SubBlock { sub_block }, &[])
     }
 
-    fn add_terminate(&mut self, block: BlockId, result_type: TypeId) -> Result<ValueId> {
-        self.check_bid(block)?;
-        self.check_tid(result_type)?;
-        Ok(ValueId(self.vid_gen.next()))
+    fn add_terminate(
+        &mut self,
+        block: BlockId,
+        unreachable_result_type: TypeId,
+    ) -> Result<ValueId> {
+        self.check_tid(unreachable_result_type)?;
+        self.add_node_checked(
+            block,
+            Op::Terminate {
+                unreachable_result_type,
+            },
+            &[],
+        )
     }
 
     fn add_new_heap_cell(&mut self, block: BlockId) -> Result<ValueId> {
-        self.check_bid(block)?;
-        Ok(ValueId(self.vid_gen.next()))
+        self.add_node_checked(block, Op::NewHeapCell, &[])
     }
 
     fn add_touch(&mut self, block: BlockId, heap_cell: ValueId) -> Result<ValueId> {
-        self.check_bid(block)?;
-        self.check_vid(heap_cell)?;
-        Ok(ValueId(self.vid_gen.next()))
+        self.add_node_checked(block, Op::Touch, &[heap_cell])
+    }
+
+    fn add_recursive_touch(&mut self, block: BlockId, object: ValueId) -> Result<ValueId> {
+        self.add_node_checked(block, Op::RecursiveTouch, &[object])
     }
 
     fn add_update(
@@ -821,18 +1029,22 @@ impl ExprContext for ExprBuilder {
         update_mode_var: UpdateModeVar,
         heap_cell: ValueId,
     ) -> Result<ValueId> {
-        self.check_bid(block)?;
-        self.check_vid(heap_cell)?;
-        if !self.update_mode_vars.insert(update_mode_var.into()) {
-            return Err(ErrorKind::DuplicateUpdateModeVar(update_mode_var.into()).into());
-        }
-        Ok(ValueId(self.vid_gen.next()))
+        let update_mode_var_id = self
+            .update_mode_vars
+            .insert(update_mode_var.into())
+            .map_err(|_| ErrorKind::DuplicateUpdateModeVar(update_mode_var.into()))?;
+        self.add_node_checked(
+            block,
+            Op::UpdateWriteOnly {
+                update_mode_var: update_mode_var_id,
+            },
+            &[heap_cell],
+        )
     }
 
     fn add_empty_bag(&mut self, block: BlockId, item_type: TypeId) -> Result<ValueId> {
-        self.check_bid(block)?;
         self.check_tid(item_type)?;
-        Ok(ValueId(self.vid_gen.next()))
+        self.add_node_checked(block, Op::EmptyBag { item_type }, &[])
     }
 
     fn add_bag_insert(
@@ -841,105 +1053,106 @@ impl ExprContext for ExprBuilder {
         bag: ValueId,
         to_insert: ValueId,
     ) -> Result<ValueId> {
-        self.check_bid(block)?;
-        self.check_vid(bag)?;
-        self.check_vid(to_insert)?;
-        Ok(ValueId(self.vid_gen.next()))
+        self.add_node_checked(block, Op::BagInsert, &[bag, to_insert])
     }
 
     fn add_bag_get(&mut self, block: BlockId, bag: ValueId) -> Result<ValueId> {
-        self.check_bid(block)?;
-        self.check_vid(bag)?;
-        Ok(ValueId(self.vid_gen.next()))
+        self.add_node_checked(block, Op::BagGet, &[bag])
     }
 
     fn add_bag_remove(&mut self, block: BlockId, bag: ValueId) -> Result<ValueId> {
-        self.check_bid(block)?;
-        self.check_vid(bag)?;
-        Ok(ValueId(self.vid_gen.next()))
+        self.add_node_checked(block, Op::BagRemove, &[bag])
     }
 
     fn add_make_tuple(&mut self, block: BlockId, field_vals: &[ValueId]) -> Result<ValueId> {
-        self.check_bid(block)?;
-        for &field_val in field_vals {
-            self.check_vid(field_val)?;
-        }
-        Ok(ValueId(self.vid_gen.next()))
+        self.add_node_checked(block, Op::MakeTuple, field_vals)
     }
 
     fn add_get_tuple_field(
         &mut self,
         block: BlockId,
         tuple: ValueId,
-        _field_idx: u32,
+        field_idx: u32,
     ) -> Result<ValueId> {
-        self.check_bid(block)?;
-        self.check_vid(tuple)?;
-        Ok(ValueId(self.vid_gen.next()))
+        self.add_node_checked(block, Op::GetTupleField { field_idx }, &[tuple])
     }
 
     fn add_make_union(
         &mut self,
         block: BlockId,
         variant_types: &[TypeId],
-        _variant_idx: u32,
+        variant_idx: u32,
         to_wrap: ValueId,
     ) -> Result<ValueId> {
-        self.check_bid(block)?;
         for &variant_type in variant_types {
             self.check_tid(variant_type)?;
         }
         // TODO: Check variant_idx in range
-        self.check_vid(to_wrap)?;
-        Ok(ValueId(self.vid_gen.next()))
+        self.add_node_checked(
+            block,
+            Op::MakeUnion {
+                variant_types: SmallVec::from_slice(variant_types),
+                variant_idx,
+            },
+            &[to_wrap],
+        )
     }
 
     fn add_unwrap_union(
         &mut self,
         block: BlockId,
         to_unwrap: ValueId,
-        _variant_idx: u32,
+        variant_idx: u32,
     ) -> Result<ValueId> {
-        self.check_bid(block)?;
-        self.check_vid(to_unwrap)?;
-        Ok(ValueId(self.vid_gen.next()))
+        self.add_node_checked(block, Op::UnwrapUnion { variant_idx }, &[to_unwrap])
     }
 
     fn add_make_named(
         &mut self,
         block: BlockId,
-        _named_mod: ModName,
-        _named: TypeName,
+        named_mod: ModName,
+        named: TypeName,
         to_wrap: ValueId,
     ) -> Result<ValueId> {
-        self.check_bid(block)?;
-        self.check_vid(to_wrap)?;
-        Ok(ValueId(self.vid_gen.next()))
+        self.add_node_checked(
+            block,
+            Op::MakeNamed {
+                named_mod: named_mod.into(),
+                named: named.into(),
+            },
+            &[to_wrap],
+        )
     }
 
     fn add_unwrap_named(
         &mut self,
         block: BlockId,
-        _named_mod: ModName,
-        _named: TypeName,
+        named_mod: ModName,
+        named: TypeName,
         to_unwrap: ValueId,
     ) -> Result<ValueId> {
-        self.check_bid(block)?;
-        self.check_vid(to_unwrap)?;
-        Ok(ValueId(self.vid_gen.next()))
+        self.add_node_checked(
+            block,
+            Op::UnwrapNamed {
+                named_mod: named_mod.into(),
+                named: named.into(),
+            },
+            &[to_unwrap],
+        )
     }
 }
 
 /// A `FuncDef` defines the signature and body of a function.
 pub struct FuncDef {
-    // We can precompute hashes for all calls because monomorphization isn't implemented yet.
-    callee_spec_vars: BTreeMap<CalleeSpecBuf, FuncSpec>,
-    update_mode_vars: BTreeSet<UpdateModeBuf>,
+    pub(crate) builder: FuncDefBuilder,
+    pub(crate) arg_type: TypeId,
+    pub(crate) ret_type: TypeId,
+    pub(crate) root: BlockExpr,
 }
 
 pub struct FuncDefBuilder {
-    expr_builder: ExprBuilder,
-    argument: ValueId,
+    pub(crate) expr_builder: ExprBuilder,
+    pub(crate) argument: ValueId,
 }
 
 impl Default for FuncDefBuilder {
@@ -974,21 +1187,23 @@ impl FuncDefBuilder {
         self.expr_builder.check_tid(ret_type)?;
         self.expr_builder.finalize_with_root(root)?;
         Ok(FuncDef {
-            callee_spec_vars: self.expr_builder.callee_spec_vars,
-            update_mode_vars: self.expr_builder.update_mode_vars,
+            builder: self,
+            arg_type,
+            ret_type,
+            root,
         })
     }
 }
 
 /// A `ConstDef` defines the type and initializer expression for a global constant.
 pub struct ConstDef {
-    // We can precompute hashes for all calls because monomorphization isn't implemented yet.
-    callee_spec_vars: BTreeMap<CalleeSpecBuf, FuncSpec>,
-    update_mode_vars: BTreeSet<UpdateModeBuf>,
+    pub(crate) builder: ConstDefBuilder,
+    pub(crate) type_: TypeId,
+    pub(crate) root: BlockExpr,
 }
 
 pub struct ConstDefBuilder {
-    expr_builder: ExprBuilder,
+    pub(crate) expr_builder: ExprBuilder,
 }
 
 impl Default for ConstDefBuilder {
@@ -1011,20 +1226,22 @@ impl ConstDefBuilder {
         self.expr_builder.check_tid(type_)?;
         self.expr_builder.finalize_with_root(root)?;
         Ok(ConstDef {
-            callee_spec_vars: self.expr_builder.callee_spec_vars,
-            update_mode_vars: self.expr_builder.update_mode_vars,
+            builder: self,
+            type_,
+            root,
         })
     }
 }
 
 /// A `ModDef` defines a module, which is a collection of named types, functions, and constants.
 pub struct ModDef {
-    func_defs: BTreeMap<FuncNameBuf, FuncDef>,
-    const_defs: BTreeMap<ConstNameBuf, ConstDef>,
+    pub(crate) type_defs: BTreeMap<TypeNameBuf, TypeDef>,
+    pub(crate) func_defs: BTreeMap<FuncNameBuf, FuncDef>,
+    pub(crate) const_defs: BTreeMap<ConstNameBuf, ConstDef>,
 }
 
 pub struct ModDefBuilder {
-    type_names: BTreeSet<TypeNameBuf>,
+    type_defs: BTreeMap<TypeNameBuf, TypeDef>,
     func_defs: BTreeMap<FuncNameBuf, FuncDef>,
     const_defs: BTreeMap<ConstNameBuf, ConstDef>,
 }
@@ -1038,7 +1255,7 @@ impl Default for ModDefBuilder {
 impl ModDefBuilder {
     pub fn new() -> Self {
         Self {
-            type_names: BTreeSet::new(),
+            type_defs: BTreeMap::new(),
             func_defs: BTreeMap::new(),
             const_defs: BTreeMap::new(),
         }
@@ -1046,13 +1263,14 @@ impl ModDefBuilder {
 
     pub fn build(self) -> Result<ModDef> {
         Ok(ModDef {
+            type_defs: self.type_defs,
             func_defs: self.func_defs,
             const_defs: self.const_defs,
         })
     }
 
-    pub fn add_named_type(&mut self, name: TypeName, _type_def: TypeDef) -> Result<()> {
-        if !self.type_names.insert(name.into()) {
+    pub fn add_named_type(&mut self, name: TypeName, type_def: TypeDef) -> Result<()> {
+        if self.type_defs.insert(name.into(), type_def).is_some() {
             return Err(ErrorKind::DuplicateTypeName(name.into()).into());
         }
         Ok(())
@@ -1078,8 +1296,14 @@ impl ModDefBuilder {
 ///
 /// Each entry point has an associated main function, which must have signature `() -> ()`.
 pub struct Program {
-    mods: BTreeMap<ModNameBuf, ModDef>,
-    entry_points: BTreeMap<EntryPointNameBuf, (ModNameBuf, FuncNameBuf)>,
+    pub(crate) mods: BTreeMap<ModNameBuf, ModDef>,
+    pub(crate) entry_points: BTreeMap<EntryPointNameBuf, (ModNameBuf, FuncNameBuf)>,
+}
+
+impl Program {
+    pub fn to_source_string(&self) -> String {
+        render_api_ir::render_program_to_string(self)
+    }
 }
 
 pub struct ProgramBuilder {
@@ -1154,14 +1378,21 @@ pub struct FuncSpec(pub [u8; SPEC_HASH_BYTES]);
 /// The solution table for an individual specialization.
 pub struct FuncSpecSolutions {
     func_def: FuncDef,
+    callee_specs: IdVec<CalleeSpecVarId, FuncSpec>,
 }
 
 impl FuncSpecSolutions {
     pub fn callee_spec(&self, var: CalleeSpecVar) -> Result<FuncSpec> {
         // TODO: The clone here is unnecessary -- avoid it!
         // (might require something like a transmute)
-        match self.func_def.callee_spec_vars.get(&var.into()) {
-            Some(spec) => Ok(*spec),
+        match self
+            .func_def
+            .builder
+            .expr_builder
+            .callee_spec_vars
+            .get_by_val(&var.into())
+        {
+            Some(id) => Ok(self.callee_specs[id]),
             None => Err(ErrorKind::CalleeSpecVarNotFound(var.into()).into()),
         }
     }
@@ -1169,10 +1400,16 @@ impl FuncSpecSolutions {
     pub fn update_mode(&self, var: UpdateModeVar) -> Result<UpdateMode> {
         // TODO: The clone here is unnecessary -- avoid it!
         // (might require something like a transmute)
-        if !self.func_def.update_mode_vars.contains(&var.into()) {
-            return Err(ErrorKind::UpdateModeVarNotFound(var.into()).into());
+        match self
+            .func_def
+            .builder
+            .expr_builder
+            .update_mode_vars
+            .get_by_val(&var.into())
+        {
+            Some(_id) => Ok(UpdateMode::Immutable),
+            None => Err(ErrorKind::UpdateModeVarNotFound(var.into()).into()),
         }
-        Ok(UpdateMode::Immutable)
     }
 }
 
@@ -1201,14 +1438,21 @@ impl FuncSolutions {
 /// Note that, unlike functions, constant definitions cannot have multiple specializations.
 pub struct ConstSolutions {
     const_def: ConstDef,
+    callee_specs: IdVec<CalleeSpecVarId, FuncSpec>,
 }
 
 impl ConstSolutions {
     pub fn callee_spec(&self, var: CalleeSpecVar) -> Result<FuncSpec> {
         // TODO: The clone here is unnecessary -- avoid it!
         // (might require something like a transmute)
-        match self.const_def.callee_spec_vars.get(&var.into()) {
-            Some(spec) => Ok(*spec),
+        match self
+            .const_def
+            .builder
+            .expr_builder
+            .callee_spec_vars
+            .get_by_val(&var.into())
+        {
+            Some(id) => Ok(self.callee_specs[id]),
             None => Err(ErrorKind::CalleeSpecVarNotFound(var.into()).into()),
         }
     }
@@ -1216,10 +1460,16 @@ impl ConstSolutions {
     pub fn update_mode(&self, var: UpdateModeVar) -> Result<UpdateMode> {
         // TODO: The clone here is unnecessary -- avoid it!
         // (might require something like a transmute)
-        if !self.const_def.update_mode_vars.contains(&var.into()) {
-            return Err(ErrorKind::UpdateModeVarNotFound(var.into()).into());
+        match self
+            .const_def
+            .builder
+            .expr_builder
+            .update_mode_vars
+            .get_by_val(&var.into())
+        {
+            Some(_id) => Ok(UpdateMode::Immutable),
+            None => Err(ErrorKind::UpdateModeVarNotFound(var.into()).into()),
         }
-        Ok(UpdateMode::Immutable)
     }
 }
 
@@ -1280,7 +1530,31 @@ impl Solutions {
     }
 }
 
+fn populate_specs(
+    callee_spec_var_ids: Count<CalleeSpecVarId>,
+    vals: &OpGraph<ValueId, Op>,
+) -> IdVec<CalleeSpecVarId, FuncSpec> {
+    let mut results = IdVec::filled_with(callee_spec_var_ids, || None);
+    for val_id in vals.count().iter() {
+        if let Op::Call {
+            callee_spec_var,
+            callee_mod,
+            callee,
+        } = vals.node(val_id).op
+        {
+            replace_none(
+                &mut results[callee_spec_var],
+                hash_func_name(callee_mod.borrowed(), callee.borrowed()),
+            )
+            .unwrap();
+        }
+    }
+    results.into_mapped(|_, spec| spec.unwrap())
+}
+
 pub fn solve(program: Program) -> Result<Solutions> {
+    preprocess::preprocess(&program).map_err(ErrorKind::PreprocessError)?;
+
     Ok(Solutions {
         mods: program
             .mods
@@ -1291,9 +1565,16 @@ pub fn solve(program: Program) -> Result<Solutions> {
                         .func_defs
                         .into_iter()
                         .map(|(func_name, func_def)| {
+                            let callee_specs = populate_specs(
+                                func_def.builder.expr_builder.callee_spec_vars.count(),
+                                &func_def.builder.expr_builder.vals,
+                            );
                             let func_sols = FuncSolutions {
                                 spec: hash_func_name(mod_name.borrowed(), func_name.borrowed()),
-                                spec_solutions: FuncSpecSolutions { func_def },
+                                spec_solutions: FuncSpecSolutions {
+                                    func_def,
+                                    callee_specs,
+                                },
                             };
                             (func_name, func_sols)
                         })
@@ -1301,7 +1582,19 @@ pub fn solve(program: Program) -> Result<Solutions> {
                     consts: mod_def
                         .const_defs
                         .into_iter()
-                        .map(|(const_name, const_def)| (const_name, ConstSolutions { const_def }))
+                        .map(|(const_name, const_def)| {
+                            let callee_specs = populate_specs(
+                                const_def.builder.expr_builder.callee_spec_vars.count(),
+                                &const_def.builder.expr_builder.vals,
+                            );
+                            (
+                                const_name,
+                                ConstSolutions {
+                                    const_def,
+                                    callee_specs,
+                                },
+                            )
+                        })
                         .collect(),
                 };
                 (mod_name, mod_sols)
diff --git a/vendor/morphic_lib/src/bindings.rs b/vendor/morphic_lib/src/bindings.rs
index c8864e30d9..c5dbb413a1 100644
--- a/vendor/morphic_lib/src/bindings.rs
+++ b/vendor/morphic_lib/src/bindings.rs
@@ -1,6 +1,8 @@
 // TODO: These bindings are incomplete
 // TODO: Add test for compatibility with `include/morphic.h`
 
+#![allow(clippy::boxed_local)]
+
 use crate::api::*;
 use std::{ffi::CString, os::raw::c_char, ptr, slice};
 
@@ -218,27 +220,28 @@ pub unsafe extern "C" fn Morphic_FuncDefBuilder_Build(
 }
 
 #[no_mangle]
-pub unsafe extern "C" fn Morphic_FuncDefBuilder_DeclareJoinPoint(
+pub unsafe extern "C" fn Morphic_FuncDefBuilder_DeclareContinuation(
     self_: *mut FuncDefBuilder,
     block: BlockId,
     arg_type: TypeId,
     ret_type: TypeId,
-    out0: *mut JoinPointId,
+    out0: *mut ContinuationId,
     out1: *mut ValueId,
 ) -> *mut Error {
-    let (join_point, value) = check_err!((*self_).declare_join_point(block, arg_type, ret_type));
-    *out0 = join_point;
+    let (continuation, value) =
+        check_err!((*self_).declare_continuation(block, arg_type, ret_type));
+    *out0 = continuation;
     *out1 = value;
     ptr::null_mut()
 }
 
 #[no_mangle]
-pub unsafe extern "C" fn Morphic_FuncDefBuilder_DefineJoinPoint(
+pub unsafe extern "C" fn Morphic_FuncDefBuilder_DefineContinuation(
     self_: *mut FuncDefBuilder,
-    join_point: JoinPointId,
+    continuation: ContinuationId,
     body: BlockExpr,
 ) -> *mut Error {
-    check_err!((*self_).define_join_point(join_point, body));
+    check_err!((*self_).define_continuation(continuation, body));
     ptr::null_mut()
 }
 
@@ -246,12 +249,12 @@ pub unsafe extern "C" fn Morphic_FuncDefBuilder_DefineJoinPoint(
 pub unsafe extern "C" fn Morphic_FuncDefBuilder_AddJump(
     self_: *mut FuncDefBuilder,
     block: BlockId,
-    join_point: JoinPointId,
+    continuation: ContinuationId,
     arg: ValueId,
     unreachable_result_type: TypeId,
     out: *mut ValueId,
 ) -> *mut Error {
-    *out = check_err!((*self_).add_jump(block, join_point, arg, unreachable_result_type));
+    *out = check_err!((*self_).add_jump(block, continuation, arg, unreachable_result_type));
     ptr::null_mut()
 }
 
diff --git a/vendor/morphic_lib/src/ir.rs b/vendor/morphic_lib/src/ir.rs
new file mode 100644
index 0000000000..b6408ebc98
--- /dev/null
+++ b/vendor/morphic_lib/src/ir.rs
@@ -0,0 +1,303 @@
+//! The core IR used for analysis.
+//!
+//! The IR uses a variant of SSA in which control-dependent values are represented using block
+//! parameters instead of phi nodes.
+
+use smallvec::SmallVec;
+
+use crate::api::{CalleeSpecVarId, UpdateModeVarId};
+use crate::name_cache::{ConstId, FuncId};
+use crate::type_cache::TypeId;
+use crate::util::blocks::Blocks;
+use crate::util::flat_slices::{FlatSlices, Slice};
+use crate::util::op_graph::OpGraph;
+use crate::util::strongly_connected::{strongly_connected, SccKind};
+
+/// The "core payload" of a node in the op graph, determining its semantics.
+///
+/// The `OpKind` of a node is *not* sufficient to uniquely determine either the ids or the types of
+/// its inputs or output.  This information is stored separately in the graph.
+#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub(crate) enum OpKind {
+    // Variadic inputs
+    // Inputs may have any type
+    // Output may have any type
+    UnknownWith,
+    // 1 input
+    // Input type must match argument type of callee
+    // Output type must match return type of callee
+    Call {
+        callee_spec_var: CalleeSpecVarId,
+        callee: FuncId,
+    },
+    // 0 inputs
+    // Output type must match type of const
+    ConstRef {
+        const_: ConstId,
+    },
+    // 0 inputs
+    // Output type is heap cell
+    NewHeapCell,
+    // 1 input
+    // Input may have any type
+    // Output type is unit
+    RecursiveTouch,
+    // 1 input
+    // Input type is heap cell
+    // Output type is unit
+    UpdateWriteOnly {
+        update_mode_var: UpdateModeVarId,
+    },
+    // 0 inputs
+    // Output type is `Bag<T>` for some `T`
+    EmptyBag,
+    // 2 inputs: `(bag, to_insert)`
+    // `bag` has type `Bag<T>`
+    // `to_insert` has type `T`
+    // Output type is `Bag<T>`
+    BagInsert,
+    // 1 input
+    // Input type is `Bag<T>`
+    // Output type is `T`
+    BagGet,
+    // 1 input
+    // Input type is `Bag<T>`
+    // Output type is `(Bag<T>, T)`
+    BagRemove,
+    // Variadic inputs
+    // Input types are `T_1, ..., T_N`
+    // Output type is `(T_1, ..., T_N)`
+    MakeTuple,
+    // 1 input
+    // Input type is `(T_1, ..., T_N)`
+    // Output type is `T_(field_idx)`
+    GetTupleField {
+        field_idx: u32,
+    },
+    // 1 input
+    // Input type is `T_(variant_idx)`
+    // Output type is `union { T_1, ..., T_N }`
+    MakeUnion {
+        variant_idx: u32,
+    },
+    // 1 input
+    // Input type is `union { T_1, ..., T_N }`
+    // Output type is `T_(variant_idx)`
+    UnwrapUnion {
+        variant_idx: u32,
+    },
+    // 1 input
+    // Input type is content type of a named type `T`
+    // Output type is `T`
+    MakeNamed,
+    // 1 input
+    // Input type is a named type `T`
+    // Output type is the content type of `T`
+    UnwrapNamed,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub(crate) enum ValueKind {
+    Op(OpKind),
+    // 0 inputs
+    BlockParam,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub(crate) struct ValueInfo {
+    pub(crate) kind: ValueKind,
+    pub(crate) result_type: TypeId,
+}
+
+id_type! {
+    pub(crate) ValueId(u32);
+}
+
+id_type! {
+    pub(crate) BlockId(u32);
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub(crate) enum JumpTarget {
+    Block(BlockId),
+    Ret,
+}
+
+pub(crate) const JUMP_TARGETS_INLINE_COUNT: usize = 8;
+
+#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub(crate) struct BlockInfo {
+    /// A block may optionally have a single parameter, which serves the same role as a phi node in
+    /// SSA.
+    ///
+    /// Invariant: If `param` is `Some`, it must point to a `BlockParam` value, not an `Op`.
+    pub(crate) param: Option<ValueId>,
+    /// List of zero or more jump targets to nondeterministically choose from.
+    pub(crate) jump_targets: SmallVec<[JumpTarget; JUMP_TARGETS_INLINE_COUNT]>,
+    /// Optional argument which will be passed to the chosen jump target.
+    pub(crate) target_arg: Option<ValueId>,
+}
+
+id_type! {
+    /// Identifies a strongly connected component in the control flow graph.
+    ///
+    /// https://en.wikipedia.org/wiki/Strongly_connected_component
+    pub(crate) SccId(u32);
+}
+
+#[derive(Clone, Debug)]
+pub(crate) struct GraphBuilder {
+    values: OpGraph<ValueId, ValueInfo>,
+    blocks: Blocks<BlockId, ValueId, BlockInfo>,
+}
+
+impl GraphBuilder {
+    pub(crate) fn new() -> Self {
+        GraphBuilder {
+            values: OpGraph::new(),
+            blocks: Blocks::new(),
+        }
+    }
+
+    pub(crate) fn add_op(
+        &mut self,
+        block: BlockId,
+        op_kind: OpKind,
+        inputs: &[ValueId],
+        result_type: TypeId,
+    ) -> ValueId {
+        let val_id = self.values.add_node(
+            ValueInfo {
+                kind: ValueKind::Op(op_kind),
+                result_type,
+            },
+            inputs,
+        );
+        self.blocks.add_value(block, val_id);
+        val_id
+    }
+
+    pub(crate) fn add_block(&mut self) -> BlockId {
+        self.blocks.add_block(
+            self.values.count().0,
+            BlockInfo {
+                param: None,
+                jump_targets: SmallVec::new(),
+                target_arg: None,
+            },
+        )
+    }
+
+    pub(crate) fn add_block_with_param(&mut self, param_type: TypeId) -> (BlockId, ValueId) {
+        let param_id = self.values.add_node(
+            ValueInfo {
+                kind: ValueKind::BlockParam,
+                result_type: param_type,
+            },
+            &[],
+        );
+        let block_id = self.blocks.add_block(
+            self.values.count().0,
+            BlockInfo {
+                param: Some(param_id),
+                jump_targets: SmallVec::new(),
+                target_arg: None,
+            },
+        );
+        (block_id, param_id)
+    }
+
+    pub(crate) fn set_jump_targets(
+        &mut self,
+        block: BlockId,
+        target_arg: Option<ValueId>,
+        jump_targets: SmallVec<[JumpTarget; JUMP_TARGETS_INLINE_COUNT]>,
+    ) {
+        let info = self.blocks.block_info_mut(block);
+        info.target_arg = target_arg;
+        info.jump_targets = jump_targets;
+    }
+
+    pub(crate) fn values(&self) -> &OpGraph<ValueId, ValueInfo> {
+        &self.values
+    }
+
+    pub(crate) fn blocks(&self) -> &Blocks<BlockId, ValueId, BlockInfo> {
+        &self.blocks
+    }
+
+    pub(crate) fn build(self, entry_block: BlockId) -> Graph {
+        debug_assert!(entry_block < self.blocks.block_count());
+        let rev_sccs = strongly_connected(self.blocks.block_count(), |block| {
+            self.blocks
+                .block_info(block)
+                .jump_targets
+                .iter()
+                .filter_map(|&jump_target| match jump_target {
+                    JumpTarget::Ret => None,
+                    JumpTarget::Block(target) => Some(target),
+                })
+        });
+        Graph {
+            values: self.values,
+            blocks: self.blocks,
+            entry_block,
+            rev_sccs,
+        }
+    }
+}
+
+#[derive(Clone, Debug)]
+pub(crate) struct Graph {
+    values: OpGraph<ValueId, ValueInfo>,
+    blocks: Blocks<BlockId, ValueId, BlockInfo>,
+    entry_block: BlockId,
+
+    // Invariant: `rev_sccs` must be stored in *reverse* topological order.  If an SCC 'A' can jump
+    // to an SCC 'B', then 'A' must appear *after* 'B' in `rev_sccs`.
+    //
+    // We don't store the SCCs in topological order because control flow graph edges point from
+    // *source block* to *target block*, so running Tarjan's algorithm on the control flow graph
+    // gives us a reverse topological sort rather than a topological sort.
+    rev_sccs: FlatSlices<SccId, SccKind, BlockId>,
+}
+
+impl Graph {
+    pub(crate) fn values(&self) -> &OpGraph<ValueId, ValueInfo> {
+        &self.values
+    }
+
+    pub(crate) fn blocks(&self) -> &Blocks<BlockId, ValueId, BlockInfo> {
+        &self.blocks
+    }
+
+    pub(crate) fn entry_block(&self) -> BlockId {
+        self.entry_block
+    }
+
+    pub(crate) fn rev_sccs(&self) -> &FlatSlices<SccId, SccKind, BlockId> {
+        &self.rev_sccs
+    }
+
+    /// Iterate over sccs in topological order.
+    ///
+    /// IF an SCC 'A' can jump to an SCC 'B', then 'A' is guaranteed to appear *before* 'B' in the
+    /// returned iterator.
+    pub(crate) fn iter_sccs(&self) -> impl Iterator<Item = Slice<'_, SccKind, BlockId>> + '_ {
+        self.rev_sccs
+            .count()
+            .iter()
+            .rev()
+            .map(move |scc_id| self.rev_sccs.get(scc_id))
+    }
+}
+
+#[derive(Clone, Debug)]
+pub(crate) struct FuncDef {
+    pub(crate) graph: Graph,
+}
+
+#[derive(Clone, Debug)]
+pub(crate) struct ConstDef {
+    pub(crate) graph: Graph,
+}
diff --git a/vendor/morphic_lib/src/lib.rs b/vendor/morphic_lib/src/lib.rs
index 3a49195dbe..a1470770dc 100644
--- a/vendor/morphic_lib/src/lib.rs
+++ b/vendor/morphic_lib/src/lib.rs
@@ -1,9 +1,15 @@
 #![allow(dead_code)]
+#![allow(clippy::too_many_arguments)]
 
 #[macro_use]
 mod util;
 
 mod api;
 mod bindings;
+mod ir;
+mod name_cache;
+mod preprocess;
+mod render_api_ir;
+mod type_cache;
 
 pub use api::*;
diff --git a/vendor/morphic_lib/src/name_cache.rs b/vendor/morphic_lib/src/name_cache.rs
new file mode 100644
index 0000000000..03e45fb821
--- /dev/null
+++ b/vendor/morphic_lib/src/name_cache.rs
@@ -0,0 +1,26 @@
+use crate::api;
+use crate::util::id_bi_map::IdBiMap;
+
+id_type! {
+    pub NamedTypeId(u32);
+}
+
+id_type! {
+    pub FuncId(u32);
+}
+
+id_type! {
+    pub ConstId(u32);
+}
+
+id_type! {
+    pub EntryPointId(u32);
+}
+
+#[derive(Clone, Debug, Default)]
+pub(crate) struct NameCache {
+    pub(crate) named_types: IdBiMap<NamedTypeId, (api::ModNameBuf, api::TypeNameBuf)>,
+    pub(crate) funcs: IdBiMap<FuncId, (api::ModNameBuf, api::FuncNameBuf)>,
+    pub(crate) consts: IdBiMap<ConstId, (api::ModNameBuf, api::ConstNameBuf)>,
+    pub(crate) entry_points: IdBiMap<EntryPointId, api::EntryPointNameBuf>,
+}
diff --git a/vendor/morphic_lib/src/preprocess.rs b/vendor/morphic_lib/src/preprocess.rs
new file mode 100644
index 0000000000..161d61ca9f
--- /dev/null
+++ b/vendor/morphic_lib/src/preprocess.rs
@@ -0,0 +1,1204 @@
+//! Before we perform any interprocedural analysis, we run a preprocessing pass to convert the "API
+//! IR" (the IR constructed by the input API) to the "analysis IR" (the IR used for all subsequent
+//! analysis).
+//!
+//! We cannot perform this preprocessing step eagerly while the API IR is being constructed, because
+//! some parts of the preprocessing (for example, typechecking) rely on nonlocal information (such
+//! as functions' type signatures) which is not available until the entire input program has been
+//! constructed.
+//!
+//! This pass currently implements the following functionality:
+//! - Scope checking for values
+//! - Typechecking
+
+use smallvec::{smallvec, SmallVec};
+
+use crate::api;
+use crate::ir;
+use crate::ir::JumpTarget;
+use crate::name_cache::{ConstId, EntryPointId, FuncId, NameCache, NamedTypeId};
+use crate::type_cache::{TypeCache, TypeData, TypeId};
+use crate::util::id_vec::IdVec;
+use crate::util::op_graph::OpGraph;
+use crate::util::replace_none::replace_none;
+
+#[derive(Clone, thiserror::Error, Debug)]
+pub(crate) enum ErrorKind {
+    #[error("could not find named type in module {0:?} with name {1:?}")]
+    NamedTypeNotFound(api::ModNameBuf, api::TypeNameBuf),
+    #[error("could not find func in module {0:?} with name {1:?}")]
+    FuncNotFound(api::ModNameBuf, api::FuncNameBuf),
+    #[error("could not find const in module {0:?} with name {1:?}")]
+    ConstNotFound(api::ModNameBuf, api::ConstNameBuf),
+    #[error("value {0:?} is not in scope")]
+    ValueNotInScope(api::ValueId),
+    #[error("continuation {0:?} is not in scope")]
+    ContinuationNotInScope(api::ContinuationId),
+    #[error("expected type '{expected}', found type '{actual}'")]
+    TypeMismatch { expected: String, actual: String },
+    #[error("expected bag type, found type '{0}'")]
+    ExpectedBagType(String),
+    #[error("expected tuple type, found type '{0}'")]
+    ExpectedTupleType(String),
+    #[error("expected union type, found type '{0}'")]
+    ExpectedUnionType(String),
+    #[error("expected named type, foudn type '{0}'")]
+    ExpectedNamedType(String),
+    #[error("tuple field index {0} out of range")]
+    TupleFieldOutOfRange(u32),
+    #[error("union variant index {0} out of range")]
+    UnionVariantOutOfRange(u32),
+    #[error("entry point does not have type '() -> ()'")]
+    EntryPointDisallowedSig,
+}
+
+/// For use in error locations
+#[derive(Clone, Debug)]
+enum DefName {
+    Type(api::TypeNameBuf),
+    Func(api::FuncNameBuf),
+    Const(api::ConstNameBuf),
+    EntryPoint(api::EntryPointNameBuf),
+}
+
+#[derive(Clone, Debug)]
+enum BindingLocation {
+    Type(api::TypeId),
+    Value(api::ValueId),
+    Continuation(api::ContinuationId),
+}
+
+#[derive(Clone, Debug)]
+pub(crate) struct Error {
+    kind: ErrorKind,
+    mod_: Option<api::ModNameBuf>,
+    def: Option<DefName>,
+    binding: Option<BindingLocation>,
+}
+
+impl Error {
+    fn annotate_mod_def<E: Into<Self>>(err: E, mod_: api::ModNameBuf, def: DefName) -> Self {
+        let mut err = err.into();
+        if err.mod_.is_none() {
+            err.mod_ = Some(mod_);
+        }
+        if err.def.is_none() {
+            err.def = Some(def);
+        }
+        err
+    }
+
+    fn annotate_type_def<E: Into<Self>>(
+        nc: &NameCache,
+        def_id: NamedTypeId,
+    ) -> impl FnOnce(E) -> Self + '_ {
+        move |err| {
+            let (mod_, name) = &nc.named_types[def_id];
+            Error::annotate_mod_def(err, mod_.clone(), DefName::Type(name.clone()))
+        }
+    }
+
+    fn annotate_func_def<E: Into<Self>>(
+        nc: &NameCache,
+        def_id: FuncId,
+    ) -> impl FnOnce(E) -> Self + '_ {
+        move |err| {
+            let (mod_, name) = &nc.funcs[def_id];
+            Error::annotate_mod_def(err, mod_.clone(), DefName::Func(name.clone()))
+        }
+    }
+
+    fn annotate_const_def<E: Into<Self>>(
+        nc: &NameCache,
+        def_id: ConstId,
+    ) -> impl FnOnce(E) -> Self + '_ {
+        move |err| {
+            let (mod_, name) = &nc.consts[def_id];
+            Error::annotate_mod_def(err, mod_.clone(), DefName::Const(name.clone()))
+        }
+    }
+
+    fn annotate_entry_point<E: Into<Self>>(
+        nc: &NameCache,
+        def_id: EntryPointId,
+    ) -> impl FnOnce(E) -> Self + '_ {
+        move |err| {
+            let mut err = err.into();
+            if err.def.is_none() {
+                err.def = Some(DefName::EntryPoint(nc.entry_points[def_id].clone()));
+            }
+            err
+        }
+    }
+
+    fn annotate_binding<E: Into<Self>>(binding: BindingLocation) -> impl FnOnce(E) -> Self {
+        move |err| {
+            let mut err = err.into();
+            if err.binding.is_none() {
+                err.binding = Some(binding);
+            }
+            err
+        }
+    }
+}
+
+impl std::fmt::Display for Error {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let mut first = true;
+        let mut loc_prefix = |f: &mut std::fmt::Formatter| -> std::fmt::Result {
+            if first {
+                write!(f, "error in ")?;
+            } else {
+                write!(f, ", ")?;
+            }
+            first = false;
+            Ok(())
+        };
+
+        if let Some(mod_) = &self.mod_ {
+            loc_prefix(f)?;
+            write!(f, "module {:?}", mod_)?;
+        }
+
+        if let Some(def) = &self.def {
+            loc_prefix(f)?;
+            match def {
+                DefName::Type(name) => {
+                    write!(f, "named type definition {:?}", name)?;
+                }
+                DefName::Func(name) => {
+                    write!(f, "function definition {:?}", name)?;
+                }
+                DefName::Const(name) => {
+                    write!(f, "constant definition {:?}", name)?;
+                }
+                DefName::EntryPoint(name) => {
+                    write!(f, "entry point definition {:?}", name)?;
+                }
+            }
+        }
+
+        if let Some(binding) = &self.binding {
+            loc_prefix(f)?;
+            match binding {
+                BindingLocation::Type(id) => {
+                    write!(f, "definition of type binding {:?}", id)?;
+                }
+                BindingLocation::Value(id) => {
+                    write!(f, "definition of value binding {:?}", id)?;
+                }
+                BindingLocation::Continuation(id) => {
+                    write!(f, "definition of continuation binding {:?}", id)?;
+                }
+            }
+        }
+
+        if !first {
+            write!(f, ": ")?;
+        }
+
+        write!(f, "{}", self.kind)
+    }
+}
+
+impl std::error::Error for Error {}
+
+impl From<ErrorKind> for Error {
+    fn from(kind: ErrorKind) -> Self {
+        Error {
+            kind,
+            mod_: None,
+            def: None,
+            binding: None,
+        }
+    }
+}
+
+fn named_type_id(
+    nc: &NameCache,
+    mod_name: &api::ModNameBuf,
+    type_name: &api::TypeNameBuf,
+) -> Result<NamedTypeId, Error> {
+    nc.named_types
+        .get_by_val(&(mod_name.clone(), type_name.clone()))
+        .ok_or_else(|| ErrorKind::NamedTypeNotFound(mod_name.clone(), type_name.clone()).into())
+}
+
+pub(crate) fn resolve_types(
+    nc: &NameCache,
+    tc: &mut TypeCache,
+    types_: &OpGraph<api::TypeId, api::TypeOp>,
+) -> Result<IdVec<api::TypeId, TypeId>, Error> {
+    let mut result = IdVec::with_capacity(types_.len());
+    for api_id in types_.count().iter() {
+        let node = types_.node(api_id);
+        let inputs: SmallVec<_> = node.inputs.iter().map(|input| result[input]).collect();
+        let type_data = match node.op {
+            api::TypeOp::Named { named_mod, named } => {
+                let named_id = named_type_id(nc, named_mod, named)
+                    .map_err(Error::annotate_binding(BindingLocation::Type(api_id)))?;
+                debug_assert_eq!(inputs.len(), 0);
+                TypeData::Named { named: named_id }
+            }
+            api::TypeOp::Tuple => TypeData::Tuple { fields: inputs },
+            api::TypeOp::Union => TypeData::Union { variants: inputs },
+            api::TypeOp::HeapCell => {
+                debug_assert_eq!(inputs.len(), 0);
+                TypeData::HeapCell
+            }
+            api::TypeOp::Bag => {
+                debug_assert_eq!(inputs.len(), 1);
+                TypeData::Bag { item: inputs[0] }
+            }
+        };
+        let type_id = tc.types.get_or_insert(type_data);
+        let pushed_id = result.push(type_id);
+        debug_assert_eq!(pushed_id, api_id);
+    }
+    debug_assert_eq!(result.count(), types_.count());
+    Ok(result)
+}
+
+#[derive(Clone, Debug)]
+struct FuncSig {
+    arg_type: TypeId,
+    ret_type: TypeId,
+}
+
+pub(crate) fn preprocess(program: &api::Program) -> Result<(), Error> {
+    let mut nc = NameCache::default();
+    let mut tc = TypeCache::default();
+
+    let mut named_types = IdVec::<NamedTypeId, &api::TypeDef>::new();
+    let mut funcs = IdVec::<FuncId, &api::FuncDef>::new();
+    let mut consts = IdVec::<ConstId, &api::ConstDef>::new();
+
+    for (mod_name, mod_) in &program.mods {
+        for (type_name, type_def) in &mod_.type_defs {
+            let nc_id = nc
+                .named_types
+                .insert((mod_name.clone(), type_name.clone()))
+                .expect("Builder API should check for duplicate names");
+            let pushed_id = named_types.push(type_def);
+            debug_assert_eq!(nc_id, pushed_id);
+        }
+        for (func_name, func_def) in &mod_.func_defs {
+            let nc_id = nc
+                .funcs
+                .insert((mod_name.clone(), func_name.clone()))
+                .expect("Builder API should check for duplicate names");
+            let pushed_id = funcs.push(func_def);
+            debug_assert_eq!(nc_id, pushed_id);
+        }
+        for (const_name, const_def) in &mod_.const_defs {
+            let nc_id = nc
+                .consts
+                .insert((mod_name.clone(), const_name.clone()))
+                .expect("Builder API should check for duplicate names");
+            let pushed_id = consts.push(const_def);
+            debug_assert_eq!(nc_id, pushed_id);
+        }
+    }
+
+    // TODO: We should ideally perform fewer passes over the program here.
+
+    let typedef_body_types = named_types.try_map(|named_id, type_def| {
+        resolve_types(&nc, &mut tc, &type_def.builder.types)
+            .map_err(Error::annotate_type_def(&nc, named_id))
+    })?;
+
+    let typedef_contents =
+        named_types.map(|named_type_id, type_def| typedef_body_types[named_type_id][type_def.root]);
+
+    let func_body_types = funcs.try_map(|func_id, func_def| {
+        resolve_types(
+            &nc,
+            &mut tc,
+            &func_def.builder.expr_builder.type_builder.types,
+        )
+        .map_err(Error::annotate_func_def(&nc, func_id))
+    })?;
+
+    let func_sigs = funcs.map(|func_id, func_def| {
+        let body_types = &func_body_types[func_id];
+        FuncSig {
+            arg_type: body_types[func_def.arg_type],
+            ret_type: body_types[func_def.ret_type],
+        }
+    });
+
+    let const_body_types = consts.try_map(|const_id, const_def| {
+        resolve_types(
+            &nc,
+            &mut tc,
+            &const_def.builder.expr_builder.type_builder.types,
+        )
+        .map_err(Error::annotate_const_def(&nc, const_id))
+    })?;
+
+    let const_sigs = consts.map(|const_id, const_def| {
+        let body_types = &const_body_types[const_id];
+        body_types[&const_def.type_]
+    });
+
+    let ctx = Context {
+        nc: &nc,
+        typedef_contents: &typedef_contents,
+        func_sigs: &func_sigs,
+        const_sigs: &const_sigs,
+    };
+
+    for (func_id, func_def) in &funcs {
+        preprocess_func_def(&mut tc, ctx, func_def, &func_body_types[func_id])
+            .map_err(Error::annotate_func_def(&nc, func_id))?;
+    }
+
+    for (const_id, const_def) in &consts {
+        preprocess_const_def(&mut tc, ctx, const_def, &const_body_types[const_id])
+            .map_err(Error::annotate_const_def(&nc, const_id))?;
+    }
+
+    let mut entry_points = IdVec::<EntryPointId, FuncId>::new();
+    for (entry_point_name, (mod_, func)) in &program.entry_points {
+        let nc_id = nc
+            .entry_points
+            .insert(entry_point_name.clone())
+            .expect("builder API should check for duplicate names");
+        // TODO: Avoid the clones here
+        let func_id = nc
+            .funcs
+            .get_by_val(&(mod_.clone(), func.clone()))
+            .ok_or_else(|| ErrorKind::FuncNotFound(mod_.clone(), func.clone()))
+            .map_err(Error::annotate_entry_point(&nc, nc_id))?;
+        let func_sig = &func_sigs[func_id];
+        let unit_type = tc.types.get_or_insert(TypeData::Tuple {
+            fields: smallvec![],
+        });
+        if func_sig.arg_type != unit_type || func_sig.ret_type != unit_type {
+            return Err(Error::annotate_entry_point(&nc, nc_id)(
+                ErrorKind::EntryPointDisallowedSig,
+            ));
+        }
+        let pushed_id = entry_points.push(func_id);
+        debug_assert_eq!(nc_id, pushed_id);
+    }
+
+    Ok(())
+}
+
+#[derive(Clone, Copy, Debug)]
+struct Context<'a> {
+    nc: &'a NameCache,
+    typedef_contents: &'a IdVec<NamedTypeId, TypeId>,
+    func_sigs: &'a IdVec<FuncId, FuncSig>,
+    const_sigs: &'a IdVec<ConstId, TypeId>,
+}
+
+#[derive(Clone, Copy, Debug)]
+struct ValueBinding {
+    id: ir::ValueId,
+    type_: TypeId,
+}
+
+#[derive(Clone, Copy, Debug)]
+struct ContinuationBinding {
+    entry_block: ir::BlockId,
+    arg_type: TypeId,
+}
+
+// TODO: Consolidate this logic with render_api_ir.rs
+fn render_byte_string(target: &mut String, bytes: &[u8]) {
+    target.push('"');
+    for &byte in bytes {
+        use std::fmt::Write;
+        write!(target, "{}", std::ascii::escape_default(byte)).unwrap();
+    }
+    target.push('"');
+}
+
+// TODO: Consolidate this logic with render_api_ir.rs
+fn render_type_rec(nc: &NameCache, tc: &TypeCache, target: &mut String, type_: TypeId) {
+    match &tc.types[type_] {
+        TypeData::Named { named } => {
+            let (mod_name, type_name) = &nc.named_types[named];
+            render_byte_string(target, &mod_name.0);
+            target.push_str("::");
+            render_byte_string(target, &type_name.0);
+        }
+
+        TypeData::Tuple { fields } => {
+            target.push('(');
+            let mut first = true;
+            for &field in fields {
+                if !first {
+                    target.push_str(", ");
+                }
+                first = false;
+                render_type_rec(nc, tc, target, field);
+            }
+            if fields.len() == 1 {
+                target.push(',');
+            }
+            target.push(')');
+        }
+
+        TypeData::Union { variants } => {
+            if variants.is_empty() {
+                // Special case to avoid extra space inside braces
+                target.push_str("union { }");
+                return;
+            }
+            target.push_str("union { ");
+            let mut first = true;
+            for &variant in variants {
+                if !first {
+                    target.push_str(", ");
+                }
+                first = false;
+                render_type_rec(nc, tc, target, variant);
+            }
+            target.push_str(" }");
+        }
+
+        TypeData::HeapCell => {
+            target.push_str("heap_cell");
+        }
+
+        TypeData::Bag { item } => {
+            target.push_str("bag<");
+            render_type_rec(nc, tc, target, *item);
+            target.push('>');
+        }
+    }
+}
+
+fn render_type(nc: &NameCache, tc: &TypeCache, type_: TypeId) -> String {
+    let mut target = String::new();
+    render_type_rec(nc, tc, &mut target, type_);
+    target
+}
+
+fn type_error<T>(
+    nc: &NameCache,
+    tc: &TypeCache,
+    expected: TypeId,
+    actual: TypeId,
+) -> Result<T, Error> {
+    debug_assert_ne!(expected, actual);
+    Err(ErrorKind::TypeMismatch {
+        expected: render_type(nc, tc, expected),
+        actual: render_type(nc, tc, actual),
+    }
+    .into())
+}
+
+fn check_type(
+    nc: &NameCache,
+    tc: &TypeCache,
+    expected: TypeId,
+    actual: TypeId,
+) -> Result<(), Error> {
+    if expected != actual {
+        type_error(nc, tc, expected, actual)
+    } else {
+        Ok(())
+    }
+}
+
+fn try_get_bag_item_type(
+    nc: &NameCache,
+    tc: &TypeCache,
+    bag_type: TypeId,
+) -> Result<TypeId, Error> {
+    if let TypeData::Bag { item } = tc.types[bag_type] {
+        Ok(item)
+    } else {
+        Err(ErrorKind::ExpectedBagType(render_type(nc, tc, bag_type)).into())
+    }
+}
+
+fn try_get_tuple_field_types<'a>(
+    nc: &NameCache,
+    tc: &'a TypeCache,
+    tuple_type: TypeId,
+) -> Result<&'a [TypeId], Error> {
+    if let TypeData::Tuple { fields } = &tc.types[tuple_type] {
+        Ok(fields)
+    } else {
+        Err(ErrorKind::ExpectedTupleType(render_type(nc, tc, tuple_type)).into())
+    }
+}
+
+fn try_get_union_variant_types<'a>(
+    nc: &NameCache,
+    tc: &'a TypeCache,
+    union_type: TypeId,
+) -> Result<&'a [TypeId], Error> {
+    if let TypeData::Union { variants } = &tc.types[union_type] {
+        Ok(variants)
+    } else {
+        Err(ErrorKind::ExpectedUnionType(render_type(nc, tc, union_type)).into())
+    }
+}
+
+fn try_get_named_type_id(
+    nc: &NameCache,
+    tc: &TypeCache,
+    named_type: TypeId,
+) -> Result<NamedTypeId, Error> {
+    if let TypeData::Named { named } = tc.types[named_type] {
+        Ok(named)
+    } else {
+        Err(ErrorKind::ExpectedNamedType(render_type(nc, tc, named_type)).into())
+    }
+}
+
+fn get_value(
+    values_in_scope: &IdVec<api::ValueId, Option<ir::ValueId>>,
+    api_value: api::ValueId,
+) -> Result<ir::ValueId, Error> {
+    values_in_scope[api_value].ok_or_else(|| ErrorKind::ValueNotInScope(api_value).into())
+}
+
+fn preprocess_block_expr(
+    tc: &mut TypeCache,
+    ctx: Context,
+    api_builder: &api::ExprBuilder,
+    body_types: &IdVec<api::TypeId, TypeId>,
+    graph_builder: &mut ir::GraphBuilder,
+    values_in_scope: &mut IdVec<api::ValueId, Option<ir::ValueId>>,
+    continuations_in_scope: &mut IdVec<api::ContinuationId, Option<ContinuationBinding>>,
+    mut block: ir::BlockId,
+    block_expr: api::BlockExpr,
+) -> Result<(ir::BlockId, ir::ValueId), Error> {
+    let api::BlockExpr(api_block, api_ret_val) = block_expr;
+
+    let mut join_info = None;
+
+    let mut api_value_ids = api_builder.blocks.block_values(api_block).peekable();
+    loop {
+        let mut continuation_group = SmallVec::<[_; 32]>::new();
+        loop {
+            if let Some(api_value_id) = api_value_ids.peek() {
+                if let api::Op::DeclareContinuation { continuation } =
+                    api_builder.vals.node(api_value_id).op
+                {
+                    let info = api_builder.continuations[continuation];
+                    let arg_type = body_types[info.arg_type];
+                    let (entry_block, arg) = graph_builder.add_block_with_param(arg_type);
+                    replace_none(
+                        &mut continuations_in_scope[continuation],
+                        ContinuationBinding {
+                            entry_block,
+                            arg_type,
+                        },
+                    )
+                    .unwrap();
+                    continuation_group.push((continuation, arg));
+                    api_value_ids.next();
+                    continue;
+                }
+            }
+            break;
+        }
+
+        for (continuation, arg) in continuation_group {
+            let ContinuationBinding {
+                entry_block,
+                arg_type: _,
+            } = continuations_in_scope[continuation].unwrap();
+
+            let api::ContinuationInfo {
+                arg_type: _,
+                ret_type: api_ret_type,
+                arg: api_arg,
+                body: api_body,
+            } = api_builder.continuations[continuation];
+
+            replace_none(&mut values_in_scope[api_arg], arg).unwrap();
+            let (continuation_final_block, continuation_ret_val) = preprocess_block_expr(
+                tc,
+                ctx,
+                api_builder,
+                body_types,
+                graph_builder,
+                values_in_scope,
+                continuations_in_scope,
+                entry_block,
+                api_body
+                    .expect("builder API should have checked that all continuations are defined"),
+            )
+            .map_err(Error::annotate_binding(BindingLocation::Continuation(
+                *continuation,
+            )))?;
+            values_in_scope[api_arg] = None;
+
+            let ret_type = body_types[api_ret_type];
+
+            check_type(
+                ctx.nc,
+                tc,
+                ret_type,
+                graph_builder
+                    .values()
+                    .node(continuation_ret_val)
+                    .op
+                    .result_type,
+            )
+            .map_err(Error::annotate_binding(BindingLocation::Continuation(
+                *continuation,
+            )))?;
+
+            let (join_block, join_val_type, _) = *join_info.get_or_insert_with(|| {
+                let (join_block, join_val) = graph_builder.add_block_with_param(ret_type);
+                (join_block, ret_type, join_val)
+            });
+
+            check_type(ctx.nc, tc, join_val_type, ret_type).map_err(Error::annotate_binding(
+                BindingLocation::Continuation(*continuation),
+            ))?;
+
+            graph_builder.set_jump_targets(
+                continuation_final_block,
+                Some(continuation_ret_val),
+                smallvec![ir::JumpTarget::Block(join_block)],
+            );
+        }
+
+        match api_value_ids.next() {
+            None => break,
+            Some(api_value_id) => {
+                let api_node = api_builder.vals.node(api_value_id);
+                debug_assert!(!matches!(&api_node.op, api::Op::DeclareContinuation { .. }));
+                let (new_block, value) = preprocess_op(
+                    tc,
+                    ctx,
+                    api_builder,
+                    body_types,
+                    graph_builder,
+                    values_in_scope,
+                    continuations_in_scope,
+                    block,
+                    &api_node.op,
+                    &api_node.inputs,
+                )
+                .map_err(Error::annotate_binding(BindingLocation::Value(
+                    api_value_id,
+                )))?;
+                block = new_block;
+                replace_none(&mut values_in_scope[api_value_id], value).unwrap();
+            }
+        }
+    }
+
+    let (final_block, final_value) = match join_info {
+        None => (block, get_value(values_in_scope, api_ret_val)?),
+        Some((join_block, join_val_type, join_val)) => {
+            let ret_val = get_value(values_in_scope, api_ret_val)?;
+            check_type(
+                ctx.nc,
+                tc,
+                join_val_type,
+                graph_builder.values().node(ret_val).op.result_type,
+            )?;
+            graph_builder.set_jump_targets(
+                block,
+                Some(ret_val),
+                smallvec![ir::JumpTarget::Block(join_block)],
+            );
+            (join_block, join_val)
+        }
+    };
+
+    for api_value_id in api_builder.blocks.block_values(api_block) {
+        match &api_builder.vals.node(api_value_id).op {
+            api::Op::DeclareContinuation { continuation } => {
+                debug_assert!(values_in_scope[api_value_id].is_none());
+                debug_assert!(continuations_in_scope[continuation].is_some());
+                continuations_in_scope[continuation] = None;
+            }
+
+            _ => {
+                debug_assert!(values_in_scope[api_value_id].is_some());
+                values_in_scope[api_value_id] = None;
+            }
+        }
+    }
+
+    Ok((final_block, final_value))
+}
+
+fn preprocess_op(
+    tc: &mut TypeCache,
+    ctx: Context,
+    api_builder: &api::ExprBuilder,
+    body_types: &IdVec<api::TypeId, TypeId>,
+    graph_builder: &mut ir::GraphBuilder,
+    values_in_scope: &mut IdVec<api::ValueId, Option<ir::ValueId>>,
+    continuations_in_scope: &mut IdVec<api::ContinuationId, Option<ContinuationBinding>>,
+    block: ir::BlockId,
+    api_op: &api::Op,
+    api_inputs: &[api::ValueId],
+) -> Result<(ir::BlockId, ir::ValueId), Error> {
+    let mut inputs = SmallVec::<[_; 32]>::with_capacity(api_inputs.len());
+    let mut input_types = SmallVec::<[_; 32]>::with_capacity(api_inputs.len());
+    for &api_input in api_inputs {
+        let id = get_value(values_in_scope, api_input)?;
+        inputs.push(id);
+        input_types.push(graph_builder.values().node(id).op.result_type);
+    }
+
+    match api_op {
+        api::Op::Arg | api::Op::ContinuationArg | api::Op::DeclareContinuation { .. } => {
+            unreachable!(
+                "op {:?} should never appear as a value binding in a block",
+                api_op,
+            )
+        }
+
+        api::Op::Jump {
+            continuation,
+            unreachable_result_type,
+        } => {
+            debug_assert_eq!(inputs.len(), 1);
+
+            let cont_binding = continuations_in_scope[continuation]
+                .ok_or_else(|| ErrorKind::ContinuationNotInScope(*continuation))?;
+
+            check_type(ctx.nc, tc, cont_binding.arg_type, input_types[0])?;
+
+            graph_builder.set_jump_targets(
+                block,
+                Some(inputs[0]),
+                smallvec![ir::JumpTarget::Block(cont_binding.entry_block)],
+            );
+
+            let (unreachable_block, unreachable_result) =
+                graph_builder.add_block_with_param(body_types[unreachable_result_type]);
+
+            Ok((unreachable_block, unreachable_result))
+        }
+
+        api::Op::UnknownWith { result_type } => {
+            let value = graph_builder.add_op(
+                block,
+                ir::OpKind::UnknownWith,
+                &inputs,
+                body_types[result_type],
+            );
+            Ok((block, value))
+        }
+
+        api::Op::Call {
+            callee_spec_var,
+            callee_mod,
+            callee,
+        } => {
+            debug_assert_eq!(inputs.len(), 1);
+
+            // TODO: Avoid the clones here
+            let callee_id = ctx
+                .nc
+                .funcs
+                .get_by_val(&(callee_mod.clone(), callee.clone()))
+                .ok_or_else(|| ErrorKind::FuncNotFound(callee_mod.clone(), callee.clone()))?;
+
+            let callee_sig = &ctx.func_sigs[callee_id];
+
+            check_type(ctx.nc, tc, callee_sig.arg_type, input_types[0])?;
+
+            let value = graph_builder.add_op(
+                block,
+                ir::OpKind::Call {
+                    callee_spec_var: *callee_spec_var,
+                    callee: callee_id,
+                },
+                &inputs,
+                callee_sig.ret_type,
+            );
+            Ok((block, value))
+        }
+
+        api::Op::ConstRef { const_mod, const_ } => {
+            debug_assert_eq!(inputs.len(), 0);
+
+            let const_id = ctx
+                .nc
+                .consts
+                .get_by_val(&(const_mod.clone(), const_.clone()))
+                .ok_or_else(|| ErrorKind::ConstNotFound(const_mod.clone(), const_.clone()))?;
+
+            let const_type = ctx.const_sigs[const_id];
+
+            let value = graph_builder.add_op(
+                block,
+                ir::OpKind::ConstRef { const_: const_id },
+                &[],
+                const_type,
+            );
+            Ok((block, value))
+        }
+
+        api::Op::Choice { cases } => {
+            debug_assert_eq!(inputs.len(), 0);
+            debug_assert_ne!(cases.len(), 0);
+
+            let case_results: SmallVec<[_; 32]> = cases
+                .iter()
+                .map(|&case| {
+                    let case_entry_block = graph_builder.add_block();
+                    let (case_final_block, case_value) = preprocess_block_expr(
+                        tc,
+                        ctx,
+                        api_builder,
+                        body_types,
+                        graph_builder,
+                        values_in_scope,
+                        continuations_in_scope,
+                        case_entry_block,
+                        case,
+                    )?;
+                    Ok((case_entry_block, case_final_block, case_value))
+                })
+                .collect::<Result<_, Error>>()?;
+
+            graph_builder.set_jump_targets(
+                block,
+                None,
+                case_results
+                    .iter()
+                    .map(|&(case_entry_block, _, _)| ir::JumpTarget::Block(case_entry_block))
+                    .collect(),
+            );
+
+            let mut result_type = None;
+            for &(_, _, case_value) in &case_results {
+                let case_value_type = graph_builder.values().node(case_value).op.result_type;
+                if let Some(prev_result_type) = result_type {
+                    check_type(ctx.nc, tc, prev_result_type, case_value_type)?;
+                } else {
+                    result_type = Some(case_value_type);
+                }
+            }
+            let result_type = result_type.expect("case_results is nonempty");
+
+            let (successor_block, choice_value) = graph_builder.add_block_with_param(result_type);
+
+            for &(_, case_final_block, case_value) in &case_results {
+                graph_builder.set_jump_targets(
+                    case_final_block,
+                    Some(case_value),
+                    smallvec![JumpTarget::Block(successor_block)],
+                );
+            }
+
+            Ok((successor_block, choice_value))
+        }
+
+        api::Op::SubBlock { sub_block } => {
+            debug_assert_eq!(inputs.len(), 0);
+            preprocess_block_expr(
+                tc,
+                ctx,
+                api_builder,
+                body_types,
+                graph_builder,
+                values_in_scope,
+                continuations_in_scope,
+                block,
+                *sub_block,
+            )
+        }
+
+        api::Op::Terminate {
+            unreachable_result_type,
+        } => {
+            debug_assert_eq!(inputs.len(), 0);
+            graph_builder.set_jump_targets(block, None, smallvec![]);
+            let (unreachable_block, unreachable_value) =
+                graph_builder.add_block_with_param(body_types[unreachable_result_type]);
+            Ok((unreachable_block, unreachable_value))
+        }
+
+        api::Op::NewHeapCell => {
+            debug_assert_eq!(inputs.len(), 0);
+            let value = graph_builder.add_op(
+                block,
+                ir::OpKind::NewHeapCell,
+                &[],
+                tc.types.get_or_insert(TypeData::HeapCell),
+            );
+            Ok((block, value))
+        }
+
+        api::Op::Touch => {
+            debug_assert_eq!(inputs.len(), 1);
+            let heap_cell_type = tc.types.get_or_insert(TypeData::HeapCell);
+            check_type(ctx.nc, tc, heap_cell_type, input_types[0])?;
+            let value = graph_builder.add_op(
+                block,
+                ir::OpKind::RecursiveTouch,
+                &inputs,
+                tc.types.get_or_insert(TypeData::Tuple {
+                    fields: smallvec![],
+                }),
+            );
+            Ok((block, value))
+        }
+
+        api::Op::RecursiveTouch => {
+            debug_assert_eq!(inputs.len(), 1);
+            let value = graph_builder.add_op(
+                block,
+                ir::OpKind::RecursiveTouch,
+                &inputs,
+                tc.types.get_or_insert(TypeData::Tuple {
+                    fields: smallvec![],
+                }),
+            );
+            Ok((block, value))
+        }
+
+        api::Op::UpdateWriteOnly { update_mode_var } => {
+            debug_assert_eq!(inputs.len(), 1);
+            let heap_cell_type = tc.types.get_or_insert(TypeData::HeapCell);
+            check_type(ctx.nc, tc, heap_cell_type, input_types[0])?;
+            let value = graph_builder.add_op(
+                block,
+                ir::OpKind::UpdateWriteOnly {
+                    update_mode_var: *update_mode_var,
+                },
+                &inputs,
+                tc.types.get_or_insert(TypeData::Tuple {
+                    fields: smallvec![],
+                }),
+            );
+            Ok((block, value))
+        }
+
+        api::Op::EmptyBag { item_type } => {
+            debug_assert_eq!(inputs.len(), 0);
+            let value = graph_builder.add_op(
+                block,
+                ir::OpKind::EmptyBag,
+                &[],
+                tc.types.get_or_insert(TypeData::Bag {
+                    item: body_types[item_type],
+                }),
+            );
+            Ok((block, value))
+        }
+
+        api::Op::BagInsert => {
+            debug_assert_eq!(inputs.len(), 2);
+            let bag_type = input_types[0];
+            let expected_item_type = try_get_bag_item_type(ctx.nc, tc, bag_type)?;
+            check_type(ctx.nc, tc, expected_item_type, input_types[1])?;
+            let value = graph_builder.add_op(block, ir::OpKind::BagInsert, &inputs, bag_type);
+            Ok((block, value))
+        }
+
+        api::Op::BagGet => {
+            debug_assert_eq!(inputs.len(), 1);
+            let item_type = try_get_bag_item_type(ctx.nc, tc, input_types[0])?;
+            let value = graph_builder.add_op(block, ir::OpKind::BagGet, &inputs, item_type);
+            Ok((block, value))
+        }
+
+        api::Op::BagRemove => {
+            debug_assert_eq!(inputs.len(), 1);
+            let bag_type = input_types[0];
+            let item_type = try_get_bag_item_type(ctx.nc, tc, bag_type)?;
+            let value = graph_builder.add_op(
+                block,
+                ir::OpKind::BagRemove,
+                &inputs,
+                tc.types.get_or_insert(TypeData::Tuple {
+                    fields: smallvec![bag_type, item_type],
+                }),
+            );
+            Ok((block, value))
+        }
+
+        api::Op::MakeTuple => {
+            let tuple_type = tc.types.get_or_insert(TypeData::Tuple {
+                fields: SmallVec::from_slice(&input_types),
+            });
+            let value = graph_builder.add_op(block, ir::OpKind::MakeTuple, &inputs, tuple_type);
+            Ok((block, value))
+        }
+
+        api::Op::GetTupleField { field_idx } => {
+            debug_assert_eq!(inputs.len(), 1);
+            let tuple_type = input_types[0];
+            let field_types = try_get_tuple_field_types(ctx.nc, tc, tuple_type)?;
+            let field_type = *field_types
+                .get(*field_idx as usize)
+                .ok_or_else(|| ErrorKind::TupleFieldOutOfRange(*field_idx))?;
+            let value = graph_builder.add_op(
+                block,
+                ir::OpKind::GetTupleField {
+                    field_idx: *field_idx,
+                },
+                &inputs,
+                field_type,
+            );
+            Ok((block, value))
+        }
+
+        api::Op::MakeUnion {
+            variant_types,
+            variant_idx,
+        } => {
+            debug_assert_eq!(inputs.len(), 1);
+            let tc_variant_types: SmallVec<_> = variant_types
+                .iter()
+                .map(|api_type_id| body_types[api_type_id])
+                .collect();
+            let this_variant_type = *tc_variant_types
+                .get(*variant_idx as usize)
+                .ok_or_else(|| ErrorKind::UnionVariantOutOfRange(*variant_idx))?;
+            check_type(ctx.nc, tc, this_variant_type, input_types[0])?;
+            let union_type = tc.types.get_or_insert(TypeData::Union {
+                variants: tc_variant_types,
+            });
+            let value = graph_builder.add_op(
+                block,
+                ir::OpKind::MakeUnion {
+                    variant_idx: *variant_idx,
+                },
+                &inputs,
+                union_type,
+            );
+            Ok((block, value))
+        }
+
+        api::Op::UnwrapUnion { variant_idx } => {
+            debug_assert_eq!(inputs.len(), 1);
+            let variant_types = try_get_union_variant_types(ctx.nc, tc, input_types[0])?;
+            let this_variant_type = *variant_types
+                .get(*variant_idx as usize)
+                .ok_or_else(|| ErrorKind::UnionVariantOutOfRange(*variant_idx))?;
+            let value = graph_builder.add_op(
+                block,
+                ir::OpKind::UnwrapUnion {
+                    variant_idx: *variant_idx,
+                },
+                &inputs,
+                this_variant_type,
+            );
+            Ok((block, value))
+        }
+
+        api::Op::MakeNamed { named_mod, named } => {
+            debug_assert_eq!(inputs.len(), 1);
+            // TODO: Avoid the clones here
+            let named_id = ctx
+                .nc
+                .named_types
+                .get_by_val(&(named_mod.clone(), named.clone()))
+                .ok_or_else(|| ErrorKind::NamedTypeNotFound(named_mod.clone(), named.clone()))?;
+            let content_type = ctx.typedef_contents[named_id];
+            check_type(ctx.nc, tc, content_type, input_types[0])?;
+            let value = graph_builder.add_op(
+                block,
+                ir::OpKind::MakeNamed,
+                &inputs,
+                tc.types.get_or_insert(TypeData::Named { named: named_id }),
+            );
+            Ok((block, value))
+        }
+
+        api::Op::UnwrapNamed { named_mod, named } => {
+            debug_assert_eq!(inputs.len(), 1);
+            let named_id = try_get_named_type_id(ctx.nc, tc, input_types[0])?;
+            let expected_named_id = ctx
+                .nc
+                .named_types
+                .get_by_val(&(named_mod.clone(), named.clone()))
+                .ok_or_else(|| ErrorKind::NamedTypeNotFound(named_mod.clone(), named.clone()))?;
+            if named_id != expected_named_id {
+                let expected = tc.types.get_or_insert(TypeData::Named {
+                    named: expected_named_id,
+                });
+                type_error(ctx.nc, tc, expected, input_types[0])?;
+            }
+            let value = graph_builder.add_op(
+                block,
+                ir::OpKind::UnwrapNamed,
+                &inputs,
+                ctx.typedef_contents[named_id],
+            );
+            Ok((block, value))
+        }
+    }
+}
+
+fn preprocess_func_def(
+    tc: &mut TypeCache,
+    ctx: Context,
+    func_def: &api::FuncDef,
+    body_types: &IdVec<api::TypeId, TypeId>,
+) -> Result<ir::FuncDef, Error> {
+    let api_builder = &func_def.builder.expr_builder;
+    let mut graph_builder = ir::GraphBuilder::new();
+    let mut values_in_scope = IdVec::filled_with(api_builder.vals.count(), || None);
+    let mut continuations_in_scope = IdVec::filled_with(api_builder.continuations.count(), || None);
+    let (entry_block, arg_val) = graph_builder.add_block_with_param(body_types[func_def.arg_type]);
+    values_in_scope[func_def.builder.argument] = Some(arg_val);
+    let (final_block, ret_val) = preprocess_block_expr(
+        tc,
+        ctx,
+        &api_builder,
+        body_types,
+        &mut graph_builder,
+        &mut values_in_scope,
+        &mut continuations_in_scope,
+        entry_block,
+        func_def.root,
+    )?;
+    check_type(
+        ctx.nc,
+        tc,
+        body_types[func_def.ret_type],
+        graph_builder.values().node(ret_val).op.result_type,
+    )?;
+    graph_builder.set_jump_targets(final_block, Some(ret_val), smallvec![ir::JumpTarget::Ret]);
+    Ok(ir::FuncDef {
+        graph: graph_builder.build(entry_block),
+    })
+}
+
+fn preprocess_const_def(
+    tc: &mut TypeCache,
+    ctx: Context,
+    const_def: &api::ConstDef,
+    body_types: &IdVec<api::TypeId, TypeId>,
+) -> Result<ir::ConstDef, Error> {
+    let api_builder = &const_def.builder.expr_builder;
+    let mut graph_builder = ir::GraphBuilder::new();
+    let mut values_in_scope = IdVec::filled_with(api_builder.vals.count(), || None);
+    let mut continuations_in_scope = IdVec::filled_with(api_builder.continuations.count(), || None);
+    let entry_block = graph_builder.add_block();
+    let (final_block, ret_val) = preprocess_block_expr(
+        tc,
+        ctx,
+        &api_builder,
+        body_types,
+        &mut graph_builder,
+        &mut values_in_scope,
+        &mut continuations_in_scope,
+        entry_block,
+        const_def.root,
+    )?;
+    check_type(
+        ctx.nc,
+        tc,
+        body_types[const_def.type_],
+        graph_builder.values().node(ret_val).op.result_type,
+    )?;
+    graph_builder.set_jump_targets(final_block, Some(ret_val), smallvec![ir::JumpTarget::Ret]);
+    Ok(ir::ConstDef {
+        graph: graph_builder.build(entry_block),
+    })
+}
diff --git a/vendor/morphic_lib/src/render_api_ir.rs b/vendor/morphic_lib/src/render_api_ir.rs
new file mode 100644
index 0000000000..dadbec892d
--- /dev/null
+++ b/vendor/morphic_lib/src/render_api_ir.rs
@@ -0,0 +1,550 @@
+use std::fmt::{Display, Formatter, Write};
+
+use crate::api::{
+    BlockExpr, CalleeSpecBuf, ConstDef, ConstNameBuf, ContinuationId, EntryPointNameBuf,
+    ExprBuilder, FuncDef, FuncNameBuf, ModDef, ModNameBuf, Op, Program, TypeDef, TypeId,
+    TypeNameBuf, TypeOp, UpdateModeBuf, ValueId,
+};
+use crate::util::id_type::Id;
+use crate::util::op_graph::{Node, OpGraph};
+
+#[derive(Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord)]
+struct RenderContext {
+    content: String,
+    spaces_per_level: usize,
+    indent_level: usize,
+    pending_indent: bool,
+}
+
+impl RenderContext {
+    fn new(spaces_per_level: usize) -> RenderContext {
+        RenderContext {
+            content: String::new(),
+            spaces_per_level,
+            indent_level: 0,
+            pending_indent: true,
+        }
+    }
+
+    fn with_indent<R>(&mut self, body: impl for<'a> FnOnce(&'a mut RenderContext) -> R) -> R {
+        self.indent_level += 1;
+        let result = body(self);
+        self.indent_level -= 1;
+        result
+    }
+
+    fn write(&mut self, to_write: impl std::fmt::Display) {
+        if self.pending_indent {
+            self.content
+                .extend((0..self.indent_level * self.spaces_per_level).map(|_| ' '));
+            self.pending_indent = false;
+        }
+        write!(&mut self.content, "{}", to_write).expect("writing to string failed");
+    }
+
+    fn writeln(&mut self, to_write: impl std::fmt::Display) {
+        self.write(to_write);
+        self.content.push('\n');
+        self.pending_indent = true;
+    }
+
+    fn finish(self) -> String {
+        self.content
+    }
+}
+
+#[derive(Clone, Copy, Debug)]
+struct Ident {
+    prefix: &'static str,
+    id: usize,
+}
+
+impl Display for Ident {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}_{}", self.prefix, self.id)
+    }
+}
+
+fn val_ident(value: ValueId) -> Ident {
+    Ident {
+        prefix: "val",
+        id: value.to_index(),
+    }
+}
+
+fn type_ident(type_: TypeId) -> Ident {
+    Ident {
+        prefix: "type",
+        id: type_.to_index(),
+    }
+}
+
+fn continuation_ident(continuation: ContinuationId) -> Ident {
+    Ident {
+        prefix: "cont",
+        id: continuation.to_index(),
+    }
+}
+
+#[derive(Clone, Copy, Debug)]
+struct ByteString<'a>(&'a [u8]);
+
+impl<'a> Display for ByteString<'a> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        write!(f, "\"")?;
+        for &byte in self.0 {
+            write!(f, "{}", std::ascii::escape_default(byte))?;
+        }
+        write!(f, "\"")?;
+        Ok(())
+    }
+}
+
+fn update_mode_string(buf: &UpdateModeBuf) -> ByteString {
+    ByteString(&buf.0)
+}
+
+fn callee_spec_string(buf: &CalleeSpecBuf) -> ByteString {
+    ByteString(&buf.0)
+}
+
+fn mod_name_string(mod_name: &ModNameBuf) -> ByteString {
+    ByteString(&mod_name.0)
+}
+
+fn func_name_string(func_name: &FuncNameBuf) -> ByteString {
+    ByteString(&func_name.0)
+}
+
+fn type_name_string(type_name: &TypeNameBuf) -> ByteString {
+    ByteString(&type_name.0)
+}
+
+fn const_name_string(const_name: &ConstNameBuf) -> ByteString {
+    ByteString(&const_name.0)
+}
+
+fn entry_point_name_string(entry_point: &EntryPointNameBuf) -> ByteString {
+    ByteString(&entry_point.0)
+}
+
+fn delimit<T>(
+    ctx: &mut RenderContext,
+    items: impl Iterator<Item = T>,
+    mut render_delim: impl for<'a> FnMut(&'a mut RenderContext),
+    mut render_item: impl for<'a> FnMut(&'a mut RenderContext, T),
+) {
+    for (i, item) in items.enumerate() {
+        if i != 0 {
+            render_delim(ctx);
+        }
+        render_item(ctx, item);
+    }
+}
+
+fn render_op(builder: &ExprBuilder, ctx: &mut RenderContext, op: &Op) {
+    match op {
+        Op::Arg | Op::ContinuationArg | Op::DeclareContinuation { .. } => {
+            ctx.write(format_args!(
+                "/* internal error: {:?} should not be rendered as a value */",
+                op
+            ));
+        }
+
+        Op::Jump {
+            continuation,
+            unreachable_result_type,
+        } => {
+            ctx.write(format_args!(
+                "jump<{}> {}",
+                type_ident(*unreachable_result_type),
+                continuation_ident(*continuation),
+            ));
+        }
+
+        Op::UnknownWith { result_type } => {
+            ctx.write(format_args!("unknown_with<{}>", type_ident(*result_type)));
+        }
+
+        Op::Call {
+            callee_spec_var,
+            callee_mod,
+            callee,
+        } => {
+            ctx.write(format_args!(
+                "call[{}] {}::{}",
+                callee_spec_string(&builder.callee_spec_vars[callee_spec_var]),
+                mod_name_string(callee_mod),
+                func_name_string(callee),
+            ));
+        }
+
+        Op::ConstRef { const_mod, const_ } => {
+            ctx.write(format_args!(
+                "const_ref {}::{}",
+                mod_name_string(const_mod),
+                const_name_string(const_),
+            ));
+        }
+
+        Op::Choice { cases } => {
+            ctx.writeln("choice {");
+            ctx.with_indent(|ctx| {
+                for &case in cases {
+                    ctx.write("case ");
+                    render_block_expr(builder, ctx, case);
+                    ctx.writeln(",");
+                }
+            });
+            ctx.write("}");
+        }
+
+        Op::SubBlock { sub_block } => {
+            render_block_expr(builder, ctx, *sub_block);
+        }
+
+        Op::Terminate {
+            unreachable_result_type,
+        } => {
+            ctx.write(format_args!(
+                "terminate<{}>",
+                type_ident(*unreachable_result_type),
+            ));
+        }
+
+        Op::NewHeapCell => {
+            ctx.write("new_heap_cell");
+        }
+
+        Op::Touch => {
+            ctx.write("touch");
+        }
+
+        Op::RecursiveTouch => {
+            ctx.write("recursive_touch");
+        }
+
+        Op::UpdateWriteOnly { update_mode_var } => {
+            ctx.write(format_args!(
+                "update_write_only[{}]",
+                update_mode_string(&builder.update_mode_vars[update_mode_var]),
+            ));
+        }
+
+        Op::EmptyBag { item_type } => {
+            ctx.write(format_args!("empty_bag<{}>", type_ident(*item_type)));
+        }
+
+        Op::BagInsert => {
+            ctx.write("bag_insert");
+        }
+
+        Op::BagGet => {
+            ctx.write("bag_get");
+        }
+
+        Op::BagRemove => {
+            ctx.write("bag_remove");
+        }
+
+        Op::MakeTuple => {
+            ctx.write("make_tuple");
+        }
+
+        Op::GetTupleField { field_idx } => {
+            ctx.write(format_args!("get_tuple_field {}", field_idx));
+        }
+
+        Op::MakeUnion {
+            variant_types,
+            variant_idx,
+        } => {
+            ctx.write("make_union<");
+            delimit(
+                ctx,
+                variant_types.iter(),
+                |ctx| {
+                    ctx.write(", ");
+                },
+                |ctx, variant_type| {
+                    ctx.write(type_ident(*variant_type));
+                },
+            );
+            ctx.write(format_args!("> {}", variant_idx));
+        }
+
+        Op::UnwrapUnion { variant_idx } => {
+            ctx.write(format_args!("unwrap_union {}", variant_idx));
+        }
+
+        Op::MakeNamed { named_mod, named } => {
+            ctx.write(format_args!(
+                "make_named {}::{}",
+                mod_name_string(named_mod),
+                type_name_string(named),
+            ));
+        }
+
+        Op::UnwrapNamed { named_mod, named } => {
+            ctx.write(format_args!(
+                "unwrap_named {}::{}",
+                mod_name_string(named_mod),
+                type_name_string(named),
+            ));
+        }
+    };
+}
+
+fn render_statement(builder: &ExprBuilder, ctx: &mut RenderContext, value_id: ValueId) {
+    match builder.vals.node(value_id) {
+        Node {
+            op: Op::DeclareContinuation { continuation },
+            inputs: [],
+        } => {
+            render_continuation_def(builder, ctx, *continuation);
+        }
+
+        Node { op, inputs } => {
+            ctx.write(format_args!("let {} = ", val_ident(value_id)));
+            render_op(builder, ctx, op);
+            ctx.write(" (");
+            delimit(
+                ctx,
+                inputs.iter(),
+                |ctx| {
+                    ctx.write(", ");
+                },
+                |ctx, input| {
+                    ctx.write(val_ident(*input));
+                },
+            );
+            ctx.writeln(");");
+        }
+    }
+}
+
+fn render_continuation_def(
+    builder: &ExprBuilder,
+    ctx: &mut RenderContext,
+    continuation: ContinuationId,
+) {
+    let info = builder.continuations[continuation];
+    ctx.write(format_args!(
+        "continuation {} ({}: {}) -> {} ",
+        continuation_ident(continuation),
+        val_ident(info.arg),
+        type_ident(info.arg_type),
+        type_ident(info.ret_type),
+    ));
+    render_block_expr(
+        builder,
+        ctx,
+        info.body
+            .expect("builder API should have verified that continuation body is defined"),
+    );
+    ctx.writeln("");
+}
+
+fn render_block_expr(builder: &ExprBuilder, ctx: &mut RenderContext, block_expr: BlockExpr) {
+    let BlockExpr(block, ret_val) = block_expr;
+    ctx.writeln("{");
+    ctx.with_indent(|ctx| {
+        for value in builder.blocks.block_values(block) {
+            render_statement(builder, ctx, value);
+        }
+        ctx.writeln(val_ident(ret_val))
+    });
+    ctx.write("}");
+}
+
+fn render_type_node(ctx: &mut RenderContext, node: Node<TypeId, TypeOp>) {
+    match node {
+        Node {
+            op: TypeOp::Named { named_mod, named },
+            inputs,
+        } => {
+            debug_assert!(inputs.is_empty());
+            ctx.write(format_args!(
+                "{}::{}",
+                mod_name_string(named_mod),
+                type_name_string(named)
+            ));
+        }
+
+        Node {
+            op: TypeOp::Tuple,
+            inputs,
+        } => {
+            ctx.write("(");
+            delimit(
+                ctx,
+                inputs.iter(),
+                |ctx| {
+                    ctx.write(", ");
+                },
+                |ctx, input| {
+                    ctx.write(type_ident(*input));
+                },
+            );
+            ctx.write(")");
+        }
+
+        Node {
+            op: TypeOp::Union,
+            inputs,
+        } => {
+            ctx.write("union { ");
+            delimit(
+                ctx,
+                inputs.iter(),
+                |ctx| {
+                    ctx.write(", ");
+                },
+                |ctx, input| {
+                    ctx.write(type_ident(*input));
+                },
+            );
+            if !inputs.is_empty() {
+                ctx.write(" ");
+            }
+            ctx.write("}");
+        }
+
+        Node {
+            op: TypeOp::HeapCell,
+            inputs,
+        } => {
+            debug_assert!(inputs.is_empty());
+            ctx.write("heap_cell");
+        }
+
+        Node {
+            op: TypeOp::Bag,
+            inputs,
+        } => {
+            debug_assert_eq!(inputs.len(), 1);
+            ctx.write(format_args!("bag<{}>", type_ident(inputs[0])));
+        }
+    }
+}
+
+fn render_type_bindings(ctx: &mut RenderContext, types: &OpGraph<TypeId, TypeOp>) {
+    ctx.writeln("{");
+    ctx.with_indent(|ctx| {
+        for type_id in types.count().iter() {
+            ctx.write(format_args!("type {} = ", type_ident(type_id)));
+            render_type_node(ctx, types.node(type_id));
+            ctx.writeln(";");
+        }
+    });
+    ctx.write("}");
+}
+
+fn render_func_def(ctx: &mut RenderContext, name: &FuncNameBuf, func_def: &FuncDef) {
+    ctx.write(format_args!(
+        "fn {} ({}: {}) -> {} ",
+        func_name_string(name),
+        val_ident(func_def.builder.argument),
+        type_ident(func_def.arg_type),
+        type_ident(func_def.ret_type),
+    ));
+    render_block_expr(&func_def.builder.expr_builder, ctx, func_def.root);
+    ctx.write(" where ");
+    render_type_bindings(ctx, &func_def.builder.expr_builder.type_builder.types);
+    ctx.writeln("");
+}
+
+fn render_const_def(ctx: &mut RenderContext, name: &ConstNameBuf, const_def: &ConstDef) {
+    ctx.write(format_args!(
+        "const {}: {} = ",
+        const_name_string(name),
+        type_ident(const_def.type_),
+    ));
+    render_block_expr(&const_def.builder.expr_builder, ctx, const_def.root);
+    ctx.write(" where ");
+    render_type_bindings(ctx, &const_def.builder.expr_builder.type_builder.types);
+    ctx.writeln("");
+}
+
+fn render_type_def(ctx: &mut RenderContext, name: &TypeNameBuf, type_def: &TypeDef) {
+    ctx.write(format_args!(
+        "type {} = {} where ",
+        type_name_string(name),
+        type_ident(type_def.root),
+    ));
+    render_type_bindings(ctx, &type_def.builder.types);
+    ctx.writeln("");
+}
+
+fn render_mod_def(ctx: &mut RenderContext, name: &ModNameBuf, mod_def: &ModDef) {
+    ctx.writeln(format_args!("mod {} {{", mod_name_string(name)));
+    ctx.with_indent(|ctx| {
+        let mut first_item = true;
+        let mut write_spacing = |ctx: &mut RenderContext| {
+            if !first_item {
+                ctx.writeln("");
+            }
+            first_item = false;
+        };
+
+        for (type_name, type_def) in &mod_def.type_defs {
+            write_spacing(ctx);
+            render_type_def(ctx, type_name, type_def);
+        }
+
+        for (const_name, const_def) in &mod_def.const_defs {
+            write_spacing(ctx);
+            render_const_def(ctx, const_name, const_def);
+        }
+
+        for (func_name, func_def) in &mod_def.func_defs {
+            write_spacing(ctx);
+            render_func_def(ctx, func_name, func_def);
+        }
+    });
+    ctx.writeln("}");
+}
+
+fn render_entry_point(
+    ctx: &mut RenderContext,
+    name: &EntryPointNameBuf,
+    entry_point: &(ModNameBuf, FuncNameBuf),
+) {
+    let (mod_, func) = entry_point;
+    ctx.writeln(format_args!(
+        "entry_point {} = {}::{};",
+        entry_point_name_string(name),
+        mod_name_string(mod_),
+        func_name_string(func)
+    ));
+}
+
+fn render_program(ctx: &mut RenderContext, program: &Program) {
+    ctx.writeln("program {");
+    ctx.with_indent(|ctx| {
+        let mut first_item = true;
+        let mut write_spacing = |ctx: &mut RenderContext| {
+            if !first_item {
+                ctx.writeln("");
+            }
+            first_item = false;
+        };
+
+        for (mod_name, mod_def) in &program.mods {
+            write_spacing(ctx);
+            render_mod_def(ctx, mod_name, mod_def);
+        }
+
+        for (entry_point_name, entry_point) in &program.entry_points {
+            write_spacing(ctx);
+            render_entry_point(ctx, entry_point_name, entry_point);
+        }
+    });
+    ctx.write("}");
+}
+
+pub fn render_program_to_string(program: &Program) -> String {
+    let mut ctx = RenderContext::new(2);
+    render_program(&mut ctx, program);
+    ctx.finish()
+}
diff --git a/vendor/morphic_lib/src/type_cache.rs b/vendor/morphic_lib/src/type_cache.rs
new file mode 100644
index 0000000000..beb16b182b
--- /dev/null
+++ b/vendor/morphic_lib/src/type_cache.rs
@@ -0,0 +1,22 @@
+use smallvec::SmallVec;
+
+use crate::name_cache::NamedTypeId;
+use crate::util::id_bi_map::IdBiMap;
+
+id_type! {
+    pub TypeId(u32);
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub enum TypeData {
+    Named { named: NamedTypeId },
+    Tuple { fields: SmallVec<[TypeId; 10]> },
+    Union { variants: SmallVec<[TypeId; 10]> },
+    HeapCell,
+    Bag { item: TypeId },
+}
+
+#[derive(Clone, Debug, Default)]
+pub struct TypeCache {
+    pub types: IdBiMap<TypeId, TypeData>,
+}
diff --git a/vendor/morphic_lib/src/util/blocks.rs b/vendor/morphic_lib/src/util/blocks.rs
new file mode 100644
index 0000000000..22c182cbfa
--- /dev/null
+++ b/vendor/morphic_lib/src/util/blocks.rs
@@ -0,0 +1,116 @@
+use crate::util::id_type::{Count, Id};
+use crate::util::id_vec::IdVec;
+use crate::util::replace_none::replace_none;
+
+id_type! {
+    BlockFragId(u32);
+}
+
+/// A "fragment" of a block, representing a run of consecutive `ValId`s and a pointer to the next
+/// fragment.
+#[derive(Clone, Debug)]
+struct BlockFrag<ValId> {
+    /// Inclusive bound
+    min_val: ValId,
+    /// Exclusive bound
+    max_val: ValId,
+    next: Option<BlockFragId>,
+}
+
+#[derive(Clone, Debug)]
+struct BlockData<BlockInfo> {
+    head: BlockFragId,
+    tail: BlockFragId,
+    info: BlockInfo,
+}
+
+/// Conceptually represents a collection of the form `IdVec<BlockId, (Vec<ValId>, BlockInfo)>`.
+///
+/// Each tuple `(Vec<ValId>, BlockInfo)` is called a "block".
+///
+/// The blocks are actually stored in a single contiguous buffer to reduce the number of heap
+/// allocations, and blocks with long runs of consecutive `ValId`s are stored in a compressed
+/// representation.
+#[derive(Clone, Debug)]
+pub struct Blocks<BlockId: Id, ValId: Id, BlockInfo> {
+    frags: IdVec<BlockFragId, BlockFrag<ValId>>,
+    blocks: IdVec<BlockId, BlockData<BlockInfo>>,
+}
+
+impl<BlockId: Id, ValId: Id, BlockInfo> Blocks<BlockId, ValId, BlockInfo> {
+    pub fn new() -> Self {
+        Self {
+            frags: IdVec::new(),
+            blocks: IdVec::new(),
+        }
+    }
+
+    pub fn len(&self) -> usize {
+        self.blocks.len()
+    }
+
+    pub fn block_count(&self) -> Count<BlockId> {
+        self.blocks.count()
+    }
+
+    pub fn add_block(&mut self, start_hint: ValId, info: BlockInfo) -> BlockId {
+        let frag = BlockFrag {
+            min_val: start_hint.clone(),
+            max_val: start_hint,
+            next: None,
+        };
+        let frag_id = self.frags.push(frag);
+        self.blocks.push(BlockData {
+            head: frag_id,
+            tail: frag_id,
+            info,
+        })
+    }
+
+    pub fn add_value(&mut self, block_id: BlockId, val_id: ValId) {
+        let block = &mut self.blocks[block_id];
+        let tail_frag = &mut self.frags[block.tail];
+        let next_val = ValId::from_index_or_panic(tail_frag.max_val.to_index() + 1);
+        if tail_frag.max_val.to_index() == val_id.to_index() {
+            tail_frag.max_val = next_val;
+        } else {
+            let new_tail = BlockFrag {
+                min_val: val_id.clone(),
+                max_val: ValId::from_index_or_panic(val_id.to_index() + 1),
+                next: None,
+            };
+            let new_tail_id = self.frags.push(new_tail);
+            replace_none(&mut self.frags[block.tail].next, new_tail_id).unwrap();
+            block.tail = new_tail_id;
+        }
+    }
+
+    pub fn block_info(&self, block_id: BlockId) -> &BlockInfo {
+        &self.blocks[block_id].info
+    }
+
+    pub fn block_info_mut(&mut self, block_id: BlockId) -> &mut BlockInfo {
+        &mut self.blocks[block_id].info
+    }
+
+    pub fn block_values(&self, block_id: BlockId) -> impl Iterator<Item = ValId> + '_ {
+        let mut frag = &self.frags[self.blocks[block_id].head];
+        let mut val = frag.min_val.clone();
+        std::iter::from_fn(move || {
+            while val.to_index() >= frag.max_val.to_index() {
+                match frag.next {
+                    Some(next) => {
+                        frag = &self.frags[next];
+                        val = frag.min_val.clone();
+                    }
+                    None => {
+                        return None;
+                    }
+                }
+            }
+            let this_val = val.clone();
+            val = ValId::from_index_unchecked(val.to_index() + 1);
+            Some(this_val)
+        })
+    }
+}
diff --git a/vendor/morphic_lib/src/util/bytes_id.rs b/vendor/morphic_lib/src/util/bytes_id.rs
index a5168c4c71..b75d618271 100644
--- a/vendor/morphic_lib/src/util/bytes_id.rs
+++ b/vendor/morphic_lib/src/util/bytes_id.rs
@@ -4,13 +4,15 @@ macro_rules! bytes_id {
         $(#[$annot_borrowed:meta])* $borrowed_vis:vis $borrowed:ident;
         $(#[$annot_owned:meta])* $owned_vis:vis $owned:ident;
     ) => {
-        #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
+        #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
         $(#[$annot_borrowed])*
         $borrowed_vis struct $borrowed<'a>($borrowed_vis &'a [u8]);
 
-        #[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+        // On 64-bit platforms we can store up to `23` bytes inline in a `SmallVec` with no space
+        // penalty.
+        #[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
         $(#[$annot_owned])*
-        $owned_vis struct $owned($owned_vis ::std::vec::Vec<u8>);
+        $owned_vis struct $owned($owned_vis ::smallvec::SmallVec<[u8; 23]>);
 
         impl $owned {
             fn borrowed<'a>(&'a self) -> $borrowed<'a> {
@@ -20,7 +22,29 @@ macro_rules! bytes_id {
 
         impl<'a> ::std::convert::From<$borrowed<'a>> for $owned {
             fn from(borrowed: $borrowed<'a>) -> Self {
-                $owned(<[u8]>::to_vec(&borrowed.0))
+                $owned(::smallvec::SmallVec::from_slice(&borrowed.0))
+            }
+        }
+
+        impl<'a> ::std::fmt::Debug for $borrowed<'a> {
+            fn fmt(&self, f: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
+                // TODO: Consolidate this with render_api_ir.rs
+                write!(f, "{}(\"", stringify!($borrowed))?;
+                for &byte in self.0 {
+                    write!(f, "{}", ::std::ascii::escape_default(byte))?;
+                }
+                write!(f, "\")")?;
+                Ok(())
+            }
+        }
+
+        impl ::std::fmt::Debug for $owned {
+            fn fmt(&self, f: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
+                // We intentionally use the name of $borrowed to render values of type $owned,
+                // because only the borrowed version of each bytestring-based identifier type are
+                // exposed/documented in the public API, and we use this debug formatting logic to
+                // render public-facing error messages.
+                write!(f, "{:?}", self.borrowed())
             }
         }
     }
diff --git a/vendor/morphic_lib/src/util/flat_slices.rs b/vendor/morphic_lib/src/util/flat_slices.rs
new file mode 100644
index 0000000000..fe72cce7e8
--- /dev/null
+++ b/vendor/morphic_lib/src/util/flat_slices.rs
@@ -0,0 +1,112 @@
+use std::borrow::Borrow;
+use std::fmt;
+
+use crate::util::id_type::{self, Count, Id};
+use crate::util::id_vec::IdVec;
+
+#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+struct SliceData<SliceInfo> {
+    info: SliceInfo,
+
+    /// Determines which slice of the `flat_data` buffer contains the items of this slice.
+    ///
+    /// If the *previous* slice has `slice_end_idx == a`, and this slice has `inputs_end_idx == b`,
+    /// then the items of this slice are given by `flat_data[a..b]`.
+    slice_end_idx: usize,
+}
+
+/// Conceptually represents a collection of the form `IdVec<SliceId, (SliceInfo, Vec<T>)>`.
+///
+/// The notional `Vec<T>` values are actually stored in a single contiguous buffer to reduce the
+/// number of heap allocations.  Because these values are all different slices of the same
+/// underlying buffer, we refer to each tuple `(SliceInfo, Vec<T>)` as a "slice" for the purposes of
+/// this data structure's API.
+///
+/// The `SliceInfo` parameter should be regarded as optional, and for some purposes it is perfectly
+/// fine (stylistically speaking) to set `SliceInfo = ()`.
+#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct FlatSlices<SliceId: Id, SliceInfo, T> {
+    flat_data: Vec<T>,
+    slices: IdVec<SliceId, SliceData<SliceInfo>>,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct Slice<'a, SliceInfo, T> {
+    pub info: &'a SliceInfo,
+    pub items: &'a [T],
+}
+
+#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct SliceMut<'a, SliceInfo, T> {
+    pub info: &'a mut SliceInfo,
+    pub items: &'a mut [T],
+}
+
+impl<SliceId: Id, SliceInfo, T> Default for FlatSlices<SliceId, SliceInfo, T> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl<SliceId: Id, SliceInfo, T> FlatSlices<SliceId, SliceInfo, T> {
+    pub fn new() -> Self {
+        Self {
+            flat_data: Vec::new(),
+            slices: IdVec::new(),
+        }
+    }
+
+    pub fn len(&self) -> usize {
+        self.slices.len()
+    }
+
+    pub fn count(&self) -> Count<SliceId> {
+        self.slices.count()
+    }
+
+    pub fn push_slice(&mut self, info: SliceInfo, slice: &[T]) -> SliceId
+    where
+        T: Clone,
+    {
+        self.flat_data.extend_from_slice(slice);
+        self.slices.push(SliceData {
+            info,
+            slice_end_idx: self.flat_data.len(),
+        })
+    }
+
+    fn data_range(&self, idx: SliceId) -> (usize, usize) {
+        let start = match id_type::decrement(idx.clone()) {
+            None => 0,
+            Some(prev_idx) => self.slices[prev_idx].slice_end_idx,
+        };
+        let end = self.slices[idx].slice_end_idx;
+        (start, end)
+    }
+
+    pub fn get<I: Borrow<SliceId>>(&self, idx: I) -> Slice<SliceInfo, T> {
+        let (start, end) = self.data_range(idx.borrow().clone());
+        Slice {
+            info: &self.slices[idx].info,
+            items: &self.flat_data[start..end],
+        }
+    }
+
+    pub fn get_mut<I: Borrow<SliceId>>(&mut self, idx: I) -> SliceMut<SliceInfo, T> {
+        let (start, end) = self.data_range(idx.borrow().clone());
+        SliceMut {
+            info: &mut self.slices[idx].info,
+            items: &mut self.flat_data[start..end],
+        }
+    }
+}
+
+impl<SliceId: Id + fmt::Debug, SliceInfo: fmt::Debug, T: fmt::Debug> fmt::Debug
+    for FlatSlices<SliceId, SliceInfo, T>
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_map()
+            .entries(self.count().iter().map(|i| (i.clone(), self.get(i))))
+            .finish()
+    }
+}
diff --git a/vendor/morphic_lib/src/util/id_bi_map.rs b/vendor/morphic_lib/src/util/id_bi_map.rs
new file mode 100644
index 0000000000..00b04c2142
--- /dev/null
+++ b/vendor/morphic_lib/src/util/id_bi_map.rs
@@ -0,0 +1,70 @@
+use std::collections::hash_map::{Entry, HashMap};
+use std::hash::Hash;
+use std::ops::Deref;
+
+use crate::util::id_type::Id;
+use crate::util::id_vec::IdVec;
+
+/// Conceptually represents a collection of the form `IdVec<K, V>` where the `V` values are unique.
+///
+/// The collection is implemented such that lookups from `V` values to `K` keys are efficient.
+#[derive(Clone, Debug)]
+pub struct IdBiMap<K: Id, V> {
+    key_to_val: IdVec<K, V>,
+    val_to_key: HashMap<V, K>,
+}
+
+impl<K: Id, V: Hash + Eq + Clone> Default for IdBiMap<K, V> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl<K: Id, V> Deref for IdBiMap<K, V> {
+    type Target = IdVec<K, V>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.key_to_val
+    }
+}
+
+impl<K: Id, V: Hash + Eq + Clone> IdBiMap<K, V> {
+    pub fn new() -> Self {
+        IdBiMap {
+            key_to_val: IdVec::new(),
+            val_to_key: HashMap::new(),
+        }
+    }
+
+    /// Insert a new unique value into the bi-map.
+    ///
+    /// If the value is not already present, returns an `Ok` value with the new index of the value.
+    ///
+    /// If the value is already present, returns an `Err` value with the existing index.
+    pub fn insert(&mut self, val: V) -> Result<K, K> {
+        match self.val_to_key.entry(val) {
+            Entry::Occupied(occupied) => Err(occupied.get().clone()),
+            Entry::Vacant(vacant) => {
+                let new_index = self.key_to_val.push(vacant.key().clone());
+                vacant.insert(new_index.clone());
+                Ok(new_index)
+            }
+        }
+    }
+
+    /// Insert a value into the bi-map, or get its key if it is already present.
+    pub fn get_or_insert(&mut self, val: V) -> K {
+        match self.val_to_key.entry(val) {
+            Entry::Occupied(occupied) => occupied.get().clone(),
+            Entry::Vacant(vacant) => {
+                let new_index = self.key_to_val.push(vacant.key().clone());
+                vacant.insert(new_index.clone());
+                new_index
+            }
+        }
+    }
+
+    pub fn get_by_val(&self, val: &V) -> Option<K> {
+        self.val_to_key.get(val).cloned()
+    }
+}
diff --git a/vendor/morphic_lib/src/util/id_type.rs b/vendor/morphic_lib/src/util/id_type.rs
new file mode 100644
index 0000000000..a0b730d746
--- /dev/null
+++ b/vendor/morphic_lib/src/util/id_type.rs
@@ -0,0 +1,111 @@
+use std::cmp::Ordering;
+
+/// Abstracts over types which wrap numerical indices.
+pub trait Id: Clone {
+    /// The maximum `usize` representable by this type.
+    const MAX_USIZE: usize;
+
+    /// Convert from `usize` to `Self`, potentially silently wrapping if the `usize` is out of
+    /// range.
+    ///
+    /// In debug builds, we may still perform a check.
+    fn from_index_unchecked(idx: usize) -> Self;
+
+    /// Check that a `usize` is in range for `Self`, panicking with an informative error message if
+    /// not.
+    fn assert_in_range(idx: usize);
+
+    /// Convert from `usize` to `Self`, panicking if the `usize` is out of range.
+    fn from_index_or_panic(idx: usize) -> Self {
+        Self::assert_in_range(idx);
+        Self::from_index_unchecked(idx)
+    }
+
+    /// Convert from `Self` to `usize`.  This should never fail.
+    fn to_index(&self) -> usize;
+}
+
+macro_rules! id_type {
+    ($(#[$annot:meta])* $id_vis:vis $name:ident($wrapped:ty); ) => {
+        $(#[$annot])*
+        #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+        $id_vis struct $name($id_vis $wrapped);
+
+        impl $crate::util::id_type::Id for $name {
+            const MAX_USIZE: usize = <$wrapped>::MAX as usize;
+
+            fn from_index_unchecked(idx: usize) -> Self {
+                if cfg!(debug_assertions) {
+                    <Self as $crate::util::id_type::Id>::assert_in_range(idx);
+                }
+                $name(idx as $wrapped)
+            }
+
+            fn assert_in_range(idx: usize) {
+                if idx > Self::MAX_USIZE {
+                    panic!(
+                        "index {} overflows range of type '{}' (backed by {})",
+                        idx,
+                        stringify!(name),
+                        stringify!($wrapped),
+                    );
+                }
+            }
+
+            fn to_index(&self) -> usize {
+                self.0 as usize
+            }
+        }
+
+        // Custom Debug impl avoid multi-line formatting when formatted with {:#?}
+        impl ::std::fmt::Debug for $name {
+            fn fmt(&self, f: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
+                write!(f, "{}({})", stringify!($name), self.0)
+            }
+        }
+
+        impl ::std::cmp::PartialEq<$crate::util::id_type::Count<$name>> for $name {
+            fn eq(&self, other: &$crate::util::id_type::Count<$name>) -> bool {
+                self.eq(&other.0)
+            }
+        }
+
+        impl ::std::cmp::PartialOrd<$crate::util::id_type::Count<$name>> for $name {
+            fn partial_cmp(
+                &self,
+                other: &$crate::util::id_type::Count<$name>,
+            ) -> Option<::std::cmp::Ordering> {
+                self.partial_cmp(&other.0)
+            }
+        }
+    }
+}
+
+/// `Count(x)` represents the range of ids `0..x`.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct Count<T>(pub T);
+
+impl<T: PartialEq> PartialEq<T> for Count<T> {
+    fn eq(&self, other: &T) -> bool {
+        self.0.eq(other)
+    }
+}
+
+impl<T: PartialOrd> PartialOrd<T> for Count<T> {
+    fn partial_cmp(&self, other: &T) -> Option<Ordering> {
+        self.0.partial_cmp(other)
+    }
+}
+
+impl<T: Id> Count<T> {
+    pub fn iter(&self) -> impl DoubleEndedIterator<Item = T> {
+        (0..self.0.to_index()).map(T::from_index_unchecked)
+    }
+}
+
+pub fn decrement<T: Id>(id: T) -> Option<T> {
+    match id.to_index() {
+        0 => None,
+        index => Some(T::from_index_unchecked(index - 1)),
+    }
+}
diff --git a/vendor/morphic_lib/src/util/id_vec.rs b/vendor/morphic_lib/src/util/id_vec.rs
new file mode 100644
index 0000000000..ab222559ea
--- /dev/null
+++ b/vendor/morphic_lib/src/util/id_vec.rs
@@ -0,0 +1,251 @@
+use std::borrow::Borrow;
+use std::fmt::{self, Debug};
+use std::iter;
+use std::marker::PhantomData;
+use std::ops::{Index, IndexMut};
+use std::slice;
+use std::vec;
+
+use crate::util::id_type::{Count, Id};
+
+#[derive(Clone, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct IdVec<K, V> {
+    key: PhantomData<K>,
+    // Invariant: items.len() <= K::MAX_USIZE
+    items: Vec<V>,
+}
+
+impl<K: Id + Debug, V: Debug> Debug for IdVec<K, V> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_map()
+            .entries(
+                self.items
+                    .iter()
+                    .enumerate()
+                    .map(|(idx, val)| (K::from_index_unchecked(idx), val)),
+            )
+            .finish()
+    }
+}
+
+#[derive(Clone, Debug)]
+pub struct IndexMapped<K, I> {
+    key: PhantomData<K>,
+    inner: I,
+}
+
+impl<K: Id, V, I: Iterator<Item = (usize, V)>> Iterator for IndexMapped<K, I> {
+    type Item = (K, V);
+
+    #[inline]
+    fn next(&mut self) -> Option<(K, V)> {
+        self.inner
+            .next()
+            .map(|(idx, val)| (K::from_index_unchecked(idx), val))
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
+    }
+
+    #[inline]
+    fn fold<Acc, G>(self, init: Acc, mut g: G) -> Acc
+    where
+        G: FnMut(Acc, Self::Item) -> Acc,
+    {
+        self.inner.fold(init, |acc, (idx, val)| {
+            g(acc, (K::from_index_unchecked(idx), val))
+        })
+    }
+}
+
+impl<K: Id, V> IntoIterator for IdVec<K, V> {
+    type Item = (K, V);
+
+    type IntoIter = IndexMapped<K, iter::Enumerate<vec::IntoIter<V>>>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        IndexMapped {
+            key: PhantomData,
+            inner: self.items.into_iter().enumerate(),
+        }
+    }
+}
+
+impl<'a, K: Id, V> IntoIterator for &'a IdVec<K, V> {
+    type Item = (K, &'a V);
+
+    type IntoIter = IndexMapped<K, iter::Enumerate<slice::Iter<'a, V>>>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        IndexMapped {
+            key: PhantomData,
+            inner: self.items.iter().enumerate(),
+        }
+    }
+}
+
+impl<'a, K: Id, V> IntoIterator for &'a mut IdVec<K, V> {
+    type Item = (K, &'a mut V);
+
+    type IntoIter = IndexMapped<K, iter::Enumerate<slice::IterMut<'a, V>>>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        IndexMapped {
+            key: PhantomData,
+            inner: self.items.iter_mut().enumerate(),
+        }
+    }
+}
+
+impl<K: Id, V> IdVec<K, V> {
+    pub fn new() -> Self {
+        IdVec {
+            key: PhantomData,
+            items: Vec::new(),
+        }
+    }
+
+    pub fn with_capacity(capacity: usize) -> Self {
+        IdVec {
+            key: PhantomData,
+            items: Vec::with_capacity(capacity),
+        }
+    }
+
+    pub fn from_items(items: Vec<V>) -> Self {
+        K::assert_in_range(items.len());
+        IdVec {
+            key: PhantomData,
+            items,
+        }
+    }
+
+    pub fn filled_with(count: Count<K>, mut f: impl FnMut() -> V) -> Self {
+        IdVec {
+            key: PhantomData,
+            items: count.iter().map(|_| f()).collect(),
+        }
+    }
+
+    pub fn items(&self) -> &[V] {
+        &self.items
+    }
+
+    pub fn items_mut(&mut self) -> &mut [V] {
+        &mut self.items
+    }
+
+    pub fn into_items(self) -> Vec<V> {
+        self.items
+    }
+
+    pub fn len(&self) -> usize {
+        self.items.len()
+    }
+
+    pub fn count(&self) -> Count<K> {
+        Count(K::from_index_unchecked(self.len()))
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.items.is_empty()
+    }
+
+    #[must_use]
+    pub fn push(&mut self, item: V) -> K {
+        let id = K::from_index_unchecked(self.len());
+        self.items.push(item);
+        K::assert_in_range(self.len());
+        id
+    }
+
+    pub fn truncate(&mut self, len: usize) {
+        self.items.truncate(len)
+    }
+
+    pub fn iter(&self) -> IndexMapped<K, iter::Enumerate<slice::Iter<V>>> {
+        self.into_iter()
+    }
+
+    pub fn iter_mut(&mut self) -> IndexMapped<K, iter::Enumerate<slice::IterMut<V>>> {
+        self.into_iter()
+    }
+
+    pub fn try_zip_exact<'a, U>(
+        &'a self,
+        other: &'a IdVec<K, U>,
+    ) -> Option<impl Iterator<Item = (K, &'a V, &'a U)>> {
+        if self.items.len() == other.items.len() {
+            Some(
+                self.items
+                    .iter()
+                    .zip(other.items.iter())
+                    .enumerate()
+                    .map(|(idx, (v1, v2))| (K::from_index_unchecked(idx), v1, v2)),
+            )
+        } else {
+            None
+        }
+    }
+
+    pub fn into_mapped<W, F: FnMut(K, V) -> W>(self, mut f: F) -> IdVec<K, W> {
+        let mapped_items = self.into_iter().map(|(idx, val)| f(idx, val)).collect();
+        IdVec::from_items(mapped_items)
+    }
+
+    pub fn try_into_mapped<W, E, F: FnMut(K, V) -> Result<W, E>>(
+        self,
+        mut f: F,
+    ) -> Result<IdVec<K, W>, E> {
+        let mapped_items = self
+            .into_iter()
+            .map(|(idx, val)| f(idx, val))
+            .collect::<Result<_, _>>()?;
+
+        Ok(IdVec::from_items(mapped_items))
+    }
+
+    pub fn map<W, F: FnMut(K, &V) -> W>(&self, mut f: F) -> IdVec<K, W> {
+        let mapped_items = self.iter().map(|(idx, val)| f(idx, val)).collect();
+        IdVec::from_items(mapped_items)
+    }
+
+    pub fn try_map<W, E, F: FnMut(K, &V) -> Result<W, E>>(
+        &self,
+        mut f: F,
+    ) -> Result<IdVec<K, W>, E> {
+        let mapped_items = self
+            .iter()
+            .map(|(idx, val)| f(idx, val))
+            .collect::<Result<_, _>>()?;
+
+        Ok(IdVec::from_items(mapped_items))
+    }
+
+    pub fn try_from_contiguous(entries: impl Iterator<Item = (K, V)>) -> Option<Self> {
+        let mut items = Vec::with_capacity(entries.size_hint().0);
+        for (idx, (key, val)) in entries.enumerate() {
+            if idx != key.to_index() {
+                return None;
+            }
+            items.push(val);
+        }
+        Some(Self::from_items(items))
+    }
+}
+
+impl<K: Id, V, I: Borrow<K>> Index<I> for IdVec<K, V> {
+    type Output = V;
+
+    fn index(&self, key: I) -> &V {
+        &self.items[key.borrow().to_index()]
+    }
+}
+
+impl<K: Id, V, I: Borrow<K>> IndexMut<I> for IdVec<K, V> {
+    fn index_mut(&mut self, key: I) -> &mut V {
+        &mut self.items[key.borrow().to_index()]
+    }
+}
diff --git a/vendor/morphic_lib/src/util/mod.rs b/vendor/morphic_lib/src/util/mod.rs
index a58e87cdac..d5fb572b08 100644
--- a/vendor/morphic_lib/src/util/mod.rs
+++ b/vendor/morphic_lib/src/util/mod.rs
@@ -1,5 +1,16 @@
+#[macro_use]
+pub mod id_type;
+
 #[macro_use]
 pub mod bytes_id;
 
 #[macro_use]
 pub mod forward_trait;
+
+pub mod blocks;
+pub mod flat_slices;
+pub mod id_bi_map;
+pub mod id_vec;
+pub mod op_graph;
+pub mod replace_none;
+pub mod strongly_connected;
diff --git a/vendor/morphic_lib/src/util/op_graph.rs b/vendor/morphic_lib/src/util/op_graph.rs
new file mode 100644
index 0000000000..46e7b38f49
--- /dev/null
+++ b/vendor/morphic_lib/src/util/op_graph.rs
@@ -0,0 +1,60 @@
+use std::borrow::Borrow;
+
+use crate::util::flat_slices::{FlatSlices, Slice};
+use crate::util::id_type::{Count, Id};
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct Node<'a, K, Op> {
+    pub op: &'a Op,
+    pub inputs: &'a [K],
+}
+
+/// Conceptually represents a collection of the form `IdVec<K, (Op, Vec<K>)>`.
+///
+/// Each tuple `(Op, Vec<K>)` is called a "node", and can be thought of as an "op" together with a
+/// list of zero or more "inputs" to that op, which are indices pointing to other nodes.
+///
+/// The input lists are actually stored in a single contiguous buffer to reduce the number of heap
+/// allocations.
+///
+/// This is essentially just a newtype wrapper around `FlatSlices`.  We use this wrapper to
+/// represent op graphs (instead of using `FlatSlices` directly) just so that the names of types,
+/// functions, and fields in the API are more meaningful for this use case.
+#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct OpGraph<K: Id, Op> {
+    inner: FlatSlices<K, Op, K>,
+}
+
+impl<K: Id, Op> Default for OpGraph<K, Op> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl<K: Id, Op> OpGraph<K, Op> {
+    pub fn new() -> Self {
+        Self {
+            inner: FlatSlices::new(),
+        }
+    }
+
+    pub fn len(&self) -> usize {
+        self.inner.len()
+    }
+
+    pub fn count(&self) -> Count<K> {
+        self.inner.count()
+    }
+
+    pub fn add_node(&mut self, op: Op, inputs: &[K]) -> K {
+        self.inner.push_slice(op, inputs)
+    }
+
+    pub fn node<I: Borrow<K>>(&self, idx: I) -> Node<K, Op> {
+        let Slice { info, items } = self.inner.get(idx);
+        Node {
+            op: info,
+            inputs: items,
+        }
+    }
+}
diff --git a/vendor/morphic_lib/src/util/replace_none.rs b/vendor/morphic_lib/src/util/replace_none.rs
new file mode 100644
index 0000000000..81e577272c
--- /dev/null
+++ b/vendor/morphic_lib/src/util/replace_none.rs
@@ -0,0 +1,13 @@
+#[derive(Clone, Debug, thiserror::Error)]
+#[error("replace_none: expected 'None' option, found value {0:?}")]
+pub struct ReplaceNoneError<T: std::fmt::Debug>(pub T);
+
+pub fn replace_none<T: std::fmt::Debug>(
+    opt: &mut Option<T>,
+    val: T,
+) -> Result<(), ReplaceNoneError<T>> {
+    match std::mem::replace(opt, Some(val)) {
+        None => Ok(()),
+        Some(prev) => Err(ReplaceNoneError(prev)),
+    }
+}
diff --git a/vendor/morphic_lib/src/util/strongly_connected.rs b/vendor/morphic_lib/src/util/strongly_connected.rs
new file mode 100644
index 0000000000..39e1f590cd
--- /dev/null
+++ b/vendor/morphic_lib/src/util/strongly_connected.rs
@@ -0,0 +1,153 @@
+use crate::util::flat_slices::FlatSlices;
+use crate::util::id_type::{Count, Id};
+use crate::util::id_vec::IdVec;
+use crate::util::replace_none::replace_none;
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub enum SccKind {
+    Acyclic,
+    Cyclic,
+}
+
+pub fn strongly_connected<SccId, NodeId, NodeSuccessors>(
+    node_count: Count<NodeId>,
+    mut node_successors: impl FnMut(NodeId) -> NodeSuccessors,
+) -> FlatSlices<SccId, SccKind, NodeId>
+where
+    SccId: Id,
+    NodeId: Id + Eq + Copy,
+    NodeSuccessors: Iterator<Item = NodeId>,
+{
+    // We use Tarjan's algorithm, performing the depth-first search using an explicit Vec-based
+    // stack instead of recursion to avoid stack overflows on large graphs.
+
+    #[derive(Clone, Copy, Debug)]
+    enum NodeState {
+        Unvisited,
+        OnSearchStack { index: u32, low_link: u32 },
+        OnSccStack { index: u32 },
+        Complete,
+    }
+
+    #[derive(Clone, Copy)]
+    enum Action<NodeId> {
+        TryVisit {
+            parent: Option<NodeId>,
+            node: NodeId,
+        },
+        FinishVisit {
+            parent: Option<NodeId>,
+            node: NodeId,
+        },
+    }
+
+    let mut sccs = FlatSlices::new();
+
+    let mut node_states = IdVec::filled_with(node_count, || NodeState::Unvisited);
+    let mut node_self_loops = IdVec::filled_with(node_count, || None);
+    let mut scc_stack = Vec::new();
+    let mut search_stack = Vec::new();
+    let mut next_index = 0;
+
+    for search_root in node_count.iter() {
+        search_stack.push(Action::TryVisit {
+            parent: None,
+            node: search_root,
+        });
+        while let Some(action) = search_stack.pop() {
+            match action {
+                Action::TryVisit { parent, node } => match node_states[node] {
+                    NodeState::Unvisited => {
+                        node_states[node] = NodeState::OnSearchStack {
+                            index: next_index,
+                            low_link: next_index,
+                        };
+                        next_index += 1;
+                        scc_stack.push(node);
+
+                        search_stack.push(Action::FinishVisit { parent, node });
+                        // We need to explicitly track self-loops so that when we obtain a size-1
+                        // SCC we can determine if it's cyclic or acyclic.
+                        let mut has_self_loop = false;
+                        for successor in node_successors(node) {
+                            if successor == node {
+                                has_self_loop = true;
+                            }
+                            search_stack.push(Action::TryVisit {
+                                parent: Some(node),
+                                node: successor,
+                            });
+                        }
+                        replace_none(&mut node_self_loops[node], has_self_loop).unwrap();
+                    }
+
+                    NodeState::OnSearchStack { index, low_link: _ }
+                    | NodeState::OnSccStack { index } => {
+                        if let Some(parent) = parent {
+                            if let NodeState::OnSearchStack {
+                                index: _,
+                                low_link: parent_low_link,
+                            } = &mut node_states[parent]
+                            {
+                                *parent_low_link = (*parent_low_link).min(index);
+                            } else {
+                                unreachable!("parent should be on search stack");
+                            }
+                        }
+                    }
+
+                    NodeState::Complete => {}
+                },
+
+                Action::FinishVisit { parent, node } => {
+                    let (index, low_link) =
+                        if let NodeState::OnSearchStack { index, low_link } = node_states[node] {
+                            (index, low_link)
+                        } else {
+                            unreachable!("node should be on search stack");
+                        };
+
+                    node_states[node] = NodeState::OnSccStack { index };
+
+                    if let Some(parent) = parent {
+                        if let NodeState::OnSearchStack {
+                            index: _,
+                            low_link: parent_low_link,
+                        } = &mut node_states[parent]
+                        {
+                            *parent_low_link = (*parent_low_link).min(low_link);
+                        } else {
+                            unreachable!("parent should be on search stack")
+                        }
+                    }
+
+                    if low_link == index {
+                        let mut scc_start = scc_stack.len();
+                        loop {
+                            scc_start -= 1;
+                            let scc_node = scc_stack[scc_start];
+                            debug_assert!(matches!(
+                                node_states[scc_node],
+                                NodeState::OnSccStack { .. }
+                            ));
+                            node_states[scc_node] = NodeState::Complete;
+                            if scc_node == node {
+                                break;
+                            }
+                        }
+                        let scc_slice = &scc_stack[scc_start..];
+                        let scc_kind = if scc_slice.len() == 1 && !node_self_loops[node].unwrap() {
+                            SccKind::Acyclic
+                        } else {
+                            SccKind::Cyclic
+                        };
+                        sccs.push_slice(scc_kind, scc_slice);
+                        scc_stack.truncate(scc_start);
+                    }
+                }
+            }
+        }
+    }
+
+    sccs
+}
diff --git a/vendor/morphic_lib/tests/basic.rs b/vendor/morphic_lib/tests/basic.rs
new file mode 100644
index 0000000000..2c7531c392
--- /dev/null
+++ b/vendor/morphic_lib/tests/basic.rs
@@ -0,0 +1,77 @@
+use morphic_lib::{
+    BlockExpr, CalleeSpecVar, EntryPointName, Error, ExprContext, FuncDefBuilder, FuncName,
+    ModDefBuilder, ModName, ProgramBuilder, TypeContext, UpdateModeVar,
+};
+
+#[test]
+fn test_basic() {
+    fn run() -> Result<(), Error> {
+        let func1_def = {
+            let mut f = FuncDefBuilder::new();
+            let b = f.add_block();
+            f.add_update(b, UpdateModeVar(b"var1"), f.get_argument())?;
+            let ret = f.add_make_tuple(b, &[])?;
+            let arg_type = f.add_heap_cell_type();
+            let ret_type = f.add_tuple_type(&[])?;
+            f.build(arg_type, ret_type, BlockExpr(b, ret))?
+        };
+
+        let func2_def = {
+            let mut f = FuncDefBuilder::new();
+            let b = f.add_block();
+            let cell = f.add_new_heap_cell(b)?;
+            let ret = f.add_call(
+                b,
+                CalleeSpecVar(b"var2"),
+                ModName(b"main_mod"),
+                FuncName(b"func1"),
+                cell,
+            )?;
+            let unit_type = f.add_tuple_type(&[])?;
+            f.build(unit_type, unit_type, BlockExpr(b, ret))?
+        };
+
+        let main_mod = {
+            let mut m = ModDefBuilder::new();
+            m.add_func(FuncName(b"func1"), func1_def)?;
+            m.add_func(FuncName(b"func2"), func2_def)?;
+            m.build()?
+        };
+
+        let program = {
+            let mut p = ProgramBuilder::new();
+            p.add_mod(ModName(b"main_mod"), main_mod)?;
+            p.add_entry_point(
+                EntryPointName(b"main"),
+                ModName(b"main_mod"),
+                FuncName(b"func2"),
+            )?;
+            p.build()?
+        };
+
+        let program_sol = morphic_lib::solve(program)?;
+
+        let (_, _, func2_spec) = program_sol.entry_point_solution(EntryPointName(b"main"))?;
+
+        let main_mod_sol = program_sol.mod_solutions(ModName(b"main_mod"))?;
+
+        let func2_sol = main_mod_sol
+            .func_solutions(FuncName(b"func2"))?
+            .spec(&func2_spec)?;
+
+        let func1_spec = func2_sol.callee_spec(CalleeSpecVar(b"var2"))?;
+
+        let func1_sol = main_mod_sol
+            .func_solutions(FuncName(b"func1"))?
+            .spec(&func1_spec)?;
+
+        let _update_mode = func1_sol.update_mode(UpdateModeVar(b"var1"))?;
+
+        Ok(())
+    }
+
+    let result = run();
+    if let Err(err) = result {
+        panic!("error: {}", err);
+    }
+}
diff --git a/vendor/morphic_lib/tests/recursive.rs b/vendor/morphic_lib/tests/recursive.rs
new file mode 100644
index 0000000000..9e50172a50
--- /dev/null
+++ b/vendor/morphic_lib/tests/recursive.rs
@@ -0,0 +1,95 @@
+use morphic_lib::{
+    BlockExpr, CalleeSpecVar, EntryPointName, Error, ExprContext, FuncDefBuilder, FuncName,
+    ModDefBuilder, ModName, ProgramBuilder, TypeContext, UpdateModeVar,
+};
+
+#[test]
+fn test_recursive() {
+    fn run() -> Result<(), Error> {
+        let rec_def = {
+            let mut f = FuncDefBuilder::new();
+            let b = f.add_block();
+            let arg = f.get_argument();
+            let case1 = {
+                let b = f.add_block();
+                BlockExpr(b, arg)
+            };
+            let case2 = {
+                let b = f.add_block();
+                f.add_update(b, UpdateModeVar(b"mode"), arg)?;
+                let new = f.add_new_heap_cell(b)?;
+                let result = f.add_call(
+                    b,
+                    CalleeSpecVar(b"call"),
+                    ModName(b"main"),
+                    FuncName(b"rec"),
+                    new,
+                )?;
+                BlockExpr(b, result)
+            };
+            let result = f.add_choice(b, &[case1, case2])?;
+            let heap_cell_type = f.add_heap_cell_type();
+            f.build(heap_cell_type, heap_cell_type, BlockExpr(b, result))?
+        };
+
+        let main_def = {
+            let mut f = FuncDefBuilder::new();
+            let b = f.add_block();
+            let init = f.add_new_heap_cell(b)?;
+            let final_ = f.add_call(
+                b,
+                CalleeSpecVar(b"call"),
+                ModName(b"main"),
+                FuncName(b"rec"),
+                init,
+            )?;
+            f.add_touch(b, final_)?;
+            let result = f.add_make_tuple(b, &[])?;
+            let unit_type = f.add_tuple_type(&[])?;
+            f.build(unit_type, unit_type, BlockExpr(b, result))?
+        };
+
+        let mod_ = {
+            let mut m = ModDefBuilder::new();
+            m.add_func(FuncName(b"rec"), rec_def)?;
+            m.add_func(FuncName(b"main"), main_def)?;
+            m.build()?
+        };
+
+        let program = {
+            let mut p = ProgramBuilder::new();
+            p.add_mod(ModName(b"main"), mod_)?;
+            p.add_entry_point(
+                EntryPointName(b"entry"),
+                ModName(b"main"),
+                FuncName(b"main"),
+            )?;
+            p.build()?
+        };
+
+        let program_sol = morphic_lib::solve(program)?;
+
+        let (_, _, main_spec) = program_sol.entry_point_solution(EntryPointName(b"entry"))?;
+
+        let main_mod_sol = program_sol.mod_solutions(ModName(b"main"))?;
+
+        let main_sol = main_mod_sol
+            .func_solutions(FuncName(b"main"))?
+            .spec(&main_spec)?;
+
+        let rec_spec = main_sol.callee_spec(CalleeSpecVar(b"call"))?;
+
+        let rec_sol = main_mod_sol
+            .func_solutions(FuncName(b"rec"))?
+            .spec(&rec_spec)?;
+
+        let _update_mode = rec_sol.update_mode(UpdateModeVar(b"mode"))?;
+
+        Ok(())
+    }
+
+    let result = run();
+    if let Err(err) = result {
+        panic!("error: {}", err);
+    }
+}
diff --git a/vendor/pretty/Cargo.toml b/vendor/pretty/Cargo.toml
index 2cc21e09b0..c44d269934 100644
--- a/vendor/pretty/Cargo.toml
+++ b/vendor/pretty/Cargo.toml
@@ -21,4 +21,4 @@ termcolor = { version = "1.1.0", optional = true }
 [dev-dependencies]
 tempfile = "3.1.0"
 difference = "2"
-criterion = "0.3"
+criterion = { git = "https://github.com/Anton-4/criterion.rs"} 
diff --git a/vendor/pretty/src/lib.rs b/vendor/pretty/src/lib.rs
index 4fdc587f7a..5471aa9429 100644
--- a/vendor/pretty/src/lib.rs
+++ b/vendor/pretty/src/lib.rs
@@ -185,9 +185,9 @@ macro_rules! impl_doc {
 
             /// Mark this document as a group.
             ///
-            /// Groups are layed out on a single line if possible.  Within a group, all basic documents with
-            /// several possible layouts are assigned the same layout, that is, they are all layed out
-            /// horizontally and combined into a one single line, or they are each layed out on their own
+            /// Groups are laid out on a single line if possible.  Within a group, all basic documents with
+            /// several possible layouts are assigned the same layout, that is, they are all laid out
+            /// horizontally and combined into a one single line, or they are each laid out on their own
             /// line.
             #[inline]
             pub fn group(self) -> Self {
@@ -714,9 +714,9 @@ where
 
     /// Mark this document as a group.
     ///
-    /// Groups are layed out on a single line if possible.  Within a group, all basic documents with
-    /// several possible layouts are assigned the same layout, that is, they are all layed out
-    /// horizontally and combined into a one single line, or they are each layed out on their own
+    /// Groups are laid out on a single line if possible.  Within a group, all basic documents with
+    /// several possible layouts are assigned the same layout, that is, they are all laid out
+    /// horizontally and combined into a one single line, or they are each laid out on their own
     /// line.
     #[inline]
     pub fn group(self) -> DocBuilder<'a, D, A> {
diff --git a/www/.gitignore b/www/.gitignore
new file mode 100644
index 0000000000..378eac25d3
--- /dev/null
+++ b/www/.gitignore
@@ -0,0 +1 @@
+build
diff --git a/www/build.sh b/www/build.sh
new file mode 100755
index 0000000000..0fc2d25909
--- /dev/null
+++ b/www/build.sh
@@ -0,0 +1,25 @@
+#!/bin/bash
+
+set -euxo pipefail
+
+# cd into the directory where this script lives.
+# This allows us to run this script from the root project directory,
+# which is what Netlify wants to do.
+SCRIPT_RELATIVE_DIR=$(dirname "${BASH_SOURCE[0]}")
+cd $SCRIPT_RELATIVE_DIR
+
+rm -rf build/
+cp -r public/ build/
+
+pushd ..
+echo 'Generating docs...'
+# We run the CLI with --no-default-features because that way we don't have the
+# "llvm" feature and therefore don't depend on LLVM being installed on the
+# system. (Netlify's build servers have Rust installed, but not LLVM.)
+#
+# We set RUSTFLAGS to -Awarnings to ignore warnings during this build,
+# because when building without "the" llvm feature (which is only ever done
+# for this exact use case), the result is lots of "unused" warnings!
+RUSTFLAGS=-Awarnings cargo run -p roc_cli --no-default-features docs compiler/builtins/docs/Bool.roc
+mv generated-docs/ www/build/builtins
+popd
diff --git a/www/netlify.toml b/www/netlify.toml
new file mode 100644
index 0000000000..0e3070a0f6
--- /dev/null
+++ b/www/netlify.toml
@@ -0,0 +1,30 @@
+# This is the file that generates and deploys https://www.roc-lang.org,
+# which is served on Netlify.
+#
+# Netlify's docs for how this configuration file works:
+# https://docs.netlify.com/routing/headers/#syntax-for-the-netlify-configuration-file
+[build]
+  publish = "build/"
+  command = "bash build.sh"
+
+[[headers]]
+  for = "/*"
+  [headers.values]
+    X-Frame-Options = "DENY"
+    X-XSS-Protection = "1; mode=block"
+    Content-Security-Policy = "default-src 'self'; img-src *;"
+    X-Content-Type-Options = "nosniff"
+
+# Redirect roc-lang.org/authors to the AUTHORS file in this repo
+#
+# This is referenced in the LICENSE file, which says to see roc-lang.org/authors
+# for a list of authors!
+[[redirects]]
+  from = "/authors"
+  to = "https://github.com/rtfeldman/roc/blob/trunk/AUTHORS"
+  force = true
+  status = 302 # TODO once the repo is public, use status = 200 and this URL:
+               # https://raw.githubusercontent.com/rtfeldman/roc/trunk/AUTHORS
+               #
+               # This way, roc-lang.org/authors will show the authors directly,
+               # proxied from the current AUTHORS file on GitHub, no redirects.
diff --git a/www/public/favicon.svg b/www/public/favicon.svg
new file mode 100644
index 0000000000..e0cff74b57
--- /dev/null
+++ b/www/public/favicon.svg
@@ -0,0 +1,4 @@
+<svg viewBox="0 0 52 53" xmlns="http://www.w3.org/2000/svg">
+  <style>polygon {fill: #5c0bff;}@media (prefers-color-scheme: dark) {polygon {fill: #7733ff;}} </style>
+  <polygon points="0,0 23.8834,3.21052 37.2438,19.0101 45.9665,16.6324 50.5,22 45,22 44.0315,26.3689 26.4673,39.3424 27.4527,45.2132 17.655,53 23.6751,22.7086"/>
+</svg>
diff --git a/www/public/index.html b/www/public/index.html
new file mode 100644
index 0000000000..442e9d266f
--- /dev/null
+++ b/www/public/index.html
@@ -0,0 +1,23 @@
+<!doctype html>
+
+<html lang="en">
+  <head>
+    <meta charset="utf-8">
+
+    <title>The Roc Programming Language</title>
+    <!-- <meta name="description" content="A language for making delightful software."> -->
+    <meta name="viewport" content="width=device-width">
+
+    <!-- <link rel="icon" href="/favicon.svg"> -->
+  </head>
+
+  <body>
+    <h1>Work in Progress</h1>
+    <p>Roc's initial release is still under development, and this website is a placeholder until that release is ready.</p>
+    <p>In the meantime, if you'd like to learn more about Roc, here are some videos:</p>
+    <ul>
+      <li><a href="https://youtu.be/FMyyYdFSOHA">Roc on Zig Showtime</a> - April 24, 2021</li>
+      <li><a href="https://youtu.be/ZnYa99QoznE?t=4790">Roc at the Berlin FP Meetup</a> - September 1, 2020 (this one has details on how to try out Roc or get involved)</li>
+    </ul>
+  </body>
+</html>
diff --git a/www/public/license/index.html b/www/public/license/index.html
new file mode 100644
index 0000000000..4eea2d8c39
--- /dev/null
+++ b/www/public/license/index.html
@@ -0,0 +1,29 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8">
+
+    <title>Roc's license: The Universal Permissive License (UPL), Version 1.0</title>
+    <meta name="description" content="The software license for the code in the Roc programming language.">
+    <meta name="viewport" content="width=device-width">
+
+    <link rel="icon" href="/favicon.svg">
+  </head>
+  <body><pre><code>Copyright (c) 2019 Richard Feldman and subsequent Roc authors &lt;<a href="https://roc-lang.org/authors">https://roc-lang.org/authors</a>&gt;
+
+<a href="https://opensource.org/licenses/UPL">The Universal Permissive License (UPL), Version 1.0</a>
+
+Subject to the condition set forth below, permission is hereby granted to any person obtaining a copy of this software, associated documentation and/or data (collectively the "Software"), free of charge and under any and all copyright rights in the Software, and any and all patent rights owned or freely licensable by each licensor hereunder covering either (i) the unmodified Software as contributed to or provided by such licensor, or (ii) the Larger Works (as defined below), to deal in both
+
+(a) the Software, and
+
+(b) any piece of software and/or hardware listed in the lrgrwrks.txt file if one is included with the Software (each a “Larger Work” to which the Software is contributed by such licensors),
+
+without restriction, including without limitation the rights to copy, create derivative works of, display, perform, and distribute the Software and make, use, sell, offer for sale, import, export, have made, and have sold the Software and the Larger Work(s), and to sublicense the foregoing rights on either these or other terms.
+
+This license is subject to the following condition:
+
+The above copyright notice and either this complete permission notice or at a minimum a reference to the UPL must be included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.</body></code></pre>
+</html>
diff --git a/www/public/logo.svg b/www/public/logo.svg
new file mode 100644
index 0000000000..bb673d5013
--- /dev/null
+++ b/www/public/logo.svg
@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg width="240" height="240" viewBox="0 0 51 53" fill="none" xmlns="http://www.w3.org/2000/svg">
+  <path d="M23.6751 22.7086L17.655 53L27.4527 45.2132L26.4673 39.3424L23.6751 22.7086Z" fill="#612BDE"/>
+  <path d="M37.2438 19.0101L44.0315 26.3689L45 22L45.9665 16.6324L37.2438 19.0101Z" fill="#8257E5"/>
+  <path d="M23.8834 3.21052L0 0L23.6751 22.7086L23.8834 3.21052Z" fill="#8257E5"/>
+  <path d="M44.0315 26.3689L23.6751 22.7086L26.4673 39.3424L44.0315 26.3689Z" fill="#8257E5"/>
+  <path d="M50.5 22L45.9665 16.6324L45 22H50.5Z" fill="#612BDE"/>
+  <path d="M23.6751 22.7086L44.0315 26.3689L37.2438 19.0101L23.8834 3.21052L23.6751 22.7086Z" fill="#612BDE"/>
+</svg>
diff --git a/www/public/search.js b/www/public/search.js
new file mode 100644
index 0000000000..0bc719563b
--- /dev/null
+++ b/www/public/search.js
@@ -0,0 +1,38 @@
+(function() {
+  let sidebar = document.getElementById("sidebar-nav");
+  let searchBox = document.getElementById("module-search");
+
+  function search() {
+    let text = searchBox.value.toLowerCase(); // Search is case-insensitive.
+
+    if (text === "") {
+      // Un-hide everything
+      sidebar.querySelectorAll(".sidebar-entry a").forEach((entry) => entry.classList.remove("hidden"));
+
+      // Re-hide all the sub-entries except for those of the first module
+      sidebar.querySelectorAll(".sidebar-entry:not(:first-of-type) .sidebar-sub-entries a").forEach((entry) => entry.classList.add("hidden"));
+    } else {
+      // First, show/hide all the sub-entries within each module (top-level functions etc.)
+      sidebar.querySelectorAll(".sidebar-sub-entries a").forEach((entry) => {
+        if (entry.textContent.toLowerCase().includes(text)) {
+          entry.classList.remove("hidden");
+        } else {
+          entry.classList.add("hidden");
+        }
+      });
+
+      // Then, show/hide modules based on whether they match, or any of their sub-entries matched
+      sidebar.querySelectorAll(".sidebar-module-link").forEach((entry) => {
+        if (entry.textContent.toLowerCase().includes(text) || entry.parentNode.querySelectorAll(".sidebar-sub-entries a:not(.hidden)").length > 0) {
+          entry.classList.remove("hidden");
+        } else {
+          entry.classList.add("hidden");
+        }
+      });
+    }
+  }
+
+  searchBox.addEventListener("input", search);
+
+  search();
+})();
diff --git a/www/public/styles.css b/www/public/styles.css
new file mode 100644
index 0000000000..27f88aacd1
--- /dev/null
+++ b/www/public/styles.css
@@ -0,0 +1,479 @@
+:root {
+  --link-color: #612bde;
+  --code-link-color: #5721d4;
+  --text-color: #333333;
+  --code-color: #222222;
+  --code-bg-color: #eeeeee;
+  --body-bg-color: #fdfdfd;
+  --border-color: #e9e9e9;
+  --faded-color: #4c4c4c;
+  --monospace-font;
+  --font-sans: -apple-system, BlinkMacSystemFont, Roboto, Helvetica, Arial, sans-serif;
+  --font-mono: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace;
+  --top-header-height: 67px;
+  --sidebar-width: 280px;
+  --top-bar-bg: #8257e5;
+  --top-bar-fg: #ffffff;
+  --nav-link-hover-color: #000000;
+}
+
+a {
+  color: #972395;
+}
+
+.logo {
+  padding: 2px 8px;
+}
+
+.logo svg {
+  height: 48px;
+  width: 48px;
+  fill: var(--top-bar-fg);
+}
+
+.logo:hover {
+  text-decoration: none;
+}
+
+.logo svg:hover {
+  fill: var(--nav-link-hover-color);
+}
+
+.pkg-full-name {
+  color: var(--text-color);
+  display: flex;
+  align-items: center;
+  font-size: 32px;
+  margin: 0 8px;
+  font-weight: normal;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  height: 100%;
+}
+
+.pkg-full-name a {
+  padding-top: 12px;
+  padding-bottom: 16px;
+}
+
+a {
+  text-decoration: none;
+}
+
+a:hover {
+  text-decoration: underline;
+}
+
+.pkg-and-logo {
+  min-width: 0; /* necessary for text-overflow: ellipsis to work in descendants */
+  display: flex;
+  align-items: center;
+  height: 100%;
+  background-color: var(--top-bar-bg);
+}
+
+.pkg-and-logo a, .pkg-and-logo a:visited {
+  color: var(--top-bar-fg);
+}
+
+.pkg-and-logo a:hover {
+  color: var(--nav-link-hover-color);
+  text-decoration: none;
+}
+
+.main-container {
+  min-width: 0; /* necessary for text-overflow: ellipsis to work in descendants */
+}
+
+.search-button {
+  flex-shrink: 0; /* always shrink the package name before these; they have a relatively constrained length */
+  padding: 12px 18px;
+  margin-right: 42px;
+  display: none; /* only show this in the mobile view */
+}
+
+.version {
+  padding: 18px 10px;
+  min-width: 48px;
+  margin-right: 8px;
+}
+
+body {
+  display: grid;
+  grid-template-columns: [before-sidebar] 1fr [sidebar] var(--sidebar-width) [main-content] fit-content(calc(1280px - var(--sidebar-width))) [end] 1fr;
+  grid-template-rows: [top-header] var(--top-header-height) [above-footer] auto [footer] auto;
+  box-sizing: border-box;
+  margin: 0;
+  padding: 0;
+  font-family: var(--font-sans);
+  color: var(--text-color);
+  background-color: var(--body-bg-color);
+}
+
+main {
+  grid-column-start: main-content;
+  grid-column-end: main-content;
+  grid-row-start: above-footer;
+  grid-row-end: above-footer;
+  box-sizing: border-box;
+  position: relative;
+  font-size: 18px;
+  line-height: 1.85em;
+  margin-top: 2px;
+  padding: 48px;
+}
+
+#sidebar-nav {
+  grid-column-start: sidebar;
+  grid-column-end: sidebar;
+  grid-row-start: above-footer;
+  grid-row-end: above-footer;
+  position: relative;
+  display: flex;
+  flex-direction: column;
+  box-sizing: border-box;
+  padding-left: 56px;
+  padding-top: 6px;
+  width: 100%;
+}
+
+.top-header-extension {
+  grid-column-start: before-sidebar;
+  grid-column-end: sidebar;
+  grid-row-start: top-header;
+  grid-row-end: top-header;
+  background-color: var(--top-bar-bg);
+}
+
+.top-header {
+  grid-column-start: sidebar;
+  grid-column-end: end;
+  grid-row-start: top-header;
+  grid-row-end: top-header;
+  display: flex;
+  flex-direction: row;
+  align-items: center;
+  flex-wrap: nowrap;
+  flex-grow: 1;
+  box-sizing: border-box;
+  font-family: var(--font-sans);
+  font-size: 24px;
+  height: 100%;
+  min-width: 0; /* necessary for text-overflow: ellipsis to work in descendants */
+}
+
+.top-header-triangle {
+  /* This used to be a clip-path, but Firefox on Android (at least as of early 2020)
+   * rendered the page extremely slowly in that version. With this approach it's super fast.
+   */
+  width: 0;
+  height: 0;
+  border-style: solid;
+  border-width: var(--top-header-height) 0 0 48px;
+  border-color: transparent transparent transparent var(--top-bar-bg);
+}
+
+p {
+    overflow-wrap: break-word;
+    margin: 24px 0;
+}
+
+footer {
+  grid-column-start: main-content;
+  grid-column-end: main-content;
+  grid-row-start: footer;
+  grid-row-end: footer;
+  max-width: var(--main-content-max-width);
+  font-size: 14px;
+  box-sizing: border-box;
+  padding: 16px;
+}
+
+footer p {
+  display: inline-block;
+  margin-top: 0;
+  margin-bottom: 8px;
+}
+
+.content {
+  box-sizing: border-box;
+  display: flex;
+  flex-direction: row;
+  justify-content: space-between;
+}
+
+.sidebar-entry ul {
+  list-style-type: none;
+  margin: 0;
+}
+
+.sidebar-entry a {
+  box-sizing: border-box;
+  min-height: 48px;
+  min-width: 48px;
+  padding: 12px 16px;
+  font-family: var(--font-mono);
+}
+
+.sidebar-sub-entries a {
+  display: block;
+  line-height: 24px;
+  width: 100%;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  padding-left: 36px;
+}
+
+.module-name {
+  font-size: 56px;
+  line-height: 1em;
+  font-family: var(--font-mono);
+  font-weight: bold;
+  margin-top: 18px;
+  margin-bottom: 48px;
+}
+
+.module-name a, .module-name a:visited {
+  color: inherit;
+}
+
+.sidebar-module-link {
+  box-sizing: border-box;
+  font-size: 18px;
+  line-height: 24px;
+  font-family: var(--font-mono);
+  font-weight: bold;
+  display: block;
+  width: 100%;
+  padding: 8px 0;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+
+a, a:visited {
+  color: var(--link-color);
+}
+
+h3 {
+  font-size: 32px;
+  margin: 48px 0 24px 0;
+}
+
+h4 {
+  font-size: 24px;
+}
+
+.type-def {
+  font-size: 24px;
+  color: var(--link-color);
+}
+
+.code-snippet {
+  padding: 12px 16px;
+  display: block;
+  box-sizing: border-box;
+  font-family: var(--font-mono);
+  background-color: var(--code-bg-color);
+}
+
+code {
+  font-family: var(--font-mono);
+  color: var(--code-color);
+  background-color: var(--code-bg-color);
+  padding: 2px 8px;
+  display: inline-block;
+}
+
+code a {
+  color: var(--code-link-color);
+}
+
+code a:visited {
+  color: var(--code-link-color);
+}
+
+pre {
+  margin: 36px 0;
+  padding: 8px;
+  box-sizing: border-box;
+  background-color: var(--code-bg-color);
+  overflow-x: auto;
+}
+
+pre code {
+  padding: 6px 8px;
+}
+
+.hidden {
+  /* Use !important to win all specificity fights. */
+  display: none !important;
+}
+
+#module-search:placeholder-shown {
+  padding: 0;
+  opacity: 0;
+  height: 0;
+}
+
+#module-search, #module-search:focus {
+  opacity: 1;
+  padding: 12px 16px;
+  height: 48px;
+}
+
+/* Show the "Search" label link when the text input has a placeholder */
+#module-search:placeholder-shown + #search-link {
+  display: flex;
+}
+
+/* Hide the "Search" label link when the text input has focus */
+#module-search:focus + #search-link {
+  display: none;
+}
+
+#module-search {
+  display: block;
+  box-sizing: border-box;
+  background-color: var(--code-bg-color);
+  width: 100%;
+  box-sizing: border-box;
+  font-size: 18px;
+  line-height: 18px;
+  margin-top: 6px;
+  border: none;
+  color: var(--faded-color);
+  background-color: var(--code-bg-color);
+  font-family: var(--font-serif);
+}
+
+#module-search::placeholder {
+  color: var(--faded-color);
+  opacity: 1;
+}
+
+#search-link {
+  box-sizing: border-box;
+  display: none;
+  align-items: center;
+  font-size: 18px;
+  line-height: 18px;
+  padding: 12px 16px;
+  height: 48px;
+  cursor: pointer;
+  color: var(--link-color);
+}
+
+#search-link:hover {
+  text-decoration: underline;
+}
+
+@media (prefers-color-scheme: dark) {
+  :root {
+    --body-bg-color: #303030;
+    --code-bg-color: #393939;
+    --border-color: #555555;
+    --code-color: #eeeeee;
+    --text-color: #cccccc;
+    --logo-solid: #777777;
+    --faded-color: #bbbbbb;
+    --link-color: #c5a8ff;
+    --code-link-color: #b894ff;
+    --top-bar-bg: #6845b9;
+    --top-bar-fg: #eeeeee;
+  }
+
+  html {
+    scrollbar-color: #444444 #2f2f2f;
+  }
+}
+
+@media only screen and (max-device-width: 480px) {
+  .search-button {
+    display: block; /* This is only visible in mobile. */
+  }
+
+  .top-header {
+    width: auto;
+  }
+
+  .pkg-full-name {
+    margin-left: 8px;
+    margin-right: 12px;
+    font-size: 24px;
+    padding-bottom: 14px;
+  }
+
+  .pkg-full-name a {
+    vertical-align: middle;
+    padding: 18px 0;
+  }
+
+  .logo {
+    padding-left: 2px;
+    width: 50px;
+    height: 54px;
+  }
+
+  .version {
+    margin: 0;
+    font-weight: normal;
+    font-size: 18px;
+    padding-bottom: 16px;
+  }
+
+  .module-name {
+    font-size: 36px;
+    margin-top: 8px;
+    margin-bottom: 8px;
+    max-width: calc(100% - 18px);
+    overflow: hidden;
+    text-overflow: ellipsis;
+  }
+
+  main {
+    padding: 18px;
+    font-size: 16px;
+  }
+
+  .container {
+    margin: 0;
+    min-width: 320px;
+    max-width: 100%;
+  }
+
+  .content {
+    flex-direction: column;
+  }
+
+  .sidebar {
+    margin-top: 0;
+    padding-left: 0;
+    width: auto;
+  }
+
+  #sidebar-heading {
+    font-size: 24px;
+    margin: 16px;
+  }
+
+  h3 {
+    font-size: 18px;
+    margin: 0;
+    padding: 0;
+  }
+
+  h4 {
+    font-size: 16px;
+  }
+
+  .top-header {
+    justify-content: space-between;
+  }
+
+  .content {
+    /* Display the sidebar below <main> without affecting tab index */
+    flex-direction: column-reverse;
+  }
+}