Stream: bugs

Topic: Type checking with question mark operator ? and branching

Matthieu Pizenberg (Dec 02 2025 at 19:33):

Type checking is failing with this example. Am I missing something or is this a bug?

get_greeting : {} -> Try(Str, _)
get_greeting = |{}| {
    match 0 {
        0 => Try.Ok(List.first(["hello"])?),
        _ => Err(Impossible)
    }
}

main! = || {
    Stdout.line!(get_greeting({}).ok_or("Error!"))
}

I get the following error message:

This expression is used in an unexpected way:
   ┌─ test/fx/question_mark_operator_branching.roc:10:5
   │
10 │     match 0 {
11 │         0 => Try.Ok(List.first(["hello"])?),
12 │         _ => Err(Impossible)
13 │     }

It has the type:
    Try(Str, [Impossible, .._others])

But the type annotation says it should have the type:
    Try(Str, [ListWasEmpty])

Richard Feldman (Dec 02 2025 at 20:27):

seems like a bug, I'll look into it!

Matthieu Pizenberg (Dec 02 2025 at 20:29):

maybe an even simpler test with "if" branching is possible. I haven’t checked if it’s "match" specific or reproducible with branching in general.

Richard Feldman (Dec 02 2025 at 20:33):

it's that we don't have polarity yet (cc @Jared Ramirez, not sure if you've started on that yet!) so it's treating [ListWasEmpty] from List.first as a closed union

Richard Feldman (Dec 02 2025 at 20:33):

I can manually annotate all the builtins as open and it should fix it

Richard Feldman (Dec 02 2025 at 20:34):

and then can put them back to the current signatures after we add polarity to the type-checker

Matthieu Pizenberg (Dec 02 2025 at 20:40):

Meanwhile, I’m just going to do a double match to make the type checker happy. Now I’m getting this for my program. Not sure where to start looking at. Any suggestions?

❯ roc day02.roc

Roc crashed: e_closure: failed to resolve capture value

Hardy (Dec 02 2025 at 20:57):

I have also seen the "e_closure" problem. It occured to me when i tried following:

    for line in test_input.split_on("\n") {
        if line.starts_with("L") {
            val = I64.from_str(line.drop_prefix("L"))
            dbg val
        }
    }

After peeking at your solution i got around that one by moving the line.drop_prefix("L") before the if. So my guess would be that there is some problem using a value in an inner scope which is already captured in an outer scope or something. Not sure how properly describe it.

Richard Feldman (Dec 02 2025 at 21:30):

can you give me a .roc file to repro?

Richard Feldman (Dec 02 2025 at 21:30):

doesn't need to be minimal, just something where I can run roc and reproduce it

Matthieu Pizenberg (Dec 02 2025 at 21:30):

yep, exactly what I was preparing

Matthieu Pizenberg (Dec 02 2025 at 21:34):

https://github.com/roc-lang/roc/issues/8547

Matthieu Pizenberg (Dec 02 2025 at 21:35):

Don’t know if Claude root cause analysis is correct, but I figured I’d leave it there for you to judge.

Richard Feldman (Dec 02 2025 at 21:48):

thanks! the original error in this thread is fixed now btw

Edwin Santos (Dec 03 2025 at 01:25):

Matthieu Pizenberg said:

Meanwhile, I’m just going to do a double match to make the type checker happy. Now I’m getting this for my program. Not sure where to start looking at. Any suggestions?

❯ roc day02.roc

Roc crashed: e_closure: failed to resolve capture value

I've also been having this issue with closures, got it with this sample program:

main! = |args| {
    identity = |a| a
    init_try : Try(U32, [SomeError, ..others])
    init_try = Err(SomeError)
    final_try = match init_try {
            Ok(ok) => Ok(ok)
            Err(e) => Err(identity(e))
    }
    was_an_error = Try.is_err(final_try)

    Stdout.line!("final_try is an error: ${was_an_error}")
    Ok({})

}

# Error: Roc crashed: e_closure: failed to resolve capture value

I tried with Strings and numerics and without type signatures they would crash, with type signatures they can succeed. But when using a Try type this always fails, with or without a type signature or changing the error type to something like [..others] or just SomeError. I also swapped Err(SomeError) with Ok(30) and still get the closure error. Was trying to get a closure like |_| Exit(101) going.

Richard Feldman (Dec 03 2025 at 02:45):

@Edwin Santos @Matthieu Pizenberg this is fixed now!

Jared Ramirez (Dec 03 2025 at 02:47):

Richard Feldman said:

it's that we don't have polarity yet (cc Jared Ramirez, not sure if you've started on that yet!) so it's treating [ListWasEmpty] from List.first as a closed union

i have polarity partially implemented, but there’s still a bit of work until its ready to go

Richard Feldman (Dec 03 2025 at 02:47):

yeah no rush! modifying the types in Builtin.roc to make the tag unions expicitly open fixed it

Last updated: Dec 21 2025 at 12:15 UTC