Free Monad

freemonad.scala

import scalaz._, Scalaz._
import scalaz.concurrent.Task

object console extends App {

  trait Op[T]
  case class Read[T](t: T) extends Op[T]
  case class Write[T](t: T) extends Op[T]

  type OpC[T] = Coyoneda[Op, T]

  def read[T](t: T) = Free.liftF[OpC, T](Coyoneda.lift(Read(t)))
  def write[T](t: T) = Free.liftF[OpC, T](Coyoneda.lift(Write(t)))

  val res = for {
    r <- read("ciao")
    w <- write("foo")
  } yield (r + w)

  def foo = new (Op ~> Task) {
    def apply[T](op: Op[T]) = op match {
      case Read(s) => Task.fork(Task.delay{
        Thread.sleep(500); println("read"); s
      })
      case Write(s) => Task.fork(Task.delay{
        Thread.sleep(700); println("write"); s
      })
    }
  }

  println(s"res: ${Free.runFC(res)(foo).run}")
}