gistfile1.scala

trait Future[+A] {
  private def apply(k: A => Unit): Unit
}

type Par[+A] = ExecutorService => Future[A]

def map2[A, B, C](pa: Par[A], pb: Par[B])(f: (A, B) => C): Par[C] = {

  ex =>
	{
	  new Future[C] {
		def apply(k: C => Unit) = {

		  var va: Option[A] = None
		  var vb: Option[B] = None

		  val actor = Actor[Either[A, B]](ex) {

			case Left(v) => {
			  if (vb.isDefined) k(f(v, vb.get))
			  else va = Some(v)
			}
			case Right(v) => {
			  if (va.isDefined) k(f(va.get, v))
			  else vb = Some(v)
			}
		  }

		  pa(ex) { v => actor ! Left(v) }
		  pb(ex) { v => actor ! Right(v) }

		}
	  }
	}
}