using scala generics in function definition

Question

I'd like to build a convinient function for making akka actors:

 def makeActor[T](id: Int): ActorRef =
   system.actorOf(Props(classOf[T], id, updater), "entity" + id)

 makeActor[SomeActor](1) ! "Some Message"

But this says "class type required but T found". Whats wrong with this code?

4lex1v · Answer 1 · 2014-03-12T17:26:41.573

3

Use scala reflection for this, e.g:

import scala.reflect._

def spawn[T <: Actor: ClassTag](id: Int) = system.actorOf(Props(classTag[T].runtimeClass, ...), s"entity$id")

edited Mar 12 '14 at 17:26

answered Mar 12 '14 at 16:45

4lex1v

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I think you meant `def spawn[T: ClassTag]`. I can't propose an edit because it's too small. – KChaloux Mar 12 '14 at 16:57

score 2 · Accepted Answer · edited May 23 '17 at 10:25

2

An answer can be found here: Scala classOf for type parameter

You can write your code for example like this:

def makeActor[T](id: Int)(implicit ev: Manifest[T]): ActorRef = system.actorOf(Props(ev.erasure, id, updater), "entity" + id)

edited May 23 '17 at 10:25

Community

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answered Mar 12 '14 at 16:48

Rado Buransky

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I used `ev.runtimeClass` because `ev.erasure` is deprecated in 2.10. I've edited your post. Thanks for this! – Themerius Mar 13 '14 at 12:10

score 2 · Answer 3 · answered Mar 12 '14 at 17:15

Fully working example:

import akka.actor.{ActorSystem, Props, Actor, ActorRef}
import scala.reflect.ClassTag

object TestActor extends App {

  val system: ActorSystem = ActorSystem("rest-service-actor-system")

  def makeActor[T <: Actor : ClassTag](id: Int): ActorRef =
    system.actorOf(Props(implicitly[ClassTag[T]].runtimeClass), "entity" + id)

  class A extends Actor {
    override def receive = {
      case anything => println("A GOT MESSAGE: " + anything)
    }
  }

  class B extends Actor {
    override def receive = {
      case anything => println("B GOT MESSAGE: " + anything)
    }
  }

  makeActor[A](1) ! "hello"
  makeActor[A](2) ! "bye"

  system.shutdown()
}

usually prints:

A GOT MESSAGE: bye
A GOT MESSAGE: hello

It forces you to have a type that is also an Actor. For instance this code will not compile:

class C

makeActor[C](3)

error you get is:

type arguments [TestActor.C] do not conform to method makeActor's type parameter bounds [T <: akka.actor.Actor]
  makeActor[C](3)
           ^

using scala generics in function definition

3 Answers3