sicp-ex-4.52

to summarize

  
  
  
 (load "evaluator-amb.scm") 
  
 (define (if-fail? exp) 
     (tagged-list? exp 'if-fail)) 
 (define (if-fail-cond exp) (cadr exp)) 
 (define (if-fail-alt exp) (caddr exp)) 
  
 (define (analyze-if-fail exp) 
     (let ((cproc (analyze (if-fail-cond exp))) 
           (aproc (analyze (if-fail-alt exp)))) 
         (lambda (env succeed fail) 
             (cproc env 
                    succeed 
                    (lambda () 
                         (aproc env succeed fail)))))) 

  
  
 ;; add this in analyze 
 ((if-fail? expr) (analyze-if-fail expr)) 
  
 ;; add those to amb evaluator 
 (define (if-fail? expr) (tagged-list? expr 'if-fail)) 
  
  (define (analyze-if-fail expr) 
   (let ((first (analyze (cadr expr))) 
         (second (analyze (caddr expr)))) 
    (lambda (env succeed fail) 
     (first env 
            (lambda (value fail2) 
             (succeed value fail)) 
            (lambda () 
             (second env succeed fail)))))) 
  

 ;; 
 ;; 
            (lambda (value fail2) 
             (succeed value fail)) 

(define try-expr cadr)
(define fail-expr caddr)

(define (analyze-if-fail exp)
  (let ((tproc (analyze (try-expr exp)))
        (fproc (analyze (fail-expr exp))))
    (lambda (env succeed fail)
      (tproc env succeed (lambda () (fproc env succeed fail))))))