练习3-68

原文

Exercise 3.68. Louis Reasoner thinks that building a stream of pairs from three parts is unnecessarily complicated. Instead of separating the pair (S0,T0) from the rest of the pairs in the first row, he proposes to work with the whole first row, as follows:

(define (pairs s t)  
   (interleave (stream-map (lambda (x) (list (stream-car s) x))               
               t)   
   (pairs (stream-cdr s) (stream-cdr t))))

Does this work? Consider what happens if we evaluate (pairs integers integers) using Louis’s definition of pairs.

分析

不会工作，这个程序将会陷入无限循环。因为没有在

(pairs (stream-cdr s) (stream-cdr t))

中使用delay进行延时求值，而这又会不断的递归，因此将进入无线循环中。