ludic/docs/language/events/kw-cancellable.md
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Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
2026-08-29 17:53:22 +03:00

1.3 KiB

id name category kind tokens sig tip order
kw-cancellable cancellable events keyword cancellable event cancellable Name { … } Marks an event whose listeners may veto it with cancel. 50

cancellable marks an event as a decision, not just a notification. A plain event tells listeners something happened; a cancellable event is fired before an action so a listener can veto it by calling cancel. The caller reads the verdict back by using emit as an expression: it yields 0 when no listener vetoed and 1 when one did, so the action is applied only on 0. This is the Bukkit / DOM preventDefault shape — observation becomes control — and a foreign mod vetoes the same way by setting the payload's trailing cancelled flag over the ABI.

program CancellableExample {
  event cancellable BeforeHurt { amount: int = 0 }

  var current_health: int = 100

  @On(BeforeHurt) handler AbsorbSmallHits { if amount <= 5 { cancel } }

  handler TakeDamage phase Update {
    if Input.key() == 'h' {
      if emit BeforeHurt(amount: 3) == 0 { current_health = current_health - 3 }
    }
  }

  handler DrawWorld phase Render {
    Screen.clear(Color.MidnightBlue)
    Screen.draw_number(x: 4, y: 4, value: current_health, color: Color.Crimson, scale: 1)
    Screen.show()
  }
}