IO.scala

trait IO[A] {
  def run: A // case class parameters may not be call-by-name
  // so we put unit in a singleton
}

object IO {
  def pure[A](input: => A): IO[A] = new IO[A] { def run: A = input }

  def >==>[A, B, C](a: A => IO[B], b: B => IO[C]): A => IO[C] = { input =>
    val pureIntput: IO[A] = pure(input)
    val pureB: IO[B] = flatMap(pureIntput)(a)
    flatMap(pureB)(b)
  }

  // again in terms of flatMap
  def map[A, B](a: IO[A])(f: A => B): IO[B] = {

    flatMap(a) { input =>
      pure(f(input))
    }
  }

  def flatMap[A, B](a: IO[A])(f: A => IO[B]): IO[B] = {
    flatMap(a)(f)
  }
}