Thanks, Tohmas and Alex!
I think I understand that simple code-walking does not help,
in many cases, for example, a loop.
(lazy-let ((a (get-a)))
(letrec ([b (lambda () (when (condition) (print a) (b)))])
(b)))
OK, I would not need a general solution,
let me think about it for a while.
Thanks for comments, and any more comments are welcome.
Daishi
At Thu, 17 Nov 2005 19:19:46 -0600,
Alex Shinn wrote:
>
> At Fri, 18 Nov 2005 08:18:11 +0900, Daishi Kato wrote:
> >
> > Would it be worth improving the lazy-let macro
> > so that it understands at least let and quote forms?
>
> Not let forms. Analyzing subforms of a macro is called code-walking,
> and the first thing you then need to do is sc-expand the body or
> lazy-let won't play well with other macros. At that point there are
> no let's remaining, and everything is in terms of lambda.
>
> But I think the problem you're trying to solve is in general
> unsolvable. Consider the form:
>
> (lazy-let ((a (get-a)))
> (list (if (condition-1) a #f)
> (if (condition-2) a #f)))
>
> If you expand this to
>
> (list (if (condition-1) (get-a) #f)
> (if (condition-2) (get-a) #f))
>
> then you may end up calling get-a twice. However, if you compute the
> value once outside the nearest enclosing form for all occurances:
>
> (let ((a (get-a)))
> (list (if (condition-1) a #f)
> (if (condition-2) a #f)))
>
> then you always compute get-a even when it may not be needed.
>
> --
> Alex
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