A little game...

history-memory-experience.lisp

;;; History

(defun make-history ()
  (let (history)
    (lambda (&optional event)
      (when (and event
                 (not (equalp event (first history))))
        (push event history))
      (first history))))

(defun happen (history event)
  (funcall history event))

(defun latest (history)
  (funcall history))

;;; Memory

(defun make-memory ()
  (let (memory)
    (lambda (&optional history)
      (when history
        (push (latest history) memory))
      memory)))

(defun record (memory history)
  (funcall memory history))

(defun recall (memory)
  (funcall memory))

;;; Experiences

(let ((h (make-history))
      (m (make-memory)))
  (happen h 1)
  (happen h 1)
  (record m h)
  (happen h 2)
  (happen h 1)
  (record m h)
  (values (latest h)
          (recall m)))

(let ((h (make-history))
      (m1 (make-memory))
      (m2 (make-memory)))
  (happen h 1)
  (record m1 h)  
  (happen h 1)
  (record m2 h)
  (happen h 2)
  (record m2 h)
  (happen h 1)
  (record m1 h)
  (values (latest h) 
          (recall m1)
          (recall m2)))