本节实现了amb求值器，题目都是扩展这个求值器，引入一些特殊的过程。我的尝试解答从4.51开始
习题4.51，要求实现permanent-set!，这个过程的副作用在遇到失败时不撤销，实现如下：
习题4.52，实现if-fail的特殊形式，在第一个表达式如果求值成功，就返回该表达式的值，否则返回第二个表达式的值，实现如下：
习题4.53，根据题意可知这个过程返回结果应该是(prime-sum-pair '(1 3 5 8) '(20 35 110))的所有结果，执行也是如此：
习题4.54，将require实现为特殊形式：
习题4.51，要求实现permanent-set!，这个过程的副作用在遇到失败时不撤销，实现如下：
;扩充analyze
((permanent - assignment? exp)
(analyze - permanent - assignment exp))
;实现
(define (permanent - assignment? 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 - assignment? exp)
(analyze - permanent - assignment exp))
;实现
(define (permanent - assignment? 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))))
习题4.52，实现if-fail的特殊形式，在第一个表达式如果求值成功，就返回该表达式的值，否则返回第二个表达式的值，实现如下：
;扩充analyze
(( if - fail? exp)
(analyze - if - fail exp))
;实现
(define ( if - fail? exp)
(tagged - list? exp ' if-fail))
(define (analyze - if - fail exp)
(let ((pproc (analyze ( if - predicate exp)))
(cproc (analyze ( if - consequent exp))))
( lambda (env succeed fail)
(pproc env ( lambda (pred - value fail2)
(succeed pred - value fail2))
( lambda () (cproc env succeed fail))))))
pproc如果执行成功，就返回结果pred-value，否则就执行fail过程
(
lambda
() (cproc env succeed fail))，测试略。
(( if - fail? exp)
(analyze - if - fail exp))
;实现
(define ( if - fail? exp)
(tagged - list? exp ' if-fail))
(define (analyze - if - fail exp)
(let ((pproc (analyze ( if - predicate exp)))
(cproc (analyze ( if - consequent exp))))
( lambda (env succeed fail)
(pproc env ( lambda (pred - value fail2)
(succeed pred - value fail2))
( lambda () (cproc env succeed fail))))))
习题4.53，根据题意可知这个过程返回结果应该是(prime-sum-pair '(1 3 5 8) '(20 35 110))的所有结果，执行也是如此：
;;; AMB
-
Eval value:
(( 8 35 ) ( 3 110 ) ( 3 20 ))
(( 8 35 ) ( 3 110 ) ( 3 20 ))
习题4.54，将require实现为特殊形式：
;扩充analyze
((require? exp)
(analyze - require exp))
;实现
(define (require? exp)
(tagged - list? exp ' require))
(define (require - predicate exp)
(cadr exp))
(define (analyze - require exp)
(let ((pproc (analyze (require - predicate exp))))
( lambda (env succeed fail)
(pproc env ( lambda (pred - value fail2)
( if ( not pred - value)
(fail2)
(succeed ' ok fail2)))
fail))))
((require? exp)
(analyze - require exp))
;实现
(define (require? exp)
(tagged - list? exp ' require))
(define (require - predicate exp)
(cadr exp))
(define (analyze - require exp)
(let ((pproc (analyze (require - predicate exp))))
( lambda (env succeed fail)
(pproc env ( lambda (pred - value fail2)
( if ( not pred - value)
(fail2)
(succeed ' ok fail2)))
fail))))
文章转自庄周梦蝶 ，原文发布时间2008-11-21
