mutable-history.scala

//historical chain of value.
import java.concurrent.ConcurrentSkipListMap

class History[Token, A, B](tk: => Token, prev: ConcurrentSkipListMap[Token, A], id: A => B){
  def apply(f: A => A): (Token, B) ={
    val token = tk
    (token, id(update(token, f)))
  }
  
  def map[C](f: B => C) = new History(prev, id andThen f)
  
  def flatMap[C](f: B => History[Token, A, C]) ={
    val entry = prev.lastEntry().getValue()
    f(id(entry))
  }
  
  protected def update(token: Token, f: A => A): A ={
    val entry = prev.lastEntry().getValue()
    val value = f(entry)
    prev.put(token, value)
    value
  }
}