state-ex.scala

import scalaz._

import scala.util.Try

final class X {
  val magic: Int = scala.util.Random.nextInt(10000)
  def isStable: Boolean = magic == 42
}

object Main extends App {
  def startStack(x: X): X = {
    val StateX = StateT.stateMonad[X]
    import StateX._ // includes whileM_, gets, put
    whileM_(gets(!_.isStable), modify(_ => new X)).exec(x)
  }

  def startHeap(x: X): X = {
    val StateX = StateT.stateTMonadState[X, Free.Trampoline]
    import StateX._ // includes whileM_, gets, put
    whileM_(gets(!_.isStable), modify(_ => new X)).exec(x).run
  }

  println(s"heap = ${startHeap(new X).magic}")
  println(s"stack = ${Try(startStack(new X).magic).getOrElse("Overflow!")}")
}