Pattern matching on tuples or custom types produces broken JavaScript · gleam-lang/gleam#5680

2026-05-03T12:53:06.000Z

The following Gleam code ```gleam pub fn main() { case #(1, 2) { #(a, _) -> a } {2 + 4} * 2 let a = 42 Nil } ``` produces this Javascript: ```js export function main() { let $ = [1, 2]; let a = $[0]; a (2 + 4) * 2; let a = 42; return undefined; } ``` Here, 2 things go wrong: 1. the `a` binding does not reserve the name in the enclosing scope, causing a parse error. 2. 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. ~ 💜

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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 a binding 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.

~ 💜

コントリビューターガイド

技術スタック: rustjavascript
領域: tooling
Issue 種別: bug
難度: 3
推定時間: 1-3 hours
活動状況: active
明確さ: clear
前提条件: Gleam basicsJavaScript output understandingFamiliarity with compiler codegen
初心者向け度: 40
調査方針: Investigate the Gleam compiler's JavaScript code generation for pattern matching. The bug involves variable scoping in let bindings and missing semicolons after expression arms. Look at the compilation of case expressions in the JavaScript backend, likely in files such as 'src/codegen/js.rs' or similar. Check how patterns are compiled and how expressions are sequenced to ensure proper semicolons and scope.