ResponderDemo.scala

import Responder._

import scalaz._

object ResponderDemo extends Application {
  import Scalaz._

    // --------------------------------------------------------------------------
    //  Uses Responder (a continuation monad) to compose asynchronous functions. 
    // --------------------------------------------------------------------------

    // asynchronously concat s1 and s2 and call k with result
    def concat(s1: String, s2: String, k: String => Unit): Unit = new Thread() {
      override def run {
        Thread sleep 1000
        k(s1 + s2)
      }
    }.start

    // concrete responder passing a concatenation result to continuation k
    class ConcatResponder(s1: String, s2: String) extends Responder[String] {
      def respond(k: (String) => Unit) = concat(s1, s2, s => k(s))
    }

    // Creates a Kleisli instance that appends s2 when applied to input String s1
    def append(s2: String, f: String => String = s => s): Kleisli[Responder, String, String] =
      kleisli((s1: String) => new ConcatResponder(s1, s2).map(f))

    // Kleisli composition of asynchronous functions
    val c1 = append("b") >=> append("c") >=> append("d") >=> append("e")

    // ... or usage in a for-comprehension
    def c2(s: String) = for {
      s1 <- append("x")(s)
      s2 <- append("y")(s1)
      s3 <- append("z")(s2)
    } yield s3

    // applications of c1 and c2 run concurrently
    c1("a") respond println // prints "abcde" after approx. 4 seconds
    c2("a") respond println // prints "axyz" after approx. 3 seconds
}