sicp-ex-3.35

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 ;; Squarer Constraint 
  
 (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)