ludic/examples/events/cancel.ludic

# cancel.ludic — EV3: cancellable (decision) events.
#
# A `cancellable` event is fired BEFORE an action so a listener can veto it. The
# listener calls `cancel`; the caller reads the verdict back — `emit E(…)` used as
# an expression yields the cancelled flag (1 = vetoed, 0 = allowed). A foreign mod
# vetoes the same way, by setting the payload's trailing `cancelled` field over
# the ABI. This is the modding headline: observation becomes control.
#
# Running it prints:  0 1 92
program Cancel {
  event cancellable BeforeHurt { amount: int = 0 }

  @On(BeforeHurt) handler Armor { if amount > 10 { cancel } }   # veto any hit over 10

  entry {
    let v1 = emit BeforeHurt(amount: 5)      # 5 <= 10 -> allowed
    print(v1)                                 # 0
    let v2 = emit BeforeHurt(amount: 15)     # 15 > 10 -> vetoed
    print(v2)                                 # 1

    # the realistic shape: only apply the effect when the decision isn't vetoed
    var hp = 100
    if emit BeforeHurt(amount: 8)  == 0 { hp = hp - 8 }    # allowed  -> 92
    if emit BeforeHurt(amount: 50) == 0 { hp = hp - 50 }   # vetoed   -> unchanged
    print(hp)                                 # 92
  }
}