# 27.1: a guard asks and changes nothing
import "ludic.base"
program MachineGuardWrites {
  enum Mood { Calm, Wary }
  property Deer { mood: Mood = Mood.Calm, fear: int = 0 }
  state Herd { deer: Table<Deer> = null }
  action Spook { @Target who: int = -1 }
  action Shout { loud: int = 1 }
  property DeerStep {
    from: Mood = Mood.Calm
    to: Mood = Mood.Calm
    on: string = ""
    guard: fn(Row<Deer>, Herd) -> bool = null
    enter: fn(Row<Deer>, Herd) -> void = null
  }
  @Machine(Deer.mood) registry DeerSteps of DeerStep
  function deer_afraid(r: Row<Deer>, h: Herd) -> bool { return r.rec.fear > 1 }
  def DeerSteps startled { from: Mood.Calm, to: Mood.Wary, on: "Spook", guard: fn deer_nervous }
  def DeerSteps settles { from: Mood.Wary, to: Mood.Calm, guard: fn deer_afraid }
  function deer_nervous(r: Row<Deer>, h: Herd) -> bool {
    r.rec.fear += 1
    return true
  }
  entry { print(1) }
}