Exhaustive Match

Exhaustive Match

Use this skill when a status, enum, union, event type, command, or error code has a finite set of known variants and missing one would create incorrect behavior.

Workflow

1. Model the variants as an enum, tagged union, sealed class, literal union, or another construct your toolchain can reason about.
2. Replace loose if chains with a match, switch, dispatch table, or strategy selection that enumerates every known case.
3. Make the default branch unreachable, absent, or fail closed. Do not use a broad default that hides newly added variants.
4. Enable the strongest available enforcement: compiler exhaustiveness, type-checker assertions, lint rules, or a test that compares handled cases to the declared variant set.
5. Add or update one test per behavior group, plus a test that proves unknown external values are rejected at the boundary.

Review Checks

• Adding a new variant creates a visible failure until behavior is implemented.
• Unknown external input is parsed separately from internal exhaustive handling.
• The fallback path is explicit and safe, not a catch-all for forgotten cases.
• The match body stays small; complex behavior is delegated to named handlers.

Anti-Patterns

• Keeping a default branch that returns a plausible value for all unknowns.
• Exhaustively matching raw strings from outside the system instead of parsing them into a known type first.
• Adding a variant without updating tests, documentation, and caller behavior.