Merge branch 'main' into repl-no-color-no-header-flags

.github/workflows/test_nightly_many_os.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ macos-12, macos-13, ubuntu-20.04, ubuntu-22.04 ]
+        os: [ macos-12, macos-13, ubuntu-20.04, ubuntu-22.04, ubuntu-24.04]
     runs-on: ${{ matrix.os }}
     timeout-minutes: 90
     steps:

Cargo.lock

@@ -2771,6 +2771,7 @@ dependencies = [
  "roc_derive_key",
  "roc_error_macros",
  "roc_late_solve",
+ "roc_lower_params",
  "roc_module",
  "roc_mono",
  "roc_packaging",
@@ -2791,6 +2792,19 @@ dependencies = [
  "ven_pretty",
 ]
 
+[[package]]
+name = "roc_lower_params"
+version = "0.0.1"
+dependencies = [
+ "bumpalo",
+ "roc_can",
+ "roc_collections",
+ "roc_module",
+ "roc_region",
+ "roc_solve_problem",
+ "roc_types",
+]
+
 [[package]]
 name = "roc_module"
 version = "0.0.1"

crates/cli/src/format.rs

@@ -263,7 +263,7 @@ mod tests {
     use std::io::Write;
     use tempfile::{tempdir, TempDir};
 
-    const FORMATTED_ROC: &str = r#"app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.14.0/dC5ceT962N_4jmoyoffVdphJ_4GlW3YMhAPyGPr-nU0.tar.br" }
+    const FORMATTED_ROC: &str = r#"app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br" }
 
 import pf.Stdout
 import pf.Task
@@ -271,7 +271,7 @@ import pf.Task
 main =
     Stdout.line! "I'm a Roc application!""#;
 
-    const UNFORMATTED_ROC: &str = r#"app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.14.0/dC5ceT962N_4jmoyoffVdphJ_4GlW3YMhAPyGPr-nU0.tar.br" }
+    const UNFORMATTED_ROC: &str = r#"app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br" }
 
 
 import pf.Stdout

crates/cli/src/lib.rs

@@ -1295,8 +1295,8 @@ fn roc_dev_native(
                         break if libc::WIFEXITED(status) {
                             libc::WEXITSTATUS(status)
                         } else {
-                            // we don't have an exit code, so we probably shouldn't make one up
-                            0
+                            // we don't have an exit code, but something went wrong if we're in this else
+                            1
                         };
                     }
                     ChildProcessMsg::Expect => {

crates/cli/tests/benchmarks/AStar.roc

@@ -1,6 +1,6 @@
-interface AStar
-    exposes [findPath, Model, initialModel, cheapestOpen, reconstructPath]
-    imports [Quicksort]
+module [findPath, Model, initialModel, cheapestOpen, reconstructPath]
+
+import Quicksort
 
 findPath = \costFn, moveFn, start, end ->
     astar costFn moveFn end (initialModel start)

crates/cli/tests/benchmarks/Base64.roc

@@ -1,4 +1,4 @@
-interface Base64 exposes [fromBytes, fromStr, toBytes, toStr] imports []
+module [fromBytes, fromStr, toBytes, toStr]
 
 import Base64.Decode
 import Base64.Encode

crates/cli/tests/benchmarks/Base64/Encode.roc

@@ -1,7 +1,4 @@
-interface Base64.Encode
-    exposes [toBytes]
-    imports []
-
+module [toBytes]
 
 import Bytes.Encode exposing [ByteEncoder]
 

crates/cli/tests/benchmarks/Bytes/Decode.roc

@@ -1,4 +1,4 @@
-interface Bytes.Decode exposes [ByteDecoder, decode, map, map2, u8, loop, Step, succeed, DecodeProblem, after, map3] imports []
+module [ByteDecoder, decode, map, map2, u8, loop, Step, succeed, DecodeProblem, after, map3]
 
 State : { bytes : List U8, cursor : U64 }
 

crates/cli/tests/benchmarks/Bytes/Encode.roc

@@ -1,4 +1,4 @@
-interface Bytes.Encode exposes [ByteEncoder, sequence, u8, u16, bytes, empty, encode] imports []
+module [ByteEncoder, sequence, u8, u16, bytes, empty, encode]
 
 Endianness : [BE, LE]
 

crates/cli/tests/benchmarks/Issue2279Help.roc

@@ -1,6 +1,4 @@
-interface Issue2279Help
-    exposes [text, asText]
-    imports []
+module [text, asText]
 
 text = "Hello, world!"
 

crates/cli/tests/benchmarks/Quicksort.roc

@@ -1,4 +1,4 @@
-interface Quicksort exposes [sortBy, sortWith, show] imports []
+module [sortBy, sortWith, show]
 
 show : List I64 -> Str
 show = \list ->

crates/cli/tests/benchmarks/cFold.roc

@@ -1,11 +1,16 @@
 app [main] { pf: platform "platform/main.roc" }
 
-import pf.Task
+import pf.PlatformTasks
 
 # adapted from https://github.com/koka-lang/koka/blob/master/test/bench/haskell/cfold.hs
-main : Task.Task {} []
+main : Task {} []
 main =
-    inputResult = Task.getInt |> Task.result!
+    { value, isError } = PlatformTasks.getInt!
+    inputResult =
+        if isError then
+            Err GetIntError
+        else
+            Ok value
 
     when inputResult is
         Ok n ->
@@ -17,10 +22,10 @@ main =
             |> Num.toStr
             |> Str.concat " & "
             |> Str.concat (Num.toStr optimized)
-            |> Task.putLine
+            |> PlatformTasks.putLine
 
         Err GetIntError ->
-            Task.putLine "Error: Failed to get Integer from stdin."
+            PlatformTasks.putLine "Error: Failed to get Integer from stdin."
 
 Expr : [
     Add Expr Expr,

crates/cli/tests/benchmarks/closure.roc

@@ -1,18 +1,15 @@
-app "closure"
-    packages { pf: "platform/main.roc" }
-    imports [pf.Task]
-    provides [main] to pf
+app [main] { pf: platform "platform/main.roc" }
 
 # see https://github.com/roc-lang/roc/issues/985
-main : Task.Task {} []
+main : Task {} []
 main = closure1 {}
 # |> Task.after (\_ -> closure2 {})
 # |> Task.after (\_ -> closure3 {})
 # |> Task.after (\_ -> closure4 {})
 # ---
-closure1 : {} -> Task.Task {} []
+closure1 : {} -> Task {} []
 closure1 = \_ ->
-    Task.succeed (foo toUnitBorrowed "a long string such that it's malloced")
+    Task.ok (foo toUnitBorrowed "a long string such that it's malloced")
     |> Task.map \_ -> {}
 
 toUnitBorrowed = \x -> Str.countUtf8Bytes x

crates/cli/tests/benchmarks/deriv.roc

@@ -1,13 +1,18 @@
 app [main] { pf: platform "platform/main.roc" }
 
-import pf.Task
+import pf.PlatformTasks
 
 # based on: https://github.com/koka-lang/koka/blob/master/test/bench/haskell/deriv.hs
-IO a : Task.Task a []
+IO a : Task a []
 
-main : Task.Task {} []
+main : Task {} []
 main =
-    inputResult = Task.getInt |> Task.result!
+    { value, isError } = PlatformTasks.getInt!
+    inputResult =
+        if isError then
+            Err GetIntError
+        else
+            Ok value
 
     when inputResult is
         Ok n ->
@@ -21,15 +26,15 @@ main =
             |> Task.map \_ -> {}
 
         Err GetIntError ->
-            Task.putLine "Error: Failed to get Integer from stdin."
+            PlatformTasks.putLine "Error: Failed to get Integer from stdin."
 
 nestHelp : I64, (I64, Expr -> IO Expr), I64, Expr -> IO Expr
 nestHelp = \s, f, m, x ->
     when m is
-        0 -> Task.succeed x
+        0 -> Task.ok x
         _ ->
-            Task.after (f (s - m) x) \w ->
-                nestHelp s f (m - 1) w
+            w = f! (s - m) x
+            nestHelp s f (m - 1) w
 
 nest : (I64, Expr -> IO Expr), I64, Expr -> IO Expr
 nest = \f, n, e -> nestHelp n f n e
@@ -162,6 +167,5 @@ deriv = \i, f ->
         Num.toStr (i + 1)
         |> Str.concat " count: "
         |> Str.concat (Num.toStr (count fprime))
-
-    Task.putLine line
-    |> Task.after \_ -> Task.succeed fprime
+    PlatformTasks.putLine! line
+    Task.ok fprime

crates/cli/tests/benchmarks/issue2279.roc

@@ -1,7 +1,7 @@
-app "issue2279"
-    packages { pf: "platform/main.roc" }
-    imports [Issue2279Help, pf.Task]
-    provides [main] to pf
+app [main] { pf: platform "platform/main.roc" }
+
+import Issue2279Help
+import pf.PlatformTasks
 
 main =
     text =
@@ -10,4 +10,4 @@ main =
         else
             Issue2279Help.asText 42
 
-    Task.putLine text
+    PlatformTasks.putLine text

crates/cli/tests/benchmarks/nQueens.roc

@@ -1,19 +1,24 @@
 app [main] { pf: platform "platform/main.roc" }
 
-import pf.Task
+import pf.PlatformTasks
 
-main : Task.Task {} []
+main : Task {} []
 main =
-    inputResult = Task.getInt |> Task.result!
+    { value, isError } = PlatformTasks.getInt!
+    inputResult =
+        if isError then
+            Err GetIntError
+        else
+            Ok value
 
     when inputResult is
         Ok n ->
             queens n # original koka 13
             |> Num.toStr
-            |> Task.putLine
+            |> PlatformTasks.putLine
 
         Err GetIntError ->
-            Task.putLine "Error: Failed to get Integer from stdin."
+            PlatformTasks.putLine "Error: Failed to get Integer from stdin."
 
 ConsList a : [Nil, Cons a (ConsList a)]
 

crates/cli/tests/benchmarks/platform/Effect.roc

@@ -1,10 +0,0 @@
-hosted Effect
-    exposes [Effect, after, map, always, forever, loop, putLine, putInt, getInt]
-    imports []
-    generates Effect with [after, map, always, forever, loop]
-
-putLine : Str -> Effect {}
-
-putInt : I64 -> Effect {}
-
-getInt : Effect { value : I64, isError : Bool }

crates/cli/tests/benchmarks/platform/PlatformTasks.roc

@@ -0,0 +1,9 @@
+hosted PlatformTasks
+    exposes [putLine, putInt, getInt]
+    imports []
+
+putLine : Str -> Task {} *
+
+putInt : I64 -> Task {} *
+
+getInt : Task { value : I64, isError : Bool } *

crates/cli/tests/benchmarks/platform/Task.roc

@@ -1,117 +0,0 @@
-module [
-    Task,
-    await,
-    succeed,
-    fail,
-    after,
-    map,
-    result,
-    putLine,
-    putInt,
-    getInt,
-    forever,
-    loop,
-    attempt,
-]
-
-import pf.Effect
-
-Task ok err : Effect.Effect (Result ok err)
-
-forever : Task val err -> Task * err
-forever = \task ->
-    looper = \{} ->
-        task
-        |> Effect.map
-            \res ->
-                when res is
-                    Ok _ -> Step {}
-                    Err e -> Done (Err e)
-
-    Effect.loop {} looper
-
-loop : state, (state -> Task [Step state, Done done] err) -> Task done err
-loop = \state, step ->
-    looper = \current ->
-        step current
-        |> Effect.map
-            \res ->
-                when res is
-                    Ok (Step newState) -> Step newState
-                    Ok (Done res2) -> Done (Ok res2)
-                    Err e -> Done (Err e)
-
-    Effect.loop state looper
-
-succeed : val -> Task val *
-succeed = \val ->
-    Effect.always (Ok val)
-
-fail : err -> Task * err
-fail = \val ->
-    Effect.always (Err val)
-
-after : Task a err, (a -> Task b err) -> Task b err
-after = \effect, transform ->
-    Effect.after
-        effect
-        \res ->
-            when res is
-                Ok a -> transform a
-                Err err -> Task.fail err
-
-await : Task a err, (a -> Task b err) -> Task b err
-await = \effect, transform ->
-    Effect.after
-        effect
-        \res ->
-            when res is
-                Ok a -> transform a
-                Err err -> Task.fail err
-
-attempt : Task a b, (Result a b -> Task c d) -> Task c d
-attempt = \task, transform ->
-    Effect.after
-        task
-        \res ->
-            when res is
-                Ok ok -> transform (Ok ok)
-                Err err -> transform (Err err)
-
-map : Task a err, (a -> b) -> Task b err
-map = \effect, transform ->
-    Effect.map
-        effect
-        \res ->
-            when res is
-                Ok a -> Ok (transform a)
-                Err err -> Err err
-
-result : Task ok err -> Task (Result ok err) *
-result = \effect ->
-    Effect.after
-        effect
-        \res -> Task.succeed res
-
-putLine : Str -> Task {} *
-putLine = \line -> Effect.map (Effect.putLine line) (\_ -> Ok {})
-
-putInt : I64 -> Task {} *
-putInt = \line -> Effect.map (Effect.putInt line) (\_ -> Ok {})
-
-getInt : Task I64 [GetIntError]
-getInt =
-    Effect.after
-        Effect.getInt
-        \{ isError, value } ->
-            if
-                isError
-            then
-                # TODO
-                # when errorCode is
-                #    # A -> Task.fail InvalidCharacter
-                #    # B -> Task.fail IOError
-                #    _ ->
-                Task.fail GetIntError
-            else
-                Task.succeed value

crates/cli/tests/benchmarks/platform/main.roc

@@ -2,7 +2,7 @@ platform "benchmarks"
     requires {} { main : Task {} [] }
     exposes []
     packages {}
-    imports [Task.{ Task }]
+    imports []
     provides [mainForHost]
 
 mainForHost : Task {} []

crates/cli/tests/benchmarks/quicksortApp.roc

@@ -1,11 +1,16 @@
 app [main] { pf: platform "platform/main.roc" }
 
-import pf.Task
+import pf.PlatformTasks
 import Quicksort
 
 main : Task.Task {} []
 main =
-    inputResult = Task.getInt |> Task.result!
+    { value, isError } = PlatformTasks.getInt!
+    inputResult =
+        if isError then
+            Err GetIntError
+        else
+            Ok value
 
     when inputResult is
         Ok n ->
@@ -19,10 +24,10 @@ main =
 
             sort unsortedList
             |> Quicksort.show
-            |> Task.putLine
+            |> PlatformTasks.putLine
 
         Err GetIntError ->
-            Task.putLine "Error: Failed to get Integer from stdin."
+            PlatformTasks.putLine "Error: Failed to get Integer from stdin."
 
 sort : List I64 -> List I64
 sort = \list ->

crates/cli/tests/benchmarks/rBTreeCk.roc

@@ -1,6 +1,6 @@
 app [main] { pf: platform "platform/main.roc" }
 
-import pf.Task
+import pf.PlatformTasks
 
 Color : [Red, Black]
 
@@ -37,9 +37,14 @@ fold = \f, tree, b ->
         Leaf -> b
         Node _ l k v r -> fold f r (f k v (fold f l b))
 
-main : Task.Task {} []
+main : Task {} []
 main =
-    inputResult = Task.getInt |> Task.result!
+    { value, isError } = PlatformTasks.getInt!
+    inputResult =
+        if isError then
+            Err GetIntError
+        else
+            Ok value
 
     when inputResult is
         Ok n ->
@@ -53,13 +58,13 @@ main =
 
                     val
                     |> Num.toStr
-                    |> Task.putLine
+                    |> PlatformTasks.putLine
 
                 Nil ->
-                    Task.putLine "fail"
+                    PlatformTasks.putLine "fail"
 
         Err GetIntError ->
-            Task.putLine "Error: Failed to get Integer from stdin."
+            PlatformTasks.putLine "Error: Failed to get Integer from stdin."
 
 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

crates/cli/tests/benchmarks/rBTreeDel.roc

@@ -1,6 +1,6 @@
 app [main] { pf: platform "platform/main.roc" }
 
-import pf.Task
+import pf.PlatformTasks
 
 Color : [Red, Black]
 
@@ -10,9 +10,14 @@ Map : Tree I64 Bool
 
 ConsList a : [Nil, Cons a (ConsList a)]
 
-main : Task.Task {} []
+main : Task {} []
 main =
-    inputResult = Task.getInt |> Task.result!
+    { value, isError } = PlatformTasks.getInt!
+    inputResult =
+        if isError then
+            Err GetIntError
+        else
+            Ok value
 
     when inputResult is
         Ok n ->
@@ -21,10 +26,10 @@ main =
 
             val
             |> Num.toStr
-            |> Task.putLine
+            |> PlatformTasks.putLine
 
         Err GetIntError ->
-            Task.putLine "Error: Failed to get Integer from stdin."
+            PlatformTasks.putLine "Error: Failed to get Integer from stdin."
 
 boom : Str -> a
 boom = \_ -> boom ""

crates/cli/tests/benchmarks/rBTreeInsert.roc

@@ -1,16 +1,15 @@
-app "rbtree-insert"
-    packages { pf: "platform/main.roc" }
-    imports [pf.Task]
-    provides [main] to pf
+app [main] { pf: platform "platform/main.roc" }
 
-main : Task.Task {} []
+import pf.PlatformTasks
+
+main : Task {} []
 main =
     tree : RedBlackTree I64 {}
     tree = insert 0 {} Empty
 
     tree
     |> show
-    |> Task.putLine
+    |> PlatformTasks.putLine
 
 show : RedBlackTree I64 {} -> Str
 show = \tree -> showRBTree tree Num.toStr (\{} -> "{}")

crates/cli/tests/benchmarks/testAStar.roc

@@ -1,20 +1,12 @@
-app "test-astar"
-    packages { pf: "platform/main.roc" }
-    imports [pf.Task, AStar]
-    provides [main] to pf
+app [main] { pf: platform "platform/main.roc" }
 
-main : Task.Task {} []
+import pf.PlatformTasks
+import AStar
+
+#main : Task {} *
 main =
-    Task.putLine (showBool test1)
+    PlatformTasks.putLine! (showBool test1)
 
-#     Task.after Task.getInt \n ->
-#         when n is
-#             1 ->
-#                 Task.putLine (showBool test1)
-#
-#             _ ->
-#                 ns = Num.toStr n
-#                 Task.putLine "No test $(ns)"
 showBool : Bool -> Str
 showBool = \b ->
     if

crates/cli/tests/benchmarks/testBase64.roc

@@ -1,18 +1,17 @@
-app "test-base64"
-    packages { pf: "platform/main.roc" }
-    imports [pf.Task, Base64]
-    provides [main] to pf
+app [main] { pf: platform "platform/main.roc" }
 
-IO a : Task.Task a []
+import Base64
+import pf.PlatformTasks
+
+IO a : Task a []
 
 main : IO {}
 main =
     when Base64.fromBytes (Str.toUtf8 "Hello World") is
-        Err _ -> Task.putLine "sadness"
+        Err _ -> PlatformTasks.putLine "sadness"
         Ok encoded ->
-            Task.after
-                (Task.putLine (Str.concat "encoded: " encoded))
-                \_ ->
-                    when Base64.toStr encoded is
-                        Ok decoded -> Task.putLine (Str.concat "decoded: " decoded)
-                        Err _ -> Task.putLine "sadness"
+            PlatformTasks.putLine! (Str.concat "encoded: " encoded)
+
+            when Base64.toStr encoded is
+                Ok decoded -> PlatformTasks.putLine (Str.concat "decoded: " decoded)
+                Err _ -> PlatformTasks.putLine "sadness"

crates/cli/tests/cli/combine-tasks.roc

@@ -1,7 +1,6 @@
-app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.14.0/dC5ceT962N_4jmoyoffVdphJ_4GlW3YMhAPyGPr-nU0.tar.br" }
+app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br" }
 
 import pf.Stdout
-import pf.Task exposing [Task]
 
 main =
     multipleIn =

crates/cli/tests/cli/countdown.roc

@@ -1,13 +1,12 @@
-app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.14.0/dC5ceT962N_4jmoyoffVdphJ_4GlW3YMhAPyGPr-nU0.tar.br" }
+app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br" }
 
 import pf.Stdin
 import pf.Stdout
-import pf.Task exposing [Task, loop]
 
 main =
     Stdout.line! "\nLet's count down from 3 together - all you have to do is press <ENTER>."
     _ = Stdin.line!
-    loop 3 tick
+    Task.loop 3 tick
 
 tick = \n ->
     if n == 0 then

crates/cli/tests/cli/echo.roc

@@ -1,8 +1,7 @@
-app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.14.0/dC5ceT962N_4jmoyoffVdphJ_4GlW3YMhAPyGPr-nU0.tar.br" }
+app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br" }
 
 import pf.Stdin
 import pf.Stdout
-import pf.Task exposing [Task]
 
 main =
     Stdout.line! "🗣  Shout into this cave and hear the echo! 👂👂👂"

crates/cli/tests/cli/env.roc

@@ -1,9 +1,8 @@
-app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.14.0/dC5ceT962N_4jmoyoffVdphJ_4GlW3YMhAPyGPr-nU0.tar.br" }
+app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br" }
 
 import pf.Stdout
 import pf.Stderr
 import pf.Env
-import pf.Task exposing [Task]
 
 main =
     task =

crates/cli/tests/cli/fileBROKEN.roc

@@ -1,7 +1,6 @@
-app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.14.0/dC5ceT962N_4jmoyoffVdphJ_4GlW3YMhAPyGPr-nU0.tar.br" }
+app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br" }
 
 import pf.Stdout
-import pf.Task exposing [Task]
 import pf.File
 import pf.Path
 import pf.Env

crates/cli/tests/cli/form.roc

@@ -1,8 +1,7 @@
-app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.14.0/dC5ceT962N_4jmoyoffVdphJ_4GlW3YMhAPyGPr-nU0.tar.br" }
+app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br" }
 
 import pf.Stdin
 import pf.Stdout
-import pf.Task exposing [await, Task]
 
 main =
     Stdout.line! "What's your first name?"

crates/cli/tests/cli/http-get.roc

@@ -1,7 +1,6 @@
-app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.14.0/dC5ceT962N_4jmoyoffVdphJ_4GlW3YMhAPyGPr-nU0.tar.br" }
+app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br" }
 
 import pf.Http
-import pf.Task exposing [Task]
 import pf.Stdout
 
 main =

crates/cli/tests/cli/ingested-file-bytes-no-ann.roc

@@ -1,4 +1,4 @@
-app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.14.0/dC5ceT962N_4jmoyoffVdphJ_4GlW3YMhAPyGPr-nU0.tar.br" }
+app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br" }
 
 import pf.Stdout
 import "test-file.txt" as testFile

crates/cli/tests/cli/ingested-file-bytes.roc

@@ -1,4 +1,4 @@
-app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.14.0/dC5ceT962N_4jmoyoffVdphJ_4GlW3YMhAPyGPr-nU0.tar.br" }
+app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br" }
 
 import pf.Stdout
 import "test-file.txt" as testFile : _ # the _ is optional

crates/cli/tests/cli/ingested-file.roc

@@ -1,4 +1,4 @@
-app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.14.0/dC5ceT962N_4jmoyoffVdphJ_4GlW3YMhAPyGPr-nU0.tar.br" }
+app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br" }
 
 import pf.Stdout
 import "ingested-file.roc" as ownCode : Str

crates/cli/tests/cli/parse-args.roc

@@ -1,9 +1,8 @@
 app [main] {
-    pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.14.0/dC5ceT962N_4jmoyoffVdphJ_4GlW3YMhAPyGPr-nU0.tar.br",
+    pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br",
 }
 
 import pf.Stdout
-import pf.Task exposing [Task]
 
 main =
     file = strParam { name: "file" }

crates/cli/tests/cli/parser-letter-counts.roc

@@ -1,5 +1,5 @@
 app [main] {
-    cli: platform "https://github.com/roc-lang/basic-cli/releases/download/0.14.0/dC5ceT962N_4jmoyoffVdphJ_4GlW3YMhAPyGPr-nU0.tar.br",
+    cli: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br",
     parser: "https://github.com/lukewilliamboswell/roc-parser/releases/download/0.5.2/9VrPjwfQQ1QeSL3CfmWr2Pr9DESdDIXy97pwpuq84Ck.tar.br",
 }
 

crates/cli/tests/cli/parser-movies-csv.roc

@@ -1,11 +1,10 @@
 app [main] {
-    pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.14.0/dC5ceT962N_4jmoyoffVdphJ_4GlW3YMhAPyGPr-nU0.tar.br",
+    pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br",
     parser: "https://github.com/lukewilliamboswell/roc-parser/releases/download/0.5.2/9VrPjwfQQ1QeSL3CfmWr2Pr9DESdDIXy97pwpuq84Ck.tar.br",
 }
 
 import pf.Stdout
 import pf.Stderr
-import pf.Task exposing [Task]
 import parser.Core exposing [map, keep]
 import parser.String exposing [strFromUtf8]
 import parser.CSV

crates/cli/tests/cli_run.rs

@@ -10,8 +10,8 @@ extern crate roc_module;
 #[cfg(test)]
 mod cli_run {
     use cli_utils::helpers::{
-        extract_valgrind_errors, file_path_from_root, fixture_file, fixtures_dir, has_error,
-        known_bad_file, run_cmd, run_roc, run_with_valgrind, Out, ValgrindError,
+        cli_testing_dir, extract_valgrind_errors, file_path_from_root, fixture_file, fixtures_dir,
+        has_error, known_bad_file, run_cmd, run_roc, run_with_valgrind, Out, ValgrindError,
         ValgrindErrorXWhat,
     };
     use const_format::concatcp;
@@ -86,11 +86,11 @@ mod cli_run {
     }
 
     fn check_compile_error(file: &Path, flags: &[&str], expected: &str) {
-        let compile_out = run_roc(
-            [CMD_CHECK, file.to_str().unwrap()].iter().chain(flags),
-            &[],
-            &[],
-        );
+        check_compile_error_with(CMD_CHECK, file, flags, expected);
+    }
+
+    fn check_compile_error_with(cmd: &str, file: &Path, flags: &[&str], expected: &str) {
+        let compile_out = run_roc([cmd, file.to_str().unwrap()].iter().chain(flags), &[], &[]);
         let err = compile_out.stdout.trim();
         let err = strip_colors(err);
 
@@ -728,6 +728,200 @@ mod cli_run {
         )
     }
 
+    #[test]
+    #[cfg_attr(windows, ignore)]
+    fn module_params() {
+        test_roc_app(
+            "crates/cli/tests/module_params",
+            "app.roc",
+            &[],
+            &[],
+            &[],
+            indoc!(
+                r#"
+                App1.baseUrl: https://api.example.com/one
+                App2.baseUrl: http://api.example.com/two
+                App3.baseUrl: https://api.example.com/three
+                App1.getUser 1: https://api.example.com/one/users/1
+                App2.getUser 2: http://api.example.com/two/users/2
+                App3.getUser 3: https://api.example.com/three/users/3
+                App1.getPost 1: https://api.example.com/one/posts/1
+                App2.getPost 2: http://api.example.com/two/posts/2
+                App3.getPost 3: https://api.example.com/three/posts/3
+                App1.getPosts [1, 2]: ["https://api.example.com/one/posts/1", "https://api.example.com/one/posts/2"]
+                App2.getPosts [3, 4]: ["http://api.example.com/two/posts/3", "http://api.example.com/two/posts/4"]
+                App2.getPosts [5, 6]: ["http://api.example.com/two/posts/5", "http://api.example.com/two/posts/6"]
+                App1.getPostComments 1: https://api.example.com/one/posts/1/comments
+                App2.getPostComments 2: http://api.example.com/two/posts/2/comments
+                App2.getPostComments 3: http://api.example.com/two/posts/3/comments
+                App1.getCompanies [1, 2]: ["https://api.example.com/one/companies/1", "https://api.example.com/one/companies/2"]
+                App2.getCompanies [3, 4]: ["http://api.example.com/two/companies/3", "http://api.example.com/two/companies/4"]
+                App2.getCompanies [5, 6]: ["http://api.example.com/two/companies/5", "http://api.example.com/two/companies/6"]
+                App1.getPostAliased 1: https://api.example.com/one/posts/1
+                App2.getPostAliased 2: http://api.example.com/two/posts/2
+                App3.getPostAliased 3: https://api.example.com/three/posts/3
+                App1.baseUrlAliased: https://api.example.com/one
+                App2.baseUrlAliased: http://api.example.com/two
+                App3.baseUrlAliased: https://api.example.com/three
+                App1.getUserSafe 1: https://api.example.com/one/users/1
+                Prod.getUserSafe 2: http://api.example.com/prod_1/users/2?safe=true
+                usersApp1: ["https://api.example.com/one/users/1", "https://api.example.com/one/users/2", "https://api.example.com/one/users/3"]
+                getUserApp3Nested 3: https://api.example.com/three/users/3
+                usersApp3Passed: ["https://api.example.com/three/users/1", "https://api.example.com/three/users/2", "https://api.example.com/three/users/3"]
+                "#
+            ),
+            UseValgrind::No,
+            TestCliCommands::Run,
+        );
+    }
+
+    #[test]
+    #[cfg_attr(windows, ignore)]
+    fn module_params_arity_mismatch() {
+        check_compile_error_with(
+            CMD_DEV,
+            &cli_testing_dir("/module_params/arity_mismatch.roc"),
+            &[],
+            indoc!(
+                r#"
+                ── TOO MANY ARGS in tests/module_params/arity_mismatch.roc ─────────────────────
+
+                The getUser function expects 1 argument, but it got 2 instead:
+
+                12│      $(Api.getUser 1 2)
+                           ^^^^^^^^^^^
+
+                Are there any missing commas? Or missing parentheses?
+
+
+                ── TOO MANY ARGS in tests/module_params/arity_mismatch.roc ─────────────────────
+
+                This value is not a function, but it was given 1 argument:
+
+                13│      $(Api.baseUrl 1)
+                           ^^^^^^^^^^^
+
+                Are there any missing commas? Or missing parentheses?
+
+
+                ── TOO FEW ARGS in tests/module_params/arity_mismatch.roc ──────────────────────
+
+                The getPostComment function expects 2 arguments, but it got only 1:
+
+                16│      $(Api.getPostComment 1)
+                           ^^^^^^^^^^^^^^^^^^
+
+                Roc does not allow functions to be partially applied. Use a closure to
+                make partial application explicit.
+
+                ────────────────────────────────────────────────────────────────────────────────
+
+                3 errors and 0 warnings found in <ignored for test> ms."#
+            ),
+        );
+    }
+
+    #[test]
+    #[cfg_attr(windows, ignore)]
+    fn module_params_unexpected_fn() {
+        check_compile_error_with(
+            CMD_DEV,
+            &cli_testing_dir("/module_params/unexpected_fn.roc"),
+            &[],
+            indoc!(
+                r#"
+                ── TYPE MISMATCH in tests/module_params/unexpected_fn.roc ──────────────────────
+
+                This argument to this string interpolation has an unexpected type:
+
+                11│      $(Api.getPost)
+                           ^^^^^^^^^^^
+
+                The argument is an anonymous function of type:
+
+                    U32 -> Str
+
+                But this string interpolation needs its argument to be:
+
+                    Str
+
+                ────────────────────────────────────────────────────────────────────────────────
+
+                1 error and 0 warnings found in <ignored for test> ms."#
+            ),
+        )
+    }
+
+    #[test]
+    #[cfg_attr(windows, ignore)]
+    fn module_params_bad_ann() {
+        check_compile_error_with(
+            CMD_DEV,
+            &cli_testing_dir("/module_params/bad_ann.roc"),
+            &[],
+            indoc!(
+                r#"
+                ── TYPE MISMATCH in tests/module_params/BadAnn.roc ─────────────────────────────
+
+                Something is off with the body of the fnAnnotatedAsValue definition:
+
+                3│   fnAnnotatedAsValue : Str
+                4│>  fnAnnotatedAsValue = /postId, commentId ->
+                5│>      "/posts/$(postId)/comments/$(Num.toStr commentId)"
+
+                The body is an anonymous function of type:
+
+                    Str, Num * -> Str
+
+                But the type annotation on fnAnnotatedAsValue says it should be:
+
+                    Str
+
+
+                ── TYPE MISMATCH in tests/module_params/BadAnn.roc ─────────────────────────────
+
+                Something is off with the body of the missingArg definition:
+
+                7│   missingArg : Str -> Str
+                8│>  missingArg = /postId, _ ->
+                9│>      "/posts/$(postId)/comments"
+
+                The body is an anonymous function of type:
+
+                    (Str, ? -> Str)
+
+                But the type annotation on missingArg says it should be:
+
+                    (Str -> Str)
+
+                Tip: It looks like it takes too many arguments. I'm seeing 1 extra.
+
+                ────────────────────────────────────────────────────────────────────────────────
+
+                2 errors and 1 warning found in <ignored for test> ms."#
+            ),
+        );
+    }
+
+    #[test]
+    #[cfg_attr(windows, ignore)]
+    fn module_params_pass_task() {
+        test_roc_app(
+            "crates/cli/tests/module_params",
+            "pass_task.roc",
+            &[],
+            &[],
+            &[],
+            indoc!(
+                r#"
+                Hi, Agus!
+                "#
+            ),
+            UseValgrind::No,
+            TestCliCommands::Run,
+        );
+    }
+
     #[test]
     #[cfg_attr(windows, ignore)]
     fn transitive_expects() {
@@ -898,8 +1092,8 @@ mod cli_run {
         )
     }
 
+    #[ignore = "likely broken because of alias analysis: https://github.com/roc-lang/roc/issues/6544"]
     #[test]
-    #[cfg_attr(target_os = "windows", ignore = "Segfault")]
     fn false_interpreter() {
         test_roc_app(
             "examples/cli/false-interpreter",
@@ -1489,9 +1683,9 @@ mod cli_run {
 
                 Something is off with the body of the main definition:
 
-                5│  main : Str -> Task {} []
-                6│  main = /_ ->
-                7│      "this is a string, not a Task {} [] function like the platform expects."
+                3│  main : Str -> Task {} []
+                4│  main = /_ ->
+                5│      "this is a string, not a Task {} [] function like the platform expects."
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
                 The body is a string of type:
@@ -1500,7 +1694,7 @@ mod cli_run {
 
                 But the type annotation on main says it should be:
 
-                    Effect.Effect (Result {} [])
+                    Task {} []
 
                 Tip: Add type annotations to functions or values to help you figure
                 this out.
@@ -1579,30 +1773,6 @@ mod cli_run {
         );
     }
 
-    #[test]
-    fn unknown_generates_with() {
-        check_compile_error(
-            &known_bad_file("UnknownGeneratesWith.roc"),
-            &[],
-            indoc!(
-                r#"
-                ── UNKNOWN GENERATES FUNCTION in tests/known_bad/UnknownGeneratesWith.roc ──────
-
-                I don't know how to generate the foobar function.
-
-                4│      generates Effect with [after, map, always, foobar]
-                                                                   ^^^^^^
-
-                Only specific functions like `after` and `map` can be generated.Learn
-                more about hosted modules at TODO.
-
-                ────────────────────────────────────────────────────────────────────────────────
-
-                1 error and 0 warnings found in <ignored for test> ms."#
-            ),
-        );
-    }
-
     #[test]
     fn format_check_good() {
         check_format_check_as_expected(&fixture_file("format", "Formatted.roc"), true);

crates/cli/tests/known_bad/TypeError.roc

@@ -1,7 +1,5 @@
 app [main] { pf: platform "../../../../examples/cli/false-interpreter/platform/main.roc" }
 
-import pf.Task exposing [Task]
-
 main : Str -> Task {} []
 main = \_ ->
     "this is a string, not a Task {} [] function like the platform expects."

crates/cli/tests/known_bad/UnknownGeneratesWith.roc

@@ -1,4 +0,0 @@
-hosted UnknownGeneratesWith
-    exposes [Effect, after, map, always]
-    imports []
-    generates Effect with [after, map, always, foobar]

crates/cli/tests/module_params/Api.roc

@@ -0,0 +1,69 @@
+module { appId, protocol } -> [
+    baseUrl,
+    getUser,
+    getPost,
+    getPosts,
+    getPostComments,
+    getCompanies,
+    baseUrlAliased,
+    getPostAliased,
+    getUserSafe,
+    getPostComment,
+]
+
+## value def referencing params
+baseUrl : Str
+baseUrl =
+    protocol "api.example.com/$(appId)"
+
+## function def referencing params
+getUser : U32 -> Str
+getUser = \userId ->
+    # purposefully not using baseUrl to test top-level fn referencing param
+    protocol "api.example.com/$(appId)/users/$(Num.toStr userId)"
+
+## function def referencing top-level value
+getPost : U32 -> Str
+getPost = \postId ->
+    "$(baseUrl)/posts/$(Num.toStr postId)"
+
+## function def passing top-level function
+getPosts : List U32 -> List Str
+getPosts = \ids ->
+    List.map ids getPost
+
+## function def calling top-level function
+getPostComments : U32 -> Str
+getPostComments = \postId ->
+    "$(getPost postId)/comments"
+
+## function def passing nested function
+getCompanies : List U32 -> List Str
+getCompanies = \ids ->
+    getCompany = \id ->
+        protocol "api.example.com/$(appId)/companies/$(Num.toStr id)"
+
+    List.map ids getCompany
+
+## aliasing top-level value
+baseUrlAliased : Str
+baseUrlAliased =
+    baseUrl
+
+## aliasing top-level fn
+getPostAliased : U32 -> Str
+getPostAliased =
+    getPost
+
+## top-level value returning functions
+getUserSafe : U32 -> Str
+getUserSafe =
+    if Str.startsWith appId "prod_" then
+        \id -> "$(getUser id)?safe=true"
+    else
+        getUser
+
+## two-argument function
+getPostComment : U32, U32 -> Str
+getPostComment = \postId, commentId ->
+    "$(getPost postId)/comments/$(Num.toStr commentId)"

crates/cli/tests/module_params/BadAnn.roc

@@ -0,0 +1,9 @@
+module { appId } -> [fnAnnotatedAsValue, missingArg]
+
+fnAnnotatedAsValue : Str
+fnAnnotatedAsValue = \postId, commentId ->
+    "/posts/$(postId)/comments/$(Num.toStr commentId)"
+
+missingArg : Str -> Str
+missingArg = \postId, _ ->
+    "/posts/$(postId)/comments"

crates/cli/tests/module_params/Menu.roc

@@ -0,0 +1,7 @@
+module { echo } -> [menu]
+
+menu = \name ->
+    indirect name
+
+indirect = \name ->
+    echo "Hi, $(name)!"

crates/cli/tests/module_params/app.roc

@@ -0,0 +1,59 @@
+app [main] {
+    pf: platform "../fixtures/multi-dep-str/platform/main.roc",
+}
+
+import Api { appId: "one", protocol: https } as App1
+import Api { appId: "two", protocol: http } as App2
+import Api { appId: "prod_1", protocol: http } as Prod
+
+https = \url -> "https://$(url)"
+http = \url -> "http://$(url)"
+
+usersApp1 =
+    # pass top-level fn in a module with params
+    List.map [1, 2, 3] App1.getUser
+
+main =
+    app3Id = "three"
+
+    import Api { appId: app3Id, protocol: https } as App3
+
+    getUserApp3Nested = \userId ->
+        # use captured params def
+        App3.getUser userId
+
+    usersApp3Passed =
+        # pass top-level fn in a nested def
+        List.map [1, 2, 3] App3.getUser
+
+    """
+    App1.baseUrl: $(App1.baseUrl)
+    App2.baseUrl: $(App2.baseUrl)
+    App3.baseUrl: $(App3.baseUrl)
+    App1.getUser 1: $(App1.getUser 1)
+    App2.getUser 2: $(App2.getUser 2)
+    App3.getUser 3: $(App3.getUser 3)
+    App1.getPost 1: $(App1.getPost 1)
+    App2.getPost 2: $(App2.getPost 2)
+    App3.getPost 3: $(App3.getPost 3)
+    App1.getPosts [1, 2]: $(Inspect.toStr (App1.getPosts [1, 2]))
+    App2.getPosts [3, 4]: $(Inspect.toStr (App2.getPosts [3, 4]))
+    App2.getPosts [5, 6]: $(Inspect.toStr (App2.getPosts [5, 6]))
+    App1.getPostComments 1: $(App1.getPostComments 1)
+    App2.getPostComments 2: $(App2.getPostComments 2)
+    App2.getPostComments 3: $(App2.getPostComments 3)
+    App1.getCompanies [1, 2]: $(Inspect.toStr (App1.getCompanies [1, 2]))
+    App2.getCompanies [3, 4]: $(Inspect.toStr (App2.getCompanies [3, 4]))
+    App2.getCompanies [5, 6]: $(Inspect.toStr (App2.getCompanies [5, 6]))
+    App1.getPostAliased 1: $(App1.getPostAliased 1)
+    App2.getPostAliased 2: $(App2.getPostAliased 2)
+    App3.getPostAliased 3: $(App3.getPostAliased 3)
+    App1.baseUrlAliased: $(App1.baseUrlAliased)
+    App2.baseUrlAliased: $(App2.baseUrlAliased)
+    App3.baseUrlAliased: $(App3.baseUrlAliased)
+    App1.getUserSafe 1: $(App1.getUserSafe 1)
+    Prod.getUserSafe 2: $(Prod.getUserSafe 2)
+    usersApp1: $(Inspect.toStr usersApp1)
+    getUserApp3Nested 3: $(getUserApp3Nested 3)
+    usersApp3Passed: $(Inspect.toStr usersApp3Passed)
+    """

crates/cli/tests/module_params/arity_mismatch.roc

@@ -0,0 +1,17 @@
+app [main] {
+    pf: platform "../fixtures/multi-dep-str/platform/main.roc",
+}
+
+import Api { appId: "one", protocol: https }
+
+https = \url -> "https://$(url)"
+
+main =
+    """
+    # too many args
+    $(Api.getUser 1 2)
+    $(Api.baseUrl 1)
+
+    # too few args
+    $(Api.getPostComment 1)
+    """

crates/cli/tests/module_params/bad_ann.roc

@@ -0,0 +1,8 @@
+app [main] {
+    pf: platform "../fixtures/multi-dep-str/platform/main.roc",
+}
+
+import BadAnn { appId: "one" }
+
+main =
+    ""

crates/cli/tests/module_params/pass_task.roc

@@ -0,0 +1,7 @@
+app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br" }
+
+import pf.Stdout
+import Menu { echo: Stdout.line }
+
+main =
+    Menu.menu "Agus"

crates/cli/tests/module_params/unexpected_fn.roc

@@ -0,0 +1,12 @@
+app [main] {
+    pf: platform "../fixtures/multi-dep-str/platform/main.roc",
+}
+
+import Api { appId: "one", protocol: https }
+
+https = \url -> "https://$(url)"
+
+main =
+    """
+    $(Api.getPost)
+    """

crates/compiler/builtins/roc/Task.roc

@@ -0,0 +1,265 @@
+module [
+    Task,
+    ok,
+    err,
+    await,
+    map,
+    mapErr,
+    onErr,
+    attempt,
+    forever,
+    loop,
+    fromResult,
+    batch,
+    sequence,
+    forEach,
+    result,
+]
+
+import List
+import Result exposing [Result]
+
+## A Task represents an effect; an interaction with state outside your Roc
+## program, such as the terminal's standard output, or a file.
+Task ok err := {} -> Result ok err
+
+## Run a task repeatedly, until it fails with `err`. Note that this task does not return a success value.
+forever : Task a err -> Task * err
+forever = \@Task task ->
+    looper = \{} ->
+        when task {} is
+            Err e -> Err e
+            Ok _ -> looper {}
+
+    @Task \{} -> looper {}
+
+## Run a task repeatedly, until it fails with `err` or completes with `done`.
+##
+## ```
+## sum =
+##     Task.loop! 0 \total ->
+##         numResult =
+##             Stdin.line
+##                 |> Task.result!
+##                 |> Result.try Str.toU64
+##
+##         when numResult is
+##             Ok num -> Task.ok (Step (total + num))
+##             Err (StdinErr EndOfFile) -> Task.ok (Done total)
+##             Err InvalidNumStr -> Task.err NonNumberGiven
+## ```
+loop : state, (state -> Task [Step state, Done done] err) -> Task done err
+loop = \state, step ->
+    looper = \current ->
+        (@Task next) = step current
+        when next {} is
+            Err e -> Err e
+            Ok (Done newResult) -> Ok newResult
+            Ok (Step newState) -> looper (newState)
+
+    @Task \{} -> looper state
+
+## Create a task that always succeeds with the value provided.
+##
+## ```
+## # Always succeeds with "Louis"
+## getName : Task.Task Str *
+## getName = Task.ok "Louis"
+## ```
+##
+ok : a -> Task a *
+ok = \a -> @Task \{} -> Ok a
+
+## Create a task that always fails with the error provided.
+##
+## ```
+## # Always fails with the tag `CustomError Str`
+## customError : Str -> Task.Task {} [CustomError Str]
+## customError = \err -> Task.err (CustomError err)
+## ```
+##
+err : a -> Task * a
+err = \a -> @Task \{} -> Err a
+
+## Transform a given Task with a function that handles the success or error case
+## and returns another task based on that. This is useful for chaining tasks
+## together or performing error handling and recovery.
+##
+## Consider the following task:
+##
+## `canFail : Task {} [Failure, AnotherFail, YetAnotherFail]`
+##
+## We can use [attempt] to handle the failure cases using the following:
+##
+## ```
+## Task.attempt canFail \result ->
+##     when result is
+##         Ok Success -> Stdout.line "Success!"
+##         Err Failure -> Stdout.line "Oops, failed!"
+##         Err AnotherFail -> Stdout.line "Ooooops, another failure!"
+##         Err YetAnotherFail -> Stdout.line "Really big oooooops, yet again!"
+## ```
+##
+## Here we know that the `canFail` task may fail, and so we use
+## `Task.attempt` to convert the task to a `Result` and then use pattern
+## matching to handle the success and possible failure cases.
+attempt : Task a b, (Result a b -> Task c d) -> Task c d
+attempt = \@Task task, transform ->
+    @Task \{} ->
+        (@Task transformed) = transform (task {})
+
+        transformed {}
+
+## Take the success value from a given [Task] and use that to generate a new [Task].
+##
+## We can [await] Task results with callbacks:
+##
+## ```
+## Task.await (Stdin.line "What's your name?") \name ->
+##     Stdout.line "Your name is: $(name)"
+## ```
+##
+## Or we can more succinctly use the `!` bang operator, which desugars to [await]:
+##
+## ```
+## name = Stdin.line! "What's your name?"
+## Stdout.line "Your name is: $(name)"
+## ```
+await : Task a b, (a -> Task c b) -> Task c b
+await = \@Task task, transform ->
+    @Task \{} ->
+        when task {} is
+            Ok a ->
+                (@Task transformed) = transform a
+                transformed {}
+
+            Err b ->
+                Err b
+
+## Take the error value from a given [Task] and use that to generate a new [Task].
+##
+## ```
+## # Prints "Something went wrong!" to standard error if `canFail` fails.
+## canFail
+## |> Task.onErr \_ -> Stderr.line "Something went wrong!"
+## ```
+onErr : Task a b, (b -> Task a c) -> Task a c
+onErr = \@Task task, transform ->
+    @Task \{} ->
+        when task {} is
+            Ok a ->
+                Ok a
+
+            Err b ->
+                (@Task transformed) = transform b
+                transformed {}
+
+## Transform the success value of a given [Task] with a given function.
+##
+## ```
+## # Succeeds with a value of "Bonjour Louis!"
+## Task.ok "Louis"
+## |> Task.map (\name -> "Bonjour $(name)!")
+## ```
+map : Task a c, (a -> b) -> Task b c
+map = \@Task task, transform ->
+    @Task \{} ->
+        when task {} is
+            Ok a -> Ok (transform a)
+            Err b -> Err b
+
+## Transform the error value of a given [Task] with a given function.
+##
+## ```
+## # Ignore the fail value, and map it to the tag `CustomError`
+## canFail
+## |> Task.mapErr \_ -> CustomError
+## ```
+mapErr : Task c a, (a -> b) -> Task c b
+mapErr = \@Task task, transform ->
+    @Task \{} ->
+        when task {} is
+            Ok a -> Ok a
+            Err b -> Err (transform b)
+
+## Use a Result among other Tasks by converting it into a [Task].
+fromResult : Result a b -> Task a b
+fromResult = \res ->
+    @Task \{} -> res
+
+## Apply a task to another task applicatively. This can be used with
+## [ok] to build a [Task] that returns a record.
+##
+## The following example returns a Record with two fields, `apples` and
+## `oranges`, each of which is a `List Str`. If it fails it returns the tag
+## `NoFruitAvailable`.
+##
+## ```
+## getFruitBasket : Task { apples : List Str, oranges : List Str } [NoFruitAvailable]
+## getFruitBasket = Task.ok {
+##     apples: <- getFruit Apples |> Task.batch,
+##     oranges: <- getFruit Oranges |> Task.batch,
+## }
+## ```
+batch : Task a c -> (Task (a -> b) c -> Task b c)
+batch = \current ->
+    \next ->
+        await next \f ->
+            map current f
+
+## Apply each task in a list sequentially, and return a list of the resulting values.
+## Each task will be awaited before beginning the next task.
+##
+## ```
+## fetchAuthorTasks : List (Task Author [DbError])
+##
+## getAuthors : Task (List Author) [DbError]
+## getAuthors = Task.sequence fetchAuthorTasks
+## ```
+##
+sequence : List (Task ok err) -> Task (List ok) err
+sequence = \taskList ->
+    Task.loop (taskList, List.withCapacity (List.len taskList)) \(tasks, values) ->
+        when tasks is
+            [task, .. as rest] ->
+                value = task!
+                Task.ok (Step (rest, List.append values value))
+
+            [] ->
+                Task.ok (Done values)
+
+## Apply a task repeatedly for each item in a list
+##
+## ```
+## authors : List Author
+## saveAuthor : Author -> Task {} [DbError]
+##
+## saveAuthors : Task (List Author) [DbError]
+## saveAuthors = Task.forEach authors saveAuthor
+## ```
+##
+forEach : List a, (a -> Task {} b) -> Task {} b
+forEach = \items, fn ->
+    List.walk items (ok {}) \state, item ->
+        state |> await \_ -> fn item
+
+## Transform a task that can either succeed with `ok`, or fail with `err`, into
+## a task that succeeds with `Result ok err`.
+##
+## This is useful when chaining tasks using the `!` suffix. For example:
+##
+## ```
+## # Path.roc
+## checkFile : Str -> Task [Good, Bad] [IOError]
+##
+## # main.roc
+## when checkFile "/usr/local/bin/roc" |> Task.result! is
+##     Ok Good -> "..."
+##     Ok Bad -> "..."
+##     Err IOError -> "..."
+## ```
+##
+result : Task ok err -> Task (Result ok err) *
+result = \@Task task ->
+    @Task \{} ->
+        Ok (task {})

crates/compiler/builtins/roc/main.roc

@@ -11,4 +11,5 @@ package [
     Hash,
     Box,
     Inspect,
+    Task,
 ] {}

crates/compiler/builtins/src/roc.rs

@@ -16,6 +16,7 @@ pub fn module_source(module_id: ModuleId) -> &'static str {
         ModuleId::DECODE => DECODE,
         ModuleId::HASH => HASH,
         ModuleId::INSPECT => INSPECT,
+        ModuleId::TASK => TASK,
         _ => internal_error!(
             "ModuleId {:?} is not part of the standard library",
             module_id
@@ -35,3 +36,4 @@ const ENCODE: &str = include_str!("../roc/Encode.roc");
 const DECODE: &str = include_str!("../roc/Decode.roc");
 const HASH: &str = include_str!("../roc/Hash.roc");
 const INSPECT: &str = include_str!("../roc/Inspect.roc");
+const TASK: &str = include_str!("../roc/Task.roc");

crates/compiler/can/src/copy.rs

@@ -290,12 +290,14 @@ fn deep_copy_expr_help<C: CopyEnv>(env: &mut C, copied: &mut Vec<Variable>, expr
         Var(sym, var) => Var(*sym, sub!(*var)),
         ParamsVar {
             symbol,
-            params,
             var,
+            params_symbol,
+            params_var,
         } => ParamsVar {
             symbol: *symbol,
-            params: *params,
             var: sub!(*var),
+            params_symbol: *params_symbol,
+            params_var: sub!(*params_var),
         },
         ImportParams(module_id, region, opt_provided) => ImportParams(
             *module_id,

crates/compiler/can/src/def.rs

@@ -22,6 +22,7 @@ use crate::pattern::{canonicalize_def_header_pattern, BindingsFromPattern, Patte
 use crate::procedure::QualifiedReference;
 use crate::procedure::References;
 use crate::scope::create_alias;
+use crate::scope::SymbolLookup;
 use crate::scope::{PendingAbilitiesInScope, Scope};
 use roc_collections::ReferenceMatrix;
 use roc_collections::VecMap;
@@ -111,6 +112,23 @@ impl Annotation {
 
         self
     }
+
+    pub fn convert_to_fn(&mut self, argument_count: usize, var_store: &mut VarStore) {
+        let mut arg_types = Vec::with_capacity(argument_count);
+
+        for _ in 0..argument_count {
+            let var = var_store.fresh();
+            self.introduced_variables.insert_inferred(Loc::at_zero(var));
+
+            arg_types.push(Type::Variable(var));
+        }
+
+        self.signature = Type::Function(
+            arg_types,
+            Box::new(Type::Variable(var_store.fresh())),
+            Box::new(self.signature.clone()),
+        );
+    }
 }
 
 #[derive(Debug)]
@@ -165,6 +183,7 @@ enum PendingValueDef<'a> {
     /// Module params from an import
     ImportParams {
         symbol: Symbol,
+        variable: Variable,
         loc_pattern: Loc<Pattern>,
         module_id: ModuleId,
         opt_provided: Option<ast::Collection<'a, Loc<AssignedField<'a, ast::Expr<'a>>>>>,
@@ -186,6 +205,7 @@ impl PendingValueDef<'_> {
             PendingValueDef::ImportParams {
                 loc_pattern,
                 symbol: _,
+                variable: _,
                 module_id: _,
                 opt_provided: _,
             } => loc_pattern,
@@ -1153,6 +1173,7 @@ fn canonicalize_value_defs<'a>(
 
                 pending_value_defs.push(PendingValueDef::ImportParams {
                     symbol: params.symbol,
+                    variable: params.variable,
                     loc_pattern: params.loc_pattern,
                     opt_provided: params.opt_provided,
                     module_id,
@@ -2400,15 +2421,17 @@ fn canonicalize_pending_value_def<'a>(
         }
         ImportParams {
             symbol,
+            variable,
             loc_pattern,
             module_id,
             opt_provided,
         } => {
             // Insert a reference to the record so that we don't report it as unused
             // If the whole module is unused, we'll report that separately
-            output
-                .references
-                .insert_value_lookup(symbol, QualifiedReference::Unqualified);
+            output.references.insert_value_lookup(
+                SymbolLookup::no_params(symbol),
+                QualifiedReference::Unqualified,
+            );
 
             let (opt_var_record, references) = match opt_provided {
                 Some(params) => {
@@ -2418,7 +2441,7 @@ fn canonicalize_pending_value_def<'a>(
                     let references = can_output.references.clone();
                     output.union(can_output);
 
-                    (Some((var_store.fresh(), Box::new(record))), references)
+                    (Some((variable, Box::new(record))), references)
                 }
                 None => (None, References::new()),
             };
@@ -2725,12 +2748,11 @@ pub fn report_unused_imports(
             for (symbol, region) in &import.exposed_symbols {
                 if !references.has_unqualified_type_or_value_lookup(*symbol)
                     && !scope.abilities_store.is_specialization_name(*symbol)
-                    && !import.is_task(env)
                 {
                     env.problem(Problem::UnusedImport(*symbol, *region));
                 }
             }
-        } else if !import.is_task(env) {
+        } else {
             env.problem(Problem::UnusedModuleImport(import.module_id, import.region));
         }
     }
@@ -3003,6 +3025,7 @@ struct PendingModuleImport<'a> {
 
 struct PendingModuleImportParams<'a> {
     symbol: Symbol,
+    variable: Variable,
     loc_pattern: Loc<Pattern>,
     opt_provided: Option<ast::Collection<'a, Loc<AssignedField<'a, ast::Expr<'a>>>>>,
 }
@@ -3013,16 +3036,6 @@ pub struct IntroducedImport {
     exposed_symbols: Vec<(Symbol, Region)>,
 }
 
-impl IntroducedImport {
-    pub fn is_task(&self, env: &Env<'_>) -> bool {
-        // Temporarily needed for `!` convenience. Can be removed when Task becomes a builtin.
-        match env.qualified_module_ids.get_name(self.module_id) {
-            Some(name) => name.as_inner().as_str() == "Task",
-            None => false,
-        }
-    }
-}
-
 #[allow(clippy::too_many_arguments)]
 fn to_pending_value_def<'a>(
     env: &mut Env<'a>,
@@ -3160,15 +3173,17 @@ fn to_pending_value_def<'a>(
             // We do this even if params weren't provided so that solve can report if they are missing
             let params_sym = scope.gen_unique_symbol();
             let params_region = module_import.params.map(|p| p.params.region).unwrap_or(region);
+            let params_var = var_store.fresh();
             let params =
                 PendingModuleImportParams {
                     symbol: params_sym,
+                    variable: params_var,
                     loc_pattern: Loc::at(params_region, Pattern::Identifier(params_sym)),
                     opt_provided: module_import.params.map(|p| p.params.value),
                 };
-            let provided_params_sym = if module_import.params.is_some() {
+            let provided_params = if module_import.params.is_some() {
                 // Only add params to scope if they are provided
-                Some(params_sym)
+                Some((params_var, params_sym))
             } else {
                 None
             };
@@ -3176,7 +3191,7 @@ fn to_pending_value_def<'a>(
             if let Err(existing_import) =
                 scope
                     .modules
-                    .insert(name_with_alias.clone(), module_id, provided_params_sym, region)
+                    .insert(name_with_alias.clone(), module_id, provided_params, region)
             {
                 env.problems.push(Problem::ImportNameConflict {
                     name: name_with_alias,

crates/compiler/can/src/env.rs

@@ -7,7 +7,8 @@ use roc_collections::{MutMap, VecSet};
 use roc_module::ident::{Ident, ModuleName};
 use roc_module::symbol::{IdentIdsByModule, ModuleId, PQModuleName, PackageModuleIds, Symbol};
 use roc_problem::can::{Problem, RuntimeError};
-use roc_region::all::{Loc, Region};
+use roc_region::all::{LineInfo, Loc, Region};
+use roc_types::subs::Variable;
 
 /// The canonicalization environment for a particular module.
 pub struct Env<'a> {
@@ -38,14 +39,24 @@ pub struct Env<'a> {
 
     pub top_level_symbols: VecSet<Symbol>,
 
+    pub home_params_record: Option<(Symbol, Variable)>,
+
     pub arena: &'a Bump,
 
     pub opt_shorthand: Option<&'a str>,
+
+    pub src: &'a str,
+
+    /// Lazily calculated line info. This data is only needed if the code contains calls to `dbg`,
+    /// otherwise we can leave it as `None` and never pay the cost of scanning the source an extra
+    /// time.
+    line_info: &'a mut Option<LineInfo>,
 }
 
 impl<'a> Env<'a> {
     pub fn new(
         arena: &'a Bump,
+        src: &'a str,
         home: ModuleId,
         module_path: &'a Path,
         dep_idents: &'a IdentIdsByModule,
@@ -54,6 +65,7 @@ impl<'a> Env<'a> {
     ) -> Env<'a> {
         Env {
             arena,
+            src,
             home,
             module_path,
             dep_idents,
@@ -64,7 +76,9 @@ impl<'a> Env<'a> {
             qualified_type_lookups: VecSet::default(),
             tailcallable_symbol: None,
             top_level_symbols: VecSet::default(),
+            home_params_record: None,
             opt_shorthand,
+            line_info: arena.alloc(None),
         }
     }
 
@@ -215,4 +229,11 @@ impl<'a> Env<'a> {
     pub fn problem(&mut self, problem: Problem) {
         self.problems.push(problem)
     }
+
+    pub fn line_info(&mut self) -> &LineInfo {
+        if self.line_info.is_none() {
+            *self.line_info = Some(LineInfo::new(self.src));
+        }
+        self.line_info.as_ref().unwrap()
+    }
 }

crates/compiler/can/src/expr.rs

@@ -18,7 +18,7 @@ use roc_error_macros::internal_error;
 use roc_module::called_via::CalledVia;
 use roc_module::ident::{ForeignSymbol, Lowercase, TagName};
 use roc_module::low_level::LowLevel;
-use roc_module::symbol::{ModuleId, Symbol};
+use roc_module::symbol::{IdentId, ModuleId, Symbol};
 use roc_parse::ast::{self, Defs, PrecedenceConflict, StrLiteral};
 use roc_parse::ident::Accessor;
 use roc_parse::pattern::PatternType::*;
@@ -111,8 +111,9 @@ pub enum Expr {
     /// Like Var, but from a module with params
     ParamsVar {
         symbol: Symbol,
-        params: Symbol,
         var: Variable,
+        params_symbol: Symbol,
+        params_var: Variable,
     },
     AbilityMember(
         /// Actual member name
@@ -320,8 +321,9 @@ impl Expr {
             &Self::Var(sym, _) => Category::Lookup(sym),
             &Self::ParamsVar {
                 symbol,
-                params: _,
                 var: _,
+                params_symbol: _,
+                params_var: _,
             } => Category::Lookup(symbol),
             &Self::AbilityMember(sym, _, _) => Category::Lookup(sym),
             Self::When { .. } => Category::When,
@@ -1207,8 +1209,20 @@ pub fn canonicalize_expr<'a>(
                 output,
             )
         }
-        ast::Expr::Dbg(_, _) => {
-            internal_error!("Dbg should have been desugared by now")
+        ast::Expr::Dbg => {
+            // Dbg was not desugared as either part of an `Apply` or a `Pizza` binop, so it's
+            // invalid.
+            env.problem(Problem::UnappliedDbg { region });
+
+            let invalid_dbg_expr = crate::desugar::desugar_invalid_dbg_expr(env, scope, region);
+
+            let (loc_expr, output) =
+                canonicalize_expr(env, var_store, scope, region, invalid_dbg_expr);
+
+            (loc_expr.value, output)
+        }
+        ast::Expr::DbgStmt(_, _) => {
+            internal_error!("DbgStmt should have been desugared by now")
         }
         ast::Expr::LowLevelDbg((source_location, source), message, continuation) => {
             let mut output = Output::default();
@@ -1554,6 +1568,8 @@ fn canonicalize_closure_body<'a>(
         &loc_body_expr.value,
     );
 
+    let mut references_top_level = false;
+
     let mut captured_symbols: Vec<_> = new_output
         .references
         .value_lookups()
@@ -1564,7 +1580,11 @@ fn canonicalize_closure_body<'a>(
         .filter(|s| !new_output.references.bound_symbols().any(|x| x == s))
         .filter(|s| bound_by_argument_patterns.iter().all(|(k, _)| s != k))
         // filter out top-level symbols those will be globally available, and don't need to be captured
-        .filter(|s| !env.top_level_symbols.contains(s))
+        .filter(|s| {
+            let is_top_level = env.top_level_symbols.contains(s);
+            references_top_level = references_top_level || is_top_level;
+            !is_top_level
+        })
         // filter out imported symbols those will be globally available, and don't need to be captured
         .filter(|s| s.module_id() == env.home)
         // filter out functions that don't close over anything
@@ -1573,6 +1593,15 @@ fn canonicalize_closure_body<'a>(
         .map(|s| (s, var_store.fresh()))
         .collect();
 
+    if references_top_level {
+        if let Some(params_record) = env.home_params_record {
+            // If this module has params and the closure references top-level symbols,
+            // we need to capture the whole record so we can pass it.
+            // The lower_params pass will take care of removing the captures for top-level fns.
+            captured_symbols.push(params_record);
+        }
+    }
+
     output.union(new_output);
 
     // Now that we've collected all the references, check to see if any of the args we defined
@@ -1918,7 +1947,7 @@ fn canonicalize_var_lookup(
             Ok(lookup) => {
                 output
                     .references
-                    .insert_value_lookup(lookup.symbol, QualifiedReference::Unqualified);
+                    .insert_value_lookup(lookup, QualifiedReference::Unqualified);
 
                 if scope.abilities_store.is_ability_member_name(lookup.symbol) {
                     AbilityMember(
@@ -1927,7 +1956,7 @@ fn canonicalize_var_lookup(
                         var_store.fresh(),
                     )
                 } else {
-                    lookup_to_expr(lookup, var_store.fresh())
+                    lookup_to_expr(var_store, lookup)
                 }
             }
             Err(problem) => {
@@ -1943,7 +1972,7 @@ fn canonicalize_var_lookup(
             Ok(lookup) => {
                 output
                     .references
-                    .insert_value_lookup(lookup.symbol, QualifiedReference::Qualified);
+                    .insert_value_lookup(lookup, QualifiedReference::Qualified);
 
                 if scope.abilities_store.is_ability_member_name(lookup.symbol) {
                     AbilityMember(
@@ -1952,7 +1981,7 @@ fn canonicalize_var_lookup(
                         var_store.fresh(),
                     )
                 } else {
-                    lookup_to_expr(lookup, var_store.fresh())
+                    lookup_to_expr(var_store, lookup)
                 }
             }
             Err(problem) => {
@@ -1971,20 +2000,21 @@ fn canonicalize_var_lookup(
 }
 
 fn lookup_to_expr(
+    var_store: &mut VarStore,
     SymbolLookup {
         symbol,
-        module_params: params,
+        module_params,
     }: SymbolLookup,
-    var: Variable,
 ) -> Expr {
-    if let Some(params) = params {
+    if let Some((params_var, params_symbol)) = module_params {
         Expr::ParamsVar {
             symbol,
-            params,
-            var,
+            var: var_store.fresh(),
+            params_symbol,
+            params_var,
         }
     } else {
-        Expr::Var(symbol, var)
+        Expr::Var(symbol, var_store.fresh())
     }
 }
 
@@ -2470,21 +2500,28 @@ pub fn is_valid_interpolation(expr: &ast::Expr<'_>) -> bool {
         | ast::Expr::AccessorFunction(_)
         | ast::Expr::RecordUpdater(_)
         | ast::Expr::Crash
+        | ast::Expr::Dbg
         | ast::Expr::Underscore(_)
         | ast::Expr::MalformedIdent(_, _)
         | ast::Expr::Tag(_)
         | ast::Expr::OpaqueRef(_)
         | ast::Expr::MalformedClosure => true,
         // Newlines are disallowed inside interpolation, and these all require newlines
-        ast::Expr::Dbg(_, _)
+        ast::Expr::DbgStmt(_, _)
         | ast::Expr::LowLevelDbg(_, _, _)
-        | ast::Expr::Defs(_, _)
         | ast::Expr::Expect(_, _)
         | ast::Expr::When(_, _)
         | ast::Expr::Backpassing(_, _, _)
         | ast::Expr::SpaceBefore(_, _)
         | ast::Expr::Str(StrLiteral::Block(_))
         | ast::Expr::SpaceAfter(_, _) => false,
+        // Desugared dbg expression
+        ast::Expr::Defs(_, loc_ret) => match loc_ret.value {
+            ast::Expr::LowLevelDbg(_, _, continuation) => {
+                is_valid_interpolation(&continuation.value)
+            }
+            _ => false,
+        },
         // These can contain subexpressions, so we need to recursively check those
         ast::Expr::Str(StrLiteral::Line(segments)) => {
             segments.iter().all(|segment| match segment {
@@ -2774,6 +2811,9 @@ pub struct Declarations {
     // used for ability member specializatons.
     pub specializes: VecMap<usize, Symbol>,
 
+    // used while lowering params.
+    arity_by_name: VecMap<IdentId, usize>,
+
     pub host_exposed_annotations: VecMap<usize, (Variable, crate::def::Annotation)>,
 
     pub function_bodies: Vec<Loc<FunctionDef>>,
@@ -2802,6 +2842,7 @@ impl Declarations {
             expressions: Vec::with_capacity(capacity),
             specializes: VecMap::default(), // number of specializations is probably low
             destructs: Vec::new(),          // number of destructs is probably low
+            arity_by_name: VecMap::with_capacity(capacity),
         }
     }
 
@@ -2840,6 +2881,9 @@ impl Declarations {
             arguments: loc_closure_data.value.arguments,
         };
 
+        self.arity_by_name
+            .insert(symbol.value.ident_id(), function_def.arguments.len());
+
         let loc_function_def = Loc::at(loc_closure_data.region, function_def);
 
         let function_def_index = Index::push_new(&mut self.function_bodies, loc_function_def);
@@ -2888,6 +2932,9 @@ impl Declarations {
             arguments: loc_closure_data.value.arguments,
         };
 
+        self.arity_by_name
+            .insert(symbol.value.ident_id(), function_def.arguments.len());
+
         let loc_function_def = Loc::at(loc_closure_data.region, function_def);
 
         let function_def_index = Index::push_new(&mut self.function_bodies, loc_function_def);
@@ -2964,6 +3011,8 @@ impl Declarations {
                 .insert(self.declarations.len(), annotation);
         }
 
+        self.arity_by_name.insert(symbol.value.ident_id(), 0);
+
         self.declarations.push(DeclarationTag::Value);
         self.variables.push(expr_var);
         self.symbols.push(symbol);
@@ -3080,6 +3129,60 @@ impl Declarations {
         }
     }
 
+    /// Convert a value def to a function def with the given arguments
+    /// Currently used in lower_params
+    pub fn convert_value_to_function(
+        &mut self,
+        index: usize,
+        new_arguments: Vec<(Variable, AnnotatedMark, Loc<Pattern>)>,
+        var_store: &mut VarStore,
+    ) {
+        match self.declarations[index] {
+            DeclarationTag::Value => {
+                let new_args_len = new_arguments.len();
+
+                let loc_body = self.expressions[index].clone();
+                let region = loc_body.region;
+
+                let closure_data = ClosureData {
+                    function_type: var_store.fresh(),
+                    closure_type: var_store.fresh(),
+                    return_type: var_store.fresh(),
+                    name: self.symbols[index].value,
+                    captured_symbols: vec![],
+                    recursive: Recursive::NotRecursive,
+                    arguments: new_arguments,
+                    loc_body: Box::new(loc_body),
+                };
+
+                let loc_closure_data = Loc::at(region, closure_data);
+
+                let function_def = FunctionDef {
+                    closure_type: loc_closure_data.value.closure_type,
+                    return_type: loc_closure_data.value.return_type,
+                    captured_symbols: loc_closure_data.value.captured_symbols,
+                    arguments: loc_closure_data.value.arguments,
+                };
+
+                let loc_function_def = Loc::at(region, function_def);
+
+                let function_def_index =
+                    Index::push_new(&mut self.function_bodies, loc_function_def);
+
+                if let Some(annotation) = &mut self.annotations[index] {
+                    annotation.convert_to_fn(new_args_len, var_store);
+                }
+
+                if let Some((_var, annotation)) = self.host_exposed_annotations.get_mut(&index) {
+                    annotation.convert_to_fn(new_args_len, var_store);
+                }
+
+                self.declarations[index] = DeclarationTag::Function(function_def_index);
+            }
+            _ => internal_error!("Expected value declaration"),
+        };
+    }
+
     pub fn len(&self) -> usize {
         self.declarations.len()
     }
@@ -3149,6 +3252,11 @@ impl Declarations {
 
         collector
     }
+
+    pub(crate) fn take_arity_by_name(&mut self) -> VecMap<IdentId, usize> {
+        // `arity_by_name` is only needed for lowering module params
+        std::mem::take(&mut self.arity_by_name)
+    }
 }
 
 roc_error_macros::assert_sizeof_default!(DeclarationTag, 8);
@@ -3205,8 +3313,9 @@ pub(crate) fn get_lookup_symbols(expr: &Expr) -> Vec<ExpectLookup> {
             Expr::Var(symbol, var)
             | Expr::ParamsVar {
                 symbol,
-                params: _,
                 var,
+                params_symbol: _,
+                params_var: _,
             }
             | Expr::RecordUpdate {
                 symbol,

crates/compiler/can/src/lib.rs

@@ -14,7 +14,6 @@ pub mod copy;
 pub mod def;
 mod derive;
 pub mod desugar;
-pub mod effect_module;
 pub mod env;
 pub mod exhaustive;
 pub mod expected;
@@ -26,6 +25,7 @@ pub mod procedure;
 pub mod scope;
 pub mod string;
 pub mod suffixed;
+pub mod task_module;
 pub mod traverse;
 
 pub use derive::DERIVED_REGION;

crates/compiler/can/src/module.rs

@@ -3,13 +3,12 @@ use std::path::Path;
 use crate::abilities::{AbilitiesStore, ImplKey, PendingAbilitiesStore, ResolvedImpl};
 use crate::annotation::{canonicalize_annotation, AnnotationFor};
 use crate::def::{canonicalize_defs, report_unused_imports, Def};
-use crate::effect_module::HostedGeneratedFunctions;
 use crate::env::Env;
 use crate::expr::{
-    AnnotatedMark, ClosureData, DbgLookup, Declarations, ExpectLookup, Expr, Output, PendingDerives,
+    ClosureData, DbgLookup, Declarations, ExpectLookup, Expr, Output, PendingDerives,
 };
 use crate::pattern::{
-    canonicalize_record_destructure, BindingsFromPattern, Pattern, PermitShadows,
+    canonicalize_record_destructs, BindingsFromPattern, Pattern, PermitShadows, RecordDestruct,
 };
 use crate::procedure::References;
 use crate::scope::Scope;
@@ -18,14 +17,14 @@ use roc_collections::{MutMap, SendMap, VecMap, VecSet};
 use roc_error_macros::internal_error;
 use roc_module::ident::Ident;
 use roc_module::ident::Lowercase;
-use roc_module::symbol::{IdentIds, IdentIdsByModule, ModuleId, PackageModuleIds, Symbol};
+use roc_module::symbol::{IdentId, IdentIds, IdentIdsByModule, ModuleId, PackageModuleIds, Symbol};
 use roc_parse::ast::{Defs, TypeAnnotation};
-use roc_parse::header::{HeaderType, ModuleParams};
+use roc_parse::header::HeaderType;
 use roc_parse::pattern::PatternType;
 use roc_problem::can::{Problem, RuntimeError};
 use roc_region::all::{Loc, Region};
 use roc_types::subs::{ExposedTypesStorageSubs, Subs, VarStore, Variable};
-use roc_types::types::{AbilitySet, Alias, AliasKind, AliasVar, Type};
+use roc_types::types::{AbilitySet, Alias, Type};
 
 /// The types of all exposed values/functions of a collection of modules
 #[derive(Clone, Debug, Default)]
@@ -138,7 +137,33 @@ pub struct Module {
     pub abilities_store: PendingAbilitiesStore,
     pub loc_expects: VecMap<Region, Vec<ExpectLookup>>,
     pub loc_dbgs: VecMap<Symbol, DbgLookup>,
-    pub params_pattern: Option<(Variable, AnnotatedMark, Loc<Pattern>)>,
+    pub module_params: Option<ModuleParams>,
+}
+
+#[derive(Debug, Clone)]
+pub struct ModuleParams {
+    pub region: Region,
+    pub whole_symbol: Symbol,
+    pub whole_var: Variable,
+    pub record_var: Variable,
+    pub record_ext_var: Variable,
+    pub destructs: Vec<Loc<RecordDestruct>>,
+    // used while lowering passed functions
+    pub arity_by_name: VecMap<IdentId, usize>,
+}
+
+impl ModuleParams {
+    pub fn pattern(&self) -> Loc<Pattern> {
+        let record_pattern = Pattern::RecordDestructure {
+            whole_var: self.record_var,
+            ext_var: self.record_ext_var,
+            destructs: self.destructs.clone(),
+        };
+        let loc_record_pattern = Loc::at(self.region, record_pattern);
+
+        let as_pattern = Pattern::As(Box::new(loc_record_pattern), self.whole_symbol);
+        Loc::at(self.region, as_pattern)
+    }
 }
 
 #[derive(Debug, Default)]
@@ -152,7 +177,7 @@ pub struct RigidVariables {
 pub struct ModuleOutput {
     pub aliases: MutMap<Symbol, Alias>,
     pub rigid_variables: RigidVariables,
-    pub params_pattern: Option<(Variable, AnnotatedMark, Loc<Pattern>)>,
+    pub module_params: Option<ModuleParams>,
     pub declarations: Declarations,
     pub exposed_imports: MutMap<Symbol, Region>,
     pub exposed_symbols: VecSet<Symbol>,
@@ -165,99 +190,6 @@ pub struct ModuleOutput {
     pub loc_dbgs: VecMap<Symbol, DbgLookup>,
 }
 
-fn validate_generate_with<'a>(
-    generate_with: &'a [Loc<roc_parse::header::ExposedName<'a>>],
-) -> (HostedGeneratedFunctions, Vec<Loc<Ident>>) {
-    let mut functions = HostedGeneratedFunctions::default();
-    let mut unknown = Vec::new();
-
-    for generated in generate_with {
-        match generated.value.as_str() {
-            "after" => functions.after = true,
-            "map" => functions.map = true,
-            "always" => functions.always = true,
-            "loop" => functions.loop_ = true,
-            "forever" => functions.forever = true,
-            other => {
-                // we don't know how to generate this function
-                let ident = Ident::from(other);
-                unknown.push(Loc::at(generated.region, ident));
-            }
-        }
-    }
-
-    (functions, unknown)
-}
-
-#[derive(Debug)]
-enum GeneratedInfo {
-    Hosted {
-        effect_symbol: Symbol,
-        generated_functions: HostedGeneratedFunctions,
-    },
-    Builtin,
-    NotSpecial,
-}
-
-impl GeneratedInfo {
-    fn from_header_type(
-        env: &mut Env,
-        scope: &mut Scope,
-        var_store: &mut VarStore,
-        header_type: &HeaderType,
-    ) -> Self {
-        match header_type {
-            HeaderType::Hosted {
-                generates,
-                generates_with,
-                name: _,
-                exposes: _,
-            } => {
-                let name: &str = generates.into();
-                let (generated_functions, unknown_generated) =
-                    validate_generate_with(generates_with);
-
-                for unknown in unknown_generated {
-                    env.problem(Problem::UnknownGeneratesWith(unknown));
-                }
-
-                let effect_symbol = scope.introduce(name.into(), Region::zero()).unwrap();
-
-                {
-                    let a_var = var_store.fresh();
-
-                    let actual =
-                        crate::effect_module::build_effect_actual(Type::Variable(a_var), var_store);
-
-                    scope.add_alias(
-                        effect_symbol,
-                        Region::zero(),
-                        vec![Loc::at_zero(AliasVar::unbound("a".into(), a_var))],
-                        vec![],
-                        actual,
-                        AliasKind::Opaque,
-                    );
-                }
-
-                GeneratedInfo::Hosted {
-                    effect_symbol,
-                    generated_functions,
-                }
-            }
-            HeaderType::Builtin {
-                generates_with,
-                name: _,
-                exposes: _,
-                opt_params: _,
-            } => {
-                debug_assert!(generates_with.is_empty());
-                GeneratedInfo::Builtin
-            }
-            _ => GeneratedInfo::NotSpecial,
-        }
-    }
-}
-
 fn has_no_implementation(expr: &Expr) -> bool {
     match expr {
         Expr::RuntimeError(RuntimeError::NoImplementationNamed { .. }) => true,
@@ -308,6 +240,7 @@ pub fn canonicalize_module_defs<'a>(
     );
     let mut env = Env::new(
         arena,
+        src,
         home,
         arena.alloc(Path::new(module_path)),
         dep_idents,
@@ -326,9 +259,6 @@ pub fn canonicalize_module_defs<'a>(
         );
     }
 
-    let generated_info =
-        GeneratedInfo::from_header_type(&mut env, &mut scope, var_store, header_type);
-
     // Desugar operators (convert them to Apply calls, taking into account
     // operator precedence and associativity rules), before doing other canonicalization.
     //
@@ -337,19 +267,11 @@ pub fn canonicalize_module_defs<'a>(
     // operators, and then again on *their* nested operators, ultimately applying the
     // rules multiple times unnecessarily.
 
-    crate::desugar::desugar_defs_node_values(
-        arena,
-        loc_defs,
-        src,
-        &mut None,
-        module_path,
-        true,
-        &mut env.problems,
-    );
+    crate::desugar::desugar_defs_node_values(&mut env, &mut scope, loc_defs, true);
 
     let mut rigid_variables = RigidVariables::default();
 
-    // Iniital scope values are treated like defs that appear before any others.
+    // Initial scope values are treated like defs that appear before any others.
     // They include builtin types that are automatically imported, and for a platform
     // package, the required values from the app.
     //
@@ -398,13 +320,13 @@ pub fn canonicalize_module_defs<'a>(
 
     let mut output = Output::default();
 
-    let params_pattern = header_type.get_params().as_ref().map(
-        |ModuleParams {
+    let module_params = header_type.get_params().as_ref().map(
+        |roc_parse::header::ModuleParams {
              pattern,
              before_arrow: _,
              after_arrow: _,
          }| {
-            let can_pattern = canonicalize_record_destructure(
+            let (destructs, _) = canonicalize_record_destructs(
                 &mut env,
                 var_store,
                 &mut scope,
@@ -415,17 +337,22 @@ pub fn canonicalize_module_defs<'a>(
                 PermitShadows(false),
             );
 
-            let loc_pattern = Loc::at(pattern.region, can_pattern);
+            let whole_symbol = scope.gen_unique_symbol();
+            env.top_level_symbols.insert(whole_symbol);
 
-            for (symbol, _) in BindingsFromPattern::new(&loc_pattern) {
-                env.top_level_symbols.insert(symbol);
+            let whole_var = var_store.fresh();
+
+            env.home_params_record = Some((whole_symbol, whole_var));
+
+            ModuleParams {
+                region: pattern.region,
+                whole_var,
+                whole_symbol,
+                record_var: var_store.fresh(),
+                record_ext_var: var_store.fresh(),
+                destructs,
+                arity_by_name: Default::default(),
             }
-
-            (
-                var_store.fresh(),
-                AnnotatedMark::new(var_store),
-                loc_pattern,
-            )
         },
     );
 
@@ -503,6 +430,11 @@ pub fn canonicalize_module_defs<'a>(
         &exposed_symbols,
     );
 
+    let module_params = module_params.map(|params| ModuleParams {
+        arity_by_name: declarations.take_arity_by_name(),
+        ..params
+    });
+
     debug_assert!(
         output.pending_derives.is_empty(),
         "I thought pending derives are only found during def introduction"
@@ -542,24 +474,6 @@ pub fn canonicalize_module_defs<'a>(
 
     report_unused_imports(imports_introduced, &output.references, &mut env, &mut scope);
 
-    if let GeneratedInfo::Hosted {
-        effect_symbol,
-        generated_functions,
-    } = generated_info
-    {
-        let mut exposed_symbols = VecSet::default();
-
-        // NOTE this currently builds all functions, not just the ones that the user requested
-        crate::effect_module::build_effect_builtins(
-            &mut scope,
-            effect_symbol,
-            var_store,
-            &mut exposed_symbols,
-            &mut declarations,
-            generated_functions,
-        );
-    }
-
     for index in 0..declarations.len() {
         use crate::expr::DeclarationTag::*;
 
@@ -580,8 +494,8 @@ pub fn canonicalize_module_defs<'a>(
                 // and which are meant to be normal definitions without a body. So for now
                 // we just assume they are hosted functions (meant to be provided by the platform)
                 if has_no_implementation(&declarations.expressions[index].value) {
-                    match generated_info {
-                        GeneratedInfo::Builtin => {
+                    match header_type {
+                        HeaderType::Builtin { .. } => {
                             match crate::builtins::builtin_defs_map(*symbol, var_store) {
                                 None => {
                                     internal_error!("A builtin module contains a signature without implementation for {:?}", symbol)
@@ -591,7 +505,7 @@ pub fn canonicalize_module_defs<'a>(
                                 }
                             }
                         }
-                        GeneratedInfo::Hosted { effect_symbol, .. } => {
+                        HeaderType::Hosted { .. } => {
                             let ident_id = symbol.ident_id();
                             let ident = scope
                                 .locals
@@ -609,13 +523,8 @@ pub fn canonicalize_module_defs<'a>(
                                 aliases: Default::default(),
                             };
 
-                            let hosted_def = crate::effect_module::build_host_exposed_def(
-                                &mut scope,
-                                *symbol,
-                                &ident,
-                                effect_symbol,
-                                var_store,
-                                annotation,
+                            let hosted_def = crate::task_module::build_host_exposed_def(
+                                &mut scope, *symbol, &ident, var_store, annotation,
                             );
 
                             declarations.update_builtin_def(index, hosted_def);
@@ -638,8 +547,8 @@ pub fn canonicalize_module_defs<'a>(
                 // and which are meant to be normal definitions without a body. So for now
                 // we just assume they are hosted functions (meant to be provided by the platform)
                 if has_no_implementation(&declarations.expressions[index].value) {
-                    match generated_info {
-                        GeneratedInfo::Builtin => {
+                    match header_type {
+                        HeaderType::Builtin { .. } => {
                             match crate::builtins::builtin_defs_map(*symbol, var_store) {
                                 None => {
                                     internal_error!("A builtin module contains a signature without implementation for {:?}", symbol)
@@ -649,7 +558,7 @@ pub fn canonicalize_module_defs<'a>(
                                 }
                             }
                         }
-                        GeneratedInfo::Hosted { effect_symbol, .. } => {
+                        HeaderType::Hosted { .. } => {
                             let ident_id = symbol.ident_id();
                             let ident = scope
                                 .locals
@@ -667,13 +576,8 @@ pub fn canonicalize_module_defs<'a>(
                                 aliases: Default::default(),
                             };
 
-                            let hosted_def = crate::effect_module::build_host_exposed_def(
-                                &mut scope,
-                                *symbol,
-                                &ident,
-                                effect_symbol,
-                                var_store,
-                                annotation,
+                            let hosted_def = crate::task_module::build_host_exposed_def(
+                                &mut scope, *symbol, &ident, var_store, annotation,
                             );
 
                             declarations.update_builtin_def(index, hosted_def);
@@ -699,18 +603,6 @@ pub fn canonicalize_module_defs<'a>(
 
     let mut aliases = MutMap::default();
 
-    if let GeneratedInfo::Hosted { effect_symbol, .. } = generated_info {
-        // Remove this from exposed_symbols,
-        // so that at the end of the process,
-        // we can see if there were any
-        // exposed symbols which did not have
-        // corresponding defs.
-        exposed_but_not_defined.remove(&effect_symbol);
-
-        let hosted_alias = scope.lookup_alias(effect_symbol).unwrap().clone();
-        aliases.insert(effect_symbol, hosted_alias);
-    }
-
     for (symbol, alias) in output.aliases {
         // Remove this from exposed_symbols,
         // so that at the end of the process,
@@ -859,7 +751,7 @@ pub fn canonicalize_module_defs<'a>(
         scope,
         aliases,
         rigid_variables,
-        params_pattern,
+        module_params,
         declarations,
         referenced_values,
         exposed_imports: can_exposed_imports,

crates/compiler/can/src/pattern.rs

@@ -623,16 +623,29 @@ pub fn canonicalize_pattern<'a>(
             }
         }
 
-        RecordDestructure(patterns) => canonicalize_record_destructure(
-            env,
-            var_store,
-            scope,
-            output,
-            pattern_type,
-            patterns,
-            region,
-            permit_shadows,
-        ),
+        RecordDestructure(patterns) => {
+            let ext_var = var_store.fresh();
+            let whole_var = var_store.fresh();
+
+            let (destructs, opt_erroneous) = canonicalize_record_destructs(
+                env,
+                var_store,
+                scope,
+                output,
+                pattern_type,
+                patterns,
+                region,
+                permit_shadows,
+            );
+
+            // If we encountered an erroneous pattern (e.g. one with shadowing),
+            // use the resulting RuntimeError. Otherwise, return a successful record destructure.
+            opt_erroneous.unwrap_or(Pattern::RecordDestructure {
+                whole_var,
+                ext_var,
+                destructs,
+            })
+        }
 
         RequiredField(_name, _loc_pattern) => {
             unreachable!("should have been handled in RecordDestructure");
@@ -779,7 +792,7 @@ pub fn canonicalize_pattern<'a>(
 }
 
 #[allow(clippy::too_many_arguments)]
-pub fn canonicalize_record_destructure<'a>(
+pub fn canonicalize_record_destructs<'a>(
     env: &mut Env<'a>,
     var_store: &mut VarStore,
     scope: &mut Scope,
@@ -788,11 +801,9 @@ pub fn canonicalize_record_destructure<'a>(
     patterns: &ast::Collection<Loc<ast::Pattern<'a>>>,
     region: Region,
     permit_shadows: PermitShadows,
-) -> Pattern {
+) -> (Vec<Loc<RecordDestruct>>, Option<Pattern>) {
     use ast::Pattern::*;
 
-    let ext_var = var_store.fresh();
-    let whole_var = var_store.fresh();
     let mut destructs = Vec::with_capacity(patterns.len());
     let mut opt_erroneous = None;
 
@@ -907,13 +918,7 @@ pub fn canonicalize_record_destructure<'a>(
         }
     }
 
-    // If we encountered an erroneous pattern (e.g. one with shadowing),
-    // use the resulting RuntimeError. Otherwise, return a successful record destructure.
-    opt_erroneous.unwrap_or(Pattern::RecordDestructure {
-        whole_var,
-        ext_var,
-        destructs,
-    })
+    (destructs, opt_erroneous)
 }
 
 /// When we detect an unsupported pattern type (e.g. 5 = 1 + 2 is unsupported because you can't

crates/compiler/can/src/procedure.rs

@@ -1,5 +1,5 @@
-use crate::expr::Expr;
 use crate::pattern::Pattern;
+use crate::{expr::Expr, scope::SymbolLookup};
 use roc_module::symbol::{ModuleId, Symbol};
 use roc_region::all::{Loc, Region};
 use roc_types::subs::Variable;
@@ -125,8 +125,18 @@ impl References {
         }
     }
 
-    pub fn insert_value_lookup(&mut self, symbol: Symbol, qualified: QualifiedReference) {
-        self.insert(symbol, qualified.flags(ReferencesBitflags::VALUE_LOOKUP));
+    pub fn insert_value_lookup(&mut self, lookup: SymbolLookup, qualified: QualifiedReference) {
+        self.insert(
+            lookup.symbol,
+            qualified.flags(ReferencesBitflags::VALUE_LOOKUP),
+        );
+
+        if let Some((_, params_symbol)) = lookup.module_params {
+            self.insert(
+                params_symbol,
+                qualified.flags(ReferencesBitflags::VALUE_LOOKUP),
+            );
+        }
     }
 
     pub fn insert_type_lookup(&mut self, symbol: Symbol, qualified: QualifiedReference) {

crates/compiler/can/src/scope.rs

@@ -466,7 +466,7 @@ impl Scope {
         self.home.register_debug_idents(&self.locals.ident_ids)
     }
 
-    /// Generates a unique, new symbol like "$1" or "$5",
+    /// Generates a unique, new symbol like "1" or "5",
     /// using the home module as the module_id.
     ///
     /// This is used, for example, during canonicalization of an Expr::Closure
@@ -475,6 +475,12 @@ impl Scope {
         Symbol::new(self.home, self.locals.gen_unique())
     }
 
+    /// Generates a unique new symbol and return the symbol's unqualified identifier name.
+    pub fn gen_unique_symbol_name(&mut self) -> &str {
+        let ident_id = self.locals.gen_unique();
+        self.locals.ident_ids.get_name(ident_id).unwrap()
+    }
+
     /// Introduce a new ignored variable (variable starting with an underscore).
     /// The underscore itself should not be included in `ident`.
     pub fn introduce_ignored_local(&mut self, ident: &str, region: Region) {
@@ -669,7 +675,7 @@ pub struct ScopeModules {
     /// Why is this module in scope?
     sources: Vec<ScopeModuleSource>,
     /// The params of a module if any
-    params: Vec<Option<Symbol>>,
+    params: Vec<Option<(Variable, Symbol)>>,
 }
 
 impl ScopeModules {
@@ -731,7 +737,7 @@ impl ScopeModules {
         &mut self,
         module_name: ModuleName,
         module_id: ModuleId,
-        params_symbol: Option<Symbol>,
+        params: Option<(Variable, Symbol)>,
         region: Region,
     ) -> Result<(), ScopeModuleSource> {
         if let Some(index) = self.names.iter().position(|name| name == &module_name) {
@@ -745,7 +751,7 @@ impl ScopeModules {
         self.ids.push(module_id);
         self.names.push(module_name);
         self.sources.push(ScopeModuleSource::Import(region));
-        self.params.push(params_symbol);
+        self.params.push(params);
         Ok(())
     }
 
@@ -768,14 +774,14 @@ impl ScopeModules {
     }
 }
 
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Copy)]
 pub struct SymbolLookup {
     pub symbol: Symbol,
-    pub module_params: Option<Symbol>,
+    pub module_params: Option<(Variable, Symbol)>,
 }
 
 impl SymbolLookup {
-    pub fn new(symbol: Symbol, params: Option<Symbol>) -> Self {
+    pub fn new(symbol: Symbol, params: Option<(Variable, Symbol)>) -> Self {
         Self {
             symbol,
             module_params: params,
@@ -789,7 +795,7 @@ impl SymbolLookup {
 
 pub struct ModuleLookup {
     pub id: ModuleId,
-    pub params: Option<Symbol>,
+    pub params: Option<(Variable, Symbol)>,
 }
 
 impl ModuleLookup {

crates/compiler/can/src/task_module.rs

@@ -0,0 +1,207 @@
+use crate::def::Def;
+use crate::expr::{AnnotatedMark, ClosureData, Expr, Recursive};
+use crate::pattern::Pattern;
+use crate::scope::Scope;
+use roc_collections::SendMap;
+use roc_module::symbol::Symbol;
+use roc_region::all::{Loc, Region};
+use roc_types::subs::{VarStore, Variable};
+use roc_types::types::{LambdaSet, OptAbleVar, Type};
+
+pub fn build_host_exposed_def(
+    scope: &mut Scope,
+    symbol: Symbol,
+    ident: &str,
+    var_store: &mut VarStore,
+    annotation: crate::annotation::Annotation,
+) -> Def {
+    let expr_var = var_store.fresh();
+    let pattern = Pattern::Identifier(symbol);
+    let mut pattern_vars = SendMap::default();
+    pattern_vars.insert(symbol, expr_var);
+
+    let mut arguments: Vec<(Variable, AnnotatedMark, Loc<Pattern>)> = Vec::new();
+    let mut linked_symbol_arguments: Vec<(Variable, Expr)> = Vec::new();
+    let mut captured_symbols: Vec<(Symbol, Variable)> = Vec::new();
+
+    let crate::annotation::Annotation {
+        introduced_variables,
+        typ,
+        aliases,
+        ..
+    } = annotation;
+
+    let def_body = {
+        match typ.shallow_structural_dealias() {
+            Type::Function(args, _, _) => {
+                for i in 0..args.len() {
+                    let name = format!("closure_arg_{ident}_{i}");
+
+                    let arg_symbol = {
+                        let ident = name.clone().into();
+                        scope.introduce(ident, Region::zero()).unwrap()
+                    };
+
+                    let arg_var = var_store.fresh();
+
+                    arguments.push((
+                        arg_var,
+                        AnnotatedMark::new(var_store),
+                        Loc::at_zero(Pattern::Identifier(arg_symbol)),
+                    ));
+
+                    captured_symbols.push((arg_symbol, arg_var));
+                    linked_symbol_arguments.push((arg_var, Expr::Var(arg_symbol, arg_var)));
+                }
+
+                let foreign_symbol_name = format!("roc_fx_{ident}");
+                let low_level_call = Expr::ForeignCall {
+                    foreign_symbol: foreign_symbol_name.into(),
+                    args: linked_symbol_arguments,
+                    ret_var: var_store.fresh(),
+                };
+
+                let task_closure_symbol = {
+                    let name = format!("task_closure_{ident}");
+
+                    let ident = name.into();
+                    scope.introduce(ident, Region::zero()).unwrap()
+                };
+
+                let task_closure = Expr::Closure(ClosureData {
+                    function_type: var_store.fresh(),
+                    closure_type: var_store.fresh(),
+                    return_type: var_store.fresh(),
+                    name: task_closure_symbol,
+                    captured_symbols,
+                    recursive: Recursive::NotRecursive,
+                    arguments: vec![(
+                        var_store.fresh(),
+                        AnnotatedMark::new(var_store),
+                        Loc::at_zero(empty_record_pattern(var_store)),
+                    )],
+                    loc_body: Box::new(Loc::at_zero(low_level_call)),
+                });
+
+                let (specialized_def_type, type_arguments, lambda_set_variables) =
+                    build_fresh_opaque_variables(var_store);
+                let body = Expr::OpaqueRef {
+                    opaque_var: var_store.fresh(),
+                    name: Symbol::TASK_TASK,
+                    argument: Box::new((var_store.fresh(), Loc::at_zero(task_closure))),
+                    specialized_def_type,
+                    type_arguments,
+                    lambda_set_variables,
+                };
+
+                Expr::Closure(ClosureData {
+                    function_type: var_store.fresh(),
+                    closure_type: var_store.fresh(),
+                    return_type: var_store.fresh(),
+                    name: symbol,
+                    captured_symbols: std::vec::Vec::new(),
+                    recursive: Recursive::NotRecursive,
+                    arguments,
+                    loc_body: Box::new(Loc::at_zero(body)),
+                })
+            }
+            _ => {
+                // not a function
+
+                let foreign_symbol_name = format!("roc_fx_{ident}");
+                let low_level_call = Expr::ForeignCall {
+                    foreign_symbol: foreign_symbol_name.into(),
+                    args: linked_symbol_arguments,
+                    ret_var: var_store.fresh(),
+                };
+
+                let task_closure_symbol = {
+                    let name = format!("task_closure_{ident}");
+
+                    let ident = name.into();
+                    scope.introduce(ident, Region::zero()).unwrap()
+                };
+
+                let task_closure = Expr::Closure(ClosureData {
+                    function_type: var_store.fresh(),
+                    closure_type: var_store.fresh(),
+                    return_type: var_store.fresh(),
+                    name: task_closure_symbol,
+                    captured_symbols,
+                    recursive: Recursive::NotRecursive,
+                    arguments: vec![(
+                        var_store.fresh(),
+                        AnnotatedMark::new(var_store),
+                        Loc::at_zero(empty_record_pattern(var_store)),
+                    )],
+                    loc_body: Box::new(Loc::at_zero(low_level_call)),
+                });
+
+                let (specialized_def_type, type_arguments, lambda_set_variables) =
+                    build_fresh_opaque_variables(var_store);
+                Expr::OpaqueRef {
+                    opaque_var: var_store.fresh(),
+                    name: Symbol::TASK_TASK,
+                    argument: Box::new((var_store.fresh(), Loc::at_zero(task_closure))),
+                    specialized_def_type,
+                    type_arguments,
+                    lambda_set_variables,
+                }
+            }
+        }
+    };
+
+    let def_annotation = crate::def::Annotation {
+        signature: typ,
+        introduced_variables,
+        aliases,
+        region: Region::zero(),
+    };
+
+    Def {
+        loc_pattern: Loc::at_zero(pattern),
+        loc_expr: Loc::at_zero(def_body),
+        expr_var,
+        pattern_vars,
+        annotation: Some(def_annotation),
+    }
+}
+
+fn build_fresh_opaque_variables(
+    var_store: &mut VarStore,
+) -> (Box<Type>, Vec<OptAbleVar>, Vec<LambdaSet>) {
+    let closure_var = var_store.fresh();
+
+    let ok_var = var_store.fresh();
+    let err_var = var_store.fresh();
+    let result_var = var_store.fresh();
+
+    let actual = Type::Function(
+        vec![Type::EmptyRec],
+        Box::new(Type::Variable(closure_var)),
+        Box::new(Type::Variable(result_var)),
+    );
+
+    let type_arguments = vec![
+        OptAbleVar {
+            var: ok_var,
+            opt_abilities: None,
+        },
+        OptAbleVar {
+            var: err_var,
+            opt_abilities: None,
+        },
+    ];
+    let lambda_set_variables = vec![roc_types::types::LambdaSet(Type::Variable(closure_var))];
+
+    (Box::new(actual), type_arguments, lambda_set_variables)
+}
+
+#[inline(always)]
+fn empty_record_pattern(var_store: &mut VarStore) -> Pattern {
+    Pattern::RecordDestructure {
+        whole_var: var_store.fresh(),
+        ext_var: var_store.fresh(),
+        destructs: vec![],
+    }
+}

crates/compiler/can/tests/helpers/mod.rs

@@ -46,6 +46,24 @@ pub fn can_expr_with(arena: &Bump, home: ModuleId, expr_str: &str) -> CanExprOut
     let var = var_store.fresh();
     let qualified_module_ids = PackageModuleIds::default();
 
+    let mut scope = Scope::new(
+        home,
+        "TestPath".into(),
+        IdentIds::default(),
+        Default::default(),
+    );
+
+    let dep_idents = IdentIds::exposed_builtins(0);
+    let mut env = Env::new(
+        arena,
+        expr_str,
+        home,
+        Path::new("Test.roc"),
+        &dep_idents,
+        &qualified_module_ids,
+        None,
+    );
+
     // Desugar operators (convert them to Apply calls, taking into account
     // operator precedence and associativity rules), before doing other canonicalization.
     //
@@ -53,21 +71,8 @@ pub fn can_expr_with(arena: &Bump, home: ModuleId, expr_str: &str) -> CanExprOut
     // visited a BinOp node we'd recursively try to apply this to each of its nested
     // operators, and then again on *their* nested operators, ultimately applying the
     // rules multiple times unnecessarily.
-    let loc_expr = desugar::desugar_expr(
-        arena,
-        &loc_expr,
-        expr_str,
-        &mut None,
-        arena.alloc("TestPath"),
-        &mut Default::default(),
-    );
+    let loc_expr = desugar::desugar_expr(&mut env, &mut scope, &loc_expr);
 
-    let mut scope = Scope::new(
-        home,
-        "TestPath".into(),
-        IdentIds::default(),
-        Default::default(),
-    );
     scope.add_alias(
         Symbol::NUM_INT,
         Region::zero(),
@@ -80,15 +85,6 @@ pub fn can_expr_with(arena: &Bump, home: ModuleId, expr_str: &str) -> CanExprOut
         roc_types::types::AliasKind::Structural,
     );
 
-    let dep_idents = IdentIds::exposed_builtins(0);
-    let mut env = Env::new(
-        arena,
-        home,
-        Path::new("Test.roc"),
-        &dep_idents,
-        &qualified_module_ids,
-        None,
-    );
     let (loc_expr, output) = canonicalize_expr(
         &mut env,
         &mut var_store,

crates/compiler/can/tests/snapshots/test_suffixed__suffixed_tests__dbg_expr.snap

@@ -0,0 +1,145 @@
+---
+source: crates/compiler/can/tests/test_suffixed.rs
+assertion_line: 463
+expression: snapshot
+---
+Defs {
+    tags: [
+        Index(2147483648),
+    ],
+    regions: [
+        @0-26,
+    ],
+    space_before: [
+        Slice(start = 0, length = 0),
+    ],
+    space_after: [
+        Slice(start = 0, length = 1),
+    ],
+    spaces: [
+        Newline,
+    ],
+    type_defs: [],
+    value_defs: [
+        Body(
+            @0-4 Identifier {
+                ident: "main",
+            },
+            @11-26 Defs(
+                Defs {
+                    tags: [
+                        Index(2147483648),
+                    ],
+                    regions: [
+                        @15-26,
+                    ],
+                    space_before: [
+                        Slice(start = 0, length = 0),
+                    ],
+                    space_after: [
+                        Slice(start = 0, length = 0),
+                    ],
+                    spaces: [],
+                    type_defs: [],
+                    value_defs: [
+                        Body(
+                            @15-26 Identifier {
+                                ident: "1",
+                            },
+                            @15-26 ParensAround(
+                                Defs(
+                                    Defs {
+                                        tags: [
+                                            Index(2147483648),
+                                        ],
+                                        regions: [
+                                            @20-25,
+                                        ],
+                                        space_before: [
+                                            Slice(start = 0, length = 0),
+                                        ],
+                                        space_after: [
+                                            Slice(start = 0, length = 0),
+                                        ],
+                                        spaces: [],
+                                        type_defs: [],
+                                        value_defs: [
+                                            Body(
+                                                @20-25 Identifier {
+                                                    ident: "0",
+                                                },
+                                                @20-25 Apply(
+                                                    @22-23 Var {
+                                                        module_name: "Num",
+                                                        ident: "add",
+                                                    },
+                                                    [
+                                                        @20-21 Num(
+                                                            "1",
+                                                        ),
+                                                        @24-25 Num(
+                                                            "1",
+                                                        ),
+                                                    ],
+                                                    BinOp(
+                                                        Plus,
+                                                    ),
+                                                ),
+                                            ),
+                                        ],
+                                    },
+                                    @15-26 LowLevelDbg(
+                                        (
+                                            "test.roc:3",
+                                            "     ",
+                                        ),
+                                        @20-25 Apply(
+                                            @20-25 Var {
+                                                module_name: "Inspect",
+                                                ident: "toStr",
+                                            },
+                                            [
+                                                @20-25 Var {
+                                                    module_name: "",
+                                                    ident: "0",
+                                                },
+                                            ],
+                                            Space,
+                                        ),
+                                        @20-25 Var {
+                                            module_name: "",
+                                            ident: "0",
+                                        },
+                                    ),
+                                ),
+                            ),
+                        ),
+                    ],
+                },
+                @11-26 LowLevelDbg(
+                    (
+                        "test.roc:2",
+                        "in =\n      ",
+                    ),
+                    @15-26 Apply(
+                        @15-26 Var {
+                            module_name: "Inspect",
+                            ident: "toStr",
+                        },
+                        [
+                            @15-26 Var {
+                                module_name: "",
+                                ident: "1",
+                            },
+                        ],
+                        Space,
+                    ),
+                    @15-26 Var {
+                        module_name: "",
+                        ident: "1",
+                    },
+                ),
+            ),
+        ),
+    ],
+}

crates/compiler/can/tests/snapshots/test_suffixed__suffixed_tests__pizza_dbg.snap

@@ -0,0 +1,143 @@
+---
+source: crates/compiler/can/tests/test_suffixed.rs
+assertion_line: 473
+expression: snapshot
+---
+Defs {
+    tags: [
+        Index(2147483648),
+    ],
+    regions: [
+        @0-51,
+    ],
+    space_before: [
+        Slice(start = 0, length = 0),
+    ],
+    space_after: [
+        Slice(start = 0, length = 1),
+    ],
+    spaces: [
+        Newline,
+    ],
+    type_defs: [],
+    value_defs: [
+        Body(
+            @0-4 Identifier {
+                ident: "main",
+            },
+            @11-51 Defs(
+                Defs {
+                    tags: [
+                        Index(2147483648),
+                    ],
+                    regions: [
+                        @11-40,
+                    ],
+                    space_before: [
+                        Slice(start = 0, length = 0),
+                    ],
+                    space_after: [
+                        Slice(start = 0, length = 0),
+                    ],
+                    spaces: [],
+                    type_defs: [],
+                    value_defs: [
+                        Body(
+                            @11-40 Identifier {
+                                ident: "1",
+                            },
+                            @11-40 Apply(
+                                @31-38 Var {
+                                    module_name: "Num",
+                                    ident: "add",
+                                },
+                                [
+                                    @11-23 Defs(
+                                        Defs {
+                                            tags: [
+                                                Index(2147483648),
+                                            ],
+                                            regions: [
+                                                @11-12,
+                                            ],
+                                            space_before: [
+                                                Slice(start = 0, length = 0),
+                                            ],
+                                            space_after: [
+                                                Slice(start = 0, length = 0),
+                                            ],
+                                            spaces: [],
+                                            type_defs: [],
+                                            value_defs: [
+                                                Body(
+                                                    @11-12 Identifier {
+                                                        ident: "0",
+                                                    },
+                                                    @11-12 Num(
+                                                        "1",
+                                                    ),
+                                                ),
+                                            ],
+                                        },
+                                        @11-23 LowLevelDbg(
+                                            (
+                                                "test.roc:2",
+                                                " ",
+                                            ),
+                                            @11-12 Apply(
+                                                @11-12 Var {
+                                                    module_name: "Inspect",
+                                                    ident: "toStr",
+                                                },
+                                                [
+                                                    @11-12 Var {
+                                                        module_name: "",
+                                                        ident: "0",
+                                                    },
+                                                ],
+                                                Space,
+                                            ),
+                                            @11-12 Var {
+                                                module_name: "",
+                                                ident: "0",
+                                            },
+                                        ),
+                                    ),
+                                    @39-40 Num(
+                                        "2",
+                                    ),
+                                ],
+                                BinOp(
+                                    Pizza,
+                                ),
+                            ),
+                        ),
+                    ],
+                },
+                @11-51 LowLevelDbg(
+                    (
+                        "test.roc:2",
+                        "  main =\n                1\n  ",
+                    ),
+                    @11-40 Apply(
+                        @11-40 Var {
+                            module_name: "Inspect",
+                            ident: "toStr",
+                        },
+                        [
+                            @11-40 Var {
+                                module_name: "",
+                                ident: "1",
+                            },
+                        ],
+                        Space,
+                    ),
+                    @11-40 Var {
+                        module_name: "",
+                        ident: "1",
+                    },
+                ),
+            ),
+        ),
+    ],
+}

crates/compiler/can/tests/test_suffixed.rs

@@ -6,22 +6,39 @@ mod suffixed_tests {
     use bumpalo::Bump;
     use insta::assert_snapshot;
     use roc_can::desugar::desugar_defs_node_values;
+    use roc_can::env::Env;
+    use roc_can::scope::Scope;
+    use roc_module::symbol::{IdentIds, ModuleIds, PackageModuleIds};
     use roc_parse::test_helpers::parse_defs_with;
+    use std::path::Path;
 
     macro_rules! run_test {
         ($src:expr) => {{
             let arena = &Bump::new();
-            let mut defs = parse_defs_with(arena, indoc!($src)).unwrap();
-            desugar_defs_node_values(
-                arena,
-                &mut defs,
-                $src,
-                &mut None,
-                "test.roc",
-                true,
-                &mut Default::default(),
+            let home = ModuleIds::default().get_or_insert(&"Test".into());
+
+            let mut scope = Scope::new(
+                home,
+                "TestPath".into(),
+                IdentIds::default(),
+                Default::default(),
             );
 
+            let dep_idents = IdentIds::exposed_builtins(0);
+            let qualified_module_ids = PackageModuleIds::default();
+            let mut env = Env::new(
+                arena,
+                $src,
+                home,
+                Path::new("test.roc"),
+                &dep_idents,
+                &qualified_module_ids,
+                None,
+            );
+
+            let mut defs = parse_defs_with(arena, indoc!($src)).unwrap();
+            desugar_defs_node_values(&mut env, &mut scope, &mut defs, true);
+
             let snapshot = format!("{:#?}", &defs);
             println!("{}", snapshot);
             assert_snapshot!(snapshot);
@@ -441,6 +458,29 @@ mod suffixed_tests {
         );
     }
 
+    #[test]
+    fn dbg_expr() {
+        run_test!(
+            r#"
+            main =
+                dbg (dbg 1 + 1)
+            "#
+        );
+    }
+
+    #[test]
+    fn pizza_dbg() {
+        run_test!(
+            r#"
+            main =
+                1
+                |> dbg
+                |> Num.add 2
+                |> dbg
+            "#
+        )
+    }
+
     #[test]
     fn apply_argument_single() {
         run_test!(

crates/compiler/checkmate/www/package-lock.json

@@ -3424,21 +3424,15 @@
       "dev": true
     },
     "node_modules/@jridgewell/trace-mapping": {
-      "version": "0.3.18",
-      "resolved": "https://registry.npmjs.org/@jridgewell/trace-mapping/-/trace-mapping-0.3.18.tgz",
-      "integrity": "sha512-w+niJYzMHdd7USdiH2U6869nqhD2nbfZXND5Yp93qIbEmnDNk7PD48o+YchRVpzMU7M6jVCbenTR7PA1FLQ9pA==",
+      "version": "0.3.25",
+      "resolved": "https://registry.npmjs.org/@jridgewell/trace-mapping/-/trace-mapping-0.3.25.tgz",
+      "integrity": "sha512-vNk6aEwybGtawWmy/PzwnGDOjCkLWSD2wqvjGGAgOAwCGWySYXfYoxt00IJkTF+8Lb57DwOb3Aa0o9CApepiYQ==",
       "dev": true,
       "dependencies": {
-        "@jridgewell/resolve-uri": "3.1.0",
-        "@jridgewell/sourcemap-codec": "1.4.14"
+        "@jridgewell/resolve-uri": "^3.1.0",
+        "@jridgewell/sourcemap-codec": "^1.4.14"
       }
     },
-    "node_modules/@jridgewell/trace-mapping/node_modules/@jridgewell/sourcemap-codec": {
-      "version": "1.4.14",
-      "resolved": "https://registry.npmjs.org/@jridgewell/sourcemap-codec/-/sourcemap-codec-1.4.14.tgz",
-      "integrity": "sha512-XPSJHWmi394fuUuzDnGz1wiKqWfo1yXecHQMRf2l6hztTO+nPru658AyDngaBe7isIxEkRsPR3FZh+s7iVa4Uw==",
-      "dev": true
-    },
     "node_modules/@jsdevtools/ono": {
       "version": "7.1.3",
       "resolved": "https://registry.npmjs.org/@jsdevtools/ono/-/ono-7.1.3.tgz",
@@ -4351,20 +4345,10 @@
         "@types/json-schema": "*"
       }
     },
-    "node_modules/@types/eslint-scope": {
-      "version": "3.7.4",
-      "resolved": "https://registry.npmjs.org/@types/eslint-scope/-/eslint-scope-3.7.4.tgz",
-      "integrity": "sha512-9K4zoImiZc3HlIp6AVUDE4CWYx22a+lhSZMYNpbjW04+YF0KWj4pJXnEMjdnFTiQibFFmElcsasJXDbdI/EPhA==",
-      "dev": true,
-      "dependencies": {
-        "@types/eslint": "*",
-        "@types/estree": "*"
-      }
-    },
     "node_modules/@types/estree": {
-      "version": "1.0.1",
-      "resolved": "https://registry.npmjs.org/@types/estree/-/estree-1.0.1.tgz",
-      "integrity": "sha512-LG4opVs2ANWZ1TJoKc937iMmNstM/d0ae1vNbnBvBhqCSezgVUOzcLCqbI5elV8Vy6WKwKjaqR+zO9VKirBBCA==",
+      "version": "1.0.5",
+      "resolved": "https://registry.npmjs.org/@types/estree/-/estree-1.0.5.tgz",
+      "integrity": "sha512-/kYRxGDLWzHOB7q+wtSUQlFrtcdUccpfy+X+9iMBpHK8QLLhx2wIPYuS5DYtR9Wa/YlZAbIovy7qVdB1Aq6Lyw==",
       "dev": true
     },
     "node_modules/@types/express": {
@@ -4932,9 +4916,9 @@
       }
     },
     "node_modules/@webassemblyjs/ast": {
-      "version": "1.11.6",
-      "resolved": "https://registry.npmjs.org/@webassemblyjs/ast/-/ast-1.11.6.tgz",
-      "integrity": "sha512-IN1xI7PwOvLPgjcf180gC1bqn3q/QaOCwYUahIOhbYUu8KA/3tw2RT/T0Gidi1l7Hhj5D/INhJxiICObqpMu4Q==",
+      "version": "1.12.1",
+      "resolved": "https://registry.npmjs.org/@webassemblyjs/ast/-/ast-1.12.1.tgz",
+      "integrity": "sha512-EKfMUOPRRUTy5UII4qJDGPpqfwjOmZ5jeGFwid9mnoqIFK+e0vqoi1qH56JpmZSzEL53jKnNzScdmftJyG5xWg==",
       "dev": true,
       "dependencies": {
         "@webassemblyjs/helper-numbers": "1.11.6",
@@ -4954,9 +4938,9 @@
       "dev": true
     },
     "node_modules/@webassemblyjs/helper-buffer": {
-      "version": "1.11.6",
-      "resolved": "https://registry.npmjs.org/@webassemblyjs/helper-buffer/-/helper-buffer-1.11.6.tgz",
-      "integrity": "sha512-z3nFzdcp1mb8nEOFFk8DrYLpHvhKC3grJD2ardfKOzmbmJvEf/tPIqCY+sNcwZIY8ZD7IkB2l7/pqhUhqm7hLA==",
+      "version": "1.12.1",
+      "resolved": "https://registry.npmjs.org/@webassemblyjs/helper-buffer/-/helper-buffer-1.12.1.tgz",
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       "bin": {

crates/compiler/collections/src/vec_set.rs

@@ -17,6 +17,10 @@ impl<T> VecSet<T> {
     pub fn into_vec(self) -> Vec<T> {
         self.elements
     }
+
+    pub fn reserve(&mut self, additional: usize) {
+        self.elements.reserve(additional)
+    }
 }
 
 impl<T: PartialEq> VecSet<T> {

crates/compiler/constrain/src/expr.rs

@@ -569,8 +569,8 @@ pub fn constrain_expr(
         Var(symbol, variable)
         | ParamsVar {
             symbol,
-            params: _,
             var: variable,
+            ..
         } => {
             // Save the expectation in the variable, then lookup the symbol's type in the environment
             let expected_type = *constraints[expected].get_type_ref();

crates/compiler/constrain/src/module.rs

@@ -3,13 +3,12 @@ use crate::pattern::{constrain_pattern, PatternState};
 use roc_can::abilities::{PendingAbilitiesStore, PendingMemberType};
 use roc_can::constraint::{Constraint, Constraints, Generalizable};
 use roc_can::expected::{Expected, PExpected};
-use roc_can::expr::{AnnotatedMark, Declarations};
+use roc_can::expr::Declarations;
+use roc_can::module::ModuleParams;
 use roc_can::pattern::Pattern;
 use roc_collections::MutMap;
-use roc_error_macros::internal_error;
 use roc_module::symbol::{ModuleId, Symbol};
 use roc_region::all::{Loc, Region};
-use roc_types::subs::Variable;
 use roc_types::types::{AnnotationSource, Category, Type, Types};
 
 pub fn constrain_module(
@@ -18,12 +17,12 @@ pub fn constrain_module(
     symbols_from_requires: Vec<(Loc<Symbol>, Loc<Type>)>,
     abilities_store: &PendingAbilitiesStore,
     declarations: &Declarations,
-    params_pattern: &Option<(Variable, AnnotatedMark, Loc<Pattern>)>,
+    opt_module_params: &Option<ModuleParams>,
     home: ModuleId,
 ) -> Constraint {
     let constraint = crate::expr::constrain_decls(types, constraints, home, declarations);
 
-    let constraint = match params_pattern {
+    let constraint = match opt_module_params {
         Some(params_pattern) => {
             constrain_params(types, constraints, home, constraint, params_pattern)
         }
@@ -51,7 +50,7 @@ fn constrain_params(
     constraints: &mut Constraints,
     home: ModuleId,
     constraint: Constraint,
-    (pattern_var, _, loc_pattern): &(Variable, AnnotatedMark, Loc<Pattern>),
+    module_params: &ModuleParams,
 ) -> Constraint {
     let mut env = Env {
         home,
@@ -59,23 +58,13 @@ fn constrain_params(
         resolutions_to_make: vec![],
     };
 
-    let index = constraints.push_variable(*pattern_var);
+    let index = constraints.push_variable(module_params.whole_var);
     let expected_params = constraints.push_pat_expected_type(PExpected::NoExpectation(index));
 
     let mut state = PatternState::default();
 
-    let closed_con = match loc_pattern.value {
-        Pattern::RecordDestructure {
-            whole_var: _,
-            ext_var,
-            destructs: _,
-        } => {
-            // Disallow record extension for module params
-            let empty_rec = constraints.push_type(types, Types::EMPTY_RECORD);
-            constraints.store(empty_rec, ext_var, file!(), line!())
-        }
-        _ => internal_error!("Only record destructures are allowed in module params. This should've been caught earlier."),
-    };
+    let empty_rec = constraints.push_type(types, Types::EMPTY_RECORD);
+    let closed_con = constraints.store(empty_rec, module_params.record_ext_var, file!(), line!());
 
     state.constraints.push(closed_con);
 
@@ -83,8 +72,8 @@ fn constrain_params(
         types,
         constraints,
         &mut env,
-        &loc_pattern.value,
-        loc_pattern.region,
+        &module_params.pattern().value,
+        module_params.region,
         expected_params,
         &mut state,
     );
@@ -100,7 +89,7 @@ fn constrain_params(
         Generalizable(true),
     );
 
-    constraints.exists([*pattern_var], cons)
+    constraints.exists([module_params.whole_var], cons)
 }
 
 fn constrain_symbols_from_requires(

crates/compiler/fmt/src/annotation.rs

@@ -38,6 +38,7 @@ pub enum Parens {
     InFunctionType,
     InApply,
     InOperator,
+    InAsPattern,
 }
 
 /// In an AST node, do we show newlines around it

crates/compiler/fmt/src/expr.rs

@@ -46,7 +46,8 @@ impl<'a> Formattable for Expr<'a> {
             | MalformedClosure
             | Tag(_)
             | OpaqueRef(_)
-            | Crash => false,
+            | Crash
+            | Dbg => false,
 
             RecordAccess(inner, _) | TupleAccess(inner, _) | TrySuffix { expr: inner, .. } => {
                 inner.is_multiline()
@@ -65,7 +66,7 @@ impl<'a> Formattable for Expr<'a> {
             Expect(condition, continuation) => {
                 condition.is_multiline() || continuation.is_multiline()
             }
-            Dbg(condition, _) => condition.is_multiline(),
+            DbgStmt(condition, _) => condition.is_multiline(),
             LowLevelDbg(_, _, _) => unreachable!(
                 "LowLevelDbg should only exist after desugaring, not during formatting"
             ),
@@ -453,8 +454,12 @@ impl<'a> Formattable for Expr<'a> {
             Expect(condition, continuation) => {
                 fmt_expect(buf, condition, continuation, self.is_multiline(), indent);
             }
-            Dbg(condition, continuation) => {
-                fmt_dbg(buf, condition, continuation, self.is_multiline(), indent);
+            Dbg => {
+                buf.indent(indent);
+                buf.push_str("dbg");
+            }
+            DbgStmt(condition, continuation) => {
+                fmt_dbg_stmt(buf, condition, continuation, self.is_multiline(), indent);
             }
             LowLevelDbg(_, _, _) => unreachable!(
                 "LowLevelDbg should only exist after desugaring, not during formatting"
@@ -1018,7 +1023,7 @@ fn fmt_when<'a>(
     }
 }
 
-fn fmt_dbg<'a>(
+fn fmt_dbg_stmt<'a>(
     buf: &mut Buf,
     condition: &'a Loc<Expr<'a>>,
     continuation: &'a Loc<Expr<'a>>,
@@ -1240,7 +1245,7 @@ fn fmt_closure<'a>(
     let mut it = loc_patterns.iter().peekable();
 
     while let Some(loc_pattern) = it.next() {
-        loc_pattern.format(buf, indent);
+        loc_pattern.format_with_options(buf, Parens::InAsPattern, Newlines::No, indent);
 
         if it.peek().is_some() {
             buf.indent(indent);

crates/compiler/fmt/src/header.rs

@@ -7,11 +7,10 @@ use crate::spaces::{fmt_comments_only, fmt_default_spaces, fmt_spaces, NewlineAt
 use crate::Buf;
 use roc_parse::ast::{Collection, CommentOrNewline, Header, Spaced, Spaces, SpacesBefore};
 use roc_parse::header::{
-    AppHeader, ExposedName, ExposesKeyword, GeneratesKeyword, HostedHeader, ImportsEntry,
-    ImportsKeyword, Keyword, KeywordItem, ModuleHeader, ModuleName, PackageEntry, PackageHeader,
-    PackageKeyword, PackageName, PackagesKeyword, PlatformHeader, PlatformKeyword,
-    PlatformRequires, ProvidesKeyword, ProvidesTo, RequiresKeyword, To, ToKeyword, TypedIdent,
-    WithKeyword,
+    AppHeader, ExposedName, ExposesKeyword, HostedHeader, ImportsEntry, ImportsKeyword, Keyword,
+    KeywordItem, ModuleHeader, ModuleName, PackageEntry, PackageHeader, PackageKeyword,
+    PackageName, PackagesKeyword, PlatformHeader, PlatformKeyword, PlatformRequires,
+    ProvidesKeyword, ProvidesTo, RequiresKeyword, To, ToKeyword, TypedIdent,
 };
 use roc_parse::ident::UppercaseIdent;
 use roc_region::all::Loc;
@@ -63,8 +62,6 @@ macro_rules! keywords {
 keywords! {
     ExposesKeyword,
     ImportsKeyword,
-    WithKeyword,
-    GeneratesKeyword,
     PackageKeyword,
     PackagesKeyword,
     RequiresKeyword,
@@ -206,9 +203,6 @@ pub fn fmt_hosted_header<'a>(buf: &mut Buf, header: &'a HostedHeader<'a>) {
     fmt_exposes(buf, header.exposes.item, indent);
     header.imports.keyword.format(buf, indent);
     fmt_imports(buf, header.imports.item, indent);
-    header.generates.format(buf, indent);
-    header.generates_with.keyword.format(buf, indent);
-    fmt_exposes(buf, header.generates_with.item, indent);
 }
 
 pub fn fmt_app_header<'a>(buf: &mut Buf, header: &'a AppHeader<'a>) {

crates/compiler/fmt/src/pattern.rs

@@ -244,9 +244,19 @@ impl<'a> Formattable for Pattern<'a> {
             }
 
             As(pattern, pattern_as) => {
+                let needs_parens = parens == Parens::InAsPattern;
+
+                if needs_parens {
+                    buf.push('(');
+                }
+
                 fmt_pattern(buf, &pattern.value, indent, parens);
 
                 pattern_as.format(buf, indent + INDENT);
+
+                if needs_parens {
+                    buf.push(')');
+                }
             }
 
             // Space

crates/compiler/load/build.rs

@@ -25,6 +25,7 @@ const MODULES: &[(ModuleId, &str)] = &[
     (ModuleId::DECODE, "Decode.roc"),
     (ModuleId::HASH, "Hash.roc"),
     (ModuleId::INSPECT, "Inspect.roc"),
+    (ModuleId::TASK, "Task.roc"),
 ];
 
 fn main() {

crates/compiler/load/src/lib.rs

@@ -256,6 +256,7 @@ fn read_cached_types() -> MutMap<ModuleId, TypeState> {
     let mod_decode = include_bytes_align_as!(u128, concat!(env!("OUT_DIR"), "/Decode.dat"));
     let mod_hash = include_bytes_align_as!(u128, concat!(env!("OUT_DIR"), "/Hash.dat"));
     let mod_inspect = include_bytes_align_as!(u128, concat!(env!("OUT_DIR"), "/Inspect.dat"));
+    let mod_task = include_bytes_align_as!(u128, concat!(env!("OUT_DIR"), "/Task.dat"));
 
     let mut output = MutMap::default();
 
@@ -279,6 +280,8 @@ fn read_cached_types() -> MutMap<ModuleId, TypeState> {
 
         output.insert(ModuleId::HASH, deserialize_help(mod_hash));
         output.insert(ModuleId::INSPECT, deserialize_help(mod_inspect));
+
+        output.insert(ModuleId::TASK, deserialize_help(mod_task));
     }
 
     output

crates/compiler/load/tests/helpers/mod.rs

@@ -51,7 +51,7 @@ pub fn infer_expr(
         exposed_by_module: &Default::default(),
         derived_module,
         function_kind: FunctionKind::LambdaSet,
-        params_pattern: None,
+        module_params: None,
         module_params_vars: Default::default(),
         #[cfg(debug_assertions)]
         checkmate: None,
@@ -161,22 +161,6 @@ pub fn can_expr_with<'a>(
     // ensure the Test module is accessible in our tests
     module_ids.get_or_insert(&PQModuleName::Unqualified("Test".into()));
 
-    // Desugar operators (convert them to Apply calls, taking into account
-    // operator precedence and associativity rules), before doing other canonicalization.
-    //
-    // If we did this *during* canonicalization, then each time we
-    // visited a BinOp node we'd recursively try to apply this to each of its nested
-    // operators, and then again on *their* nested operators, ultimately applying the
-    // rules multiple times unnecessarily.
-    let loc_expr = desugar::desugar_expr(
-        arena,
-        &loc_expr,
-        expr_str,
-        &mut None,
-        arena.alloc("TestPath"),
-        &mut Default::default(),
-    );
-
     let mut scope = Scope::new(
         home,
         "TestPath".into(),
@@ -187,12 +171,23 @@ pub fn can_expr_with<'a>(
     let dep_idents = IdentIds::exposed_builtins(0);
     let mut env = Env::new(
         arena,
+        expr_str,
         home,
         Path::new("Test.roc"),
         &dep_idents,
         &module_ids,
         None,
     );
+
+    // Desugar operators (convert them to Apply calls, taking into account
+    // operator precedence and associativity rules), before doing other canonicalization.
+    //
+    // If we did this *during* canonicalization, then each time we
+    // visited a BinOp node we'd recursively try to apply this to each of its nested
+    // operators, and then again on *their* nested operators, ultimately applying the
+    // rules multiple times unnecessarily.
+    let loc_expr = desugar::desugar_expr(&mut env, &mut scope, &loc_expr);
+
     let (loc_expr, output) = canonicalize_expr(
         &mut env,
         &mut var_store,

crates/compiler/load/tests/test_reporting.rs

@@ -3793,9 +3793,9 @@ mod test_reporting {
     of these?
 
         Set
+        Task
         List
         Dict
-        Hash
 
     ── SYNTAX PROBLEM in /code/proj/Main.roc ───────────────────────────────────────
 
@@ -5760,28 +5760,6 @@ mod test_reporting {
     "#
     );
 
-    test_report!(
-        dbg_without_final_expression,
-        indoc!(
-            r"
-            dbg 42
-            "
-        ),
-        @r#"
-    ── INDENT ENDS AFTER EXPRESSION in tmp/dbg_without_final_expression/Test.roc ───
-
-    I am partway through parsing a dbg statement, but I got stuck here:
-
-    4│      dbg 42
-                  ^
-
-    I was expecting a final expression, like so
-
-        dbg 42
-        "done"
-    "#
-    );
-
     test_report!(
         expect_without_final_expression,
         indoc!(
@@ -6318,16 +6296,17 @@ All branches in an `if` must have the same type!
         )
     }
 
+    // TODO: this test seems out of date (what is the `effects` clause?) and as such should be removed
     #[test]
     fn platform_requires_rigids() {
         report_header_problem_as(
             indoc!(
                 r#"
                 platform "folkertdev/foo"
-                    requires { main : Effect {} }
+                    requires { main : Task {} [] }
                     exposes []
                     packages {}
-                    imports [Task]
+                    imports []
                     provides [mainForHost]
                     effects fx.Effect
                          {
@@ -6344,13 +6323,13 @@ All branches in an `if` must have the same type!
                 I am partway through parsing a header, but I got stuck here:
 
                 1│  platform "folkertdev/foo"
-                2│      requires { main : Effect {} }
+                2│      requires { main : Task {} [] }
                                    ^
 
                 I am expecting a list of type names like `{}` or `{ Model }` next. A full
                 `requires` definition looks like
 
-                    requires { Model, Msg } {main : Effect {}}
+                    requires { Model, Msg } {main : Task {} []}
             "#
             ),
         )
@@ -8116,7 +8095,7 @@ All branches in an `if` must have the same type!
         unimported_modules_reported,
         indoc!(
             r#"
-            alt : Task.Task {} []
+            alt : Unimported.CustomType
             alt = "whatever man you don't even know my type"
             alt
             "#
@@ -8124,18 +8103,18 @@ All branches in an `if` must have the same type!
         @r"
     ── MODULE NOT IMPORTED in /code/proj/Main.roc ──────────────────────────────────
 
-    The `Task` module is not imported:
+    The `Unimported` module is not imported:
 
-    4│      alt : Task.Task {} []
-                  ^^^^^^^^^^^^^^^
+    4│      alt : Unimported.CustomType
+                  ^^^^^^^^^^^^^^^^^^^^^
 
     Is there an import missing? Perhaps there is a typo. Did you mean one
     of these?
 
-        Hash
+        Encode
+        Inspect
+        Dict
         List
-        Num
-        Box
     "
     );
 
@@ -14516,6 +14495,76 @@ All branches in an `if` must have the same type!
     "
     );
 
+    test_report!(
+        dbg_unapplied,
+        indoc!(
+            r"
+            1 + dbg + 2
+            "
+        ),
+    @r"
+    ── UNAPPLIED DBG in /code/proj/Main.roc ────────────────────────────────────────
+
+    This `dbg` doesn't have a value given to it:
+
+    4│      1 + dbg + 2
+                ^^^
+
+    `dbg` must be passed a value to print at the exact place it's used. `dbg`
+    can't be used as a value that's passed around, like functions can be -
+    it must be applied immediately!
+
+    ── TYPE MISMATCH in /code/proj/Main.roc ────────────────────────────────────────
+
+    This 2nd argument to + has an unexpected type:
+
+    4│      1 + dbg + 2
+                ^^^
+
+    This value is a:
+
+        {}
+
+    But + needs its 2nd argument to be:
+
+        Num *
+    "
+    );
+
+    test_report!(
+        dbg_overapplied,
+        indoc!(
+            r#"
+            1 + dbg "" "" + 2
+            "#
+        ),
+    @r#"
+    ── OVERAPPLIED DBG in /code/proj/Main.roc ──────────────────────────────────────
+
+    This `dbg` has too many values given to it:
+
+    4│      1 + dbg "" "" + 2
+                    ^^^^^
+
+    `dbg` must be given exactly one value to print.
+
+    ── TYPE MISMATCH in /code/proj/Main.roc ────────────────────────────────────────
+
+    This 2nd argument to + has an unexpected type:
+
+    4│      1 + dbg "" "" + 2
+                ^^^^^^^^^
+
+    This value is a:
+
+        {}
+
+    But + needs its 2nd argument to be:
+
+        Num *
+    "#
+    );
+
     // TODO: add the following tests after built-in Tasks are added
     // https://github.com/roc-lang/roc/pull/6836
 

crates/compiler/load_internal/Cargo.toml

@@ -33,6 +33,7 @@ roc_tracing = { path = "../../tracing" }
 roc_types = { path = "../types" }
 roc_unify = { path = "../unify" }
 roc_worker = { path = "../worker" }
+roc_lower_params = { path = "../lower_params" }
 
 ven_pretty = { path = "../../vendor/pretty" }
 

crates/compiler/load_internal/src/file.rs

@@ -18,7 +18,7 @@ use roc_can::constraint::{Constraint as ConstraintSoa, Constraints, TypeOrVar};
 use roc_can::expr::{DbgLookup, Declarations, ExpectLookup, PendingDerives};
 use roc_can::module::{
     canonicalize_module_defs, ExposedByModule, ExposedForModule, ExposedModuleTypes, Module,
-    ResolvedImplementations, TypeState,
+    ModuleParams, ResolvedImplementations, TypeState,
 };
 use roc_collections::{default_hasher, BumpMap, MutMap, MutSet, VecMap, VecSet};
 use roc_constrain::module::constrain_module;
@@ -251,6 +251,7 @@ fn start_phase<'a>(
 
                 let mut aliases = MutMap::default();
                 let mut abilities_store = PendingAbilitiesStore::default();
+                let mut imported_module_params = VecMap::default();
 
                 for imported in parsed.available_modules.keys() {
                     match state.module_cache.aliases.get(imported) {
@@ -293,6 +294,10 @@ fn start_phase<'a>(
                                 .union(import_store.closure_from_imported(exposed_symbols));
                         }
                     }
+
+                    if let Some(params) = state.module_cache.module_params.get(imported) {
+                        imported_module_params.insert(*imported, params.clone());
+                    }
                 }
 
                 let skip_constraint_gen = {
@@ -310,6 +315,8 @@ fn start_phase<'a>(
                     abilities_store,
                     skip_constraint_gen,
                     exposed_module_ids: state.exposed_modules,
+                    exec_mode: state.exec_mode,
+                    imported_module_params,
                 }
             }
 
@@ -356,6 +363,7 @@ fn start_phase<'a>(
                     declarations,
                     state.cached_types.clone(),
                     derived_module,
+                    state.exec_mode,
                     //
                     #[cfg(debug_assertions)]
                     checkmate,
@@ -889,6 +897,8 @@ enum BuildTask<'a> {
         abilities_store: PendingAbilitiesStore,
         exposed_module_ids: &'a [ModuleId],
         skip_constraint_gen: bool,
+        exec_mode: ExecutionMode,
+        imported_module_params: VecMap<ModuleId, ModuleParams>,
     },
     Solve {
         module: Module,
@@ -905,6 +915,7 @@ enum BuildTask<'a> {
         dep_idents: IdentIdsByModule,
         cached_subs: CachedTypeState,
         derived_module: SharedDerivedModule,
+        exec_mode: ExecutionMode,
 
         #[cfg(debug_assertions)]
         checkmate: Option<roc_checkmate::Collector>,
@@ -2318,6 +2329,7 @@ fn update<'a>(
                 extend_module_with_builtin_import(parsed, ModuleId::DECODE);
                 extend_module_with_builtin_import(parsed, ModuleId::HASH);
                 extend_module_with_builtin_import(parsed, ModuleId::INSPECT);
+                extend_module_with_builtin_import(parsed, ModuleId::TASK);
             }
 
             state
@@ -2395,11 +2407,11 @@ fn update<'a>(
                 .pending_abilities
                 .insert(module_id, constrained_module.module.abilities_store.clone());
 
-            if let Some(params_pattern) = constrained_module.module.params_pattern.clone() {
+            if let Some(module_params) = constrained_module.module.module_params.clone() {
                 state
                     .module_cache
-                    .param_patterns
-                    .insert(module_id, params_pattern);
+                    .module_params
+                    .insert(module_id, module_params);
             }
 
             state
@@ -3557,7 +3569,6 @@ fn load_builtin_module_help<'a>(
                 header_type: HeaderType::Builtin {
                     name: header::ModuleName::new(name_stem),
                     exposes: unspace(arena, header.exposes.items),
-                    generates_with: &[],
                     opt_params: header.params,
                 },
                 module_comments: comments,
@@ -3640,6 +3651,7 @@ fn load_module<'a>(
         "Decode", ModuleId::DECODE
         "Hash", ModuleId::HASH
         "Inspect", ModuleId::INSPECT
+        "Task", ModuleId::TASK
     }
 
     let (filename, opt_shorthand) = module_name_to_path(src_dir, &module_name, arc_shorthands);
@@ -3875,8 +3887,6 @@ fn parse_header<'a>(
                 header_type: HeaderType::Hosted {
                     name: header.name.value,
                     exposes: unspace(arena, header.exposes.item.items),
-                    generates: header.generates.item,
-                    generates_with: unspace(arena, header.generates_with.item.items),
                 },
                 module_comments: comments,
                 header_imports: Some(header.imports),
@@ -4311,6 +4321,7 @@ impl<'a> BuildTask<'a> {
         declarations: Declarations,
         cached_subs: CachedTypeState,
         derived_module: SharedDerivedModule,
+        exec_mode: ExecutionMode,
 
         #[cfg(debug_assertions)] checkmate: Option<roc_checkmate::Collector>,
     ) -> Self {
@@ -4334,6 +4345,7 @@ impl<'a> BuildTask<'a> {
             module_timing,
             cached_subs,
             derived_module,
+            exec_mode,
 
             #[cfg(debug_assertions)]
             checkmate,
@@ -4618,6 +4630,7 @@ struct SolveResult {
     exposed_vars_by_symbol: Vec<(Symbol, Variable)>,
     problems: Vec<TypeError>,
     abilities_store: AbilitiesStore,
+    imported_modules_with_params: Vec<ModuleId>,
 
     #[cfg(debug_assertions)]
     checkmate: Option<roc_checkmate::Collector>,
@@ -4642,7 +4655,7 @@ fn run_solve_solve(
         aliases,
         rigid_variables,
         abilities_store: pending_abilities,
-        params_pattern,
+        module_params,
         ..
     } = module;
 
@@ -4663,6 +4676,11 @@ fn run_solve_solve(
         &mut imported_flex_vars,
     );
 
+    let imported_modules_with_params = imported_param_vars
+        .keys()
+        .copied()
+        .collect::<Vec<ModuleId>>();
+
     let actual_constraint = constraints.let_import_constraint(
         imported_rigid_vars,
         imported_flex_vars,
@@ -4690,7 +4708,7 @@ fn run_solve_solve(
             derived_module,
             #[cfg(debug_assertions)]
             checkmate,
-            params_pattern: params_pattern.map(|(_, _, pattern)| pattern.value),
+            module_params,
             module_params_vars: imported_param_vars,
         };
 
@@ -4745,6 +4763,7 @@ fn run_solve_solve(
         exposed_vars_by_symbol,
         problems: errors,
         abilities_store: resolved_abilities_store,
+        imported_modules_with_params,
 
         #[cfg(debug_assertions)]
         checkmate,
@@ -4766,6 +4785,7 @@ fn run_solve<'a>(
     dep_idents: IdentIdsByModule,
     cached_types: CachedTypeState,
     derived_module: SharedDerivedModule,
+    exec_mode: ExecutionMode,
 
     #[cfg(debug_assertions)] checkmate: Option<roc_checkmate::Collector>,
 ) -> Msg<'a> {
@@ -4776,10 +4796,8 @@ fn run_solve<'a>(
     // TODO remove when we write builtins in roc
     let aliases = module.aliases.clone();
 
-    let opt_params_var = module
-        .params_pattern
-        .as_ref()
-        .map(|(params_var, _, _)| *params_var);
+    let opt_params_var = module.module_params.as_ref().map(|params| params.whole_var);
+    let home_has_params = opt_params_var.is_some();
 
     let mut module = module;
     let loc_expects = std::mem::take(&mut module.loc_expects);
@@ -4814,6 +4832,7 @@ fn run_solve<'a>(
                     exposed_vars_by_symbol,
                     problems: vec![],
                     abilities_store: abilities,
+                    imported_modules_with_params: vec![],
 
                     #[cfg(debug_assertions)]
                     checkmate: None,
@@ -4841,8 +4860,9 @@ fn run_solve<'a>(
         solved: mut solved_subs,
         solved_implementations,
         exposed_vars_by_symbol,
-        problems,
+        mut problems,
         abilities_store,
+        imported_modules_with_params,
 
         #[cfg(debug_assertions)]
         checkmate,
@@ -4857,6 +4877,20 @@ fn run_solve<'a>(
         &abilities_store,
     );
 
+    match exec_mode {
+        ExecutionMode::Check => {
+            // Params are not lowered in check mode
+        }
+        ExecutionMode::Executable | ExecutionMode::ExecutableIfCheck | ExecutionMode::Test => {
+            roc_lower_params::type_error::remove_module_param_arguments(
+                &mut problems,
+                home_has_params,
+                &decls.symbols,
+                imported_modules_with_params,
+            );
+        }
+    }
+
     let solved_module = SolvedModule {
         exposed_vars_by_symbol,
         problems,
@@ -5006,6 +5040,8 @@ fn canonicalize_and_constrain<'a>(
     parsed: ParsedModule<'a>,
     skip_constraint_gen: bool,
     exposed_module_ids: &[ModuleId],
+    exec_mode: ExecutionMode,
+    imported_module_params: VecMap<ModuleId, ModuleParams>,
 ) -> CanAndCon {
     let canonicalize_start = Instant::now();
 
@@ -5032,7 +5068,7 @@ fn canonicalize_and_constrain<'a>(
 
     let mut var_store = VarStore::default();
 
-    let module_output = canonicalize_module_defs(
+    let mut module_output = canonicalize_module_defs(
         arena,
         parsed_defs,
         &header_type,
@@ -5091,6 +5127,26 @@ fn canonicalize_and_constrain<'a>(
     // _before has an underscore because it's unused in --release builds
     let _before = roc_types::types::get_type_clone_count();
 
+    match exec_mode {
+        ExecutionMode::Check => {
+            // No need to lower params for `roc check` and lang server
+            // If we did, we'd have to update the language server to exclude the extra arguments
+        }
+        ExecutionMode::Executable | ExecutionMode::ExecutableIfCheck | ExecutionMode::Test => {
+            // We need to lower params only if the current module has any or imports at least one with params
+            if module_output.module_params.is_some() || !imported_module_params.is_empty() {
+                roc_lower_params::lower::lower(
+                    module_id,
+                    &module_output.module_params,
+                    imported_module_params,
+                    &mut module_output.declarations,
+                    &mut module_output.scope.locals.ident_ids,
+                    &mut var_store,
+                );
+            }
+        }
+    }
+
     let mut constraints = Constraints::new();
 
     let constraint = if skip_constraint_gen {
@@ -5102,7 +5158,7 @@ fn canonicalize_and_constrain<'a>(
             module_output.symbols_from_requires,
             &module_output.scope.abilities_store,
             &module_output.declarations,
-            &module_output.params_pattern,
+            &module_output.module_params,
             module_id,
         )
     };
@@ -5140,6 +5196,7 @@ fn canonicalize_and_constrain<'a>(
                         | ModuleId::SET
                         | ModuleId::HASH
                         | ModuleId::INSPECT
+                        | ModuleId::TASK
                 );
 
                 if !name.is_builtin() || should_include_builtin {
@@ -5159,7 +5216,7 @@ fn canonicalize_and_constrain<'a>(
         abilities_store: module_output.scope.abilities_store,
         loc_expects: module_output.loc_expects,
         loc_dbgs: module_output.loc_dbgs,
-        params_pattern: module_output.params_pattern,
+        module_params: module_output.module_params,
     };
 
     let constrained_module = ConstrainedModule {
@@ -6209,6 +6266,8 @@ fn run_task<'a>(
             abilities_store,
             skip_constraint_gen,
             exposed_module_ids,
+            exec_mode,
+            imported_module_params,
         } => {
             let can_and_con = canonicalize_and_constrain(
                 arena,
@@ -6220,6 +6279,8 @@ fn run_task<'a>(
                 parsed,
                 skip_constraint_gen,
                 exposed_module_ids,
+                exec_mode,
+                imported_module_params,
             );
 
             Ok(Msg::CanonicalizedAndConstrained(can_and_con))
@@ -6239,6 +6300,7 @@ fn run_task<'a>(
             dep_idents,
             cached_subs,
             derived_module,
+            exec_mode,
 
             #[cfg(debug_assertions)]
             checkmate,
@@ -6257,6 +6319,7 @@ fn run_task<'a>(
             dep_idents,
             cached_subs,
             derived_module,
+            exec_mode,
             //
             #[cfg(debug_assertions)]
             checkmate,

crates/compiler/load_internal/src/lib.rs

@@ -25,4 +25,5 @@ pub const BUILTIN_MODULES: &[(ModuleId, &str)] = &[
     (ModuleId::DECODE, "Decode"),
     (ModuleId::HASH, "Hash"),
     (ModuleId::INSPECT, "Inspect"),
+    (ModuleId::TASK, "Task"),
 ];

crates/compiler/load_internal/src/module_cache.rs

@@ -4,13 +4,13 @@ use crate::module::{
     ModuleHeader, ParsedModule, TypeCheckedModule,
 };
 use roc_can::abilities::PendingAbilitiesStore;
-use roc_can::expr::AnnotatedMark;
+use roc_can::module::ModuleParams;
 use roc_collections::{MutMap, MutSet, VecMap};
 use roc_module::ident::ModuleName;
 use roc_module::symbol::{ModuleId, PQModuleName, Symbol};
 use roc_mono::ir::ExternalSpecializations;
 use roc_problem::Severity;
-use roc_region::all::{Loc, Region};
+use roc_region::all::Region;
 use roc_solve_problem::TypeError;
 use roc_types::subs::Variable;
 use roc_types::types::Alias;
@@ -27,8 +27,7 @@ pub(crate) struct ModuleCache<'a> {
     pub(crate) aliases: MutMap<ModuleId, MutMap<Symbol, (bool, Alias)>>,
     pub(crate) pending_abilities: MutMap<ModuleId, PendingAbilitiesStore>,
     pub(crate) constrained: MutMap<ModuleId, ConstrainedModule>,
-    pub(crate) param_patterns:
-        MutMap<ModuleId, (Variable, AnnotatedMark, Loc<roc_can::pattern::Pattern>)>,
+    pub(crate) module_params: MutMap<ModuleId, ModuleParams>,
     pub(crate) typechecked: MutMap<ModuleId, TypeCheckedModule<'a>>,
     pub(crate) checked: MutMap<ModuleId, CheckedModule>,
     pub(crate) found_specializations: MutMap<ModuleId, FoundSpecializationsModule<'a>>,
@@ -94,6 +93,7 @@ impl Default for ModuleCache<'_> {
             DECODE,
             HASH,
             INSPECT,
+            TASK,
         }
 
         Self {
@@ -103,7 +103,7 @@ impl Default for ModuleCache<'_> {
             aliases: Default::default(),
             pending_abilities: Default::default(),
             constrained: Default::default(),
-            param_patterns: Default::default(),
+            module_params: Default::default(),
             typechecked: Default::default(),
             checked: Default::default(),
             found_specializations: Default::default(),

crates/compiler/load_internal/tests/test_load.rs

@@ -2151,7 +2151,7 @@ fn roc_file_no_extension() {
         indoc!(
             r#"
             app "helloWorld"
-                packages { pf: "https://github.com/roc-lang/basic-cli/releases/download/0.14.0/dC5ceT962N_4jmoyoffVdphJ_4GlW3YMhAPyGPr-nU0.tar.br" }
+                packages { pf: "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br" }
                 imports [pf.Stdout]
                 provides [main] to pf
 

crates/compiler/lower_params/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "roc_lower_params"
+description = "Lowers module params by extending functions and their calls"
+
+authors.workspace = true
+edition.workspace = true
+license.workspace = true
+version.workspace = true
+
+[dependencies]
+roc_can = { path = "../can" }
+roc_module = { path = "../module" }
+roc_region = { path = "../region" }
+roc_types = { path = "../types" }
+roc_collections = { path = "../collections" }
+roc_solve_problem = { path = "../solve_problem" }
+
+bumpalo.workspace = true

crates/compiler/lower_params/src/lib.rs

@@ -0,0 +1,2 @@
+pub mod lower;
+pub mod type_error;

crates/compiler/lower_params/src/lower.rs

@@ -0,0 +1,566 @@
+use roc_can::{
+    expr::{
+        AnnotatedMark, ClosureData,
+        DeclarationTag::*,
+        Declarations,
+        Expr::{self, *},
+    },
+    module::ModuleParams,
+    pattern::{Pattern, RecordDestruct},
+};
+use roc_collections::VecMap;
+use roc_module::symbol::{IdentId, IdentIds, ModuleId, Symbol};
+use roc_region::all::Loc;
+use roc_types::{
+    subs::{VarStore, Variable},
+    types::Type,
+};
+use std::iter::once;
+
+struct LowerParams<'a> {
+    home_id: ModuleId,
+    home_params: &'a Option<ModuleParams>,
+    imported_params: VecMap<ModuleId, ModuleParams>,
+    var_store: &'a mut VarStore,
+    ident_ids: &'a mut IdentIds,
+    top_level_idents: Vec<IdentId>,
+}
+
+pub fn lower(
+    home_id: ModuleId,
+    home_params: &Option<ModuleParams>,
+    imported_params: VecMap<ModuleId, ModuleParams>,
+    decls: &mut Declarations,
+    ident_ids: &mut IdentIds,
+    var_store: &mut VarStore,
+) {
+    let top_level_idents = decls
+        .symbols
+        .iter()
+        .map(|symbol| symbol.value.ident_id())
+        .collect();
+
+    let mut env = LowerParams {
+        home_id,
+        home_params,
+        imported_params,
+        ident_ids,
+        var_store,
+        top_level_idents,
+    };
+
+    env.lower_decls(decls);
+}
+
+impl<'a> LowerParams<'a> {
+    fn lower_decls(&mut self, decls: &mut Declarations) {
+        let home_param_symbols = match self.home_params {
+            Some(params) => params
+                .destructs
+                .iter()
+                .map(|destruct| destruct.value.symbol)
+                .chain(once(params.whole_symbol))
+                .collect::<Vec<Symbol>>(),
+            None => vec![],
+        };
+
+        for index in 0..decls.len() {
+            let tag = decls.declarations[index];
+
+            match tag {
+                Value => {
+                    self.lower_expr(&mut decls.expressions[index].value);
+
+                    if let Some(new_arg) = self.home_params_argument() {
+                        // This module has params, and this is a top-level value,
+                        // so we need to convert it into a function that takes them.
+
+                        decls.convert_value_to_function(index, vec![new_arg], self.var_store);
+                    }
+                }
+                Function(fn_def_index) | Recursive(fn_def_index) | TailRecursive(fn_def_index) => {
+                    if let Some((var, mark, pattern)) = self.home_params_argument() {
+                        // This module has params, and this is a top-level function,
+                        // so we need to extend its definition to take them.
+
+                        let function_body = &mut decls.function_bodies[fn_def_index.index()].value;
+                        function_body.arguments.push((var, mark, pattern));
+
+                        // Remove home params from the captured symbols, only nested lambdas need them.
+                        function_body
+                            .captured_symbols
+                            .retain(|(sym, _)| !home_param_symbols.contains(sym));
+
+                        if let Some(ann) = &mut decls.annotations[index] {
+                            if let Type::Function(args, _, _) = &mut ann.signature {
+                                args.push(Type::Variable(var));
+                            }
+                        }
+                    }
+
+                    self.lower_expr(&mut decls.expressions[index].value);
+                }
+
+                Destructure(_) | Expectation | ExpectationFx => {
+                    self.lower_expr(&mut decls.expressions[index].value);
+                }
+                MutualRecursion { .. } => {}
+            }
+        }
+    }
+
+    fn lower_expr(&mut self, expr: &mut Expr) {
+        let mut expr_stack = vec![expr];
+
+        while let Some(expr) = expr_stack.pop() {
+            match expr {
+                // Nodes to lower
+                ParamsVar {
+                    symbol,
+                    var,
+                    params_symbol,
+                    params_var,
+                } => {
+                    // The module was imported with params, but it might not actually expect them.
+                    // We should only lower if it does to prevent confusing type errors.
+                    if let Some(arity) = self.get_imported_def_arity(symbol) {
+                        *expr = self.lower_naked_params_var(
+                            arity,
+                            *symbol,
+                            *var,
+                            *params_symbol,
+                            *params_var,
+                        );
+                    }
+                }
+                Var(symbol, var) => {
+                    if let Some((params, arity)) = self.params_extended_home_symbol(symbol) {
+                        *expr = self.lower_naked_params_var(
+                            arity,
+                            *symbol,
+                            *var,
+                            params.whole_symbol,
+                            params.whole_var,
+                        );
+                    }
+                }
+                Call(fun, args, _called_via) => {
+                    expr_stack.reserve(args.len() + 1);
+
+                    match fun.1.value {
+                        ParamsVar {
+                            symbol,
+                            var,
+                            params_var,
+                            params_symbol,
+                        } => {
+                            // Calling an imported function with params
+
+                            match self.get_imported_def_arity(&symbol) {
+                                Some(0) => {
+                                    // We are calling a function but the top-level declaration has no arguments.
+                                    // This can either be a function alias or a top-level def that returns functions
+                                    // under multiple branches.
+                                    // We call the value def with params, and apply the returned function to the original arguments.
+                                    fun.1.value = self.call_value_def_with_params(
+                                        symbol,
+                                        var,
+                                        params_symbol,
+                                        params_var,
+                                    );
+                                }
+                                Some(_) => {
+                                    // The module expects params and they were provided, we need to extend the call.
+                                    fun.1.value = Var(symbol, var);
+
+                                    args.push((
+                                        params_var,
+                                        Loc::at_zero(Var(params_symbol, params_var)),
+                                    ));
+                                }
+                                None => {
+                                    // The module expects no params, do not extend to prevent confusing type errors.
+                                    fun.1.value = Var(symbol, var);
+                                }
+                            }
+                        }
+                        Var(symbol, var) => {
+                            if let Some((params, arity)) = self.params_extended_home_symbol(&symbol)
+                            {
+                                if arity == 0 {
+                                    // Calling the result of a top-level value def in the current module
+                                    fun.1.value = self.call_value_def_with_params(
+                                        symbol,
+                                        var,
+                                        params.whole_symbol,
+                                        params.whole_var,
+                                    );
+                                } else {
+                                    // Calling a top-level function in the current module with params
+                                    args.push((
+                                        params.whole_var,
+                                        Loc::at_zero(Var(params.whole_symbol, params.whole_var)),
+                                    ));
+                                }
+                            }
+                        }
+                        _ => expr_stack.push(&mut fun.1.value),
+                    }
+
+                    for (_, arg) in args.iter_mut() {
+                        expr_stack.push(&mut arg.value);
+                    }
+                }
+
+                // Nodes to walk
+                Closure(ClosureData {
+                    loc_body,
+                    captured_symbols: _,
+                    name: _,
+                    function_type: _,
+                    closure_type: _,
+                    return_type: _,
+                    recursive: _,
+                    arguments: _,
+                }) => {
+                    expr_stack.push(&mut loc_body.value);
+                }
+
+                LetNonRec(def, cont) => {
+                    expr_stack.reserve(2);
+                    expr_stack.push(&mut def.loc_expr.value);
+                    expr_stack.push(&mut cont.value);
+                }
+
+                LetRec(defs, cont, _cycle_mark) => {
+                    expr_stack.reserve(defs.len() + 1);
+
+                    for def in defs {
+                        expr_stack.push(&mut def.loc_expr.value);
+                    }
+
+                    expr_stack.push(&mut cont.value);
+                }
+                When {
+                    loc_cond,
+                    branches,
+                    cond_var: _,
+                    expr_var: _,
+                    region: _,
+                    branches_cond_var: _,
+                    exhaustive: _,
+                } => {
+                    expr_stack.reserve(branches.len() + 1);
+                    expr_stack.push(&mut loc_cond.value);
+
+                    for branch in branches.iter_mut() {
+                        expr_stack.push(&mut branch.value.value);
+                    }
+                }
+                If {
+                    branches,
+                    final_else,
+                    cond_var: _,
+                    branch_var: _,
+                } => {
+                    expr_stack.reserve(branches.len() * 2 + 1);
+
+                    for (cond, ret) in branches.iter_mut() {
+                        expr_stack.push(&mut cond.value);
+                        expr_stack.push(&mut ret.value);
+                    }
+
+                    expr_stack.push(&mut final_else.value);
+                }
+                RunLowLevel {
+                    args,
+                    op: _,
+                    ret_var: _,
+                }
+                | ForeignCall {
+                    foreign_symbol: _,
+                    args,
+                    ret_var: _,
+                } => {
+                    expr_stack.extend(args.iter_mut().map(|(_, arg)| arg));
+                }
+                List {
+                    elem_var: _,
+                    loc_elems,
+                } => {
+                    expr_stack.extend(loc_elems.iter_mut().map(|loc_elem| &mut loc_elem.value));
+                }
+                Record {
+                    record_var: _,
+                    fields,
+                } => {
+                    expr_stack.extend(
+                        fields
+                            .iter_mut()
+                            .map(|(_, field)| &mut field.loc_expr.value),
+                    );
+                }
+                Tuple {
+                    tuple_var: _,
+                    elems,
+                } => {
+                    expr_stack.extend(elems.iter_mut().map(|(_, elem)| &mut elem.value));
+                }
+                ImportParams(_, _, Some((_, params_expr))) => {
+                    expr_stack.push(params_expr);
+                }
+                Crash { msg, ret_var: _ } => {
+                    expr_stack.push(&mut msg.value);
+                }
+                RecordAccess {
+                    loc_expr,
+                    record_var: _,
+                    ext_var: _,
+                    field_var: _,
+                    field: _,
+                } => expr_stack.push(&mut loc_expr.value),
+                TupleAccess {
+                    loc_expr,
+                    tuple_var: _,
+                    ext_var: _,
+                    elem_var: _,
+                    index: _,
+                } => expr_stack.push(&mut loc_expr.value),
+                RecordUpdate {
+                    updates,
+                    record_var: _,
+                    ext_var: _,
+                    symbol: _,
+                } => expr_stack.extend(
+                    updates
+                        .iter_mut()
+                        .map(|(_, field)| &mut field.loc_expr.value),
+                ),
+                Tag {
+                    arguments,
+                    tag_union_var: _,
+                    ext_var: _,
+                    name: _,
+                } => expr_stack.extend(arguments.iter_mut().map(|(_, arg)| &mut arg.value)),
+                OpaqueRef {
+                    argument,
+                    opaque_var: _,
+                    name: _,
+                    specialized_def_type: _,
+                    type_arguments: _,
+                    lambda_set_variables: _,
+                } => expr_stack.push(&mut argument.1.value),
+                Expect {
+                    loc_condition,
+                    loc_continuation,
+                    lookups_in_cond: _,
+                } => {
+                    expr_stack.reserve(2);
+                    expr_stack.push(&mut loc_condition.value);
+                    expr_stack.push(&mut loc_continuation.value);
+                }
+                ExpectFx {
+                    loc_condition,
+                    loc_continuation,
+                    lookups_in_cond: _,
+                } => {
+                    expr_stack.reserve(2);
+                    expr_stack.push(&mut loc_condition.value);
+                    expr_stack.push(&mut loc_continuation.value);
+                }
+                Dbg {
+                    loc_message,
+                    loc_continuation,
+                    source_location: _,
+                    source: _,
+                    variable: _,
+                    symbol: _,
+                } => {
+                    expr_stack.reserve(2);
+                    expr_stack.push(&mut loc_message.value);
+                    expr_stack.push(&mut loc_continuation.value);
+                }
+                RecordAccessor(_)
+                | ImportParams(_, _, None)
+                | ZeroArgumentTag {
+                    closure_name: _,
+                    variant_var: _,
+                    ext_var: _,
+                    name: _,
+                }
+                | OpaqueWrapFunction(_)
+                | EmptyRecord
+                | TypedHole(_)
+                | RuntimeError(_)
+                | Num(_, _, _, _)
+                | Int(_, _, _, _, _)
+                | Float(_, _, _, _, _)
+                | Str(_)
+                | SingleQuote(_, _, _, _)
+                | IngestedFile(_, _, _)
+                | AbilityMember(_, _, _) => { /* terminal */ }
+            }
+        }
+    }
+
+    fn unique_symbol(&mut self) -> Symbol {
+        Symbol::new(self.home_id, self.ident_ids.gen_unique())
+    }
+
+    fn home_params_argument(&mut self) -> Option<(Variable, AnnotatedMark, Loc<Pattern>)> {
+        match &self.home_params {
+            Some(module_params) => {
+                let destructs: Vec<Loc<RecordDestruct>> = module_params
+                    .destructs
+                    .iter()
+                    .map(|destructure| {
+                        destructure.map(|d| RecordDestruct {
+                            symbol: d.symbol,
+                            var: self.var_store.fresh(),
+                            label: d.label.clone(),
+                            typ: d.typ.clone(),
+                        })
+                    })
+                    .collect();
+
+                let record_pattern = Pattern::RecordDestructure {
+                    whole_var: module_params.record_var,
+                    ext_var: module_params.record_ext_var,
+                    destructs,
+                };
+                let loc_record_pattern = Loc::at(module_params.region, record_pattern);
+                let as_pattern =
+                    Pattern::As(Box::new(loc_record_pattern), module_params.whole_symbol);
+                let loc_pattern = Loc::at(module_params.region, as_pattern);
+
+                Some((
+                    self.var_store.fresh(),
+                    AnnotatedMark::new(self.var_store),
+                    loc_pattern,
+                ))
+            }
+            None => None,
+        }
+    }
+
+    fn params_extended_home_symbol(&self, symbol: &Symbol) -> Option<(&ModuleParams, usize)> {
+        if symbol.module_id() == self.home_id {
+            self.home_params.as_ref().and_then(|params| {
+                params
+                    .arity_by_name
+                    .get(&symbol.ident_id())
+                    .map(|arity| (params, *arity))
+            })
+        } else {
+            None
+        }
+    }
+
+    fn get_imported_def_arity(&self, symbol: &Symbol) -> Option<usize> {
+        self.imported_params
+            .get(&symbol.module_id())
+            .and_then(|params| params.arity_by_name.get(&symbol.ident_id()).copied())
+    }
+
+    fn lower_naked_params_var(
+        &mut self,
+        arity: usize,
+        symbol: Symbol,
+        var: Variable,
+        params_symbol: Symbol,
+        params_var: Variable,
+    ) -> Expr {
+        if arity == 0 {
+            // We are passing a top-level value that takes params, so we need to replace the Var
+            // with a call that passes the params to get the final result.
+            //
+            // value = \#params -> #params.x * 2
+            // record = \... #params -> { doubled: value }
+            //                                       ↓
+            //                                     value #params
+            self.call_value_def_with_params(symbol, var, params_symbol, params_var)
+        } else {
+            // We are passing a top-level function that takes params, so we need to replace
+            // the Var with a closure that captures the params and passes them to the function.
+            //
+            // fn1 = \arg #params -> #params.x * arg
+            // fn2 = \... #params -> List.map [1, 2] fn1
+            //                                        ↓
+            //                                       (\#1 -> fn1 #1 #params)
+            //
+            let mut arguments = Vec::with_capacity(arity);
+            let mut call_arguments = Vec::with_capacity(arity + 1);
+
+            for _ in 0..arity {
+                let sym = self.unique_symbol();
+                let var = self.var_store.fresh();
+
+                arguments.push((
+                    var,
+                    AnnotatedMark::new(self.var_store),
+                    Loc::at_zero(Pattern::Identifier(sym)),
+                ));
+                call_arguments.push((var, Loc::at_zero(Var(sym, var))));
+            }
+
+            let params_arg = (params_var, Loc::at_zero(Var(params_symbol, params_var)));
+
+            call_arguments.push(params_arg);
+
+            let call_fn = Box::new((
+                self.var_store.fresh(),
+                Loc::at_zero(Var(symbol, var)),
+                self.var_store.fresh(),
+                self.var_store.fresh(),
+            ));
+
+            let body = Call(
+                call_fn,
+                call_arguments,
+                roc_module::called_via::CalledVia::NakedParamsVar,
+            );
+
+            let captured_symbols = if symbol.module_id() == self.home_id
+                || !self.top_level_idents.contains(&params_symbol.ident_id())
+            {
+                vec![(params_symbol, params_var)]
+            } else {
+                vec![]
+            };
+
+            Closure(ClosureData {
+                function_type: self.var_store.fresh(),
+                closure_type: self.var_store.fresh(),
+                return_type: self.var_store.fresh(),
+                name: self.unique_symbol(),
+                captured_symbols,
+                recursive: roc_can::expr::Recursive::NotRecursive,
+                arguments,
+                loc_body: Box::new(Loc::at_zero(body)),
+            })
+        }
+    }
+
+    fn call_value_def_with_params(
+        &mut self,
+        symbol: Symbol,
+        var: Variable,
+        params_symbol: Symbol,
+        params_var: Variable,
+    ) -> Expr {
+        let params_arg = (params_var, Loc::at_zero(Var(params_symbol, params_var)));
+
+        let call_fn = Box::new((
+            self.var_store.fresh(),
+            Loc::at_zero(Var(symbol, var)),
+            self.var_store.fresh(),
+            self.var_store.fresh(),
+        ));
+
+        Call(
+            call_fn,
+            vec![params_arg],
+            roc_module::called_via::CalledVia::NakedParamsVar,
+        )
+    }
+}

crates/compiler/lower_params/src/type_error.rs

@@ -0,0 +1,208 @@
+use roc_can::{expected::Expected, pattern::Pattern};
+use roc_module::symbol::{ModuleId, Symbol};
+use roc_region::all::Loc;
+use roc_solve_problem::TypeError;
+use roc_types::types::{ErrorType, Reason};
+
+/// Removes the arguments added as a result of lowering params
+pub fn remove_module_param_arguments(
+    errors: &mut Vec<TypeError>,
+    home_has_params: bool,
+    home_top_level_symbols: &[Loc<Symbol>],
+    imported_modules_with_params: Vec<ModuleId>,
+) {
+    if errors.is_empty() || (!home_has_params && imported_modules_with_params.is_empty()) {
+        return;
+    }
+
+    let home_top_level_symbols = if home_has_params {
+        home_top_level_symbols
+            .iter()
+            .map(|loc| loc.value)
+            .collect::<Vec<_>>()
+    } else {
+        vec![]
+    };
+
+    let env = Env {
+        home_top_level_symbols,
+        imported_modules_with_params,
+    };
+
+    for error in errors {
+        match error {
+            TypeError::BadExpr(_, _, found, Expected::ForReason(reason, expected, _)) => {
+                remove_for_reason(&env, found, reason, expected);
+            }
+
+            TypeError::BadExpr(
+                _,
+                _,
+                found,
+                Expected::FromAnnotation(
+                    Loc {
+                        value: Pattern::Identifier(name),
+                        region: _,
+                    },
+                    arity,
+                    _,
+                    expected,
+                ),
+            ) if env.is_extended(name) => {
+                if *arity > 1 {
+                    *arity -= 1;
+                }
+
+                drop_last_argument(found);
+                drop_last_argument(expected);
+
+                if let (
+                    ErrorType::Function(found_args, _, _),
+                    ErrorType::Function(expected_args, _, _),
+                ) = (found, expected)
+                {
+                    if found_args.len() > expected_args.len() {
+                        // If the found arity doesn't match  the annotation's, the params
+                        // record would appear in the error, so we replace it with `?`.
+                        if let Some(last) = found_args.last_mut() {
+                            *last = ErrorType::Error;
+                        }
+                    }
+                }
+            }
+
+            TypeError::BadExpr(_, _, _, Expected::FromAnnotation(_, _, _, _))
+            | TypeError::BadExpr(_, _, _, Expected::NoExpectation(_)) => {}
+
+            // Irrelevant
+            TypeError::BadPattern(_, _, _, _)
+            | TypeError::CircularType(_, _, _)
+            | TypeError::CircularDef(_)
+            | TypeError::UnexposedLookup(_, _)
+            | TypeError::UnfulfilledAbility(_)
+            | TypeError::BadExprMissingAbility(_, _, _, _)
+            | TypeError::BadPatternMissingAbility(_, _, _, _)
+            | TypeError::Exhaustive(_)
+            | TypeError::StructuralSpecialization {
+                region: _,
+                typ: _,
+                ability: _,
+                member: _,
+            }
+            | TypeError::WrongSpecialization {
+                region: _,
+                ability_member: _,
+                expected_opaque: _,
+                found_opaque: _,
+            }
+            | TypeError::IngestedFileBadUtf8(_, _)
+            | TypeError::IngestedFileUnsupportedType(_, _)
+            | TypeError::UnexpectedModuleParams(_, _)
+            | TypeError::MissingModuleParams(_, _, _)
+            | TypeError::ModuleParamsMismatch(_, _, _, _) => {}
+        }
+    }
+}
+
+struct Env {
+    home_top_level_symbols: Vec<Symbol>,
+    imported_modules_with_params: Vec<ModuleId>,
+}
+
+impl Env {
+    fn is_extended(&self, symbol: &Symbol) -> bool {
+        self.home_top_level_symbols.contains(symbol)
+            || self
+                .imported_modules_with_params
+                .contains(&symbol.module_id())
+    }
+}
+
+fn remove_for_reason(
+    env: &Env,
+    found_type: &mut ErrorType,
+    reason: &mut Reason,
+    expected_type: &mut ErrorType,
+) {
+    match reason {
+        Reason::FnCall {
+            name: Some(name),
+            arity,
+            called_via: _,
+        } if env.is_extended(name) => {
+            *arity -= 1;
+            debug_assert_ne!(0, *arity);
+
+            drop_last_argument(found_type);
+            drop_last_argument(expected_type);
+        }
+
+        Reason::FnCall { .. } => {}
+
+        Reason::FnArg { .. } | Reason::TypedArg { .. } => {
+            // I believe these don't need to be touched because reporting only
+            // shows the type of the arguments, not the whole function.
+        }
+
+        // Irrelevant
+        Reason::LowLevelOpArg {
+            op: _,
+            arg_index: _,
+        }
+        | Reason::ForeignCallArg {
+            foreign_symbol: _,
+            arg_index: _,
+        }
+        | Reason::FloatLiteral
+        | Reason::IntLiteral
+        | Reason::NumLiteral
+        | Reason::StrInterpolation
+        | Reason::WhenBranches
+        | Reason::WhenBranch { index: _ }
+        | Reason::WhenGuard
+        | Reason::ExpectCondition
+        | Reason::IfCondition
+        | Reason::IfBranch {
+            index: _,
+            total_branches: _,
+        }
+        | Reason::ElemInList { index: _ }
+        | Reason::RecordUpdateValue(_)
+        | Reason::RecordUpdateKeys(_, _)
+        | Reason::RecordDefaultField(_)
+        | Reason::NumericLiteralSuffix
+        | Reason::InvalidAbilityMemberSpecialization {
+            member_name: _,
+            def_region: _,
+            unimplemented_abilities: _,
+        }
+        | Reason::GeneralizedAbilityMemberSpecialization {
+            member_name: _,
+            def_region: _,
+        }
+        | Reason::CrashArg
+        | Reason::ImportParams(_) => {}
+    }
+}
+
+fn drop_last_argument(err_type: &mut ErrorType) {
+    match err_type {
+        ErrorType::Function(arguments, _, _) => {
+            arguments.pop();
+        }
+        // Irrelevant
+        ErrorType::Infinite
+        | ErrorType::Type(_, _)
+        | ErrorType::FlexVar(_)
+        | ErrorType::RigidVar(_)
+        | ErrorType::FlexAbleVar(_, _)
+        | ErrorType::RigidAbleVar(_, _)
+        | ErrorType::Record(_, _)
+        | ErrorType::Tuple(_, _)
+        | ErrorType::TagUnion(_, _, _)
+        | ErrorType::RecursiveTagUnion(_, _, _, _)
+        | ErrorType::Alias(_, _, _, _)
+        | ErrorType::Range(_)
+        | ErrorType::Error => {}
+    }
+}

crates/compiler/module/src/called_via.rs

@@ -99,6 +99,9 @@ pub enum CalledVia {
     /// This call is the result of desugaring a Result.try from `?` syntax
     /// e.g. Dict.get? items "key" becomes Result.try (Dict.get items "key") \item -> ...
     QuestionSuffix,
+
+    /// This call is a result of lowering a reference to a module-params-extended def
+    NakedParamsVar,
 }
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]

crates/compiler/module/src/symbol.rs

@@ -123,6 +123,10 @@ impl Symbol {
                 .any(|(_, (s, _))| *s == self)
     }
 
+    pub fn is_generated(self, interns: &Interns) -> bool {
+        self.ident_ids(interns).is_generated_id(self.ident_id())
+    }
+
     pub fn module_string<'a>(&self, interns: &'a Interns) -> &'a ModuleName {
         interns
             .module_ids
@@ -137,24 +141,15 @@ impl Symbol {
     }
 
     pub fn as_str(self, interns: &Interns) -> &str {
-        let ident_ids = interns
-            .all_ident_ids
-            .get(&self.module_id())
+        self.ident_ids(interns)
+            .get_name(self.ident_id())
             .unwrap_or_else(|| {
                 internal_error!(
-                    "ident_string could not find IdentIds for module {:?} in {:?}",
-                    self.module_id(),
-                    interns
+                    "ident_string's IdentIds did not contain an entry for {} in module {:?}",
+                    self.ident_id().0,
+                    self.module_id()
                 )
-            });
-
-        ident_ids.get_name(self.ident_id()).unwrap_or_else(|| {
-            internal_error!(
-                "ident_string's IdentIds did not contain an entry for {} in module {:?}",
-                self.ident_id().0,
-                self.module_id()
-            )
-        })
+            })
     }
 
     pub const fn as_u64(self) -> u64 {
@@ -186,6 +181,19 @@ impl Symbol {
     pub fn contains(self, needle: &str) -> bool {
         format!("{self:?}").contains(needle)
     }
+
+    fn ident_ids(self, interns: &Interns) -> &IdentIds {
+        interns
+            .all_ident_ids
+            .get(&self.module_id())
+            .unwrap_or_else(|| {
+                internal_error!(
+                    "ident_string could not find IdentIds for module {:?} in {:?}",
+                    self.module_id(),
+                    interns
+                )
+            })
+    }
 }
 
 /// Rather than displaying as this:
@@ -707,6 +715,12 @@ impl IdentIds {
         IdentId(self.interner.insert_index_str() as u32)
     }
 
+    pub fn is_generated_id(&self, id: IdentId) -> bool {
+        self.interner
+            .try_get(id.0 as usize)
+            .map_or(false, |str| str.starts_with(|c: char| c.is_ascii_digit()))
+    }
+
     #[inline(always)]
     pub fn get_id(&self, ident_name: &str) -> Option<IdentId> {
         self.interner
@@ -1666,6 +1680,23 @@ define_builtins! {
         32 INSPECT_TO_INSPECTOR: "toInspector"
         33 INSPECT_TO_STR: "toStr"
     }
+    15 TASK: "Task" => {
+        0 TASK_TASK: "Task" exposed_type=true // the Task.Task opaque type
+        1 TASK_FOREVER: "forever"
+        2 TASK_LOOP: "loop"
+        3 TASK_OK: "ok"
+        4 TASK_ERR: "err"
+        5 TASK_ATTEMPT: "attempt"
+        6 TASK_AWAIT: "await"
+        7 TASK_ON_ERR: "onErr"
+        8 TASK_MAP: "map"
+        9 TASK_MAP_ERR: "mapErr"
+        10 TASK_FROM_RESULT: "fromResult"
+        11 TASK_BATCH: "batch"
+        12 TASK_SEQUENCE: "sequence"
+        13 TASK_FOR_EACH: "forEach"
+        14 TASK_RESULT: "result"
+    }
 
-    num_modules: 15 // Keep this count up to date by hand! (TODO: see the mut_map! macro for how we could determine this count correctly in the macro)
+    num_modules: 16 // Keep this count up to date by hand! (TODO: see the mut_map! macro for how we could determine this count correctly in the macro)
 }

crates/compiler/mono/src/ir.rs

@@ -2870,7 +2870,6 @@ fn pattern_to_when(
     body: Loc<roc_can::expr::Expr>,
 ) -> (Symbol, Loc<roc_can::expr::Expr>) {
     use roc_can::expr::Expr::*;
-    use roc_can::expr::{WhenBranch, WhenBranchPattern};
     use roc_can::pattern::Pattern::{self, *};
 
     match &pattern.value {
@@ -2888,7 +2887,7 @@ fn pattern_to_when(
             (*new_symbol, Loc::at_zero(RuntimeError(error)))
         }
 
-        As(_, _) => todo!("as bindings are not supported yet"),
+        As(pattern, symbol) => pattern_to_when_help(pattern_var, pattern, body_var, body, *symbol),
 
         UnsupportedPattern(region) => {
             // create the runtime error here, instead of delegating to When.
@@ -2915,28 +2914,7 @@ fn pattern_to_when(
         | TupleDestructure { .. }
         | UnwrappedOpaque { .. } => {
             let symbol = env.unique_symbol();
-
-            let wrapped_body = When {
-                cond_var: pattern_var,
-                expr_var: body_var,
-                region: Region::zero(),
-                loc_cond: Box::new(Loc::at_zero(Var(symbol, pattern_var))),
-                branches: vec![WhenBranch {
-                    patterns: vec![WhenBranchPattern {
-                        pattern,
-                        degenerate: false,
-                    }],
-                    value: body,
-                    guard: None,
-                    // If this type-checked, it's non-redundant
-                    redundant: RedundantMark::known_non_redundant(),
-                }],
-                branches_cond_var: pattern_var,
-                // If this type-checked, it's exhaustive
-                exhaustive: ExhaustiveMark::known_exhaustive(),
-            };
-
-            (symbol, Loc::at_zero(wrapped_body))
+            pattern_to_when_help(pattern_var, &pattern, body_var, body, symbol)
         }
 
         Pattern::List { .. } => todo!(),
@@ -2960,6 +2938,38 @@ fn pattern_to_when(
     }
 }
 
+fn pattern_to_when_help(
+    pattern_var: Variable,
+    pattern: &Loc<roc_can::pattern::Pattern>,
+    body_var: Variable,
+    body: Loc<roc_can::expr::Expr>,
+    symbol: Symbol,
+) -> (Symbol, Loc<roc_can::expr::Expr>) {
+    use roc_can::expr::{Expr, WhenBranch, WhenBranchPattern};
+
+    let wrapped_body = Expr::When {
+        cond_var: pattern_var,
+        expr_var: body_var,
+        region: Region::zero(),
+        loc_cond: Box::new(Loc::at_zero(Expr::Var(symbol, pattern_var))),
+        branches: vec![WhenBranch {
+            patterns: vec![WhenBranchPattern {
+                pattern: pattern.to_owned(),
+                degenerate: false,
+            }],
+            value: body,
+            guard: None,
+            // If this type-checked, it's non-redundant
+            redundant: RedundantMark::known_non_redundant(),
+        }],
+        branches_cond_var: pattern_var,
+        // If this type-checked, it's exhaustive
+        exhaustive: ExhaustiveMark::known_exhaustive(),
+    };
+
+    (symbol, Loc::at_zero(wrapped_body))
+}
+
 fn specialize_suspended<'a>(
     env: &mut Env<'a, '_>,
     procs: &mut Procs<'a>,
@@ -4444,7 +4454,7 @@ pub fn with_hole<'a>(
             specialize_naked_symbol(env, variable, procs, layout_cache, assigned, hole, symbol)
         }
         ParamsVar { .. } => {
-            unimplemented!("module params code generation")
+            internal_error!("ParamsVar should've been lowered to Var")
         }
         ImportParams(_, _, Some((_, value))) => {
             with_hole(env, *value, variable, procs, layout_cache, assigned, hole)

crates/compiler/parse/src/ast.rs

@@ -497,7 +497,10 @@ pub enum Expr<'a> {
 
     Backpassing(&'a [Loc<Pattern<'a>>], &'a Loc<Expr<'a>>, &'a Loc<Expr<'a>>),
     Expect(&'a Loc<Expr<'a>>, &'a Loc<Expr<'a>>),
-    Dbg(&'a Loc<Expr<'a>>, &'a Loc<Expr<'a>>),
+
+    Dbg,
+    DbgStmt(&'a Loc<Expr<'a>>, &'a Loc<Expr<'a>>),
+
     // This form of debug is a desugared call to roc_dbg
     LowLevelDbg(&'a (&'a str, &'a str), &'a Loc<Expr<'a>>, &'a Loc<Expr<'a>>),
 
@@ -663,9 +666,9 @@ pub fn is_expr_suffixed(expr: &Expr) -> bool {
         Expr::Tag(_) => false,
         Expr::OpaqueRef(_) => false,
         Expr::Backpassing(_, _, _) => false, // TODO: we might want to check this?
-        Expr::Expect(a, b) | Expr::Dbg(a, b) => {
-            is_expr_suffixed(&a.value) || is_expr_suffixed(&b.value)
-        }
+        Expr::Expect(a, b) => is_expr_suffixed(&a.value) || is_expr_suffixed(&b.value),
+        Expr::Dbg => false,
+        Expr::DbgStmt(a, b) => is_expr_suffixed(&a.value) || is_expr_suffixed(&b.value),
         Expr::LowLevelDbg(_, a, b) => is_expr_suffixed(&a.value) || is_expr_suffixed(&b.value),
         Expr::UnaryOp(a, _) => is_expr_suffixed(&a.value),
         Expr::When(cond, branches) => {
@@ -958,9 +961,17 @@ impl<'a, 'b> RecursiveValueDefIter<'a, 'b> {
                     expr_stack.push(&a.value);
                     expr_stack.push(&b.value);
                 }
-                Expect(condition, cont)
-                | Dbg(condition, cont)
-                | LowLevelDbg(_, condition, cont) => {
+                Expect(condition, cont) => {
+                    expr_stack.reserve(2);
+                    expr_stack.push(&condition.value);
+                    expr_stack.push(&cont.value);
+                }
+                DbgStmt(condition, cont) => {
+                    expr_stack.reserve(2);
+                    expr_stack.push(&condition.value);
+                    expr_stack.push(&cont.value);
+                }
+                LowLevelDbg(_, condition, cont) => {
                     expr_stack.reserve(2);
                     expr_stack.push(&condition.value);
                     expr_stack.push(&cont.value);
@@ -1032,6 +1043,7 @@ impl<'a, 'b> RecursiveValueDefIter<'a, 'b> {
                 | Var { .. }
                 | Underscore(_)
                 | Crash
+                | Dbg
                 | Tag(_)
                 | OpaqueRef(_)
                 | MalformedIdent(_, _)
@@ -2518,8 +2530,9 @@ impl<'a> Malformed for Expr<'a> {
             Closure(args, body) => args.iter().any(|arg| arg.is_malformed()) || body.is_malformed(),
             Defs(defs, body) => defs.is_malformed() || body.is_malformed(),
             Backpassing(args, call, body) => args.iter().any(|arg| arg.is_malformed()) || call.is_malformed() || body.is_malformed(),
-            Expect(condition, continuation) |
-            Dbg(condition, continuation) => condition.is_malformed() || continuation.is_malformed(),
+            Expect(condition, continuation) => condition.is_malformed() || continuation.is_malformed(),
+            Dbg => false,
+            DbgStmt(condition, continuation) => condition.is_malformed() || continuation.is_malformed(),
             LowLevelDbg(_, condition, continuation) => condition.is_malformed() || continuation.is_malformed(),
             Apply(func, args, _) => func.is_malformed() || args.iter().any(|arg| arg.is_malformed()),
             BinOps(firsts, last) => firsts.iter().any(|(expr, _)| expr.is_malformed()) || last.is_malformed(),

crates/compiler/parse/src/expr.rs

@@ -194,6 +194,7 @@ fn loc_term_or_underscore_or_conditional<'a>(
         )),
         loc(specialize_err(EExpr::Closure, closure_help(options))),
         loc(crash_kw()),
+        loc(specialize_err(EExpr::Dbg, dbg_kw())),
         loc(underscore_expression()),
         loc(record_literal_help()),
         loc(specialize_err(EExpr::List, list_literal_help())),
@@ -215,6 +216,7 @@ fn loc_term_or_underscore<'a>(
             positive_number_literal_help()
         )),
         loc(specialize_err(EExpr::Closure, closure_help(options))),
+        loc(specialize_err(EExpr::Dbg, dbg_kw())),
         loc(underscore_expression()),
         loc(record_literal_help()),
         loc(specialize_err(EExpr::List, list_literal_help())),
@@ -232,6 +234,7 @@ fn loc_term<'a>(options: ExprParseOptions) -> impl Parser<'a, Loc<Expr<'a>>, EEx
             positive_number_literal_help()
         )),
         loc(specialize_err(EExpr::Closure, closure_help(options))),
+        loc(specialize_err(EExpr::Dbg, dbg_kw())),
         loc(record_literal_help()),
         loc(specialize_err(EExpr::List, list_literal_help())),
         ident_seq(),
@@ -541,7 +544,7 @@ fn stmt_start<'a>(
         )),
         loc(specialize_err(
             EExpr::Dbg,
-            dbg_help(options, preceding_comment)
+            dbg_stmt_help(options, preceding_comment)
         )),
         loc(specialize_err(EExpr::Import, map(import(), Stmt::ValueDef))),
         map(
@@ -2170,7 +2173,8 @@ fn expr_to_pattern_help<'a>(arena: &'a Bump, expr: &Expr<'a>) -> Result<Pattern<
         | Expr::If(_, _)
         | Expr::When(_, _)
         | Expr::Expect(_, _)
-        | Expr::Dbg(_, _)
+        | Expr::Dbg
+        | Expr::DbgStmt(_, _)
         | Expr::LowLevelDbg(_, _, _)
         | Expr::MalformedClosure
         | Expr::MalformedSuffixed(..)
@@ -2647,7 +2651,7 @@ fn expect_help<'a>(
     }
 }
 
-fn dbg_help<'a>(
+fn dbg_stmt_help<'a>(
     options: ExprParseOptions,
     preceding_comment: Region,
 ) -> impl Parser<'a, Stmt<'a>, EExpect<'a>> {
@@ -2672,7 +2676,17 @@ fn dbg_help<'a>(
 
         Ok((MadeProgress, stmt, state))
     })
-    .trace("dbg_help")
+    .trace("dbg_stmt_help")
+}
+
+fn dbg_kw<'a>() -> impl Parser<'a, Expr<'a>, EExpect<'a>> {
+    (move |arena: &'a Bump, state: State<'a>, min_indent: u32| {
+        let (_, _, next_state) =
+            parser::keyword(keyword::DBG, EExpect::Dbg).parse(arena, state, min_indent)?;
+
+        Ok((MadeProgress, Expr::Dbg, next_state))
+    })
+    .trace("dbg_kw")
 }
 
 fn import<'a>() -> impl Parser<'a, ValueDef<'a>, EImport<'a>> {
@@ -2986,14 +3000,17 @@ fn stmts_to_expr<'a>(
                     arena.alloc(e).before(space)
                 }
             }
-            Stmt::ValueDef(ValueDef::Dbg { .. }) => {
-                return Err(EExpr::Dbg(
-                    EExpect::Continuation(
-                        arena.alloc(EExpr::IndentEnd(loc_stmt.region.end())),
-                        loc_stmt.region.end(),
-                    ),
-                    loc_stmt.region.start(),
-                ));
+            Stmt::ValueDef(ValueDef::Dbg { condition, .. }) => {
+                // If we parse a `dbg` as the last thing in a series of statements then it's
+                // actually an expression.
+                Expr::Apply(
+                    arena.alloc(Loc {
+                        value: Expr::Dbg,
+                        region: loc_stmt.region,
+                    }),
+                    arena.alloc([condition]),
+                    CalledVia::Space,
+                )
             }
             Stmt::ValueDef(ValueDef::Expect { .. }) => {
                 return Err(EExpr::Expect(
@@ -3145,13 +3162,19 @@ fn stmts_to_defs<'a>(
                 } = vd
                 {
                     if exprify_dbg {
-                        if i + 1 >= stmts.len() {
-                            return Err(EExpr::DbgContinue(sp_stmt.item.region.end()));
-                        }
-
-                        let rest = stmts_to_expr(&stmts[i + 1..], arena)?;
-
-                        let e = Expr::Dbg(arena.alloc(condition), arena.alloc(rest));
+                        let e = if i + 1 < stmts.len() {
+                            let rest = stmts_to_expr(&stmts[i + 1..], arena)?;
+                            Expr::DbgStmt(arena.alloc(condition), arena.alloc(rest))
+                        } else {
+                            Expr::Apply(
+                                arena.alloc(Loc {
+                                    value: Expr::Dbg,
+                                    region: sp_stmt.item.region,
+                                }),
+                                arena.alloc([condition]),
+                                CalledVia::Space,
+                            )
+                        };
 
                         let e = if sp_stmt.before.is_empty() {
                             e

crates/compiler/parse/src/header.rs

@@ -6,20 +6,20 @@ use crate::ast::{
 };
 use crate::blankspace::{space0_around_ee, space0_before_e, space0_e};
 use crate::expr::merge_spaces;
-use crate::ident::{self, lowercase_ident, unqualified_ident, uppercase, UppercaseIdent};
+use crate::ident::{self, lowercase_ident, unqualified_ident, UppercaseIdent};
 use crate::parser::Progress::{self, *};
 use crate::parser::{
     and, backtrackable, byte, collection_trailing_sep_e, increment_min_indent, loc, map,
     map_with_arena, optional, reset_min_indent, skip_first, skip_second, specialize_err, succeed,
-    then, two_bytes, zero_or_more, EExposes, EGenerates, EGeneratesWith, EHeader, EImports,
-    EPackageEntry, EPackageName, EPackages, EParams, EProvides, ERequires, ETypedIdent, Parser,
-    SourceError, SpaceProblem, SyntaxError,
+    then, two_bytes, zero_or_more, EExposes, EHeader, EImports, EPackageEntry, EPackageName,
+    EPackages, EParams, EProvides, ERequires, ETypedIdent, Parser, SourceError, SpaceProblem,
+    SyntaxError,
 };
 use crate::pattern::record_pattern_fields;
 use crate::state::State;
 use crate::string_literal::{self, parse_str_literal};
 use crate::type_annotation;
-use roc_module::symbol::{ModuleId, Symbol};
+use roc_module::symbol::ModuleId;
 use roc_region::all::{Loc, Position, Region};
 
 fn end_of_file<'a>() -> impl Parser<'a, (), SyntaxError<'a>> {
@@ -188,8 +188,6 @@ fn hosted_header<'a>() -> impl Parser<'a, HostedHeader<'a>, EHeader<'a>> {
         name: loc(module_name_help(EHeader::ModuleName)),
         exposes: specialize_err(EHeader::Exposes, exposes_values_kw()),
         imports: specialize_err(EHeader::Imports, imports()),
-        generates: specialize_err(EHeader::Generates, generates()),
-        generates_with: specialize_err(EHeader::GeneratesWith, generates_with()),
     })
     .trace("hosted_header")
 }
@@ -761,43 +759,6 @@ fn packages_collection<'a>(
     )
 }
 
-#[inline(always)]
-fn generates<'a>(
-) -> impl Parser<'a, KeywordItem<'a, GeneratesKeyword, UppercaseIdent<'a>>, EGenerates> {
-    record!(KeywordItem {
-        keyword: spaces_around_keyword(
-            GeneratesKeyword,
-            EGenerates::Generates,
-            EGenerates::IndentGenerates,
-            EGenerates::IndentTypeStart
-        ),
-        item: specialize_err(|(), pos| EGenerates::Identifier(pos), uppercase())
-    })
-}
-
-#[inline(always)]
-fn generates_with<'a>() -> impl Parser<
-    'a,
-    KeywordItem<'a, WithKeyword, Collection<'a, Loc<Spaced<'a, ExposedName<'a>>>>>,
-    EGeneratesWith,
-> {
-    record!(KeywordItem {
-        keyword: spaces_around_keyword(
-            WithKeyword,
-            EGeneratesWith::With,
-            EGeneratesWith::IndentWith,
-            EGeneratesWith::IndentListStart
-        ),
-        item: collection_trailing_sep_e(
-            byte(b'[', EGeneratesWith::ListStart),
-            exposes_entry(EGeneratesWith::Identifier),
-            byte(b',', EGeneratesWith::ListEnd),
-            byte(b']', EGeneratesWith::ListEnd),
-            Spaced::SpaceBefore
-        )
-    })
-}
-
 #[inline(always)]
 fn imports<'a>() -> impl Parser<
     'a,
@@ -977,14 +938,11 @@ pub enum HeaderType<'a> {
     Hosted {
         name: ModuleName<'a>,
         exposes: &'a [Loc<ExposedName<'a>>],
-        generates: UppercaseIdent<'a>,
-        generates_with: &'a [Loc<ExposedName<'a>>],
     },
     /// Only created during canonicalization, never actually parsed from source
     Builtin {
         name: ModuleName<'a>,
         exposes: &'a [Loc<ExposedName<'a>>],
-        generates_with: &'a [Symbol],
         opt_params: Option<ModuleParams<'a>>,
     },
     Package {
@@ -1045,7 +1003,6 @@ impl<'a> HeaderType<'a> {
                 opt_params,
                 name: _,
                 exposes: _,
-                generates_with: _,
             } => opt_params,
             Self::App {
                 provides: _,
@@ -1059,8 +1016,6 @@ impl<'a> HeaderType<'a> {
             | Self::Hosted {
                 name: _,
                 exposes: _,
-                generates: _,
-                generates_with: _,
             }
             | Self::Platform {
                 opt_app_module_id: _,
@@ -1083,7 +1038,6 @@ impl<'a> HeaderType<'a> {
             } if module_id.is_builtin() => HeaderType::Builtin {
                 name,
                 exposes,
-                generates_with: &[],
                 opt_params,
             },
             _ => self,
@@ -1202,8 +1156,6 @@ macro_rules! keywords {
 
 keywords! {
     ExposesKeyword => "exposes",
-    WithKeyword => "with",
-    GeneratesKeyword => "generates",
     PackageKeyword => "package",
     PackagesKeyword => "packages",
     RequiresKeyword => "requires",
@@ -1247,10 +1199,6 @@ pub struct HostedHeader<'a> {
     pub exposes: KeywordItem<'a, ExposesKeyword, Collection<'a, Loc<Spaced<'a, ExposedName<'a>>>>>,
 
     pub imports: KeywordItem<'a, ImportsKeyword, Collection<'a, Loc<Spaced<'a, ImportsEntry<'a>>>>>,
-
-    pub generates: KeywordItem<'a, GeneratesKeyword, UppercaseIdent<'a>>,
-    pub generates_with:
-        KeywordItem<'a, WithKeyword, Collection<'a, Loc<Spaced<'a, ExposedName<'a>>>>>,
 }
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
@@ -1329,7 +1277,7 @@ pub enum ImportsEntry<'a> {
 
 /// e.g.
 ///
-/// printLine : Str -> Effect {}
+/// printLine : Str -> Result {} *
 #[derive(Copy, Clone, Debug, PartialEq)]
 pub struct TypedIdent<'a> {
     pub ident: Loc<&'a str>,

crates/compiler/parse/src/highlight.rs

@@ -365,7 +365,7 @@ fn fast_forward_to(
     tokens.push(Loc::at(Region::between(start, state.pos()), Token::Error));
 }
 
-pub const HEADER_KEYWORDS: [&str; 14] = [
+pub const HEADER_KEYWORDS: [&str; 12] = [
     "interface",
     "app",
     "package",
@@ -373,8 +373,6 @@ pub const HEADER_KEYWORDS: [&str; 14] = [
     "hosted",
     "exposes",
     "imports",
-    "with",
-    "generates",
     "package",
     "packages",
     "requires",

crates/compiler/parse/src/module.rs

@@ -0,0 +1,905 @@
+use crate::ast::{Collection, CommentOrNewline, Defs, Header, Module, Spaced, Spaces};
+use crate::blankspace::{space0_around_ee, space0_before_e, space0_e};
+use crate::expr::merge_spaces;
+use crate::header::{
+    package_entry, package_name, AppHeader, ExposedName, ExposesKeyword, HostedHeader,
+    ImportsCollection, ImportsEntry, ImportsKeyword, ImportsKeywordItem, Keyword, KeywordItem,
+    ModuleHeader, ModuleName, ModuleParams, PackageEntry, PackageHeader, PackagesKeyword,
+    PlatformHeader, PlatformRequires, ProvidesKeyword, ProvidesTo, RequiresKeyword, To, ToKeyword,
+    TypedIdent,
+};
+use crate::ident::{self, lowercase_ident, unqualified_ident, UppercaseIdent};
+use crate::parser::Progress::{self, *};
+use crate::parser::{
+    and, backtrackable, byte, collection_trailing_sep_e, increment_min_indent, loc, map,
+    map_with_arena, optional, reset_min_indent, skip_first, skip_second, specialize_err, succeed,
+    two_bytes, zero_or_more, EExposes, EHeader, EImports, EPackages, EParams, EProvides, ERequires,
+    ETypedIdent, Parser, SourceError, SpaceProblem, SyntaxError,
+};
+use crate::pattern::record_pattern_fields;
+use crate::state::State;
+use crate::string_literal::{self, parse_str_literal};
+use crate::type_annotation;
+use roc_region::all::{Loc, Position, Region};
+
+fn end_of_file<'a>() -> impl Parser<'a, (), SyntaxError<'a>> {
+    |_arena, state: State<'a>, _min_indent: u32| {
+        if state.has_reached_end() {
+            Ok((NoProgress, (), state))
+        } else {
+            Err((NoProgress, SyntaxError::NotEndOfFile(state.pos())))
+        }
+    }
+}
+
+pub fn parse_module_defs<'a>(
+    arena: &'a bumpalo::Bump,
+    state: State<'a>,
+    defs: Defs<'a>,
+) -> Result<Defs<'a>, SyntaxError<'a>> {
+    let min_indent = 0;
+    match crate::expr::parse_top_level_defs(arena, state.clone(), defs) {
+        Ok((_, defs, state)) => match end_of_file().parse(arena, state, min_indent) {
+            Ok(_) => Ok(defs),
+            Err((_, fail)) => Err(fail),
+        },
+        Err((_, fail)) => Err(SyntaxError::Expr(fail, state.pos())),
+    }
+}
+
+pub fn parse_header<'a>(
+    arena: &'a bumpalo::Bump,
+    state: State<'a>,
+) -> Result<(Module<'a>, State<'a>), SourceError<'a, EHeader<'a>>> {
+    let min_indent = 0;
+    match header().parse(arena, state.clone(), min_indent) {
+        Ok((_, module, state)) => Ok((module, state)),
+        Err((_, fail)) => Err(SourceError::new(fail, &state)),
+    }
+}
+
+pub fn header<'a>() -> impl Parser<'a, Module<'a>, EHeader<'a>> {
+    use crate::parser::keyword;
+
+    record!(Module {
+        comments: space0_e(EHeader::IndentStart),
+        header: one_of![
+            map(
+                skip_first(
+                    keyword("module", EHeader::Start),
+                    increment_min_indent(module_header())
+                ),
+                Header::Module
+            ),
+            map(
+                skip_first(
+                    keyword("interface", EHeader::Start),
+                    increment_min_indent(interface_header())
+                ),
+                Header::Module
+            ),
+            map(
+                skip_first(
+                    keyword("app", EHeader::Start),
+                    increment_min_indent(one_of![app_header(), old_app_header()])
+                ),
+                Header::App
+            ),
+            map(
+                skip_first(
+                    keyword("package", EHeader::Start),
+                    increment_min_indent(one_of![package_header(), old_package_header()])
+                ),
+                Header::Package
+            ),
+            map(
+                skip_first(
+                    keyword("platform", EHeader::Start),
+                    increment_min_indent(platform_header())
+                ),
+                Header::Platform
+            ),
+            map(
+                skip_first(
+                    keyword("hosted", EHeader::Start),
+                    increment_min_indent(hosted_header())
+                ),
+                Header::Hosted
+            ),
+        ]
+    })
+}
+
+#[inline(always)]
+fn module_header<'a>() -> impl Parser<'a, ModuleHeader<'a>, EHeader<'a>> {
+    record!(ModuleHeader {
+        after_keyword: space0_e(EHeader::IndentStart),
+        params: optional(specialize_err(EHeader::Params, module_params())),
+        exposes: specialize_err(EHeader::Exposes, exposes_list()),
+        interface_imports: succeed(None)
+    })
+    .trace("module_header")
+}
+
+fn module_params<'a>() -> impl Parser<'a, ModuleParams<'a>, EParams<'a>> {
+    record!(ModuleParams {
+        params: specialize_err(EParams::Pattern, record_pattern_fields()),
+        before_arrow: skip_second(
+            space0_e(EParams::BeforeArrow),
+            loc(two_bytes(b'-', b'>', EParams::Arrow))
+        ),
+        after_arrow: space0_e(EParams::AfterArrow),
+    })
+}
+
+// TODO does this need to be a macro?
+macro_rules! merge_n_spaces {
+    ($arena:expr, $($slice:expr),*) => {
+        {
+            let mut merged = bumpalo::collections::Vec::with_capacity_in(0 $(+ $slice.len())*, $arena);
+            $(merged.extend_from_slice($slice);)*
+            merged.into_bump_slice()
+        }
+    };
+}
+
+/// Parse old interface headers so we can format them into module headers
+#[inline(always)]
+fn interface_header<'a>() -> impl Parser<'a, ModuleHeader<'a>, EHeader<'a>> {
+    let after_keyword = map_with_arena(
+        and(
+            skip_second(
+                space0_e(EHeader::IndentStart),
+                loc(module_name_help(EHeader::ModuleName)),
+            ),
+            specialize_err(EHeader::Exposes, exposes_kw()),
+        ),
+        |arena: &'a bumpalo::Bump,
+         (before_name, kw): (&'a [CommentOrNewline<'a>], Spaces<'a, ExposesKeyword>)| {
+            merge_n_spaces!(arena, before_name, kw.before, kw.after)
+        },
+    );
+
+    record!(ModuleHeader {
+        after_keyword: after_keyword,
+        params: succeed(None),
+        exposes: specialize_err(EHeader::Exposes, exposes_list()).trace("exposes_list"),
+        interface_imports: map(
+            specialize_err(EHeader::Imports, imports()),
+            imports_none_if_empty
+        )
+        .trace("imports"),
+    })
+    .trace("interface_header")
+}
+
+fn imports_none_if_empty(value: ImportsKeywordItem<'_>) -> Option<ImportsKeywordItem<'_>> {
+    if value.item.is_empty() {
+        None
+    } else {
+        Some(value)
+    }
+}
+
+#[inline(always)]
+fn hosted_header<'a>() -> impl Parser<'a, HostedHeader<'a>, EHeader<'a>> {
+    record!(HostedHeader {
+        before_name: space0_e(EHeader::IndentStart),
+        name: loc(module_name_help(EHeader::ModuleName)),
+        exposes: specialize_err(EHeader::Exposes, exposes_values_kw()),
+        imports: specialize_err(EHeader::Imports, imports()),
+    })
+    .trace("hosted_header")
+}
+
+fn chomp_module_name(buffer: &[u8]) -> Result<&str, Progress> {
+    use encode_unicode::CharExt;
+
+    let mut chomped = 0;
+
+    if let Ok((first_letter, width)) = char::from_utf8_slice_start(&buffer[chomped..]) {
+        if first_letter.is_uppercase() {
+            chomped += width;
+        } else {
+            return Err(Progress::NoProgress);
+        }
+    }
+
+    while let Ok((ch, width)) = char::from_utf8_slice_start(&buffer[chomped..]) {
+        // After the first character, only these are allowed:
+        //
+        // * Unicode alphabetic chars - you might include `鹏` if that's clear to your readers
+        // * ASCII digits - e.g. `1` but not `¾`, both of which pass .is_numeric()
+        // * A '.' separating module parts
+        if ch.is_alphabetic() || ch.is_ascii_digit() {
+            chomped += width;
+        } else if ch == '.' {
+            chomped += width;
+
+            if let Ok((first_letter, width)) = char::from_utf8_slice_start(&buffer[chomped..]) {
+                if first_letter.is_uppercase() {
+                    chomped += width;
+                } else if first_letter == '{' {
+                    // the .{ starting a `Foo.{ bar, baz }` importing clauses
+                    chomped -= width;
+                    break;
+                } else {
+                    return Err(Progress::MadeProgress);
+                }
+            }
+        } else {
+            // we're done
+            break;
+        }
+    }
+
+    let name = unsafe { std::str::from_utf8_unchecked(&buffer[..chomped]) };
+
+    Ok(name)
+}
+
+#[inline(always)]
+fn module_name<'a>() -> impl Parser<'a, ModuleName<'a>, ()> {
+    |_, mut state: State<'a>, _min_indent: u32| match chomp_module_name(state.bytes()) {
+        Ok(name) => {
+            let width = name.len();
+            state = state.advance(width);
+
+            Ok((MadeProgress, ModuleName::new(name), state))
+        }
+        Err(progress) => Err((progress, ())),
+    }
+}
+
+#[inline(always)]
+fn app_header<'a>() -> impl Parser<'a, AppHeader<'a>, EHeader<'a>> {
+    record!(AppHeader {
+        before_provides: space0_e(EHeader::IndentStart),
+        provides: specialize_err(EHeader::Exposes, exposes_list()),
+        before_packages: space0_e(EHeader::IndentStart),
+        packages: specialize_err(EHeader::Packages, loc(packages_collection())),
+        old_imports: succeed(None),
+        old_provides_to_new_package: succeed(None),
+    })
+    .trace("app_header")
+}
+
+struct OldAppHeader<'a> {
+    pub before_name: &'a [CommentOrNewline<'a>],
+    pub packages: Option<Loc<OldAppPackages<'a>>>,
+    pub imports: Option<KeywordItem<'a, ImportsKeyword, ImportsCollection<'a>>>,
+    pub provides: ProvidesTo<'a>,
+}
+
+type OldAppPackages<'a> =
+    KeywordItem<'a, PackagesKeyword, Collection<'a, Loc<Spaced<'a, PackageEntry<'a>>>>>;
+
+#[inline(always)]
+fn old_app_header<'a>() -> impl Parser<'a, AppHeader<'a>, EHeader<'a>> {
+    let old = record!(OldAppHeader {
+        before_name: skip_second(
+            space0_e(EHeader::IndentStart),
+            loc(crate::parser::specialize_err(
+                EHeader::AppName,
+                string_literal::parse_str_literal()
+            ))
+        ),
+        packages: optional(specialize_err(EHeader::Packages, loc(packages()))),
+        imports: optional(specialize_err(EHeader::Imports, imports())),
+        provides: specialize_err(EHeader::Provides, provides_to()),
+    });
+
+    map_with_arena(old, |arena: &'a bumpalo::Bump, old: OldAppHeader<'a>| {
+        let mut before_packages: &'a [CommentOrNewline] = &[];
+
+        let packages = match old.packages {
+            Some(packages) => {
+                before_packages = merge_spaces(
+                    arena,
+                    packages.value.keyword.before,
+                    packages.value.keyword.after,
+                );
+
+                if let To::ExistingPackage(platform_shorthand) = old.provides.to.value {
+                    packages.map(|coll| {
+                        coll.item.map_items(arena, |loc_spaced_pkg| {
+                            if loc_spaced_pkg.value.item().shorthand == platform_shorthand {
+                                loc_spaced_pkg.map(|spaced_pkg| {
+                                    spaced_pkg.map(arena, |pkg| {
+                                        let mut new_pkg = *pkg;
+                                        new_pkg.platform_marker = Some(merge_spaces(
+                                            arena,
+                                            old.provides.to_keyword.before,
+                                            old.provides.to_keyword.after,
+                                        ));
+                                        new_pkg
+                                    })
+                                })
+                            } else {
+                                *loc_spaced_pkg
+                            }
+                        })
+                    })
+                } else {
+                    packages.map(|kw| kw.item)
+                }
+            }
+            None => Loc {
+                region: Region::zero(),
+                value: Collection::empty(),
+            },
+        };
+
+        let provides = match old.provides.types {
+            Some(types) => {
+                let mut combined_items = bumpalo::collections::Vec::with_capacity_in(
+                    old.provides.entries.items.len() + types.items.len(),
+                    arena,
+                );
+
+                combined_items.extend_from_slice(old.provides.entries.items);
+
+                for loc_spaced_type_ident in types.items {
+                    combined_items.push(loc_spaced_type_ident.map(|spaced_type_ident| {
+                        spaced_type_ident.map(arena, |type_ident| {
+                            ExposedName::new(From::from(*type_ident))
+                        })
+                    }));
+                }
+
+                let value_comments = old.provides.entries.final_comments();
+                let type_comments = types.final_comments();
+
+                let mut combined_comments = bumpalo::collections::Vec::with_capacity_in(
+                    value_comments.len() + type_comments.len(),
+                    arena,
+                );
+                combined_comments.extend_from_slice(value_comments);
+                combined_comments.extend_from_slice(type_comments);
+
+                Collection::with_items_and_comments(
+                    arena,
+                    combined_items.into_bump_slice(),
+                    combined_comments.into_bump_slice(),
+                )
+            }
+            None => old.provides.entries,
+        };
+
+        AppHeader {
+            before_provides: merge_spaces(
+                arena,
+                old.before_name,
+                old.provides.provides_keyword.before,
+            ),
+            provides,
+            before_packages: merge_spaces(
+                arena,
+                before_packages,
+                old.provides.provides_keyword.after,
+            ),
+            packages,
+            old_imports: old.imports.and_then(imports_none_if_empty),
+            old_provides_to_new_package: match old.provides.to.value {
+                To::NewPackage(new_pkg) => Some(new_pkg),
+                To::ExistingPackage(_) => None,
+            },
+        }
+    })
+}
+
+#[inline(always)]
+fn package_header<'a>() -> impl Parser<'a, PackageHeader<'a>, EHeader<'a>> {
+    record!(PackageHeader {
+        before_exposes: space0_e(EHeader::IndentStart),
+        exposes: specialize_err(EHeader::Exposes, exposes_module_collection()),
+        before_packages: space0_e(EHeader::IndentStart),
+        packages: specialize_err(EHeader::Packages, loc(packages_collection())),
+    })
+    .trace("package_header")
+}
+
+#[derive(Debug, Clone, PartialEq)]
+struct OldPackageHeader<'a> {
+    before_name: &'a [CommentOrNewline<'a>],
+    exposes: KeywordItem<'a, ExposesKeyword, Collection<'a, Loc<Spaced<'a, ModuleName<'a>>>>>,
+    packages:
+        Loc<KeywordItem<'a, PackagesKeyword, Collection<'a, Loc<Spaced<'a, PackageEntry<'a>>>>>>,
+}
+
+#[inline(always)]
+fn old_package_header<'a>() -> impl Parser<'a, PackageHeader<'a>, EHeader<'a>> {
+    map_with_arena(
+        record!(OldPackageHeader {
+            before_name: skip_second(
+                space0_e(EHeader::IndentStart),
+                specialize_err(EHeader::PackageName, package_name())
+            ),
+            exposes: specialize_err(EHeader::Exposes, exposes_modules()),
+            packages: specialize_err(EHeader::Packages, loc(packages())),
+        }),
+        |arena: &'a bumpalo::Bump, old: OldPackageHeader<'a>| {
+            let before_exposes = merge_n_spaces!(
+                arena,
+                old.before_name,
+                old.exposes.keyword.before,
+                old.exposes.keyword.after
+            );
+            let before_packages = merge_spaces(
+                arena,
+                old.packages.value.keyword.before,
+                old.packages.value.keyword.after,
+            );
+
+            PackageHeader {
+                before_exposes,
+                exposes: old.exposes.item,
+                before_packages,
+                packages: old.packages.map(|kw| kw.item),
+            }
+        },
+    )
+    .trace("old_package_header")
+}
+
+#[inline(always)]
+fn platform_header<'a>() -> impl Parser<'a, PlatformHeader<'a>, EHeader<'a>> {
+    record!(PlatformHeader {
+        before_name: space0_e(EHeader::IndentStart),
+        name: loc(specialize_err(EHeader::PlatformName, package_name())),
+        requires: specialize_err(EHeader::Requires, requires()),
+        exposes: specialize_err(EHeader::Exposes, exposes_modules()),
+        packages: specialize_err(EHeader::Packages, packages()),
+        imports: specialize_err(EHeader::Imports, imports()),
+        provides: specialize_err(EHeader::Provides, provides_exposed()),
+    })
+    .trace("platform_header")
+}
+
+fn provides_to_package<'a>() -> impl Parser<'a, To<'a>, EProvides<'a>> {
+    one_of![
+        specialize_err(
+            |_, pos| EProvides::Identifier(pos),
+            map(lowercase_ident(), To::ExistingPackage)
+        ),
+        specialize_err(EProvides::Package, map(package_name(), To::NewPackage))
+    ]
+}
+
+#[inline(always)]
+fn provides_to<'a>() -> impl Parser<'a, ProvidesTo<'a>, EProvides<'a>> {
+    record!(ProvidesTo {
+        provides_keyword: spaces_around_keyword(
+            ProvidesKeyword,
+            EProvides::Provides,
+            EProvides::IndentProvides,
+            EProvides::IndentListStart
+        ),
+        entries: collection_trailing_sep_e(
+            byte(b'[', EProvides::ListStart),
+            exposes_entry(EProvides::Identifier),
+            byte(b',', EProvides::ListEnd),
+            byte(b']', EProvides::ListEnd),
+            Spaced::SpaceBefore
+        ),
+        types: optional(backtrackable(provides_types())),
+        to_keyword: spaces_around_keyword(
+            ToKeyword,
+            EProvides::To,
+            EProvides::IndentTo,
+            EProvides::IndentListStart
+        ),
+        to: loc(provides_to_package()),
+    })
+    .trace("provides_to")
+}
+
+fn provides_exposed<'a>() -> impl Parser<
+    'a,
+    KeywordItem<'a, ProvidesKeyword, Collection<'a, Loc<Spaced<'a, ExposedName<'a>>>>>,
+    EProvides<'a>,
+> {
+    record!(KeywordItem {
+        keyword: spaces_around_keyword(
+            ProvidesKeyword,
+            EProvides::Provides,
+            EProvides::IndentProvides,
+            EProvides::IndentListStart
+        ),
+        item: collection_trailing_sep_e(
+            byte(b'[', EProvides::ListStart),
+            exposes_entry(EProvides::Identifier),
+            byte(b',', EProvides::ListEnd),
+            byte(b']', EProvides::ListEnd),
+            Spaced::SpaceBefore
+        ),
+    })
+}
+
+#[inline(always)]
+fn provides_types<'a>(
+) -> impl Parser<'a, Collection<'a, Loc<Spaced<'a, UppercaseIdent<'a>>>>, EProvides<'a>> {
+    skip_first(
+        // We only support spaces here, not newlines, because this is not intended
+        // to be the design forever. Someday it will hopefully work like Elm,
+        // where platform authors can provide functions like Browser.sandbox which
+        // present an API based on ordinary-looking type variables.
+        zero_or_more(byte(
+            b' ',
+            // HACK: If this errors, EProvides::Provides is not an accurate reflection
+            // of what went wrong. However, this is both skipped and zero_or_more,
+            // so this error should never be visible to anyone in practice!
+            EProvides::Provides,
+        )),
+        collection_trailing_sep_e(
+            byte(b'{', EProvides::ListStart),
+            provides_type_entry(EProvides::Identifier),
+            byte(b',', EProvides::ListEnd),
+            byte(b'}', EProvides::ListEnd),
+            Spaced::SpaceBefore,
+        ),
+    )
+}
+
+fn provides_type_entry<'a, F, E>(
+    to_expectation: F,
+) -> impl Parser<'a, Loc<Spaced<'a, UppercaseIdent<'a>>>, E>
+where
+    F: Fn(Position) -> E,
+    F: Copy,
+    E: 'a,
+{
+    loc(map(
+        specialize_err(move |_, pos| to_expectation(pos), ident::uppercase()),
+        Spaced::Item,
+    ))
+}
+
+fn exposes_entry<'a, F, E>(
+    to_expectation: F,
+) -> impl Parser<'a, Loc<Spaced<'a, ExposedName<'a>>>, E>
+where
+    F: Fn(Position) -> E,
+    F: Copy,
+    E: 'a,
+{
+    loc(map(
+        specialize_err(move |_, pos| to_expectation(pos), unqualified_ident()),
+        |n| Spaced::Item(ExposedName::new(n)),
+    ))
+}
+
+#[inline(always)]
+fn requires<'a>(
+) -> impl Parser<'a, KeywordItem<'a, RequiresKeyword, PlatformRequires<'a>>, ERequires<'a>> {
+    record!(KeywordItem {
+        keyword: spaces_around_keyword(
+            RequiresKeyword,
+            ERequires::Requires,
+            ERequires::IndentRequires,
+            ERequires::IndentListStart
+        ),
+        item: platform_requires(),
+    })
+}
+
+#[inline(always)]
+fn platform_requires<'a>() -> impl Parser<'a, PlatformRequires<'a>, ERequires<'a>> {
+    record!(PlatformRequires {
+        rigids: skip_second(requires_rigids(), space0_e(ERequires::ListStart)),
+        signature: requires_typed_ident()
+    })
+}
+
+#[inline(always)]
+fn requires_rigids<'a>(
+) -> impl Parser<'a, Collection<'a, Loc<Spaced<'a, UppercaseIdent<'a>>>>, ERequires<'a>> {
+    collection_trailing_sep_e(
+        byte(b'{', ERequires::ListStart),
+        specialize_err(
+            |_, pos| ERequires::Rigid(pos),
+            loc(map(ident::uppercase(), Spaced::Item)),
+        ),
+        byte(b',', ERequires::ListEnd),
+        byte(b'}', ERequires::ListEnd),
+        Spaced::SpaceBefore,
+    )
+}
+
+#[inline(always)]
+fn requires_typed_ident<'a>() -> impl Parser<'a, Loc<Spaced<'a, TypedIdent<'a>>>, ERequires<'a>> {
+    skip_first(
+        byte(b'{', ERequires::ListStart),
+        skip_second(
+            reset_min_indent(space0_around_ee(
+                specialize_err(ERequires::TypedIdent, loc(typed_ident())),
+                ERequires::ListStart,
+                ERequires::ListEnd,
+            )),
+            byte(b'}', ERequires::ListStart),
+        ),
+    )
+}
+
+#[inline(always)]
+fn exposes_values_kw<'a>() -> impl Parser<
+    'a,
+    KeywordItem<'a, ExposesKeyword, Collection<'a, Loc<Spaced<'a, ExposedName<'a>>>>>,
+    EExposes,
+> {
+    record!(KeywordItem {
+        keyword: exposes_kw(),
+        item: exposes_list()
+    })
+}
+
+#[inline(always)]
+fn exposes_kw<'a>() -> impl Parser<'a, Spaces<'a, ExposesKeyword>, EExposes> {
+    spaces_around_keyword(
+        ExposesKeyword,
+        EExposes::Exposes,
+        EExposes::IndentExposes,
+        EExposes::IndentListStart,
+    )
+}
+
+#[inline(always)]
+fn exposes_list<'a>() -> impl Parser<'a, Collection<'a, Loc<Spaced<'a, ExposedName<'a>>>>, EExposes>
+{
+    collection_trailing_sep_e(
+        byte(b'[', EExposes::ListStart),
+        exposes_entry(EExposes::Identifier),
+        byte(b',', EExposes::ListEnd),
+        byte(b']', EExposes::ListEnd),
+        Spaced::SpaceBefore,
+    )
+}
+
+pub fn spaces_around_keyword<'a, K: Keyword, E>(
+    keyword_item: K,
+    expectation: fn(Position) -> E,
+    indent_problem1: fn(Position) -> E,
+    indent_problem2: fn(Position) -> E,
+) -> impl Parser<'a, Spaces<'a, K>, E>
+where
+    E: 'a + SpaceProblem,
+{
+    map(
+        and(
+            skip_second(
+                // parse any leading space before the keyword
+                backtrackable(space0_e(indent_problem1)),
+                // parse the keyword
+                crate::parser::keyword(K::KEYWORD, expectation),
+            ),
+            // parse the trailing space
+            space0_e(indent_problem2),
+        ),
+        move |(before, after)| Spaces {
+            before,
+            item: keyword_item,
+            after,
+        },
+    )
+}
+
+#[inline(always)]
+fn exposes_modules<'a>() -> impl Parser<
+    'a,
+    KeywordItem<'a, ExposesKeyword, Collection<'a, Loc<Spaced<'a, ModuleName<'a>>>>>,
+    EExposes,
+> {
+    record!(KeywordItem {
+        keyword: spaces_around_keyword(
+            ExposesKeyword,
+            EExposes::Exposes,
+            EExposes::IndentExposes,
+            EExposes::IndentListStart
+        ),
+        item: exposes_module_collection(),
+    })
+}
+
+fn exposes_module_collection<'a>(
+) -> impl Parser<'a, Collection<'a, Loc<Spaced<'a, ModuleName<'a>>>>, EExposes> {
+    collection_trailing_sep_e(
+        byte(b'[', EExposes::ListStart),
+        exposes_module(EExposes::Identifier),
+        byte(b',', EExposes::ListEnd),
+        byte(b']', EExposes::ListEnd),
+        Spaced::SpaceBefore,
+    )
+}
+
+fn exposes_module<'a, F, E>(
+    to_expectation: F,
+) -> impl Parser<'a, Loc<Spaced<'a, ModuleName<'a>>>, E>
+where
+    F: Fn(Position) -> E,
+    F: Copy,
+    E: 'a,
+{
+    loc(map(
+        specialize_err(move |_, pos| to_expectation(pos), module_name()),
+        Spaced::Item,
+    ))
+}
+
+#[inline(always)]
+fn packages<'a>() -> impl Parser<
+    'a,
+    KeywordItem<'a, PackagesKeyword, Collection<'a, Loc<Spaced<'a, PackageEntry<'a>>>>>,
+    EPackages<'a>,
+> {
+    record!(KeywordItem {
+        keyword: packages_kw(),
+        item: packages_collection()
+    })
+}
+
+#[inline(always)]
+fn packages_kw<'a>() -> impl Parser<'a, Spaces<'a, PackagesKeyword>, EPackages<'a>> {
+    spaces_around_keyword(
+        PackagesKeyword,
+        EPackages::Packages,
+        EPackages::IndentPackages,
+        EPackages::IndentListStart,
+    )
+}
+
+#[inline(always)]
+fn packages_collection<'a>(
+) -> impl Parser<'a, Collection<'a, Loc<Spaced<'a, PackageEntry<'a>>>>, EPackages<'a>> {
+    collection_trailing_sep_e(
+        byte(b'{', EPackages::ListStart),
+        specialize_err(EPackages::PackageEntry, loc(package_entry())),
+        byte(b',', EPackages::ListEnd),
+        byte(b'}', EPackages::ListEnd),
+        Spaced::SpaceBefore,
+    )
+}
+
+fn imports<'a>() -> impl Parser<
+    'a,
+    KeywordItem<'a, ImportsKeyword, Collection<'a, Loc<Spaced<'a, ImportsEntry<'a>>>>>,
+    EImports,
+> {
+    record!(KeywordItem {
+        keyword: spaces_around_keyword(
+            ImportsKeyword,
+            EImports::Imports,
+            EImports::IndentImports,
+            EImports::IndentListStart
+        ),
+        item: collection_trailing_sep_e(
+            byte(b'[', EImports::ListStart),
+            loc(imports_entry()),
+            byte(b',', EImports::ListEnd),
+            byte(b']', EImports::ListEnd),
+            Spaced::SpaceBefore
+        )
+    })
+    .trace("imports")
+}
+
+#[inline(always)]
+pub fn typed_ident<'a>() -> impl Parser<'a, Spaced<'a, TypedIdent<'a>>, ETypedIdent<'a>> {
+    // e.g.
+    //
+    // printLine : Str -> Task {} *
+    map(
+        and(
+            and(
+                loc(specialize_err(
+                    |_, pos| ETypedIdent::Identifier(pos),
+                    lowercase_ident(),
+                )),
+                space0_e(ETypedIdent::IndentHasType),
+            ),
+            skip_first(
+                byte(b':', ETypedIdent::HasType),
+                space0_before_e(
+                    specialize_err(
+                        ETypedIdent::Type,
+                        reset_min_indent(type_annotation::located(true)),
+                    ),
+                    ETypedIdent::IndentType,
+                ),
+            ),
+        ),
+        |((ident, spaces_before_colon), ann)| {
+            Spaced::Item(TypedIdent {
+                ident,
+                spaces_before_colon,
+                ann,
+            })
+        },
+    )
+}
+
+fn shortname<'a>() -> impl Parser<'a, &'a str, EImports> {
+    specialize_err(|_, pos| EImports::Shorthand(pos), lowercase_ident())
+}
+
+pub fn module_name_help<'a, F, E>(to_expectation: F) -> impl Parser<'a, ModuleName<'a>, E>
+where
+    F: Fn(Position) -> E,
+    E: 'a,
+    F: 'a,
+{
+    specialize_err(move |_, pos| to_expectation(pos), module_name())
+}
+
+#[inline(always)]
+fn imports_entry<'a>() -> impl Parser<'a, Spaced<'a, ImportsEntry<'a>>, EImports> {
+    type Temp<'a> = (
+        (Option<&'a str>, ModuleName<'a>),
+        Option<Collection<'a, Loc<Spaced<'a, ExposedName<'a>>>>>,
+    );
+
+    let spaced_import = |((opt_shortname, module_name), opt_values): Temp<'a>| {
+        let exposed_values = opt_values.unwrap_or_else(Collection::empty);
+
+        let entry = match opt_shortname {
+            Some(shortname) => ImportsEntry::Package(shortname, module_name, exposed_values),
+
+            None => ImportsEntry::Module(module_name, exposed_values),
+        };
+
+        Spaced::Item(entry)
+    };
+
+    one_of!(
+        map(
+            and(
+                and(
+                    // e.g. `pf.`
+                    optional(backtrackable(skip_second(
+                        shortname(),
+                        byte(b'.', EImports::ShorthandDot)
+                    ))),
+                    // e.g. `Task`
+                    module_name_help(EImports::ModuleName)
+                ),
+                // e.g. `.{ Task, after}`
+                optional(skip_first(
+                    byte(b'.', EImports::ExposingDot),
+                    collection_trailing_sep_e(
+                        byte(b'{', EImports::SetStart),
+                        exposes_entry(EImports::Identifier),
+                        byte(b',', EImports::SetEnd),
+                        byte(b'}', EImports::SetEnd),
+                        Spaced::SpaceBefore
+                    )
+                ))
+            ),
+            spaced_import
+        )
+        .trace("normal_import"),
+        map(
+            and(
+                and(
+                    // e.g. "filename"
+                    // TODO: str literal allows for multiline strings. We probably don't want that for file names.
+                    specialize_err(|_, pos| EImports::StrLiteral(pos), parse_str_literal()),
+                    // e.g. as
+                    and(
+                        and(
+                            space0_e(EImports::AsKeyword),
+                            two_bytes(b'a', b's', EImports::AsKeyword)
+                        ),
+                        space0_e(EImports::AsKeyword)
+                    )
+                ),
+                // e.g. file : Str
+                specialize_err(|_, pos| EImports::TypedIdent(pos), typed_ident())
+            ),
+            |((file_name, _), typed_ident)| {
+                // TODO: look at blacking block strings during parsing.
+                Spaced::Item(ImportsEntry::IngestedFile(file_name, typed_ident))
+            }
+        )
+        .trace("ingest_file_import")
+    )
+    .trace("imports_entry")
+}

crates/compiler/parse/src/normalize.rs

@@ -13,19 +13,18 @@ use crate::{
         TypeDef, TypeHeader, ValueDef, WhenBranch,
     },
     header::{
-        AppHeader, ExposedName, ExposesKeyword, GeneratesKeyword, HostedHeader, ImportsEntry,
-        ImportsKeyword, KeywordItem, ModuleHeader, ModuleName, ModuleParams, PackageEntry,
-        PackageHeader, PackageKeyword, PackageName, PackagesKeyword, PlatformHeader,
-        PlatformKeyword, PlatformRequires, ProvidesKeyword, ProvidesTo, RequiresKeyword, To,
-        ToKeyword, TypedIdent, WithKeyword,
+        AppHeader, ExposedName, ExposesKeyword, HostedHeader, ImportsEntry, ImportsKeyword,
+        KeywordItem, ModuleHeader, ModuleName, ModuleParams, PackageEntry, PackageHeader,
+        PackageKeyword, PackageName, PackagesKeyword, PlatformHeader, PlatformKeyword,
+        PlatformRequires, ProvidesKeyword, ProvidesTo, RequiresKeyword, To, ToKeyword, TypedIdent,
     },
     ident::{BadIdent, UppercaseIdent},
     parser::{
-        EAbility, EClosure, EExpect, EExposes, EExpr, EGenerates, EGeneratesWith, EHeader, EIf,
-        EImport, EImportParams, EImports, EInParens, EList, EPackageEntry, EPackageName, EPackages,
-        EParams, EPattern, EProvides, ERecord, ERequires, EString, EType, ETypeAbilityImpl,
-        ETypeApply, ETypeInParens, ETypeInlineAlias, ETypeRecord, ETypeTagUnion, ETypedIdent,
-        EWhen, PInParens, PList, PRecord, SyntaxError,
+        EAbility, EClosure, EExpect, EExposes, EExpr, EHeader, EIf, EImport, EImportParams,
+        EImports, EInParens, EList, EPackageEntry, EPackageName, EPackages, EParams, EPattern,
+        EProvides, ERecord, ERequires, EString, EType, ETypeAbilityImpl, ETypeApply, ETypeInParens,
+        ETypeInlineAlias, ETypeRecord, ETypeTagUnion, ETypedIdent, EWhen, PInParens, PList,
+        PRecord, SyntaxError,
     },
 };
 
@@ -59,8 +58,6 @@ macro_rules! keywords {
 keywords! {
     ExposesKeyword,
     ImportsKeyword,
-    WithKeyword,
-    GeneratesKeyword,
     PackageKeyword,
     PackagesKeyword,
     RequiresKeyword,
@@ -179,8 +176,6 @@ impl<'a> Normalize<'a> for Header<'a> {
                 name: header.name.normalize(arena),
                 exposes: header.exposes.normalize(arena),
                 imports: header.imports.normalize(arena),
-                generates: header.generates.normalize(arena),
-                generates_with: header.generates_with.normalize(arena),
             }),
         }
     }
@@ -784,7 +779,8 @@ impl<'a> Normalize<'a> for Expr<'a> {
                 arena.alloc(a.normalize(arena)),
                 arena.alloc(b.normalize(arena)),
             ),
-            Expr::Dbg(a, b) => Expr::Dbg(
+            Expr::Dbg => Expr::Dbg,
+            Expr::DbgStmt(a, b) => Expr::DbgStmt(
                 arena.alloc(a.normalize(arena)),
                 arena.alloc(b.normalize(arena)),
             ),
@@ -1587,38 +1583,6 @@ impl<'a> Normalize<'a> for EAbility<'a> {
     }
 }
 
-impl<'a> Normalize<'a> for EGeneratesWith {
-    fn normalize(&self, _arena: &'a Bump) -> Self {
-        match self {
-            EGeneratesWith::Open(_) => EGeneratesWith::Open(Position::zero()),
-            EGeneratesWith::With(_) => EGeneratesWith::With(Position::zero()),
-            EGeneratesWith::IndentWith(_) => EGeneratesWith::IndentWith(Position::zero()),
-            EGeneratesWith::IndentListStart(_) => EGeneratesWith::IndentListStart(Position::zero()),
-            EGeneratesWith::IndentListEnd(_) => EGeneratesWith::IndentListEnd(Position::zero()),
-            EGeneratesWith::ListStart(_) => EGeneratesWith::ListStart(Position::zero()),
-            EGeneratesWith::ListEnd(_) => EGeneratesWith::ListEnd(Position::zero()),
-            EGeneratesWith::Identifier(_) => EGeneratesWith::Identifier(Position::zero()),
-            EGeneratesWith::Space(inner_err, _) => {
-                EGeneratesWith::Space(*inner_err, Position::zero())
-            }
-        }
-    }
-}
-
-impl<'a> Normalize<'a> for EGenerates {
-    fn normalize(&self, _arena: &'a Bump) -> Self {
-        match self {
-            EGenerates::Open(_) => EGenerates::Open(Position::zero()),
-            EGenerates::Generates(_) => EGenerates::Generates(Position::zero()),
-            EGenerates::IndentGenerates(_) => EGenerates::IndentGenerates(Position::zero()),
-            EGenerates::Identifier(_) => EGenerates::Identifier(Position::zero()),
-            EGenerates::Space(inner_err, _) => EGenerates::Space(*inner_err, Position::zero()),
-            EGenerates::IndentTypeStart(_) => EGenerates::IndentTypeStart(Position::zero()),
-            EGenerates::IndentTypeEnd(_) => EGenerates::IndentTypeEnd(Position::zero()),
-        }
-    }
-}
-
 impl<'a> Normalize<'a> for EPackages<'a> {
     fn normalize(&self, arena: &'a Bump) -> Self {
         match self {
@@ -1658,12 +1622,6 @@ impl<'a> Normalize<'a> for EHeader<'a> {
             EHeader::Packages(inner_err, _) => {
                 EHeader::Packages(inner_err.normalize(arena), Position::zero())
             }
-            EHeader::Generates(inner_err, _) => {
-                EHeader::Generates(inner_err.normalize(arena), Position::zero())
-            }
-            EHeader::GeneratesWith(inner_err, _) => {
-                EHeader::GeneratesWith(inner_err.normalize(arena), Position::zero())
-            }
             EHeader::Space(inner_err, _) => EHeader::Space(*inner_err, Position::zero()),
             EHeader::Start(_) => EHeader::Start(Position::zero()),
             EHeader::ModuleName(_) => EHeader::ModuleName(Position::zero()),

crates/compiler/parse/src/parser.rs

@@ -83,8 +83,6 @@ impl_space_problem! {
     EExpect<'a>,
     EExposes,
     EExpr<'a>,
-    EGenerates,
-    EGeneratesWith,
     EHeader<'a>,
     EIf<'a>,
     EImport<'a>,
@@ -122,8 +120,6 @@ pub enum EHeader<'a> {
     Imports(EImports, Position),
     Requires(ERequires<'a>, Position),
     Packages(EPackages<'a>, Position),
-    Generates(EGenerates, Position),
-    GeneratesWith(EGeneratesWith, Position),
 
     Space(BadInputError, Position),
     Start(Position),
@@ -251,30 +247,6 @@ pub enum EImports {
     StrLiteral(Position),
 }
 
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub enum EGenerates {
-    Open(Position),
-    Generates(Position),
-    IndentGenerates(Position),
-    Identifier(Position),
-    Space(BadInputError, Position),
-    IndentTypeStart(Position),
-    IndentTypeEnd(Position),
-}
-
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub enum EGeneratesWith {
-    Open(Position),
-    With(Position),
-    IndentWith(Position),
-    IndentListStart(Position),
-    IndentListEnd(Position),
-    ListStart(Position),
-    ListEnd(Position),
-    Identifier(Position),
-    Space(BadInputError, Position),
-}
-
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum BadInputError {
     HasTab,
@@ -856,8 +828,7 @@ where
         }
 
         // This should be enough for anyone. Right? RIGHT?
-        let indent_text =
-            "| ; : ! | ; : ! | ; : ! | ; : ! | ; : ! | ; : ! | ; : ! | ; : ! | ; : ! ";
+        let indent_text = "| ; : ! ".repeat(20);
 
         let cur_indent = INDENT.with(|i| *i.borrow());