Stream: compiler development

Topic: bang(!) desugaring

Brendan Hansknecht (Jul 29 2024 at 05:52):

Just want to double check, this should work eventually, right?

main =
    Arg.list! {}
    |> List.dropFirst 1
    |> List.mapTry Str.toU8
    |> Task.fromResult!
    |> List.sum
    |> \total -> "Sum of numbers: $(Num.toStr total)"
    |> Stdout.line!

Or do we not plan to allow ! in pipes? I would assume it should be allowed.

Luke Boswell (Jul 29 2024 at 06:21):

It's discussed in the Proposal: Chaining Syntax in section Pipelines.

Luke Boswell (Jul 29 2024 at 06:24):

I would expect what you have written to desguar to effectively the following (simplified)

main =
    Arg.list! {}
    |> List.dropFirst 1
    |> List.mapTry Str.toU8Checked
    |> Task.fromResult!
    |> List.sum
    |> \total -> "Sum of numbers: $(Num.toStr total)"
    |> Stdout.line!

main =
    Task.await (Arg.list {}) \answer1 ->
        Task.await
            (
                answer1
                |> List.dropFirst 1
                |> List.mapTry Str.toU8Checked
                |> Task.fromResult
            )
            \answer2 ->
                answer2
                    |> List.sum
                    |> \total -> "Sum of numbers: $(Num.toStr total)"
                    |> Stdout.line

Brendan Hansknecht (Jul 29 2024 at 06:27):

Makes sense. Not sure what is missing today, but I guess it is a reasonable bug case.

Brendan Hansknecht (Jul 29 2024 at 06:27):

relatively minimal

Luke Boswell (Jul 29 2024 at 06:28):

It should work today

Luke Boswell (Jul 29 2024 at 06:28):

I'll make an issue while I have it open

Luke Boswell (Jul 29 2024 at 06:29):

https://github.com/roc-lang/basic-cli/issues/234

Kiryl Dziamura (Jul 29 2024 at 06:30):

I’ll take a look today

Brendan Hansknecht (Jul 29 2024 at 06:31):

Also, it can be cut to be a bit more minimal:

main =
    Arg.list! {}
    |> List.len
    |> Num.toStr
    |> Stdout.line!

Kiryl Dziamura (Jul 29 2024 at 06:31):

Ah, have you tried to change the indentation?

Kiryl Dziamura (Jul 29 2024 at 06:32):

Add one space to each pipe, does it help?

Brendan Hansknecht (Jul 29 2024 at 06:32):

yeah, this works:

main =
    Arg.list! {}
        |> List.len
        |> Num.toStr
        |> Stdout.line!

Brendan Hansknecht (Jul 29 2024 at 06:33):

So ! requires extra tabbing currently...interesting.

Brendan Hansknecht (Jul 29 2024 at 06:34):

With the more nested version, it is still broken:

main =
    Arg.list! {}
        |> List.dropFirst 1
        |> List.mapTry Str.toU8
        |> Task.fromResult!
            |> List.sum
            |> \total -> "Sum of numbers: $(Num.toStr total)"
            |> Stdout.line!

Kiryl Dziamura (Jul 29 2024 at 06:34):

Yeah, it’s a known bug in parser. I assume it will go away with @Joshua Warner refactoring, but will give it another look today anyway

Luke Boswell (Jul 29 2024 at 06:36):

I just tested with the block changes -- it's still there

Luke Boswell (Jul 29 2024 at 06:39):

I'm not so sure it's a bug. I think it's a deliberate design choice/tradeoff we made.

Might be worth coming up with ideas to improve the experience here.

But could be tricky if it doesnt even parse I guess.

Kiryl Dziamura (Jul 29 2024 at 06:41):

But the suffix shouldn’t break parsing. If the pipeline operator is expected to be on the same indent level - it’s expected to work in each case

Richard Feldman (Jul 29 2024 at 06:44):

I think it's a bug - it should work without indentation!

Richard Feldman (Jul 29 2024 at 06:45):

and we talked in some other thread about how the formatter shouldn't indent it in this scenario

Luke Boswell (Jul 29 2024 at 07:28):

Luke Boswell said:

I just tested with the block changes -- it's still there

Please ignore this comment.

The error message we see is from Can, so it's definitely possible this is fixed, or the parsing parts in Josh's work.

I'm running off my stale memory of this issue. Need to dig a bit more before I comment next time.

Joshua Warner (Jul 30 2024 at 00:33):

Just tested this on the current main (after my block parsing PR landed), and the syntax tree at least looks correct to me:

main =
    Arg.list! {}
    |> List.dropFirst 1
    |> List.mapTry Str.toU8
    |> Task.fromResult!
    |> List.sum
    |> \total -> "Sum of numbers: $(Num.toStr total)"
    |> Stdout.line!

(which is to say, it parses fine and gives a sequence of exprs joined by pizza operators, all at the same level... does that sound right?)

That's not true prior to my PR (just checked that as well) - it used to choke on the first pizza operator. So there was definitely an improvement here.

Joshua Warner (Jul 30 2024 at 00:33):

I _think_ that's what you were saying Luke; just wanted to make it explicit / make sure we're on the same page...

Luke Boswell (Jul 30 2024 at 04:03):

Thank you, yes that's what I was trying to say.

Last updated: Jul 26 2025 at 12:14 UTC