scratch-io-monad

IO.scala

sealed trait IO[+A] {

  import IO._

  def map[B](f: A => B): IO[B] = this match {
    case Map(original, g, index) if index != 128 =>
      Map(original, g.andThen(f), index + 1)
    case _ => Map(this, f, 0)
  }

  def flatMap[B](f: A => IO[B]): IO[B] = FlatMap(this, f)
}

object IO {

  def pure[A](a: A): IO[A] = Pure(a)

  def apply[A](a: => A): IO[A] = delay(a)

  def delay[A](a: => A): IO[A] = Delay(a _)

  def raiseError[A](err: Throwable): IO[A] = RaiseError(err)

  final case class Pure[+A](a: A) extends IO[A]
  final case class Delay[+A](thunk: () => A) extends IO[A]
  final case class RaiseError(err: Throwable) extends IO[Nothing]
  final case class FlatMap[E, +A](original: IO[E], f: E => IO[A]) extends IO[A]
  final case class Map[E, +A](original: IO[E], f: E => A, index: Int)
      extends IO[A]
      with (E => IO[A])
}