Quitting of actor without error message - scala

Question

This is a problem from the chat I'm developing.
There is a main/gui object with the main method, an Actor sender, that sends the messages and a empfänger, that receives them.
The Problem is to make the empfänger use a function of the main/gui object to show the incoming messages. Because it didn't work, I simplified it, but I still didn't get the problem.
May anyone tell my why this returns nothing at the terminal?
What am I doing wrong?
import scala.actors._
object main_object {
def infoterm(msg: String) = {
println(msg)
}
def main(args: Array[String]) = {
println("hallo")
empfänger ! "foo"
}
}
object empfänger extends Actor{
var port = 50042
var name = "local"
var continuevar = true
def foo(in: String) = {
println("foo-empfänger" + in)
}
def act() {
println("ydfjskj")
test2.infoterm("tut")
println("Empfänger gestartet")
while(continuevar) {
react {
case msg:String => {
println("empfänger" + msg)
test2.infoterm(msg)
foo("empfänger" + msg)
}
}
}
}
}
Thanks for you help, but I still don't get on.
I modified like you told me but it's still not working.
Only by defining the Actor empfänger the whole program stops working, like this:
import scala.actors._
object test2 {
def infoterm(msg: String) = {
println(msg)
}
def main(args: Array[String]) = {
println("hallo")
}
}
object empfänger extends Actor{
def act() {
// test2.infoterm("tut")
// println("Empfänger gestartet")
loop {
react {
case msg:String => {
if(msg == "Stop") exit()
else {
println("empfänger" + msg)
// test2.infoterm(msg)
}
}
}
}
}
}
What am I doing wrong?

Answer 1

There are two ways from within an actor of looping upon receiving events:
while-receive
loop-react
You are using a mixture of the two (i.e. while-react). This simply cannot work for the following reasons:
receive
Receive blocks the current thread of execution waiting for a message to process. It can be used in a while-loop; although you should rarely use this as it ties an actor to a single thread - i.e. it is not scalable because you use 1 thread per actor.
react
Notice that the signature of the react method says that its return type is Nothing. This means that the method cannot terminate normally; it must either never return or throw an exception. In fact the latter happens; it uses exceptions as a flow-control mechanism, which only works when you use react inside loop (assuming you want to continually handle events). So your act method should look like this:
def act() {
loop {
react {
case msg: String =>
}
}
}
It's well worth a look at the actor source code to see how the exception flow control works as it is a thing of beauty.
Error Messages: Linking actors
This is something I have struggled with for the actor framework. If an actor's reactions throw an exception, the actor exits and it's quite common to have no record of this is sys-out or sys-err. It can be extremely frustrating.
I strongly recommend linking canary actors to each system actor to listen for exits as follows:
def act() {
import Actor._
self link actor {
case Exit(from, msg) => /* your logging here */
}
loop { ...
}
}
}
However, this does not work in your case (again, extremely frustrating) because your whole act method throws an exception.

Answer 2

You're using a while loop inside the act method. The actor works asynchronously and gets activated, when it receives a message. Use loop {} instead of while and it should work. To stop an actor you have to send it a message like 'stop and react to it in your act method with exit.

Answer 3

Actors are not automagically started, that would be a bad thing. You have to explicitly call the 'start' method, ie, in your main() empänger.start()