SICP Exercise 4.4

SICP Exercise 4.4

a）因为数据导向的方式增加起来比较方便，所以我们继续采用联系4.3中的方式：

(define (eval-and exp env)  (define (and-loop exps env)    (if (null? exps)        true        (if (eval (car exps) env)            (and-loop (cdr exps) env)            false)))  (and-loop (cdr exp) env))(define (eval-or exp env)  (define (or-loop exps env)    (if (null? exps)        false        (if (eval (car exps) env)            true            (or-loop (cdr exps) env))))  (or-loop (cdr exp) env))

然后就是把这两个过程添加到求值表中即可：

(put 'and eval-and)(put 'or eval-or)

b）下面采用派生表达式的方式实现and和or：

(define (and->if exp)  (define (expand-clauses clauses)    (if (null? clauses)        true        (let ((first (car clauses))              (rest (cdr clauses)))          (make-if first                   (expand-clauses rest)                   false))))  (expand-clauses (cdr exp)))(define (eval-and exp env)  (eval (and->if exp) env))(define (or->if exp)  (define (expand-clauses clauses)    (if (null? clauses)        false        (let ((first (car clauses))              (rest (cdr clauses)))          (make-if first                   true                   (expand-clauses rest)))))  (expand-clauses (cdr exp)))(define (eval-or exp env)  (eval (or->if exp) env))

测试结果如下：

;;; M-Eval input:(and true true false);;; M-Eval value:#f;;; M-Eval input:(or false true);;; M-Eval value:#t