IO

import scala.annotation.tailrec

object MyIo2 {

  sealed trait IO[+A] {
    def flatMap[B](f: A => IO[B]): IO[B] = this match {
      case FlatMap(x, g) => FlatMap(x, (a: Any) => g(a).flatMap(f))
      case x => FlatMap(x, f)
    }

    def map[B](f: A => B): IO[B] =
      FlatMap[A, B](this, a => Return(f(a)))

    def run: A = IO.run(this)
  }

  object IO {
    def apply[A](a: => A): IO[A] = Suspend(() => Return[A](a))

    def join[A](ffa: IO[IO[A]]) = ffa.flatMap(identity)

    @tailrec def run[A](io: IO[A]): A = io match {
      case Return(a) => a
      case Suspend(r) => run(r())
      case FlatMap(x, f) => x match {
        case Return(a) => run(f(a))
        case Suspend(r) => run(r() flatMap f)
        case FlatMap(y, g) => run(y flatMap (a => g(a) flatMap f))
      }
    }
  }

  case class Return[A](a: A) extends IO[A]

  case class Suspend[A](resume: () => IO[A]) extends IO[A]

  case class FlatMap[A, B](sub: IO[A], k: A => IO[B]) extends IO[B]

  def PrintLine(msg: String): IO[Unit] = IO(println(msg))

  val actions: Stream[IO[Unit]] =
    Stream.fill(1000000)(PrintLine("Still going..."))

  val composite: IO[Unit] =
    actions.foldLeft(Return(()): IO[Unit]) {
      (acc, a) => acc.flatMap {
        _ => a
      }
    }
}