squarter.scm

(define (squarter 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)
(define (square x) (* x x))