Exercise 2.22 · 习题

Exercise 2.22: Louis Reasoner tries to rewrite
the first square-list procedure of Exercise 2.21 so that it
evolves an iterative process:

(define (square-list items)
(define (iter things answer)
(if (null? things)
answer
(iter (cdr things)
(cons (square (car things))
answer))))
(iter items nil))

Unfortunately, defining square-list this way produces the answer list in
the reverse order of the one desired. Why?

Louis then tries to fix his bug by interchanging the arguments to cons:

(define (square-list items)
(define (iter things answer)
(if (null? things)
answer
(iter (cdr things)
(cons answer
(square
(car things))))))
(iter items nil))

This doesn’t work either. Explain.

练习 2.22：Louis Reasoner 试图重写练习 2.21 中的第一个 square-list 过程，使其产生一个迭代型计算过程：

(define (square-list items)
(define (iter things answer)
(if (null? things)
answer
(iter (cdr things)
(cons (square (car things))
answer))))
(iter items nil))

不幸的是，这样定义的 square-list 所产生的答案表的顺序与期望的相反。为什么？

Louis 随后试图通过交换传给 cons 的两个参数来修正这个错误：

(define (square-list items)
(define (iter things answer)
(if (null? things)
answer
(iter (cdr things)
(cons answer
(square
(car things))))))
(iter items nil))

这样同样不行。请解释原因。