gleam-lang/gleam
Add better error message when pattern matching on string prefix and suffix
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#5,695 opened on May 9, 2026
good first issuehelp wanted
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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.