sealed trait IO[+A] { self =>
  def run: A

  def map[B](f: A => B): IO[B] =
    flatMap(a => IO.unit(f(a)))

  def flatMap[B](f: A => IO[B]): IO[B] =
    new IO[B] {
      def run = f(self.run).run
    }
}

object IO {
  def unit[A](a: A): IO[A] = new IO[A] { def run = a }
}

case object ReadLine extends IO[String] {
  def run = { readLine }
}

case class WriteLine(msg: String) extends IO[Unit] {
  def run = { println(msg) }
}

abstract class Application {
  def universe: IO[Unit]
  def main(args: Array[String]) {
    universe.run
  }
}

object Main extends Application {
  def universe =
    for {
      l <- ReadLine
      _ <- WriteLine(l)
    } yield ()
}