Pattern-match with `match`

Goal: branch on the variants of a sum type (or a Result/Option), binding each variant’s payload.

Match on a sum type

match requires an arm for every variant. Name a variant to match it; bind its payload by naming the fields:

commons shop {
  type Status =
    | Pending
    | Shipped(tracking: String)
    | Cancelled(reason: String)

  fn describe(s: Status) -> String {
    match s {
      Pending => "awaiting shipment"
      Shipped(tracking: t) => t
      Cancelled(reason: r) => r
    }
  }
}

Omit a variant and the program does not compile — there is no accidental fall-through. A match is an expression: its value is the value of the matched arm.

Match on Result and Option

The same form works for the built-in sum types:

fn label(o: Option[Int]) -> String {
  match o {
    Some(n) => "present"
    None => "absent"
  }
}

Related

• For a one-branch test that yields a Bool, see Narrow and bind with is.
• Reference: type system.