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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Beschreibung

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.

~ 💜

Contributor Guide

Tech Stack: rustjavascript
Domain: tooling
Issue Type: bug
Schwierigkeit: 3
Geschätzte Zeit: 1-3 hours
Aktivitätsstatus: active
Klarheit: clear
Voraussetzungen: Gleam basicsJavaScript output understandingFamiliarity with compiler codegen
Einsteigerfreundlichkeit: 40
Research-Richtung: 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.