41 Nested patterns

A pattern may match a variant inside another variant's payload.

enum Delivery {
  Pending,
  Sent(String code)
}

enum Event {
  Updated(Delivery delivery),
  Ignored
}

String describe(Event event) {
  return switch event {
    case Updated(Sent(String code)) { break code }
    case Updated(_) { break "waiting" }
    case Ignored { break "ignored" }
  }
}

main() {
  printLine(describe(event: Event.Updated(delivery: Delivery.Sent(code: "N-42"))))
  printLine(describe(event: Event.Updated(delivery: Delivery.Pending)))
}

Expected output:

N-42
waiting

The first case extracts the code only when an Updated event contains Sent. The next case handles other Updated deliveries; the final case handles Ignored. Cases are tested in order without falling through.

Try it: Swap the first two cases and observe whether the compiler flags an unreachable case.

Precise rules: Reference.