SICP 問題 4.52

利用者が失敗を捉えることができるif-failを実装する．

(define (analyze exp)
  (cond ((self-evaluating? exp) (analyze-self-evaluating exp))
        ((quoted? exp) (analyze-quoted exp))
        ((variable? exp) (analyze-variable exp))
        ((assignment? exp) (analyze-assignment exp))
        ((permanent-assignment? exp) (analyze-permanent-assignment exp))
        ((definition? exp) (analyze-definition exp))
        ((amb? exp) (analyze-amb exp))
        ((ramb? exp) (analyze-ramb exp))
        ((if? exp) (analyze-if exp))
        ((if-fail? exp) (analyze-if-fail exp))
        ((lambda? exp) (analyze-lambda exp))
        ((let? exp) (analyze (let->combination exp)))
        ((begin? exp) (analyze-sequence (begin-actions exp)))
        ((cond? exp) (analyze (cond->if exp)))
        ((application? exp) (analyze-application exp))
        (else (error "Unknown expression type: ANALYZE" exp))))

(define (if-fail? exp) (tagged-list? exp 'if-fail))

(define (analyze-if-fail exp)
  (let ((proc (analyze (cadr exp)))
        (fail-proc (analyze (caddr exp))))
    (lambda (env succeed fail)
      (proc env succeed
            (lambda ()
              (announce-output output-prompt)
              (user-print (fail-proc env succeed fail))
              (driver-loop))))))

test

;;; 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