ludic/examples/events/answer.ludic

# answer.ludic — one listener per kind: each answers only for its own and returns early for
# every other. A `return` ends that listener, never the dispatch, so the listener for kind 2
# still hears the event after the one for kind 1 has returned.
#
# Running it prints:  10 20 0
program Answer {
  event cancellable Ask { kind: int }
  state AnswerState {
    answer: int = 0
  }

  @On(Ask) handler One(answer_st: mut AnswerState) { if kind != 1 { return };  answer_st.answer = 10;  cancel }
  @On(Ask) handler Two(answer_st: mut AnswerState) { if kind != 2 { return };  answer_st.answer = 20;  cancel }

  function ask(answer_st: mut AnswerState, k: int) -> int {
    answer_st.answer = 0
    if emit Ask(kind: k) != 0 { return answer_st.answer }
    return 0
  }

  entry (answer_st: mut AnswerState) {
    print(ask(answer_st, 1))
    print(ask(answer_st, 2))
    print(ask(answer_st, 3))
  }
}