Stream: beginners

Topic: Formatting a closure in a record

Brendan Hansknecht (Jun 22 2023 at 17:39):

How should we format a closure in a record?

Here is a concrete example that I wrote recently that breaks the formatter:

{
    key: "data",
    value:
        value <- Inspect.dict data Dict.walk Inspect.str
        when value is
            Thing i -> Inspect.tag "Thing" [Inspect.i32 i]
            Stuff -> Inspect.tag "Stuff" []
        ,
}

Note, it can also be written without the backpassing and maybe that is clearer, but it still will break the formatter:

{
    key: "data",
    value:
        Inspect.dict data Dict.walk Inspect.str \value ->
            when value is
                Thing i -> Inspect.tag "Thing" [Inspect.i32 i]
                Stuff -> Inspect.tag "Stuff" []
        ,
}

Thoughts on:

1. What should be considered valid formatting here?
2. How should roc format handle the code examples above?

Anton (Jun 22 2023 at 17:49):

Is it the "free" comma that is causing trouble?

Brendan Hansknecht (Jun 22 2023 at 17:52):

Formatter fails with or without the comma.

Brendan Hansknecht (Jun 22 2023 at 17:53):

Parse error was: Expr(Closure(Body(List(End(@948), @714), @689), @430), @120)

Anton (Jun 22 2023 at 18:03):

For (1.) I think both snippets should be valid (parseable)
For (2.) I think the comma should be put here: "Stuff" [],

Richard Feldman (Jun 22 2023 at 18:24):

part of me thinks we should make the comma optional and format without it, since it's syntactically unambiguous anyway

Richard Feldman (Jun 22 2023 at 18:25):

e.g. if you put another field anywhere after that last line, like foo:, it couldn't possibly mean anything other than that you're ready to define the next record field

Richard Feldman (Jun 22 2023 at 18:25):

however, that doesn't help if what you want to do next is punning, e.g.

{
    key: "data",
    value:
        Inspect.dict data Dict.walk Inspect.str \value ->
            when value is
                Thing i -> Inspect.tag "Thing" [Inspect.i32 i]
                Stuff -> Inspect.tag "Stuff" []
        ,
    foo
}

Richard Feldman (Jun 22 2023 at 18:26):

without the comma, or indentation-awareness, that wouldn't work

Last updated: Jul 26 2025 at 12:14 UTC