Re: Strange behavior with delayed objects

[Linas Vepstas]

BTW, this is guile version 1.8.7, for me.

--linas

On 10 May 2010 11:26, Linas Vepstas <address@hidden> wrote:
> On 7 May 2010 03:09, user8472 <address@hidden> wrote:
>>
>> Please find the code for streams and the integration below.
>> ;; The troublesome procedure
>> (define (solve f y0 dt)
>>  (define y (integral (delay dy) y0 dt))
>>  (define dy (stream-map f y))
>>  y)
>>
>> ;; This works
>> (define y (integral (delay dy) 1 0.001))
>> (define dy (stream-map (lambda (x) x) y))
>> (stream-ref y 1000)
>>
>> ;; This doesn't work
>> ;(stream-ref (solve (lambda (x) x) 1 0.001) 1000)
>> </code>
>
> Well, you modified your code enough so that let* now works fine,
> at least for me:
>
> guile> (define (solve f y0 dt)
> ... (let* ((y (integral (delay dy) y0 dt))
> ... (dy (stream-map f y))
> ... )
> ... y))
> guile> (stream-ref (solve (lambda (x) x) 1 0.001) 1000)
> 2.7169239322359
>
> Superficially, I want to say "this is why let* was invented, to
> ensure ordering of definitions".  In practice, I see that dy is
> used in both, so there's a circular reference going on here.
> The thing that saves you, and allows the  let* to work, is the
> (delay dy) which avoids evaluating dy (which is undefined,
> at the time its encountered).  Without the delay, the circular
> references would cause the whole thing would fall apart,
> and not even letrec would save you.
>
> As proof of this, here's another example, without let*, which
> does enforce ordering (and which depends on the (delay dy)
> to avoid the "use of variable dy before its defined" error):
>
> guile> (define (solve f y0 dt)
> ... (define y (integral (delay dy) y0 dt))
> ... (let ((dy (stream-map f y)))
> ... y))
> guile> (stream-ref (solve (lambda (x) x) 1 0.001) 1000)
> 2.7169239322359
> guile>
>
>
> I'm an amateur at scheme, but I beleive what I say is more
> or less correct ...
>
> --linas
>