Stolen from the scala-user mailing list

Var.scala

case class Var[T](i: T) extends AtomicReference[T](i) {
  def mutate(m: T => T): T = {
     @tailrec def apply(): T = {
        val c = get
        val n = m(c)
        if (compareAndSet(c, n)) n else apply()
     }
     apply()
  }
}