gleam-lang/gleam

Error's type printing: Consider only annotated type variables for the current definition rather than the whole module

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#3,786 opened on Nov 4, 2024

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help wanted

Description

I talked about this on Discord with Gears, and he thinks this is actually a bug worth reporting 😄

When the compiler introduces auto-generated type variable names for reporting errors (and in the LSP), 1.5.1 always starts at a. The nightly release of the compiler instead seems to reserve explicitly annotated type variable names used elsewhere in the module. This means that the names generated will not usually start with a, but rather with the next free letter.

import gleam/io

type Wibble(a, b, c, d, e, f, g) {
  Wibble(value: a)
}

fn wibble(value: h) {
  io.debug(value)
  Nil
}

fn wobble() {
  let x: List(i) = []
  wibble(x)
  wibble(fn() -> List(j) { [] })
}

pub fn main() {
  let x: Nil = []
}

produces:

error: Type mismatch
   ┌─ C:\Users\joshua\Documents\globe\src\globe.gleam:19:16
   │
18 │   let x: Nil = []
   │                ^^

Expected type:

    Nil

Found type:

    List(k)

type variables a..j are reserved because of the other usages above.

I ran git bisect and figured out this was introduced in https://github.com/gleam-lang/gleam/commit/8c5834bcdc0115abeea16c7e1ee9826d42462564

~ 💜

Contributor guide

Tech stack
rust
Domain
backendtooling
Issue type
bug
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.
1-2 days
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
understanding of Gleam syntaxfamiliarity with compiler error reporting
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
35
Research direction
Examine the commit 8c5834bcdc0115abeea16c7e1ee9826d42462564 in the Gleam compiler repository to understand how auto generated type variable names are now reserved. Look at the error type printing code, likely in the compiler's formatting or type checking module, to see how the naming logic works. The issue is to change it so that only type variables from the current definition are considered, not the entire module.