Question

Suppose I have to write some GUI code as follows:

widget1.addListener(event1 =>
   handle1(event1)
   widget2.addListener(event2 =>
     handle2(event2)
     widget3.addListener(event3 => handle3(event3))
   )
)

How would you write it in CPS-style using Scala continuations?

Answer 1

Just wanted to provide working example in addition to other answers. With Scala continuations it can look like this:

import scala.util.continuations._

object ContinuationsExample extends App {
  val widget1 = Widget()
  val widget2 = Widget()

  reset {
    val event1 = widget1.getEvent
    println("Handling first event: " + event1)
    val event2 = widget2.getEvent
    println("Handling second event: " + event2)
  }

  widget2 fireEvent Event("should not be handled")
  widget1 fireEvent Event("event for first widget")
  widget2 fireEvent Event("event for second widget")
  widget1 fireEvent Event("one more event")
}

case class Event(text: String)

case class Widget() {
  type Listener = Event => Unit
  var listeners : List[Listener] = Nil

  def getEvent = shift { (l: Listener) =>
    listeners = l +: listeners
  }

  def fireEvent(event: Event) = listeners.foreach(_(event))
}

This code actually compiles and runs, so you can try it yourself. You should receive following output:

Handling first event: Event(event for first widget)
Handling second event: Event(event for second widget)
Handling first event: Event(one more event) 

If you will compile this example, then don’t forget to enable continuations by providing -P:continuations:enable argument for the Scala compiler.