State exercise

gistfile1.scala

case class State[S, +A](run: S ⇒ (A, S))  {
  def map[B](f: A => B): State[S, B] =
    State(s ⇒ {
      val (a, t) = run(s)
      (f(a), t)
    })

  def flatMap[B](f: A ⇒ State[S, B]): State[S, B] =
    State(s ⇒ {
      val (a, t) = run(s)
      f(a) run t
    })

  def modify(f: S ⇒ S): State[S, A] =
    State(s ⇒ {
      val (a, t) = run(s)
      (a, f(s))
    })

  def put[S, A](s: S): State[S, Unit] = State(_ ⇒ ((), s))

object State {
  def point[S, A](a: => A): State[S, A] =
    State(s ⇒ (a, s))
}