Extending Scala Actor

Question

I'm new to scala. When learning Actor, I tried to extend it to save one line of def:

import scala.actors.Actor
import Actor._
class Actoo(actoo: =>Unit) extends Actor {
    def act() {actoo}
}
object run extends Application {
    /* 
    // this one runs well
    val a = new Actor {
        def act() {
            receive { case 1 => println("1") }
        }
    }
    */
    val a = new Actoo {
        receive { case 1 => println("1") }
    }
    a.start
    a ! 1
}

Then the exception trace looks like this:

java.lang.AssertionError: assertion failed: receive from channel belonging to other actor
    at scala.Predef$.assert(Predef.scala:92)
    at scala.actors.Actor$class.receive(Actor.scala:424)
    at Actoo.receive(actoo.scala:3)
    at run$$anon$1.<init>(actoo.scala:16)
    at run$.<init>(actoo.scala:15)
    at run$.<clinit>(actoo.scala)
    at run.main(actoo.scala)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at scala.tools.nsc.ObjectRunner$$anonfun$run$1.apply(ObjectRunner.scala:75)
    at scala.tools.nsc.ObjectRunner$.withContextClassLoader(ObjectRunner.scala:49)
    at scala.tools.nsc.ObjectRunner$.run(ObjectRunner.scala:74)
    at scala.tools.nsc.MainGenericRunner$.main(MainGenericRunner.scala:154)
    at scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala)

There may be many alternatives which can do the same thing, but it's better for me to know the reason why the code above doesn't work.

Answer 1

It's pretty easy. This behavior is not due to actors library, in fact. The piece of code

val a = new Actoo {
        receive { case 1 => println("1") }
    }

gets interpreted by a compiler as "create new instance of Actoo" with an initialization body receive {...} and val actoo becomes equal to (). Thus, your code is equivivalent to

val a = new Actoo() {
        receive { case 1 => println("1") }
    }

In order to fix the code, you need to write

val a = new Actoo ({
        receive { case 1 => println("1") }
    })

Answer 2

There's also an actor method in the Actor singleton that does what you want. It even calls start for you automatically.

import scala.actors.Actor
import Actor._

val a = actor {
  receive { case 1 => println("1") }
}

a ! 1

Answer 3

Your are actually trying to receive from mailbox on the current native thread (native threads are Actors as well).

Witness the following:

Welcome to Scala version 2.7.5.final (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_13).
Type in expressions to have them evaluated.
Type :help for more information.

scala> import scala.actors.Actor._
import scala.actors.Actor._

scala> self ! 123

scala> receive { case x => println(x) }
123

Now, what you want to accomplish is in the library already (Actor.actor):

val a = actor {
  receive { case x => println(x) }
}
// no need to start a 
a ! 123

BTW, it is a very bad idea to extend Application. Use def main(args: Array[String]) instead.