gleam-lang/gleam

Add better error message when pattern matching on string prefix and suffix

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#5695 opened on May 9, 2026

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

Description

When attempting to pattern match on both the suffix and prefix of a string, the error message could be friendlier.

For example, with

case "my string!" {
  "my " <> object <> "!" -> todo
  start <> "!" -> todo
}

The current error is:

error: Syntax error
  ┌─ /private/tmp/match_string_end/src/match_string_end.gleam:5:21
  │
5 │     "my " <> object <> "!" -> todo
  │                     ^^ I was not expecting this

Found `<>`, expected one of:
- `->`

Hint: Did you mean to wrap a multi line clause in curly braces?

Compare to the error if you don't try to match on the string's prefix:

error: Syntax error
  ┌─ /private/tmp/match_string_end/src/match_string_end.gleam:6:5
  │
6 │     start <> "!" -> todo
  │     ^^^^^ This must be a string literal

We can't tell what size this prefix should be so we don't know
how to handle this pattern.

If you want to match one character consider using `pop_grapheme`
from the stdlib's `gleam/string` module.

Thanks to @the_tank_man_ from the discord for suggesting this :]

Also, I can start working on this.

Contributor guide

Tech stack
rust
Domain
developer experience
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.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Familiarity with Gleam syntaxBasic Rust programming
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
65
Research direction
The issue involves improving the error message when pattern matching on string prefix and suffix in the Gleam compiler. The error is generated during parsing, likely in a file related to syntax error reporting. Look for the error message 'I was not expecting this' in the compiler source code (e.g., in the parser or error module). The improvement should detect when both prefix and suffix patterns are used and provide a hint about using string pattern matching limitations or alternatives. The GitHub issue includes examples and a reference to a Discord suggestion. Start by finding the relevant error generation code and crafting a more descriptive message.