match
match value { 0, 1 => … _ => … }
A match replaces an if/else ladder that tests one value against several constants. It evaluates the subject once, then takes the first arm whose pattern matches; an arm lists one or more literal patterns separated by commas and points at a body with =>, and a lone _ arm is the catch-all default. Patterns are compile-time constants — integers, char literals like 'w', or enum variants such as Action.Guard — which makes match ideal for dispatching on a key press, a tile code, or a mode. Each arm's body is a single statement or a { … } block; matching lowers to plain branches, so it is as cheap as the if chain it replaces.
When the subject is a **tagged-union enum** (an enum whose variants carry payloads), match also *destructures* it: an arm names a variant and binds its payload — Door(n) => …, Portal(x, y) => … — with n, x, y in scope for that arm's body. A tagged match must be **exhaustive**: it covers every variant or ends with a _, or the compiler rejects it, so a newly added variant flags every match that must handle it. See [enum](../structure/kw-enum) for the full picture.
Example
program Steering {
property Velocity { delta_x: int = 0, delta_y: int = 0 }
model Player { Velocity }
handler Boot phase Start {
spawn Hero { Velocity { delta_x: 0, delta_y: 0 } }
}
handler ReadKeys phase Input {
let pressed = Input.key()
for (velocity) in query [Velocity, {Player}] {
match pressed {
'w' => velocity.delta_y = -1
's' => velocity.delta_y = 1
'a', 'h' => velocity.delta_x = -1
'd', 'l' => velocity.delta_x = 1
_ => velocity.delta_x = 0
}
}
}
}