MemoizingActor.scala

/* imports suppressed */

class MemoizingActor(actual: ActorRef) extends Actor {

  case class DropState(message: Any)

  override def receive: Actor.Receive = actualReceive(Map())

  def actualReceive(data: Map[Any, Future[Any]]): Receive = {
    case DropState(any) =>
      context become actualReceive(data - any)

    case any: Any =>
      val future = data getOrElse (any, {
        (actual ? any) andThen { case _ => self ! DropState(any) }
      }) pipeTo sender

      context become actualReceive(data + (any -> future))
  }
}