io-monad.scala

import scala.io.StdIn.readLine

trait IO[+A] {
  def run(): A
  def flatMap[B](f: A => IO[B]): IO[B] = IO(f(run()).run())
  def map[B](f: A => B): IO[B] = flatMap(a => IO(f(a)))
}

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

def PrintLn(x: String): IO[Unit] = IO(println(x))

def ReadLn: IO[String] = IO(readLine())

val prog = for {
  name <- ReadLn
  _ <- PrintLn(s"Hello, $name!")
} yield ()