[plt-scheme] Re: delay & stream

>(define (cons-stream a b)
>  (cons a (delay b)))

You can't define cons-stream like this. Scheme is call-by-value, which
means it will evaluate b before passing it to cons-stream.

>(define (delay object) (memo-proc (lambda() object)))

The same for delay.

So please try:

(define-syntax cons-stream
  (syntax-rules ()
    ((cons-stream a b)
     (cons a (delay b)))))

(define-syntax delay
  (syntax-rules ()
    ((delay object)
     (memo-proc (lambda() object)))))


BTW: Your orginal email used a meanless/confusing subject.

Sincerely,
Chongkai Zhu  

======= At 2006-03-15, 08:55:19 geb a wrote: =======

>Hello,
>
>The following code is taken from SICP's section on
>streams verbatim.  While I realize there is an
>infinite recursion generating a stream of numbers,
>apparently the authors felt the code would work but
>drscheme seems to evaluate each infinite stream first.
> Does this have something to do with lazy evaluation
>(a term I know nothing about?).  Is there some
>underlying reason for this that I don't know about or
>have I made some mistake with the code.
>
>Thanks in advance for your time...
>
>Dan Anderson
>
>(define (stream-car stream) (car stream))
>
>(define (stream-cdr stream) (force (cdr stream)))
>
>(define stream-null? null?)
>
>(define the-empty-stream '())
>
>(define (memo-proc proc)
>  (let ((already-run? false) (result false))
>    (lambda ()
>      (if (not already-run?)
>          (begin (set! result (proc))
>                 (set! already-run? true)
>                 result)
>          result))))
>
>(define (delay object) (memo-proc (lambda() object)))
>
>(define (cons-stream a b)
>  (cons a (delay b)))
>
>
>(define (force delayed-object)
>  (delayed-object))
>
>
>(define (stream-ref s n)
>  (if (= n 0)
>      (stream-car s)
>      (stream-ref (stream-cdr s) (- n 1))))
>
>(define (stream-map proc s)
>  (if (stream-null? s)
>      the-empty-stream
>      (cons-stream (proc (stream-car s))
>                   (stream-map proc (stream-cdr s)))))
>
>(define (stream-for-each proc s)
>  (if (stream-null? s)
>      'done
>      (begin (proc (stream-car s))
>             (stream-for-each proc (stream-cdr s)))))
>
>(define (display-stream s)
>  (stream-for-each display-line s))
>
>(define (display-line x)
>  (newline)
>  (display x))
>
>(define (stream-enumerate-interval low high)
>  (if (> low high)
>      the-empty-stream
>      (cons-stream
>       low
>       (stream-enumerate-interval (+ low 1) high))))
>
>
>
>(define (stream-filter pred stream)
>  (cond ((stream-null? stream) the-empty-stream)
>        ((pred (stream-car stream))
>         (cons-stream (stream-car stream)
>                      (stream-filter pred
>                                     (stream-cdr
>stream))))
>        (else (stream-filter pred (stream-cdr
>stream)))))
>
>
>(define (integers-starting-from n)
>  (cons-stream n (integers-starting-from (+ n 1))))
>;
>(define integers (integers-starting-from 1))
>;
>(define (divisible? x y) (= (remainder x y) 0))
>;
>(define no-sevens
>  (stream-filter (lambda (x) (not (divisible? x 7)))
>                 integers))
>(define (sieve stream)
>  (cons-stream
>   (stream-car stream)
>   (sieve (stream-filter
>           (lambda (x)
>             (not (divisible? x (stream-car stream))))
>           (stream-cdr stream)))))
>(stream-ref no-sevens 100)
>
>
>
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