match finds the first matching pattern and returns the resulting expression.

Syntax

(match ⟨expr⟩
  (⟨pattern⟩ ⟨body⟩...)
  ...)

Semantics

  • Each branch pairs a pattern with a body.
  • Patterns can destructure constructors, tuples, literals, and wildcards (both variables and _).
  • Constructor patterns must always be wrapped in parentheses. None is a variable wildcard pattern while (None) is a constructor pattern
  • match is the main way to consume algebraic data types.
  • Non-exhaustive patterns warn at compile time. The behavior of falling through without a wildcard is undefined.
  • ⟨body⟩ forms an implicit progn.

Example

(match opt
  ((Some x) x)
  ((None)   0))

;; Not intended, almost surely wrong:
(match opt
 ((Some x) x)
 (None     0))