gleam-lang/gleam

Improve error message for invalid externals

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#5561 opened on Apr 3, 2026

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 (2 comments) (0 reactions) (0 assignees)Rust (21,417 stars) (960 forks)batch import
help wanted

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

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.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Rust programmingGleam syntax basicsCompiler error handling
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60
Research direction
Examine the error reporting code in the Gleam compiler, likely in the parser module. Look at issue #5557 for a similar pattern. The error should detect incomplete @external attribute and provide a hint to the external documentation at https://tour.gleam.run/advanced features/externals/.