chickadee » sicp » multiplier

A constraint that multiplies two numbers

a1

Multiplier

a2

Multiplicand

sum

Product

(define (multiplier m1 m2 product)
  (define (process-new-value)
    (cond ((or (and (has-value? m1) (= (get-value m1) 0))
               (and (has-value? m2) (= (get-value m2) 0)))
           (set-value! product 0 me))
          ((and (has-value? m1) (has-value? m2))
           (set-value! product (* (get-value m1) (get-value m2)) me))
          ((and (has-value? product) (has-value? m1))
           (set-value! m2 (/ (get-value product) (get-value m1)) me))
          ((and (has-value? product) (has-value? m2))
           (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)
    (case request
      ((I-have-a-value) (process-new-value))
      ((I-lost-my-value) (process-forget-value))
      (else (error "Unknown request: MULTIPLIER" request))))
  (connect m1 me)
  (connect m2 me)
  (connect product me)
  me)