Stream: beginners

Topic: Simple result with I/O

Mark J. Reed (Jun 20 2025 at 18:16):

How do I write the value that Stdout.write! and friends return on success? I want my own routine to return a compatible value in both branches of a conditional, one of which does some output while the other does not...

Anton (Jun 20 2025 at 18:22):

Hi @Mark J. Reed,
Stdout.write! returns Ok({}) on success, so in your other branch you'll have to end with Ok({}) as well.

Anton (Jun 20 2025 at 18:22):

You can figure this out like this btw:

main! : List Arg => Result {} _
main! = |_args|
    res = Stdout.line!("Hello, World!")
    Stdout.line!(Inspect.to_str(res))

Mark J. Reed (Jun 20 2025 at 18:30):

Huh. I tried that, and got this:

This OK tag application has the type:

    [OK {}]

but the then branch has the type:

    Result {} [StdoutErr [
        AlreadyExists,
        BrokenPipe,
        Interrupted,
        NotFound,
        Other Str,
        OutOfMemory,
        PermissionDenied,
        Unsupported,
    ]]

All branches in an if must have the same type!```

Richard Feldman (Jun 20 2025 at 19:14):

it's a capitalization issue - should be Ok over OK :smiley:

Richard Feldman (Jun 20 2025 at 19:15):

that might be a good special-case compiler error to include

Kilian Vounckx (Jun 20 2025 at 21:13):

Might not even need to be special case?
Couldn't we check for tags which should be valid with different casing in general?

Richard Feldman (Jun 20 2025 at 22:11):

yeah we could do that after unrolling the type alias :thumbs_up:

Mark J. Reed (Jun 20 2025 at 22:15):

Ack! You'd think after 40 years of programming I'd be better at spotting a case issue. That's just embarrassing.

Richard Feldman (Jun 21 2025 at 00:45):

in fairness, OK is the usual way it's spelled; nobody really spells it Ok except for programming languages :stuck_out_tongue:

Anton (Jun 21 2025 at 08:08):

There is an issue for better error messages for OK #6630

Last updated: Jul 26 2025 at 12:14 UTC