A simple instance of a trait which reflects side-effecting stateful changes that can be undone.

gistfile1.txt

//Unlike a State Monad or some derived type, does not retain the previous state. Instead, just retains a means of "undoing" some stateful
//side effect. Still not sure if I completely like it as it doesn't indicate that it is intended to be used with an IO like action.

trait Change[+A]{
  def map[B](f: A => B): Change[B]
  def flatMap[B](f: A => B): Change[B]
}

case class Reversible[A+](value: A, undo: List[() => Unit]) extends Change[A]{
  def map[B](f: A => B) = Reversible(f(value), undo)
  def flatMap[B](f: A => Something[B]) = f(value) match{
    case Reversible(x, u) => Reversible(x, u :: undo)
    case Irreversible(x) => Reversible(x, undo)
  }
}

case class Irreversible[A+](value: A) extends Change[A]{
  def map[B](f: A => B) = Irreversible(f(value))
  def flatMap[B](f: A => Something[B]) = f(value)
}

Damn it, forgot to log in... AAaarrggg...

Fork of it: https://gist.github.com/4484330