akka-actor based object orientated HelloWorld in Scala

HelloWorld.scala

import akka.actor.Actor
import akka.actor.Actor.actorOf

object HelloWorld extends App {
  val a = actorOf[Hello]
  a.start()
  val resp = (a !! World) getOrElse sys.error("no response")
  a.stop()
  
  println(resp)
}

trait Message
case object World extends Message

class Hello extends Actor {
  def receive = {
    case World => self reply "Hello "+World
  }
}