Pattern matching

match scrutinee {
    pattern1 => expr1,
    pattern2 if guard => expr2,
    _ => default
}

A match is an expression. Arms are tried top-to-bottom; the first match wins. The scrutinee is evaluated once. All arm right-hand sides must have the same type.

Patterns

Pattern	Matches
integer literal	the value (e.g. 0, 1, -42)
char literal	the codepoint as i32 (e.g. 'A')
true / false	the corresponding bool
identifier n	binds the scrutinee to a fresh name n
_	wildcard, no binding

There are no constructor patterns, no struct destructuring, no tensor-shape patterns. These arrive with ADTs / records.

Guards

An arm may carry an if guard after its pattern:

match x {
    n if n > 0 => 1,
    _ => -1
}

The arm fires only when both pattern matches and the guard evaluates to true. The bound name (n here) is in scope inside the guard and the right-hand side.

Exhaustiveness

The type checker does not verify exhaustiveness today. A match whose patterns do not collectively cover every possible input compiles fine, but feeding it an unmatched value at runtime is undefined.

Always end a match with a wildcard arm until exhaustiveness checking lands. See planned: roadmap.

Typing

• The scrutinee has some type T.
• Each pattern must be compatible with T: integer patterns require i32, bool patterns require bool, wildcard / binding patterns match any T.
• Each guard must have type bool.
• All right-hand sides have a common type, which is the type of the whole match expression.