Replacement effects with partial functions

This is a very crude mockup of how replacement effects could look in Laterna Magica. Instead of any impressive effect, I'm just modifying a variable that represents a player's life total, but it does quite a lot for only being about 30 lines of code:

//a replaceable event
trait Event { def execute(): Unit }
//an effect that replaces an event
type ReplacementEffect = PartialFunction[Event, Event]

//applies all effects to the event. If a replacement doesn't match, the
//event remains unchanged
def replace(event: Event, effects: ReplacementEffect*) =
  effects.foldLeft(event) { case (event, effect) => effect.applyOrElse(event, identity[Event] _) }

var activeEffects: Seq[ReplacementEffect] = Nil
def execute(event: Event) = replace(event, activeEffects: _*).execute()

//example: A player's life points
var _life = 20
def life = _life

def gainLife(life: Int) = execute(GainLife(life))

case class GainLife(life: Int) extends Event {
  def execute() = _life += life
}

//execution:

//gain three life
gainLife(3)
println(life) //23

//gain twice as much life
activeEffects = List({ case GainLife(x) => GainLife(x * 2) })

//gain six life
gainLife(3)
println(life) //29

The trait Event is used to represent replaceable events, and effects are simply "partial functions": functions that can only be applied to some of the values that its parameter type would allow. For example, at the bottom there's the partial function:

{ case GainLife(x) => GainLife(x * 2) }

While ReplacementEffect is defined for all Events, this one only accepts GainLife instances. Using PartialFunction.applyOrElse, I handle the cases where a replacement effect does not apply to an event.

The example is self-contained, so you should be able to run it yourself. In any case, the output is next to the print statements: The first time, you only get 3 life, but after registering the effect, the amount is indeed doubled. In reality, execute (and replace?) needs to be a bit smarter to recognize the right effects to apply, but otherwise this could stay almost as it is.