Chapter 4, Metalinguistic Abstraction
Exercise 4.52
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(define (if-fail? exp) (tagged-list? exp 'if-fail))
(define (if-fail-first exp) (cadr exp))
(define (if-fail-second exp) (caddr exp))
(define (analyze-if-fail exp)
(let ((first (analyze (if-fail-first exp)))
(second (analyze (if-fail-second exp))))
(lambda (env succeed fail)
(first env
succeed
(lambda()
(second env succeed fail))))))
For testing, First I defined these functions in global environment:
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(define (require p)
(if (not p) (amb)))
(define (an-element-of l)
(require (not (null? l)))
(amb (car l) (an-element-of (cdr l))))
(define (even? x) (= (remainder x 2) 0))
(define (odd? x) (not (even? x)))
Now, testing:
(Check last interesting example which i tested additionaly apart from the given in book)
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;;; Amb-Eval input:
(if-fail (let ((x (an-element-of '(1 3 5))))
(require (even? x))
x)
'all-odd)
;;; Starting a new problem
;;; Amb-Eval value:
all-odd
;;; Amb-Eval input:
(if-fail (let ((x (an-element-of '(1 3 5 8))))
(require (even? x))
x)
'all-odd)
;;; Starting a new problem
;;; Amb-Eval value:
8
;;; Amb-Eval input:
(if-fail (let ((x (an-element-of '(1 3 5))))
(require (even? x))
x)
(let ((x (an-element-of '(1 3 5 18))))
(require (even? x))
x))
;;; Starting a new problem
;;; Amb-Eval value:
18
;;; Amb-Eval input:
(if-fail (let ((x (an-element-of '(1 3 5))))
(require (even? x))
x)
(let ((x (an-element-of '(1 3 5))))
(require (even? x))
x))
;;; Starting a new problem
;;; There are no more values of
(if-fail (let ((x (an-element-of (quote (1 3 5))))) (require (even? x)) x) (let ((x (an-element-of (quote (1 3 5))))) (require (even? x)) x))
;;; Amb-Eval input: