Chapter 3, Modularity, Objects, and State

Exercise 3.68

Let me first write the Louis code:

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

Note that pairs procedure requires invocation of interleave. To invoke interleave, first it is required to evaluate the arguments passed to it.

Thus (pairs (stream-cdr s) (stream-cdr t)) is called. Now again, inside the pairs procedure to invoke interleave, procedure pairs is called again and which will again call interleave and so on. Thus it will go into infinite loop without ever entering into interleave procedure!

In the book’s version this does not happen because the calls to interleave is inside the cdr part of the cons-stream. Thus this call will happen only when needed.

Well, I think this infinite loop happens only because of applicative order evaluation(see chapter 1, check ex-1.5). If the arguments were first expanded then reduced or normal order evaluation then it would have worked fine!