gleam-lang/gleam
Pattern matching on tuples or custom types produces broken JavaScript
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#5,680 opened on 2026/05/03
help wantedhigh priority
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説明
The following Gleam code
pub fn main() {
case #(1, 2) {
#(a, _) -> a
}
{2 + 4} * 2
let a = 42
Nil
}
produces this Javascript:
export function main() {
let $ = [1, 2];
let a = $[0];
a
(2 + 4) * 2;
let a = 42;
return undefined;
}
Here, 2 things go wrong:
- the
abinding does not reserve the name in the enclosing scope, causing a parse error. - The expression inside the arm of the pattern is not followed by a semicolon. The line after it is constructed in such a way that this is now parsed as a function call, changing the meaning of this program.
Both of these can be triggered independently.
~ 💜