【SICP练习】148 练习4.4

练习4-4

原文

Exercise 4.4. Recall the definitions of the special forms and and or from chapter 1:

● and: The expressions are evaluated from left to right. If any expression evaluates to false, false is returned; any remaining expressions are not evaluated. If all the expressions evaluate to true values, the value of the last expression is returned. If there are no expressions then true is returned.

● or: The expressions are evaluated from left to right. If any expression evaluates to a true value, that value is returned; any remaining expressions are not evaluated. If all expressions evaluate to false, or if there are no expressions, then false is returned.

Install and and or as new special forms for the evaluator by defining appropriate syntax procedures and evaluation procedures eval-and and eval-or. Alternatively, show how to implement and and or as derived expressions.

代码

 ((and? expr) (evaln (and->if expr) env)) 

 (define (and->if expr) (expand-and-clauses (and-clauses expr))) 
 (define (expand-and-clauses clauses) (if (null? clauses) (make-if 'true 'true 'false) (let ((first (car clauses)) (rest (cdr clauses))) (if (null? rest) (make-if first first 'false) (make-if first (expand-and-clauses rest) 'false))))) 

 ((or? expr) (evaln (or->if expr) env))  
 (define (or->if expr) (expand-or-clauses (or-clauses expr))) 
 (define (expand-or-clauses clauses) (if (null? clauses) (make-if 'true 'false 'true) (let ((first (car clauses)) (rest (cdr clauses))) (make-if first 'true (expand-or-clauses rest))))) 

---

---

感谢您的访问，希望对您有所帮助。

欢迎大家关注或收藏、评论或点赞。

---

为使本文得到斧正和提问，转载请注明出处：
http://blog.youkuaiyun.com/nomasp

---

版权声明：本文为 NoMasp柯于旺 原创文章，未经许可严禁转载！欢迎访问我的博客：http://blog.youkuaiyun.com/nomasp

转载于:https://my.oschina.net/nomasp/blog/503353