gleam-lang/gleam

Qualified constants cannot be referenced inside bit array size segment patterns.

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#5607 opened on Apr 11, 2026

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good first issuehelp wanted

Description

When pattern matching on a bit array, the compiler produces a syntax error if a qualified imported constant is referenced inside a size segment, for example:

import example/internal

const lap_data_size = 50

pub fn parse(bits:BitArray)->Result(_,Nil){
  case bits {
    <<
      lap_data:bytes-size(internal.maximum_cars * lap_data_size),
      time_trial_personal_best_car:unsigned-int-8,
      time_trial_rival_car:unsigned-int-8,
    >> -> todo
    _ -> todo
  }
}

This code produces the following error pointing to internal.:

Syntax error

I was not expecting this.

Found `.`, expected one of: 
- `)`

Contributor guide

Tech stack
None
Domain
tooling
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.
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
Gleam syntaxRust programmingcompiler parsing
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Examine the parser's bit array pattern parsing code in the Gleam compiler repository. Identify where size expressions are parsed and add support for qualified constant names, currently causing a syntax error on the dot character.