how is the sicp cons-stream implemented?

Question

I'm working through the streams section of the scip and am stuck on how to define a stream.

The following is my code:

(define (memo-func function)
  (let ((already-run? false)
        (result false))
    (lambda ()
      (if (not already-run?)
          (begin (set! result (function))
                 (set! already-run? true)
                 result)
          result))))


(define (delay exp)
  (memo-func (lambda () exp)))

(define (force function)
  (function))

(define the-empty-stream '())
(define (stream-null? stream) (null? stream))
(define (stream-car stream) (car stream))
(define (stream-cdr stream) (force (cdr stream)))

(define (cons-stream a b) (cons a (memo-func (lambda () b))))

If I define integers the way that the book descibes:

(define (integers-starting-from n)
   (cons-stream n (integers-starting-from (+ n 1))))
(define integers (integers-starting-from 1))

I get a message saying: Aborting!: maximum recursion depth exceeded.

I'm guessing that the delay function is not working but I don't know how to fix it. I am running the MIT scheme on my Mac.

update 1

So now with cons-stream as a macro, the integers can be defined.

But then I've got another error.

(define (stream-take n s)
  (cond ((or (stream-null? s)
             (= n 0)) the-empty-stream)
        (else (cons-stream (stream-car s)
                           (stream-take (- n 1) (stream-cdr s))))))

(stream-take 10 integers)
;ERROR - Variable reference to a syntactic keyword: cons-stream

update 2

Please ignore update 1 above

Answer 1

cons-stream needs to be a macro in order for your sample code to work correctly. Otherwise the invocation of cons-stream will evaluate all its arguments eagerly.

Try this (not tested):

(define-syntax cons-stream
  (syntax-rules ()
    ((cons-stream a b)
     (cons a (memo-func (lambda () b))))))

P.S. Your delay needs to be a macro also, for similar reasons. Then after you fix delay, you can make your cons-stream use delay directly.

Answer 2

You cannot define delay as a function, since prior to calling it, Scheme will evaluate its argument - which is exactly what you're trying to postpone. SICP says this explicitly that delay should be a special form.