gleam-lang/gleam

Improve error message for invalid externals

Open

#5,561 opened on Apr 3, 2026

View on GitHub
 (2 comments) (0 reactions) (0 assignees)Rust (960 forks)batch import
help wanted

Repository metrics

Stars
 (21,417 stars)
PR merge metrics
 (Avg merge 8d 6h) (59 merged PRs in 30d)

Description

Similar to #5557.

Assume following code:

@external
pub fn wibble()

The build tool gives this vague message:

error: Syntax error
  ┌─ C:\Users\user\projects\test_gleam\src\test_gleam.gleam:2:1
  │
2 │ pub fn wibble()
  │ ^^^ I was not expecting this

Found the keyword `pub`, expected one of:
- `(`

It could be improved like:

error: Syntax error
  ┌─ C:\Users\user\projects\test_gleam\src\test_gleam.gleam:1:1
  │
1 │ @external
  │ ^^^^^^^^^ This attribute is incomplete

Found the keyword `pub`, expected one of:
- `(`
Hint: See https://tour.gleam.run/advanced-features/externals/

Contributor guide

Research direction
Inspect the error reporting mechanism for attributes in the Gleam parser. Trace how `@external` is parsed and where the syntax error is produced, then modify to output a more helpful message with a hint.
Tech stack
rust
Domain
developer experiencetooling
Issue type
Feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
GitRust
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70