gleam-lang/gleam

Emphasize mismatching part of complex type mismatches

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#4,578 opened on May 9, 2025

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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

Research direction
Investigate the compiler's type error reporting module and implement a method to highlight mismatching types.
Tech stack
rust
Domain
developer experiencecompilers
Issue type
Feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
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
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50