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)
          


        

Answer 1

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.

Answer 2

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.