3.33.rkt

; connector implement
(define (make-connector)
  (let ((value #f) (informant #f) (constraints '()))
    (define (set-my-value newval setter)
      (cond ((not (has-value? me))
             (set! value newval)
             (set! informant setter)
             (for-each-except setter
                              inform-about-value
                              constraints))
            ((not (= value newval))
             (error "Constradiction" (list value newval)))
            (else 'ignored)))
    (define (forget-my-value retractor)
      (if (eq? retractor informant)
          (begin (set! informant #f)
                 (for-each-except retractor
                                  inform-about-no-value
                                  constraints))
          'ignored))
    (define (connect new-constraint)
      (if (not (memq new-constraint constraints))
          (set! constraints
                (cons new-constraint constraints)))
      (if (has-value? me)
          (inform-about-value new-constraint))
      'done)
    (define (me request)
      (cond ((eq? request 'has-value?)
             (if informant #t #f))
            ((eq? request 'value) value)
            ((eq? request 'set-value!) set-my-value)
            ((eq? request 'forget) forget-my-value)
            ((eq? request 'connect) connect)
            (else (error "Unknown operation -- CONNECTOR"
                         request))))
    me))

(define (has-value? connector)
  (connector 'has-value?))

(define (get-value connector)
  (connector 'value))

(define (set-value! connector new-value informant)
  ((connector 'set-value!) new-value informant))

(define (forget-value! connector retractor)
  ((connector 'forget) retractor))

(define (connect connector new-constraint)
  ((connector 'connect) new-constraint))

(define (for-each-except exception procedure list)
  (define (loop items)
    (cond ((null? items) 'done)
          ((eq? (car items) exception) (loop (cdr items)))
          (else (procedure (car items))
                (loop (cdr items)))))
  (loop list))

(define (inform-about-value constraints)
  (constraints 'I-have-a-value))

(define (inform-about-no-value constraints)
  (constraints 'I-lost-my-value))

; adder
(define (adder a1 a2 sum)
  (define (process-new-value)
    (cond ((and (has-value? a1) (has-value? a2))
           (set-value! sum
                       (+ (get-value a1) (get-value a2))
                       me))
          ((and (has-value? a1) (has-value? sum))
           (set-value! a2
                       (- (get-value sum) (get-value a1))
                       me))
          ((and (has-value? a2) (has-value? sum))
           (set-value! a1
                       (- (get-value sum) (get-value a2))
                       me))))
  (define (process-forget-value)
    (forget-value! sum me)
    (forget-value! a1 me)
    (forget-value! a2 me)
    (process-new-value))
  (define (me request)
    (cond ((eq? request 'I-have-a-value)
           (process-new-value))
          ((eq? request 'I-lost-my-value)
           (process-forget-value))
          (else
           (error "Unknown request -- ADDER" request))))
  (connect a1 me)
  (connect a2 me)
  (connect sum me)
  me)

; multiplier
(define (multiplier m1 m2 product)
  (define (process-new-value)
    (cond ((and (has-value? m1) (has-value? m2))
           (set-value! product
                       (* (get-value a1) (get-value a2))
                       me))
          ((and (has-value? m1) (has-value? product))
           (set-value! m2
                       (/ (get-value product) (get-value m1))
                       me))
          ((and (has-value? m2) (has-value? product))
           (set-value! m1
                       (/ (get-value product) (get-value m2))
                       me))))
  (define (process-forget-value)
    (forget-value! product me)
    (forget-value! m1 me)
    (forget-value! m2 me)
    (process-new-value))
  (define (me request)
    (cond ((eq? request 'I-have-a-value)
           (process-new-value))
          ((eq? request 'I-lost-my-value)
           (process-forget-value))
          (else
           (error "Unknown request -- ADDER" request))))
  (connect m1 me)
  (connect m2 me)
  (connect product me)
  me)

; constant
(define (constant value connector)
  (define (me request)
    (error "Unknown request -- CONSTANT" request))
  (connect connector me)
  (set-value! connector value me)
  me)

; averager
(define (average a b c)
  (let ((u (make-connector))
        (x (make-connector)))
    (adder a b u)
    (constant 2 x)
    (multiplier c x u)))

(define a (make-connector))
(define b (make-connector))
(define c (make-connector))
(average a b c)
(set-value! a 10 'tt)
(set-value! b 20 'tt)
(get-value c)