fram-lang/dbl

Pretty-print infix constructors in pattern counterexamples using infix notation

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#325 opened on Feb 21, 2026

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Description

Pretty-print infix constructors in pattern counterexamples using infix notation

When pattern matching is non-exhaustive, the compiler shows a counterexample. Currently, infix constructors like :: are printed in prefix notation with excessive parentheses:

fatal error: This pattern-matching is not exhaustive.
Here is an example of a case that is not matched:
  (::) _ ((::) _ _)

This should be printed as:

  _ :: _ :: _

Details

  • Infix constructors are stored internally as "(::)" (parentheses-wrapped)
  • Precedence is determined by first character (: -> level 70, + -> 80, * -> 90, etc.)

Contributor guide

Research direction
Examine the code that prints counterexamples for non exhaustive pattern matching. Locate where constructors like (::) are printed in prefix form. Implement logic to detect infix constructors and format them with proper precedence and associativity.
Tech stack
None
Domain
backendcompilers
Issue type
Bug
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.
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
OCamlcompiler internals understanding
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60