chickadee » sicp » adder

A constraint that adds two numbers

a1

Addend

a2

Augend

sum

Sum

(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)
    (case request
      ((I-have-a-value) (process-new-value))
      ((I-lost-my-value) (process-forget-value))
      (else (error "Unknown request: ADDER" request))))
  (connect a1 me)
  (connect a2 me)
  (connect sum me)
  me)