Ex-3.35

code from book for constraint network.

(define (squarer a b)
  (define (process-new-value)
    (if (has-value? b)
        (if (< (get-value b) 0)
            (error "square less than 0 -- SQUARER" (get-value b))
            (set-value! a (sqrt (get-value b)) me))
        (if (has-value? a)
            (set-value! b (square (get-value a)) me))))
  (define (process-forget-value)
    (forget-value! a me)
    (forget-value! b 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 -- SQUARER" request))))
  (connect a me)
  (connect b me)
  me)

;;tests
(define A (make-connector))
(define B (make-connector))
(probe "A" A)
(probe "B" B)

(squarer A B)

(set-value! A 3 'user)
;Probe: B = 9
;Probe: A = 3
;Value: done

(forget-value! A 'user)
;Probe: B = ?
;Probe: A = ?
;Value: done

(set-value! B 25 'user)
;Probe: A = 5
;Probe: B = 25
;Value: done