SimpleIOMonad

IO.scala

class IO[+T](run: => T) {
  def map[R](f: T => R): IO[R] = {
    IO(f(run))
  }
  
  def flatMap[R](f: T => IO[R]): IO[R] = {
    IO(f(run).unsafeRun())
  }
  
  def unsafeRun(): T = run
}

object IO {
  def apply[T](run: => T) = new IO(run)
  
  def log(a: Any): IO[Unit] = IO {
    println(a)
  }
  
  def random: IO[Double] = IO {
    Math.random()
  }
  
  def randomInt: IO[Int] = {
    random map (n => (n * 100).toInt)
  }
}

val program = for {
  rnd <- IO.randomInt
  _ <- IO.log(rnd)
  rnd2 <- IO.randomInt
  _ <- IO.log(rnd2 - rnd)
} yield ()

program.unsafeRun()