The argument is an anonymous function of type:

    { … }, Str => { … }

But walk needs its 3rd argument to be:

    { … }, Str -> { … }

app [main!] { pf: platform "../platform/main.roc" }

import pf.Stdout
import pf.File

dir_list : List Str

main! = \_args ->
    foo = List.walk dir_list { file: [], dir: [] } \state, elem ->
        when File.is_file! elem is
            Ok is_file if is_file ->
                { state & file: List.append state.file elem }

            Ok _ ->
                { state & dir: List.append state.dir elem }

            _ ->
                # Just ignore failures
                state

    Stdout.line! "Hello, World!"

This 3rd argument to walk has an unexpected type:

 9│>      foo = List.walk dir_list { file: [], dir: [] } \state, elem ->
10│>          when File.is_file! elem is
11│>              Ok is_file if is_file ->
12│>                  { state & file: List.append state.file elem }
13│>
14│>              Ok _ ->
15│>                  { state & dir: List.append state.dir elem }
16│>
17│>              _ ->
18│>                  # Just ignore failures
19│>                  state

The argument is an anonymous function of type:

    { … }, Str => { … }

But walk needs its 3rd argument to be:

    { … }, Str -> { … }

module [broken, effectful!]

effectful! : Str => Result Str []

broken = List.map [] effectful!

── TYPE MISMATCH in test.roc ───────────────────────────────────────────────────

This 2nd argument to map has an unexpected type:

5│  broken = List.map [] effectful!
                         ^^^^^^^^^^

This effectful! value is a:

    Str => Result Str []

But map needs its 2nd argument to be:

    Str -> Result Str []

────────────────────────────────────────────────────────────────────────────────

1 error and 0 warnings found in 30 ms.