[plt-scheme] stream-delay question

In srfi-40, stream-delay is just the compose of delay and force.

To understand why stream-delay is needed, "from0" is not a good example.

Let's consider another example. Suppose you stream should be:

(if (some-complex-condition)
     (stream-cons x y)
     (stream-cons u v))

(where the the value of x y u v might be bound when doing 
"some-complex-condition")

Then the difference shows up. If you say

(define my-stream
   (stream-delay
    (if (some-complex-condition)
        (stream-cons x y)
        (stream-cons u v))))

everything is fine. But if you omit the stream-delay, the 
"some-complex-condition" will be evaluated immediately, which is not 
what we went when define a stream.

Chongkai



Robert Nikander wrote:
> Hi,
> 
> I'm trying to learn srfi-40 (streams), and I don't understand 
> stream-delay.  The srfi doc uses it in an example stream `from0'.  Can 
> someone give me an example where from0 and from0/no-delay behave 
> differently?
> 
> 
> (require (lib "40.ss" "srfi"))
> 
> (define from0
>   (let loop ((x 0))
>     (stream-delay
>      (stream-cons x (loop (+ x 1))))))
> 
> (define from0/no-delay
>   (let loop ((x 0))
>     (stream-cons x (loop (+ x 1)))))
> 
> 
> thanks,
> 
> Rob
> 
> 
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