gleam-lang/gleam

Emphasize mismatching part of complex type mismatches

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#4578 opened on May 9, 2025

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

Description

When a type mismatch is complex, and some types are unbound (is this the correct term?), it can be hard to see where the error actually is. Here is a concrete example I actually wasted quite a bit of time on:

Expected type:

    drift.Step(List(String), Input, Output, Nil)

Found type:

    drift.Step(List(string), Input, a, b)

It could be made clearer by emphasizing where the type mismatch was exactly:

Expected type:

    drift.Step(List(String), Input, Output, Nil)
                    ^^^^^^

Found type:

    drift.Step(List(string), Input, a, b)
                    ^^^^^^

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
Rust programmingGleam type system basicsCompiler error reporting
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70
Research direction
Look at the Gleam compiler's error reporting code, likely in the 'src/error' or 'compiler core/error' directory. Find where type mismatch errors are displayed and modify the formatting to highlight the mismatching types with carets. Refer to existing error formatting patterns.