traverse.scala

class BlockOp(implicit ec: ExecutionContext) {
  def blockOp(number: Int) = Future {
    Thread.sleep(2000)
    println(s"$number after sleep, time is now ${System.currentTimeMillis() / 1000}")
    number
  }
}

sealed trait TestTraverse[A]
case class BlockingOp(number: Int) extends TestTraverse[Int]

class TestTraverseInterpreter(implicit ec: ExecutionContext) extends (TestTraverse ~> Future) {
  override def apply[A](fa: TestTraverse[A]): Future[A] = fa match {
    case BlockingOp(number) => new BlockOp().blockOp(number)
  }
}

// here the traverse doesn't work in parallel

object FreeTraverse {
  def main(args: Array[String]) = {
    implicit val ec = ExecutionContext.fromExecutor(Executors.newFixedThreadPool(5))

    val prog = (1 to 10).toList.traverse(x => Free.liftF[TestTraverse, Int](BlockingOp(x)))
    val result = prog.foldMap(new TestTraverseInterpreter)
    val delayedResult = Await.result(result, 30.seconds)
    println(delayedResult)
  }
}

// here the traverse works in parallel

object FutureTraverse {
  def main(args: Array[String]) = {
    implicit val ec = ExecutionContext.fromExecutor(Executors.newFixedThreadPool(5))

    val result = (1 to 10).toList.traverse(new BlockOp().blockOp)
    val delayedResult = Await.result(result, 30.seconds)
    println(delayedResult)
  }
}