protected class EventHandler[Event, +Result](callback: Event => Result) {
  def apply(event: Event) = // [... code omitted ...]

  def applyFromHistory(event: Event) = callback(event)
}

protected def handler[A, B](callback: A => B) = new EventHandler(callback)

implicit protected def handlerToPartialFunction[A, B](handler: EventHandler[A, B])(implicit m: Manifest[A]) =
  new PartialFunction[AnyRef, B] {
    def isDefinedAt(event: AnyRef) = m.erasure.isInstance(event)

    def apply(event: AnyRef) = handler.applyFromHistory(event.asInstanceOf[A])
  }