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問題3.35

(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))
        (set-value! b
                    (square (get-value a))
                    me)))
  (define (process-forget-value)
    (forget-value! a me)
    (forget-value! b me))
  (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)

実行結果

(define A (make-connector))
(define B (make-connector))
(squarer A B)
(probe "A" A)
(probe "B" B)

(set-value! A 3 'user)
gosh> 
Probe: A = 3
Probe: B = 9done
(forget-value! A 'user)
gosh> 
Probe: A = ?
Probe: B = ?done
(set-value! B 16 'user)
gosh> 
Probe: B = 16
Probe: A = 4.0done

計算機プログラムの構造と解釈

posted with amazlet at 08.11.07

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