CallbacksToFutures.scala

// Traditional callback (using a continuation)

def program(a: A, k: B => Unit): Unit

// Let's start moving this towards a Future
def program(a: A)(B => Unit): Unit

// now introduce the following type alias
type Future[+T] = (T => Unit) => Unit
def program(a: A): Future[B]

// we could do a better implemetation of Future as
type Future[+T] = (Try[T] => Unit) => Unit

// we could make this closer to the real Scala Future by
// reifying the type alias into a Trait
trait Future[+T] extends (Try[T] => Unit) => Unit {
  def apply(k: Try[T] => Unit): Unit
}

// now just renaming 'apply' to 'onComplete' and this is 
// close to the real definition of a Future
trait Future[+T] extends (Try[T] => Unit) => Unit {
  def onComplete(k: Try[T] => Unit): Unit
}