Main.scala

package example

case class IO[A](private val run: () => A) {
  def unsafeRun(): A = run()
  def map[B](f: A => B): IO[B] = IO(() => f(run()))
  def flatMap[B](f: A => IO[B]): IO[B] = IO(() => f(run()).run())
}

case class Ref[A](private var value: A) {
  def get: IO[A] = IO(() => value)
  def set(x: A): IO[Unit] = IO(() => { value = x ; () })
  def update(f: A => A): IO[Unit] = {
    for {
      x <- get
      _ <- set(f(x))
    } yield ()
  }
}

object Ref {
  def unsafe[A](a: A): Ref[A] = Ref(a)
  def safe[A](a: A): IO[Ref[A]] = IO(() => Ref(a))
}

object Main {
  def main(args: Array[String]): Unit = {
    val a1 = {
      val x = Ref.unsafe(0)
      for {
        _ <- x.update(_ + 1)
        res <- x.get
      } yield res
    }
    val a2 = {
      for {
        x <- Ref.safe(0)
        _ <- x.update(_ + 1)
        res <- x.get
      } yield res
    }

    println(a1.unsafeRun)
    println(a1.unsafeRun)
    println(a1.unsafeRun)

    println(a2.unsafeRun)
    println(a2.unsafeRun)
    println(a2.unsafeRun)
  }
}