diff --git a/Cargo.lock b/Cargo.lock
index 84a6839e12..8a1d024080 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1871,9 +1871,9 @@ checksum = "90953f308a79fe6d62a4643e51f848fbfddcd05975a38e69fdf4ab86a7baf7ca"
 
 [[package]]
 name = "insta"
-version = "1.19.0"
+version = "1.20.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f37dc501f12ec0c339b385787fa89ffda3d5d2caa62e558da731134c24d6e0c4"
+checksum = "58a931b01c76064c5be919faa2ef0dc570e9a889dcd1e5fef08a8ca6eb4d6c0b"
 dependencies = [
  "console",
  "linked-hash-map",
@@ -2776,9 +2776,9 @@ checksum = "19b17cddbe7ec3f8bc800887bab5e717348c95ea2ca0b1bf0837fb964dc67099"
 
 [[package]]
 name = "peg"
-version = "0.8.0"
+version = "0.8.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "af728fe826811af3b38c37e93de6d104485953ea373d656eebae53d6987fcd2c"
+checksum = "a07f2cafdc3babeebc087e499118343442b742cc7c31b4d054682cc598508554"
 dependencies = [
  "peg-macros",
  "peg-runtime",
@@ -2786,9 +2786,9 @@ dependencies = [
 
 [[package]]
 name = "peg-macros"
-version = "0.8.0"
+version = "0.8.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4536be147b770b824895cbad934fccce8e49f14b4c4946eaa46a6e4a12fcdc16"
+checksum = "4a90084dc05cf0428428e3d12399f39faad19b0909f64fb9170c9fdd6d9cd49b"
 dependencies = [
  "peg-runtime",
  "proc-macro2",
@@ -2797,9 +2797,9 @@ dependencies = [
 
 [[package]]
 name = "peg-runtime"
-version = "0.8.0"
+version = "0.8.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f9b0efd3ba03c3a409d44d60425f279ec442bcf0b9e63ff4e410da31c8b0f69f"
+checksum = "9fa00462b37ead6d11a82c9d568b26682d78e0477dc02d1966c013af80969739"
 
 [[package]]
 name = "percent-encoding"
diff --git a/crates/cli/tests/cli_run.rs b/crates/cli/tests/cli_run.rs
index 1720f94876..75b379c2a6 100644
--- a/crates/cli/tests/cli_run.rs
+++ b/crates/cli/tests/cli_run.rs
@@ -87,6 +87,7 @@ mod cli_run {
         executable_filename: &'a str,
         stdin: &'a [&'a str],
         arguments: &'a [Arg<'a>],
+        env: &'a [(&'a str, &'a str)],
         expected_ending: &'a str,
         use_valgrind: bool,
     }
@@ -136,12 +137,14 @@ mod cli_run {
         compile_out
     }
 
+    #[allow(clippy::too_many_arguments)]
     fn check_output_with_stdin(
         file: &Path,
         stdin: &[&str],
         executable_filename: &str,
         flags: &[&str],
         app_args: &[String],
+        extra_env: &[(&str, &str)],
         expected_ending: &str,
         use_valgrind: bool,
     ) {
@@ -214,16 +217,31 @@ mod cli_run {
                             file.with_file_name(executable_filename).to_str().unwrap(),
                             stdin.iter().copied(),
                             app_args,
+                            extra_env.iter().copied(),
                         )
                     }
                 }
-                CliMode::Roc => run_roc_on(file, flags.clone(), stdin, app_args),
-                CliMode::RocRun => run_roc_on(
-                    file,
-                    iter::once(CMD_RUN).chain(flags.clone()),
-                    stdin,
-                    app_args,
-                ),
+                CliMode::Roc => {
+                    if !extra_env.is_empty() {
+                        // TODO: environment is not currently forwarded by Roc to the target
+                        // binary, so this would fail!
+                        continue;
+                    }
+                    run_roc_on(file, flags.clone(), stdin, app_args)
+                }
+                CliMode::RocRun => {
+                    if !extra_env.is_empty() {
+                        // TODO: environment is not currently forwarded by Roc to the target
+                        // binary, so this would fail!
+                        continue;
+                    }
+                    run_roc_on(
+                        file,
+                        iter::once(CMD_RUN).chain(flags.clone()),
+                        stdin,
+                        app_args,
+                    )
+                }
             };
 
             if !&out.stdout.ends_with(expected_ending) {
@@ -367,6 +385,7 @@ mod cli_run {
                         example.executable_filename,
                         &custom_flags,
                         &app_args,
+                        example.env,
                         example.expected_ending,
                         example.use_valgrind,
                     );
@@ -381,6 +400,7 @@ mod cli_run {
                         example.executable_filename,
                         &custom_flags,
                         &app_args,
+                        example.env,
                         example.expected_ending,
                         example.use_valgrind,
                     );
@@ -394,6 +414,7 @@ mod cli_run {
                             example.executable_filename,
                             &[LINKER_FLAG, "legacy"],
                             &app_args,
+                            example.env,
                             example.expected_ending,
                             example.use_valgrind,
                         );
@@ -431,6 +452,7 @@ mod cli_run {
             executable_filename: "helloWorld",
             stdin: &[],
             arguments: &[],
+            env: &[],
             expected_ending:"Hello, World!\n",
             use_valgrind: true,
         },
@@ -439,6 +461,7 @@ mod cli_run {
             executable_filename: "rocLovesPlatforms",
             stdin: &[],
             arguments: &[],
+            env: &[],
             expected_ending:"Which platform am I running on now?\n",
             use_valgrind: true,
         },
@@ -458,6 +481,7 @@ mod cli_run {
             executable_filename: "rocLovesRust",
             stdin: &[],
             arguments: &[],
+            env: &[],
             expected_ending:"Roc <3 Rust!\n",
             use_valgrind: true,
         },
@@ -466,6 +490,7 @@ mod cli_run {
             executable_filename: "rocLovesSwift",
             stdin: &[],
             arguments: &[],
+            env: &[],
             expected_ending:"Roc <3 Swift!\n",
             use_valgrind: true,
         },
@@ -474,6 +499,7 @@ mod cli_run {
             executable_filename: "rocLovesWebAssembly",
             stdin: &[],
             arguments: &[],
+            env: &[],
             expected_ending:"Roc <3 Web Assembly!\n",
             use_valgrind: true,
         },
@@ -482,6 +508,7 @@ mod cli_run {
             executable_filename: "rocLovesZig",
             stdin: &[],
             arguments: &[],
+            env: &[],
             expected_ending:"Roc <3 Zig!\n",
             use_valgrind: true,
         },
@@ -490,6 +517,7 @@ mod cli_run {
             executable_filename: "libhello",
             stdin: &[],
             arguments: &[],
+            env: &[],
             expected_ending:"",
             use_valgrind: true,
         },
@@ -498,6 +526,7 @@ mod cli_run {
             executable_filename: "fibonacci",
             stdin: &[],
             arguments: &[],
+            env: &[],
             expected_ending:"55\n",
             use_valgrind: true,
         },
@@ -506,6 +535,7 @@ mod cli_run {
             executable_filename: "hello-gui",
             stdin: &[],
             arguments: &[],
+            env: &[],
             expected_ending: "",
             use_valgrind: false,
         },
@@ -514,6 +544,7 @@ mod cli_run {
             executable_filename: "breakout",
             stdin: &[],
             arguments: &[],
+            env: &[],
             expected_ending: "",
             use_valgrind: false,
         },
@@ -522,6 +553,7 @@ mod cli_run {
             executable_filename: "quicksort",
             stdin: &[],
             arguments: &[],
+            env: &[],
             expected_ending: "[0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2]\n",
             use_valgrind: true,
         },
@@ -538,14 +570,28 @@ mod cli_run {
             executable_filename: "args",
             stdin: &[],
             arguments: &[Arg::PlainText("log"), Arg::PlainText("-b"), Arg::PlainText("3"), Arg::PlainText("--num"), Arg::PlainText("81")],
+            env: &[],
             expected_ending: "4\n",
             use_valgrind: false,
         },
+        env:"interactive" => Example {
+            filename: "env.roc",
+            executable_filename: "env",
+            stdin: &[],
+            arguments: &[],
+            env: &[("EDITOR", "roc-editor"), ("SHLVL", "3"), ("LETTERS", "a,c,e,j")],
+            expected_ending: (
+                "Your favorite editor is roc-editor!\n\
+                Your current shell level is 3!\n\
+                Your favorite letters are: a c e j\n"),
+            use_valgrind: false,
+        },
         effects:"interactive" => Example {
             filename: "effects.roc",
             executable_filename: "effects",
             stdin: &["hi there!"],
             arguments: &[],
+            env: &[],
             expected_ending: "hi there!\nIt is known\n",
             use_valgrind: true,
         },
@@ -562,6 +608,7 @@ mod cli_run {
             executable_filename: "form",
             stdin: &["Giovanni\n", "Giorgio\n"],
             arguments: &[],
+            env: &[],
             expected_ending: "Hi, Giovanni Giorgio! 👋\n",
             use_valgrind: false,
         },
@@ -570,6 +617,7 @@ mod cli_run {
             executable_filename: "tui",
             stdin: &["foo\n"], // NOTE: adding more lines leads to memory leaks
             arguments: &[],
+            env: &[],
             expected_ending: "Hello Worldfoo!\n",
             use_valgrind: true,
         },
@@ -595,6 +643,7 @@ mod cli_run {
                 executable_filename: "false",
                 stdin: &[],
                 arguments: &[Arg::ExamplePath("examples/hello.false")],
+                env: &[],
                 expected_ending:"Hello, World!\n",
                 use_valgrind: false,
             }
@@ -604,6 +653,7 @@ mod cli_run {
             executable_filename: "swiftui",
             stdin: &[],
             arguments: &[],
+            env: &[],
             expected_ending: "",
             use_valgrind: false,
         },
@@ -613,6 +663,7 @@ mod cli_run {
                 executable_filename: "static-site",
                 stdin: &[],
                 arguments: &[Arg::ExamplePath("input"), Arg::ExamplePath("output")],
+                env: &[],
                 expected_ending: "Processed 3 files with 3 successes and 0 errors\n",
                 use_valgrind: false,
             }
@@ -623,6 +674,7 @@ mod cli_run {
                 executable_filename: "parse-movies-csv",
                 stdin: &[],
                 arguments: &[],
+                env: &[],
                 expected_ending: "Parse success!\n",
                 use_valgrind: false,
             }
@@ -671,6 +723,7 @@ mod cli_run {
                             benchmark.executable_filename,
                             &[],
                             &app_args,
+                            benchmark.env,
                             benchmark.expected_ending,
                             benchmark.use_valgrind,
                         );
@@ -689,6 +742,7 @@ mod cli_run {
                             benchmark.executable_filename,
                             &[PREBUILT_PLATFORM],
                             &app_args,
+                            benchmark.env,
                             benchmark.expected_ending,
                             benchmark.use_valgrind,
                         );
@@ -700,6 +754,7 @@ mod cli_run {
                         benchmark.executable_filename,
                         &[PREBUILT_PLATFORM, OPTIMIZE_FLAG],
                         &app_args,
+                        benchmark.env,
                         benchmark.expected_ending,
                         benchmark.use_valgrind,
                     );
@@ -813,6 +868,7 @@ mod cli_run {
                 executable_filename: "nqueens",
                 stdin: &["6"],
                 arguments: &[],
+                env: &[],
                 expected_ending: "4\n",
                 use_valgrind: true,
             },
@@ -821,6 +877,7 @@ mod cli_run {
                 executable_filename: "cfold",
                 stdin: &["3"],
                 arguments: &[],
+                env: &[],
                 expected_ending: "11 & 11\n",
                 use_valgrind: true,
             },
@@ -829,6 +886,7 @@ mod cli_run {
                 executable_filename: "deriv",
                 stdin: &["2"],
                 arguments: &[],
+                env: &[],
                 expected_ending: "1 count: 6\n2 count: 22\n",
                 use_valgrind: true,
             },
@@ -837,6 +895,7 @@ mod cli_run {
                 executable_filename: "rbtree-ck",
                 stdin: &["100"],
                 arguments: &[],
+                env: &[],
                 expected_ending: "10\n",
                 use_valgrind: true,
             },
@@ -845,6 +904,7 @@ mod cli_run {
                 executable_filename: "rbtree-insert",
                 stdin: &[],
                 arguments: &[],
+                env: &[],
                 expected_ending: "Node Black 0 {} Empty Empty\n",
                 use_valgrind: true,
             },
@@ -853,6 +913,7 @@ mod cli_run {
     //            executable_filename: "rbtree-del",
     //            stdin: &["420"],
     //            arguments: &[],
+    //            env: &[],
     //            expected_ending: "30\n",
     //            use_valgrind: true,
     //        },
@@ -861,6 +922,7 @@ mod cli_run {
                 executable_filename: "test-astar",
                 stdin: &[],
                 arguments: &[],
+                env: &[],
                 expected_ending: "True\n",
                 use_valgrind: false,
             },
@@ -869,6 +931,7 @@ mod cli_run {
                 executable_filename: "test-base64",
                 stdin: &[],
                 arguments: &[],
+                env: &[],
                 expected_ending: "encoded: SGVsbG8gV29ybGQ=\ndecoded: Hello World\n",
                 use_valgrind: true,
             },
@@ -877,6 +940,7 @@ mod cli_run {
                 executable_filename: "closure",
                 stdin: &[],
                 arguments: &[],
+                env: &[],
                 expected_ending: "",
                 use_valgrind: false,
             },
@@ -885,6 +949,7 @@ mod cli_run {
                 executable_filename: "issue2279",
                 stdin: &[],
                 arguments: &[],
+                env: &[],
                 expected_ending: "Hello, world!\n",
                 use_valgrind: true,
             },
@@ -893,6 +958,7 @@ mod cli_run {
                 executable_filename: "quicksortapp",
                 stdin: &[],
                 arguments: &[],
+                env: &[],
                 expected_ending: "todo put the correct quicksort answer here",
                 use_valgrind: true,
             },
@@ -973,6 +1039,7 @@ mod cli_run {
             "multi-dep-str",
             &[],
             &[],
+            &[],
             "I am Dep2.str2\n",
             true,
         );
@@ -987,6 +1054,7 @@ mod cli_run {
             "multi-dep-str",
             &[OPTIMIZE_FLAG],
             &[],
+            &[],
             "I am Dep2.str2\n",
             true,
         );
@@ -1001,6 +1069,7 @@ mod cli_run {
             "multi-dep-thunk",
             &[],
             &[],
+            &[],
             "I am Dep2.value2\n",
             true,
         );
@@ -1015,6 +1084,7 @@ mod cli_run {
             "multi-dep-thunk",
             &[OPTIMIZE_FLAG],
             &[],
+            &[],
             "I am Dep2.value2\n",
             true,
         );
diff --git a/crates/cli_utils/src/bench_utils.rs b/crates/cli_utils/src/bench_utils.rs
index c9f971266d..8afb84be99 100644
--- a/crates/cli_utils/src/bench_utils.rs
+++ b/crates/cli_utils/src/bench_utils.rs
@@ -48,12 +48,12 @@ fn check_cmd_output(
     let out = if cmd_str.contains("cfold") {
         let child = thread::Builder::new()
             .stack_size(CFOLD_STACK_SIZE)
-            .spawn(move || run_cmd(&cmd_str, [stdin_str], &[]))
+            .spawn(move || run_cmd(&cmd_str, [stdin_str], &[], []))
             .unwrap();
 
         child.join().unwrap()
     } else {
-        run_cmd(&cmd_str, [stdin_str], &[])
+        run_cmd(&cmd_str, [stdin_str], &[], [])
     };
 
     if !&out.stdout.ends_with(expected_ending) {
@@ -96,13 +96,14 @@ fn bench_cmd<T: Measurement>(
 
     if let Some(bench_group) = bench_group_opt {
         bench_group.bench_function(&format!("Benchmarking {:?}", executable_filename), |b| {
-            b.iter(|| run_cmd(black_box(&cmd_str), black_box([stdin_str]), &[]))
+            b.iter(|| run_cmd(black_box(&cmd_str), black_box([stdin_str]), &[], []))
         });
     } else {
         run_cmd(
             black_box(file.with_file_name(executable_filename).to_str().unwrap()),
             black_box([stdin_str]),
             &[],
+            [],
         );
     }
 }
diff --git a/crates/cli_utils/src/helpers.rs b/crates/cli_utils/src/helpers.rs
index f2f37bd3cd..020e890c37 100644
--- a/crates/cli_utils/src/helpers.rs
+++ b/crates/cli_utils/src/helpers.rs
@@ -161,10 +161,11 @@ where
     }
 }
 
-pub fn run_cmd<'a, I: IntoIterator<Item = &'a str>>(
+pub fn run_cmd<'a, I: IntoIterator<Item = &'a str>, E: IntoIterator<Item = (&'a str, &'a str)>>(
     cmd_name: &str,
     stdin_vals: I,
     args: &[String],
+    env: E,
 ) -> Out {
     let mut cmd = Command::new(cmd_name);
 
@@ -172,6 +173,10 @@ pub fn run_cmd<'a, I: IntoIterator<Item = &'a str>>(
         cmd.arg(arg);
     }
 
+    for (env, val) in env.into_iter() {
+        cmd.env(env, val);
+    }
+
     let mut child = cmd
         .stdin(Stdio::piped())
         .stdout(Stdio::piped())
diff --git a/crates/compiler/load_internal/src/file.rs b/crates/compiler/load_internal/src/file.rs
index 352ac88b5c..b85db78390 100644
--- a/crates/compiler/load_internal/src/file.rs
+++ b/crates/compiler/load_internal/src/file.rs
@@ -3732,14 +3732,21 @@ fn send_header<'a>(
         // Also build a list of imported_values_to_expose (like `bar` above.)
         for (qualified_module_name, exposed_idents, region) in imported.into_iter() {
             let cloned_module_name = qualified_module_name.module.clone();
-            let pq_module_name = match qualified_module_name.opt_package {
-                None => match opt_shorthand {
-                    Some(shorthand) => {
-                        PQModuleName::Qualified(shorthand, qualified_module_name.module)
-                    }
-                    None => PQModuleName::Unqualified(qualified_module_name.module),
-                },
-                Some(package) => PQModuleName::Qualified(package, cloned_module_name),
+            let pq_module_name = if qualified_module_name.is_builtin() {
+                // If this is a builtin, it must be unqualified, and we should *never* prefix it
+                // with the package shorthand! The user intended to import the module as-is here.
+                debug_assert!(qualified_module_name.opt_package.is_none());
+                PQModuleName::Unqualified(qualified_module_name.module)
+            } else {
+                match qualified_module_name.opt_package {
+                    None => match opt_shorthand {
+                        Some(shorthand) => {
+                            PQModuleName::Qualified(shorthand, qualified_module_name.module)
+                        }
+                        None => PQModuleName::Unqualified(qualified_module_name.module),
+                    },
+                    Some(package) => PQModuleName::Qualified(package, cloned_module_name),
+                }
             };
 
             let module_id = module_ids.get_or_insert(&pq_module_name);
diff --git a/crates/compiler/solve/Cargo.toml b/crates/compiler/solve/Cargo.toml
index 2bd339c042..4e085f2da1 100644
--- a/crates/compiler/solve/Cargo.toml
+++ b/crates/compiler/solve/Cargo.toml
@@ -37,4 +37,4 @@ tempfile = "3.2.0"
 bumpalo = { version = "3.11.0", features = ["collections"] }
 regex = "1.5.5"
 lazy_static = "1.4.0"
-insta = "1.19.0"
+insta = "1.20.0"
diff --git a/crates/compiler/test_derive/Cargo.toml b/crates/compiler/test_derive/Cargo.toml
index 11a3622eee..2b03ef57d4 100644
--- a/crates/compiler/test_derive/Cargo.toml
+++ b/crates/compiler/test_derive/Cargo.toml
@@ -29,4 +29,4 @@ lazy_static = "1.4.0"
 indoc = "1.0.7"
 ven_pretty = { path = "../../vendor/pretty" }
 pretty_assertions = "1.3.0"
-insta = "1.19.0"
+insta = "1.20.0"
diff --git a/crates/docs/Cargo.toml b/crates/docs/Cargo.toml
index e613e30065..a8723f3c6b 100644
--- a/crates/docs/Cargo.toml
+++ b/crates/docs/Cargo.toml
@@ -22,7 +22,7 @@ roc_highlight = { path = "../highlight"}
 roc_reporting = { path = "../reporting"}
 bumpalo = { version = "3.11.0", features = ["collections"] }
 snafu = { version = "0.7.1", features = ["backtraces"] }
-peg = "0.8.0"
+peg = "0.8.1"
 
 [dev-dependencies]
 pretty_assertions = "1.3.0"
diff --git a/crates/highlight/Cargo.toml b/crates/highlight/Cargo.toml
index 5c4f498091..8277ed2cb9 100644
--- a/crates/highlight/Cargo.toml
+++ b/crates/highlight/Cargo.toml
@@ -7,5 +7,5 @@ edition = "2021"
 description = "For syntax highlighting, starts with a string and returns our markup nodes."
 
 [dependencies]
-peg = "0.8.0"
+peg = "0.8.1"
 roc_code_markup = { path = "../code_markup"}
diff --git a/crates/reporting/Cargo.toml b/crates/reporting/Cargo.toml
index 75df48621b..0e548f7fcd 100644
--- a/crates/reporting/Cargo.toml
+++ b/crates/reporting/Cargo.toml
@@ -34,4 +34,4 @@ roc_test_utils = { path = "../test_utils" }
 roc_solve = { path = "../compiler/solve" }
 pretty_assertions = "1.3.0"
 indoc = "1.0.7"
-insta = "1.19.0"
+insta = "1.20.0"
diff --git a/examples/interactive/.gitignore b/examples/interactive/.gitignore
index 084ea28578..8e260e400a 100644
--- a/examples/interactive/.gitignore
+++ b/examples/interactive/.gitignore
@@ -6,3 +6,4 @@ form
 tui
 http-get
 file-io
+env
diff --git a/examples/interactive/cli-platform/Env.roc b/examples/interactive/cli-platform/Env.roc
index 12a520b974..dcd08cda6b 100644
--- a/examples/interactive/cli-platform/Env.roc
+++ b/examples/interactive/cli-platform/Env.roc
@@ -1,6 +1,5 @@
 interface Env
-    exposes [cwd, dict, var]
-    imports [Task.{ Task }, Path.{ Path }, InternalPath, Effect, InternalTask]
+    exposes [cwd, dict, var, decode] imports [Task.{ Task }, Path.{ Path }, InternalPath, Effect, InternalTask, EnvDecoding]
 
 ## Reads the [current working directory](https://en.wikipedia.org/wiki/Working_directory)
 ## from the environment. File operations on relative [Path]s are relative to this directory.
@@ -33,31 +32,40 @@ var = \name ->
     |> Effect.map (\result -> Result.mapErr result \{} -> VarNotFound)
     |> InternalTask.fromEffect
 
-# ## Reads the given environment variable and attempts to decode it.
-# ##
-# ## The type being decoded into will be determined by type inference. For example,
-# ## if this ends up being used like a `Task U16 …` then the environment variable
-# ## will be decoded as a string representation of a `U16`.
-# ##
-# ##     getU16Var : Str -> Task U16 [VarNotFound, DecodeErr DecodeError]* [Env]*
-# ##     getU16Var = \var -> Env.decode var
-# ##     # If the environment contains a variable NUM_THINGS=123, then calling
-# ##     # (getU16Var "NUM_THINGS") would return a task which succeeds with the U16 number 123.
-# ##     #
-# ##     # However, if the NUM_THINGS environment variable was set to 1234567, then
-# ##     # (getU16Var "NUM_THINGS") would fail because that number is too big to fit in a U16.
-# ##
-# ## Supported types:
-# ## - strings
-# ## - numbers, as long as they contain only numeric digits, up to one `.`, and an optional `-` at the front for negative numbers
-# ## - comma-separated lists (of either strings or numbers), as long as there are no spaces after the commas
-# ##
-# ## Trying to decode into any other types will always fail with a `DecodeErr`.
-# decode : Str -> Task val [VarNotFound, DecodeErr DecodeError]* [Env]*
-#     | val has Decode
-# decode = \var ->
-#     Effect.envVar var
-#     |> InternalTask.fromEffect
+## Reads the given environment variable and attempts to decode it.
+##
+## The type being decoded into will be determined by type inference. For example,
+## if this ends up being used like a `Task U16 …` then the environment variable
+## will be decoded as a string representation of a `U16`.
+##
+##     getU16Var : Str -> Task U16 [VarNotFound, DecodeErr DecodeError]* [Env]*
+##     getU16Var = \var -> Env.decode var
+##     # If the environment contains a variable NUM_THINGS=123, then calling
+##     # (getU16Var "NUM_THINGS") would return a task which succeeds with the U16 number 123.
+##     #
+##     # However, if the NUM_THINGS environment variable was set to 1234567, then
+##     # (getU16Var "NUM_THINGS") would fail because that number is too big to fit in a U16.
+##
+## Supported types:
+## - strings
+## - numbers, as long as they contain only numeric digits, up to one `.`, and an optional `-` at the front for negative numbers
+## - comma-separated lists (of either strings or numbers), as long as there are no spaces after the commas
+##
+## Trying to decode into any other types will always fail with a `DecodeErr`.
+decode : Str -> Task val [VarNotFound, DecodeErr DecodeError]* [Env]* | val has Decoding
+decode = \name ->
+    Effect.envVar name
+    |> Effect.map
+        (
+            \result ->
+                result
+                |> Result.mapErr (\{} -> VarNotFound)
+                |> Result.try
+                    (\varStr ->
+                        Decode.fromBytes (Str.toUtf8 varStr) (EnvDecoding.format {})
+                        |> Result.mapErr (\_ -> DecodeErr TooShort)))
+    |> InternalTask.fromEffect
+
 ## Reads all the process's environment variables into a [Dict].
 ##
 ## If any key or value contains invalid Unicode, the [Unicode replacement character](https://unicode.org/glossary/#replacement_character)
diff --git a/examples/interactive/cli-platform/EnvDecoding.roc b/examples/interactive/cli-platform/EnvDecoding.roc
new file mode 100644
index 0000000000..62b25f3f9d
--- /dev/null
+++ b/examples/interactive/cli-platform/EnvDecoding.roc
@@ -0,0 +1,95 @@
+interface EnvDecoding exposes [EnvFormat, format] imports []
+
+EnvFormat := {} has [
+         DecoderFormatting {
+             u8: envU8,
+             u16: envU16,
+             u32: envU32,
+             u64: envU64,
+             u128: envU128,
+             i8: envI8,
+             i16: envI16,
+             i32: envI32,
+             i64: envI64,
+             i128: envI128,
+             f32: envF32,
+             f64: envF64,
+             dec: envDec,
+             bool: envBool,
+             string: envString,
+             list: envList,
+             record: envRecord,
+         },
+     ]
+
+format : {} -> EnvFormat
+format = \{} -> @EnvFormat {}
+
+decodeBytesToNum = \bytes, transformer ->
+    when Str.fromUtf8 bytes is
+        Ok s ->
+            when transformer s is
+                Ok n -> { result: Ok n, rest: [] }
+                Err _ -> { result: Err TooShort, rest: bytes }
+
+        Err _ -> { result: Err TooShort, rest: bytes }
+
+envU8 = Decode.custom \bytes, @EnvFormat {} -> decodeBytesToNum bytes Str.toU8
+envU16 = Decode.custom \bytes, @EnvFormat {} -> decodeBytesToNum bytes Str.toU16
+envU32 = Decode.custom \bytes, @EnvFormat {} -> decodeBytesToNum bytes Str.toU32
+envU64 = Decode.custom \bytes, @EnvFormat {} -> decodeBytesToNum bytes Str.toU64
+envU128 = Decode.custom \bytes, @EnvFormat {} -> decodeBytesToNum bytes Str.toU128
+envI8 = Decode.custom \bytes, @EnvFormat {} -> decodeBytesToNum bytes Str.toI8
+envI16 = Decode.custom \bytes, @EnvFormat {} -> decodeBytesToNum bytes Str.toI16
+envI32 = Decode.custom \bytes, @EnvFormat {} -> decodeBytesToNum bytes Str.toI32
+envI64 = Decode.custom \bytes, @EnvFormat {} -> decodeBytesToNum bytes Str.toI64
+envI128 = Decode.custom \bytes, @EnvFormat {} -> decodeBytesToNum bytes Str.toI128
+envF32 = Decode.custom \bytes, @EnvFormat {} -> decodeBytesToNum bytes Str.toF32
+envF64 = Decode.custom \bytes, @EnvFormat {} -> decodeBytesToNum bytes Str.toF64
+envDec = Decode.custom \bytes, @EnvFormat {} -> decodeBytesToNum bytes Str.toDec
+envBool = Decode.custom \bytes, @EnvFormat {} ->
+    when Str.fromUtf8 bytes is
+        Ok "true" -> { result: Ok Bool.true, rest: [] }
+        Ok "false" -> { result: Ok Bool.false, rest: [] }
+        _ -> { result: Err TooShort, rest: bytes }
+envString = Decode.custom \bytes, @EnvFormat {} ->
+    when Str.fromUtf8 bytes is
+        Ok s -> { result: Ok s, rest: [] }
+        Err _ -> { result: Err TooShort, rest: bytes }
+
+envList = \decodeElem -> Decode.custom \bytes, @EnvFormat {} ->
+        # Per our supported methods of decoding, this is either a list of strings or
+        # a list of numbers; in either case, the list of bytes must be Utf-8
+        # decodable. So just parse it as a list of strings and pass each chunk to
+        # the element decoder. By construction, our element decoders expect to parse
+        # a whole list of bytes anyway.
+        decodeElems = \allBytes, accum ->
+            { toParse, remainder } =
+                when List.splitFirst allBytes (Num.toU8 ',') is
+                    Ok { before, after } ->
+                        { toParse: before, remainder: Some after }
+
+                    Err NotFound ->
+                        { toParse: allBytes, remainder: None }
+
+            when Decode.decodeWith toParse decodeElem (@EnvFormat {}) is
+                { result, rest } ->
+                    when result is
+                        Ok val ->
+                            when remainder is
+                                Some restBytes -> decodeElems restBytes (List.append accum val)
+                                None -> Done (List.append accum val)
+
+                        Err e -> Errored e rest
+
+        when decodeElems bytes [] is
+            Errored e rest -> { result: Err e, rest }
+            Done vals ->
+                { result: Ok vals, rest: [] }
+
+# TODO: we must currently annotate the arrows here so that the lambda sets are
+# exercised, and the solver can find an ambient lambda set for the
+# specialization.
+envRecord : _, (_, _ -> [Keep (Decoder _ _), Skip]), (_ -> _) -> Decoder _ _
+envRecord = \_initialState, _stepField, _finalizer -> Decode.custom \bytes, @EnvFormat {} ->
+        { result: Err TooShort, rest: bytes }
diff --git a/examples/interactive/env.roc b/examples/interactive/env.roc
new file mode 100644
index 0000000000..c53f7d7b26
--- /dev/null
+++ b/examples/interactive/env.roc
@@ -0,0 +1,25 @@
+app "env"
+    packages { pf: "cli-platform/main.roc" }
+    imports [pf.Stdout, pf.Env, pf.Task, pf.Program.{ Program }]
+    provides [main] to pf
+
+main : Program
+main =
+    Env.decode "EDITOR"
+    |> Task.await (\editor -> Stdout.line "Your favorite editor is \(editor)!")
+    |> Task.await (\{} -> Env.decode "SHLVL")
+    |> Task.await
+        (\lvl ->
+            when lvl is
+                1u8 -> Stdout.line "You're running this in a root shell!"
+                n ->
+                    lvlStr = Num.toStr n
+
+                    Stdout.line "Your current shell level is \(lvlStr)!")
+    |> Task.await (\{} -> Env.decode "LETTERS")
+    |> Task.await
+        (\letters ->
+            joinedLetters = Str.joinWith letters " "
+
+            Stdout.line "Your favorite letters are: \(joinedLetters)")
+    |> Program.quick