Stream: beginners

Topic: Formatter inconsistently drops parens in the type definition

Kiryl Dziamura (Jan 25 2024 at 18:47):

Formatter transforms this type

x : (a, (b -> c)) -> d

to

x : (a, b -> c) -> d

Originally, (a, (b -> c)) is a tuple with two items a and b -> c
But after formatting it becomes (a, b -> c) which is a function with two arguments

Created an issue
https://github.com/roc-lang/roc/issues/6426

Brian Teague (Jan 25 2024 at 19:20):

The above example is still only one argument as a tuple.
x : (a, b->c) is equivalent to x: (a, (b->c)) which is why the parenthesis were dropped.

x has one argument, tuple, with values a and function b->c

Two separate arguments would look like this.
x: a, (b->c) -> d

Kiryl Dziamura (Jan 25 2024 at 19:25):

The parser sees it differently (as I described, it assumes ((a, b) -> c) -> d). So it's a bug for the parser then

Brian Teague (Jan 25 2024 at 19:32):

You're right. This grammar is ambiguous.
(a, b-> c) -> d
Is it a tuple with type a and type function b -> c, or is it a function with two args a, b -> c

Brian Teague (Jan 25 2024 at 19:34):

Those extra parenthesis should not be dropped.

Brendan Hansknecht (Jan 25 2024 at 19:35):

Your original type is correct. So formatter bug. It needs to put parens around closure type definitions if they aren't top level.

Brendan Hansknecht (Jan 25 2024 at 19:36):

I guess this was missed ambiguity that got added when we added the tuple syntax

Brian Teague (Jan 26 2024 at 01:00):

Just to document an additional edge cases:
x: (a, (b, c -> d)) -> e
x: (a, b, (c->d)) -> e
x: a, (b, c -> d) -> e
x: a, b, (c -> d) -> e

Folkert de Vries (Jan 26 2024 at 22:39):

the actual bug would be in the parser, right?

Brendan Hansknecht (Jan 26 2024 at 23:06):

It is the formatter failing.

Slightly simpler example:

Original: x: (A, (B -> C))
Formatted x : (A, B -> C)

It correctly parses the first expession as a tuple that contains a value A and a function B -> C.
When formatting and printing that back out, it needs to put the function B -> C into parens otherwise, it will be invalid.

Annotation(
    Identifier(
        "x",
    ),
    Tuple {
        elems: [
            Apply(
                "",
                "A",
                [],
            ),
            Function(
                [
                    Apply(
                        "",
                        "B",
                        [],
                    ),
                ],
                Apply(
                    "",
                    "C",
                    [],
                ),
            ),
        ],
        ext: None,
    },
),

Formatting failed and gave us (A, B -> C), that said, it does parse the wrong expression correctly.
(A, B -> C) is a function A and B to C.

Annotation(
    Identifier(
        "x",
    ),
    Function(
        [
            Apply(
                "",
                "A",
                [],
            ),
            Apply(
                "",
                "B",
                [],
            ),
        ],
        Apply(
            "",
            "C",
            [],
        ),
    ),
),

Last updated: Jul 26 2025 at 12:14 UTC