;;; These definitions for PRINT and SIZE are
;;; already supplied by
(require 'yasos)

(define-operation (print obj port)
  (format port
          (if (instance? obj) "#<instance>" "~s")
          obj))

(define-operation (size obj)
  (cond
   ((vector? obj) (vector-length obj))
   ((list?   obj) (length obj))
   ((pair?   obj) 2)
   ((string? obj) (string-length obj))
   ((char?   obj) 1)
   (else
    (slib:error "Operation not supported: size" obj))))

(define-predicate cell?)
(define-operation (fetch obj))
(define-operation (store! obj newValue))

(define (make-cell value)
  (object
   ((cell? self) #t)
   ((fetch self) value)
   ((store! self newValue)
    (set! value newValue)
    newValue)
   ((size self) 1)
   ((print self port)
    (format port "#<Cell: ~s>" (fetch self)))))

(define-operation (discard obj value)
  (format #t "Discarding ~s~%" value))

(define (make-filtered-cell value filter)
  (object-with-ancestors
   ((cell (make-cell value)))
   ((store! self newValue)
   (if (filter newValue)
       (store! cell newValue)
       (discard self newValue)))))

(define-predicate array?)
(define-operation (array-ref array index))
(define-operation (array-set! array index value))

(define (make-array num-slots)
  (let ((anArray (make-vector num-slots)))
    (object
     ((array? self) #t)
     ((size self) num-slots)
     ((array-ref self index)
      (vector-ref  anArray index))
     ((array-set! self index newValue)
      (vector-set! anArray index newValue))
     ((print self port)
      (format port "#<Array ~s>" (size self))))))

(define-operation (position obj))
(define-operation (discarded-value obj))

(define (make-cell-with-history value filter size)
  (let ((pos 0) (most-recent-discard #f))
    (object-with-ancestors
     ((cell (make-filtered-call value filter))
      (sequence (make-array size)))
     ((array? self) #f)
     ((position self) pos)
     ((store! self newValue)
      (operate-as cell store! self newValue)
      (array-set! self pos newValue)
      (set! pos (+ pos 1)))
     ((discard self value)
      (set! most-recent-discard value))
     ((discarded-value self) most-recent-discard)
     ((print self port)
      (format port "#<Cell-with-history ~s>"
              (fetch self))))))

(define-access-operation fetch)
(add-setter fetch store!)
(define foo (make-cell 1))
(print foo #f)
⇒ "#<Cell: 1>"
(set (fetch foo) 2)
⇒
(print foo #f)
⇒ "#<Cell: 2>"
(fetch foo)
⇒ 2