;; GOAL: create a counter class using
;;       regular Scheme


;;;;;;;;;;;;;;;;;;
;; Step1: instance variable


(define make-counter
 (lambda ()
  (let ((num 1))
    (lambda ()
      (set! num (+ num 1))
      num))))



;;;;;;;;;;;;;;;;;;
;; Step2: class variable

(define make-counter
  (let ((total-num 0))
    (lambda ()
      (let ((num 1))
	(lambda ()
	  (set! total-num (+ total-num 1))
	  (set! num (+ num 1))
	  num)))))



;;;;;;;;;;;;;;;;;;
;; Step3: accepting message

(define make-counter
  (let ((total-num 0))
    (lambda ()
      (let ((num 1))
	(define (dispatch m)
	  (cond ((eq? m 'count)
		   (set! total-num (+ total-num 1))
		   (set! num (+ num 1))
		   num))
		(else
		 'unknown-message)))
	dispatch)))


;;;;;;;;;;;;;;;;;
;; Step4: method

(define make-counter
  (let ((total-num 0))
    (lambda ()
      (let ((num 1))
	
	(define (increase n)
	  (set! num (+ num n))
	  num)
	(define (count)
	  (set! num (+ num 1))
	  (set! total-num (+ total-num 1))
	  num)

	(define (dispatch m)
	  (cond ((eq? m 'count) count)
		((eq? m 'increase) increase)
		(else 'unknown-message)))
	dispatch)
	  
      )))