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問題4.51

変数を元の値に戻す失敗継続（p257 analyze-assignment の注釈*2*）を、元の値に戻さずにそのまま失敗を継続させる。

(define (analyze exp)
  (cond ((self-evaluating? exp)
         (analyze-self-evaluating exp))
        ;; 省略
        ((passignment? exp) (analyze-permanent-assignment exp))
        ;; 省略
        (else
          (error "Unknown expression type -- ANALYZE" exp))))

(define (passignment? exp) (tagged-list? exp 'permanent-set!))

(define (analyze-permanent-assignment exp)
  (let ((var (assignment-variable exp))
        (vproc (analyze (assignment-value exp))))
       (lambda (env succeed fail)
               (vproc env
                      (lambda (val fail2)
                              (set-variable-value! var val env)
                              (succeed 'ok fail2))
                      fail))))

permanent-set! を使った場合の実行結果

;;; Amb-Eval input:
(define (require p)
  (if (not p) (amb)))

;;; Starting a new problem 
;;; Amb-Eval value:
ok

;;; Amb-Eval input:
(define (an-element-of items)
  (require (not (null? items)))
  (amb (car items) (an-element-of (cdr items))))

;;; Starting a new problem 
;;; Amb-Eval value:
ok

;;; Amb-Eval input:
(define count 0)

;;; Starting a new problem 
;;; Amb-Eval value:
ok

;;; Amb-Eval input:
(let ((x (an-element-of '(a b c)))
      (y (an-element-of '(a b c))))
     (permanent-set! count (+ count 1))
     (require (not (eq? x y)))
     (list x y count))

;;; Starting a new problem 
;;; Amb-Eval value:
(a b 2)

;;; Amb-Eval input:
try-again

;;; Amb-Eval value:
(a c 3)

;;; Amb-Eval input:
try-again

;;; Amb-Eval value:
(b a 4)

;;; Amb-Eval input:
try-again

;;; Amb-Eval value:
(b c 6)

;;; Amb-Eval input:
try-again

;;; Amb-Eval value:
(c a 7)

;;; Amb-Eval input:
try-again

;;; Amb-Eval value:
(c b 8)

;;; Amb-Eval input:
try-again

;;; There are no more values of
(let ((x (an-element-of '(a b c))) (y (an-element-of '(a b c)))) (permanent-set! count (+ count 1)) (require (not (eq? x y))) (list x y count))

set! を使った場合の実行結果

;;; Amb-Eval input:
(let ((x (an-element-of '(a b c)))
      (y (an-element-of '(a b c))))
     (set! count (+ count 1))
     (require (not (eq? x y)))
     (list x y count))

;;; Starting a new problem 
;;; Amb-Eval value:
(a b 1)

;;; Amb-Eval input:
try-again

;;; Amb-Eval value:
(a c 1)

;;; Amb-Eval input:
try-again

;;; Amb-Eval value:
(b a 1)

;;; Amb-Eval input:
try-again

;;; Amb-Eval value:
(b c 1)

;;; Amb-Eval input:
try-again

;;; Amb-Eval value:
(c a 1)

;;; Amb-Eval input:
try-again

;;; Amb-Eval value:
(c b 1)

;;; Amb-Eval input:
try-again

;;; There are no more values of
(let ((x (an-element-of '(a b c))) (y (an-element-of '(a b c)))) (set! count (+ count 1)) (require (not (eq? x y))) (list x y count))

計算機プログラムの構造と解釈

posted with amazlet at 08.11.07

ジェラルド・ジェイ サスマン ジュリー サスマン ハロルド エイブルソン
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