本節實作了amb求值器，題目都是擴充這個求值器，引入一些特殊的過程。我的嘗試解答從4.51開始
習題4.51，要求實作permanent-set!，這個過程的副作用在遇到失敗時不撤銷，實作如下：
;擴充analyze
((permanent-assignment? exp)
(analyze-permanent-assignment exp))
;實作
(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))))
習題4.52，實作if-fail的特殊形式，在第一個表達式如果求值成功，就傳回該表達式的值，否則傳回第二個表達式的值，實作如下：
;擴充analyze
((if-fail? exp)
(analyze-if-fail exp))
(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如果執行成功，就傳回結果pred-value，否則就執行fail過程(lambda() (cproc env succeed fail))，測試略。
習題4.53，根據題意可知這個過程傳回結果應該是(prime-sum-pair '(1 3 5 8) '(20 35 110))的所有結果，執行也是如此：
;;; amb-eval value:
((8 35) (3 110) (3 20))
習題4.54，将require實作為特殊形式：
((require? exp)
(analyze-require exp))
(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))))
文章轉自莊周夢蝶 ，原文釋出時間2008-11-21