Wildcard Pattern Suppresses Exhaustiveness Checks in F# Discriminated Unions

Question

Wildcard Pattern Impact on Exhaustiveness
In F#, discriminated unions (DUs) are a core construct for representing state. The compiler enforces exhaustive pattern matching, issuing warnings when not all cases are handled. However, a wildcard pattern (_) can silence these warnings, allowing a match expression to compile even if new DU cases are added later.
When a new case is introduced, every existing match that uses a wildcard must be revisited to preserve correctness. The silence of the compiler warning can lead to logic errors that surface only at runtime.
Key questions for deciding how to manage this behavior:

Is there a compiler option or attribute that forces exhaustiveness checks even when a wildcard pattern is present?
Can we enforce a coding standard that disallows _ in public API match expressions to maintain safety?
What tooling or static analysis can automatically flag or refactor wildcard patterns in large codebases?

Tasadduq Burney · Answer

AnswerNo, the F# compiler does not expose any flag or attribute that forces exhaustiveness checks when a wildcard pattern (_) is present; the wildcard simply suppresses the warning.Why the wildcard silences the checkWhen a match includes a _ pattern, the compiler treats it as a catch‑all for any remaining union cases. Because the pattern is guaranteed to match, the compiler considers the match exhaustive and does not emit the usual warning about missing cases.Practical steps to retain safetyTurn on /warnaserror (or --warnaserror) so that all other warnings become errors. This does not restore the exhaustiveness check for wildcards, but it helps catch other issues.Define a project‑wide coding standard that disallows _ in match expressions that belong to public APIs.Enforce the standard with a linter. FSharpLint provides the rule NoWildcardInPublicMatch; add it to your .fs lint configuration and run it in CI.If a wildcard is truly needed (e.g., a genuine catch‑all that is documented and safe), place a comment explaining the intent and consider moving the match to an internal helper where the rule can be relaxed.When a new union case is added, rebuild the project. Matches without a wildcard will now trigger a warning (or error if /warnaserror is set), prompting you to update them.Missing diagnostic detailAre the match expressions you are concerned about part of a public API? If they are internal only, the risk of future breakage is lower and the prohibition on wildcards can be relaxed accordingly.

Wildcard Pattern Suppresses Exhaustiveness Checks in F# Discriminated Unions

Wildcard Pattern Impact on Exhaustiveness

1 answer

Answer

Why the wildcard silences the check

Practical steps to retain safety

Missing diagnostic detail

Your perspective belongs here.

0 question comments