-- iteration 24: call — send that waits. The caller parks until the
-- receive's return value comes back; same-shard here (the runner pins
-- WO_SHARDS=1), so the reply path is the direct unpark. The actor's
-- state update across two calls proves the mailbox kept its order.
class Q {
  n: Int
}

class Doubler {
  seen: Int
  fn receive(msg: Q) -> Int {
    self.seen = self.seen + 1;
    return msg.n * 2;
  }
}

fn main() -> Int {
  let d: actor Q = spawn Doubler { seen: 0 };
  let a = call(d, Q { n: 21 });
  print("a=${a}");
  let b = call(d, Q { n: a });
  print("b=${b}");
  return 0;
}