gistfile1.scala

  class Box[A] {    
    private var waiters = Queue[A => Unit]()
    def watch(f:A => Unit) = waiters = waiters :+ f
    def put(a:A) = waiters.foreach(_(a))
  }
  
  val b = new Box[String]

  for (i <- 1 to 5) 
    b.watch(s => println(s + " " + i))
 
  b.put("foo")