Exercise 3.14. The following procedure is quite useful, although obscure:
(define (mystery x)
(define (loop x y)
(if (null? x)
y
(let ((temp (cdr x)))
(set-cdr! x y)
(loop temp x))))
(loop x '()))
Loop uses the ‘‘temporary’’ variable temp to hold the old value of the cdr of x, since the set-cdr! on the next line destroys the cdr. Explain what mystery does in general. Suppose v is defined by (define v (list ‘a ‘b ‘c ‘d)). Draw the box-and-pointer diagram that represents the list to which v is bound. Suppose that we now evaluate (define w (mystery v)). Draw box-and-pointer diagrams that show the structures v and w after evaluating this expression. What would be printed as the values of v and w ?
因为 loop 使用迭代的方式完成的这个操作，我们可以把参数写出来：
(loop '(a b c d) '())
(loop '(b c d) '(a))
(loop '(c d) '(b a))
(loop '(d) '(c b a))
(loop '() '(d c b a))可以看出，最后得到了逆序。因为 loop 中使用 set-cdr! 来改变链表，这使得传递进来的参数也发生了改变。
刚开始时，v 是
v +---+---+ +---+---+ +---+---+ +---+---+
----->| | +----->| | +----->| | +----->| | / |
+-+-+---+ +-+-+---+ +-+-+---+ +-+-+---+
| | | |
| | | |
| | | |
v v v v
+---+ +---+ +---+ +---+
| a | | b | | c | | d |
+---+ +---+ +---+ +---+经过 mystery 过程之后，变为
w v
+---+---+ +---+---+ +---+---+ +---+---+
----->| | +----->| | +----->| | +----->| | / |
+-+-+---+ +-+-+---+ +-+-+---+ +-+-+---+
| | | |
| | | |
| | | |
v v v v
+---+ +---+ +---+ +---+
| d | | c | | b | | a |
+---+ +---+ +---+ +---+结果即为：w 是 (d c b a)，v 为 (a)