IntS.scala

trait Monad[F[_], A] {
  def map[B](f: A => B): F[B]
  def flatMap[B](f: A => F[B]): F[B]
}

case class IntS[A](k: Int => (Int, A)) extends Monad[IntS, A] {
  def map[B](f: A => B): IntS[B] = IntS((i: Int) => {
    k(i) match {
      case (s, a) => (s, f(a))
    }
  })

  def flatMap[B](f: A => IntS[B]): IntS[B] = IntS((i: Int) => {
    k(i) match {
      case (s, a) => f(a).k(s)
    }
  })

  def print = k(0) match {
    case (s, a) => println("state " + s + " value " + a)
  }
}


object Main {
  def main(args: Array[String]) {
    val initial = IntS((i: Int) => (5, "test"))
    initial.print
    val second = initial.map(f=> f + "test")

    second.print
    val third = second.flatMap(f => IntS((i: Int) => (6, f)))
    third.print

  }
}

scala console output

state 5 value test
state 5 value testtest
state 6 value testtest