An observer pattern library by family polymorphism

gistfile1.scala

//library code
//references: http://lamp.epfl.ch/~odersky/papers/ScalableComponent.pdf
abstract class SubjectObserver {
  type S <: Subject
  type O <: Observer
  abstract class Subject { this: S =>
    private var observers: List[O] = List()
    def subscribe(obs: O) =
      observers = obs :: observers
    def publish =
      for (obs <- observers) obs.notify(this)
  }
  abstract class Observer {
    def notify(sub: S): Unit
  }
}

//user code
object SensorReader extends SubjectObserver {
  type S = Sensor
  type O = Display
  abstract class Sensor extends Subject {
    val label: String
    var value: Double = 0.0
    def changeValue(v: Double) = {
      value = v
      publish
    }
  }
  class Display extends Observer {
    def println(s: String) = Predef.println(s)
    def notify(sub: Sensor) =
      println(sub.label + " has value " + sub.value)
  }
}

object Test {
  import SensorReader._
  val s1 = new Sensor { val label = "sensor1" }
  val s2 = new Sensor { val label = "sensor2" }
  def main(args: Array[String]) = {
    val d1 = new Display
    val d2 = new Display
    s1.subscribe(d1)
    s1.subscribe(d2)
    s2.subscribe(d1)
    s1.changeValue(2)
    s2.changeValue(3)
  }
}