error type unification when using an opaque type

[Anton-4]

This error is from a while ago but we forgot to make an issue for it

app "command-line-args"
    packages {
        pf: "https://github.com/roc-lang/basic-cli/releases/download/0.4.0/DI4lqn7LIZs8ZrCDUgLK-tHHpQmxGF1ZrlevRKq5LXk.tar.br",
    }
    imports [
        pf.Stdout,
        pf.Stderr,
        pf.File,
        pf.Path,
        pf.Task,
        pf.Arg,
    ]
    provides [main] to pf

main : Task.Task {} U32
main =
    finalTask =
        # try to read the first command line argument
        pathArg <- Task.await readFirstArgT

        readFileToStr (Path.fromStr pathArg)

    finalResult <- Task.attempt finalTask

    when finalResult is

        Err ZeroArgsGiven ->
            {} <- Stderr.line "Error ZeroArgsGiven:\n\tI expected one argument, but I got none.\n\tRun the app like this: `roc command-line-args.roc -- path/to/input.txt`" |> Task.await

            Task.fail 1

        Err (ReadFileErr errMsg) ->
            indentedErrMsg = indentLines errMsg
            {} <- Stderr.line "Error ReadFileErr:\n\(indentedErrMsg)" |> Task.await

            Task.fail 1

        Ok fileContentStr ->
            Stdout.line "file content: \(fileContentStr)"

# Task to read the first CLI arg (= Str)
readFirstArgT : Task.Task Str [ZeroArgsGiven]
readFirstArgT =
    # read all command line arguments
    args <- Arg.list |> Task.await

    # get the second argument, the first is the executable's path
    List.get args 1 |> Result.mapErr (\_ -> ZeroArgsGiven) |> Task.fromResult

# reads a file and puts all lines in one Str
readFileToStr : Path.Path -> Task.Task Str [ReadFileErr Str]
readFileToStr = \path ->
    path
    |> File.readUtf8
    # Make a nice error message 
    |> Task.mapFail
        (\fileReadErr ->
            pathStr = Path.display path
            # TODO use FileReadErrToErrMsg when it is implemented: https://github.com/roc-lang/basic-cli/issues/44
            when fileReadErr is
                FileReadErr _ readErr ->
                    readErrStr = File.readErrToStr readErr
                    ReadFileErr "Failed to read file at:\n\t\(pathStr)\n\(readErrStr)"

                FileReadUtf8Err _ _ ->
                    ReadFileErr "I could not read the file:\n\t\(pathStr)\nIt contains charcaters that are not valid UTF-8:\n\t- Check if the file is encoded using a different format and convert it to UTF-8.\n\t- Check if the file is corrupted.\n\t- Find the characters that are not valid UTF-8 and fix or remove them."
        )

# indent all lines in a Str with a single tab
indentLines: Str -> Str
indentLines = \inputStr ->
    Str.split inputStr "\n"
    |> List.map (\line -> Str.concat "\t" line)
    |> Str.joinWith "\n

Results in:

❯ ./roc_nightly/roc check examples/CommandLineArgs/main.roc 

── TYPE MISMATCH ─────────────────────────── examples/CommandLineArgs/main.roc ─

This 2nd argument to await has an unexpected type:

20│>          pathArg <- Task.await readFirstArgT
21│>
22│>          readFileToStr (Path.fromStr pathArg)

The argument is an anonymous function of type:

    Str -> Task Str [ReadFileErr Str]

But await needs its 2nd argument to be:

    Str -> Task Str [ZeroArgsGiven]


── TYPE MISMATCH ─────────────────────────── examples/CommandLineArgs/main.roc ─

This 2nd argument to attempt has an unexpected type:

24│>      finalResult <- Task.attempt finalTask
25│>
26│>      when finalResult is
27│>
28│>          Err ZeroArgsGiven ->
29│>              {} <- Stderr.line "Error ZeroArgsGiven:\n\tI expected one argument, but I got none.\n\tRun the app like this: `roc command-line-args.roc -- path/to/input.txt`" |> Task.await
30│>
31│>              Task.fail 1
32│>
33│>          Err (ReadFileErr errMsg) ->
34│>              indentedErrMsg = indentLines errMsg
35│>              {} <- Stderr.line "Error ReadFileErr:\n\(indentedErrMsg)" |> Task.await
36│>
37│>              Task.fail 1
38│>
39│>          Ok fileContentStr ->
40│>              Stdout.line "file content: \(fileContentStr)"

The argument is an anonymous function of type:

    [
        Err [
            ReadFileErr Str,
            ZeroArgsGiven,
        ],
        Ok Str,
    ] -> Task {} (Num *)

But attempt needs its 2nd argument to be:

    Result Str [ZeroArgsGiven]* -> Task {} (Num *)

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

2 errors and 0 warnings found in 40 ms.

Using roc from commit 8a3d07c

No errors should be returned here.

Explanation and workaround by Ayaz:

it's because Task.Task is an opaque type, and not all tag unions can be open-by-default-in-output position under an opaque type (but type parameters like the tags here should be). I'll file an issue, but in the meantime you can work around this by saying [ZeroArgsGiven]_ and [ReadFileErr Str]_.

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