match (Keyword)

match selects a case and gives names to its values. Each arm produces the same result type. Pattern names belong to their arm: an enclosing variable with the same name keeps its original value after the match.

Example

type Choice = Some(int) | None
fn describe(choice) = {
    let value = 100
    let selected = match choice {
        Some(value) => value
        None => value
    }
    "${selected}:${value}"
}
print("${describe(Some(2))} / ${describe(None)}")

type Choice = Some int | None
describe choice =
    value = 100
    selected = match choice
        Some value => value
        None => value
    "${selected}:${value}"
print "${describe (Some 2)} / ${describe None}"

Both examples print 2:100 / 100:100. The Some arm uses its payload; the None arm uses the enclosing value. Nested matches restore their enclosing bindings too. A closure created inside an arm retains the binding it captured there, and a function extracted by a pattern keeps its parameter and return types.

Patterns also select Result variants, list shapes and record fields. See the pattern matching contract for the full rules.