On Fri, Jan 4, 2013 at 2:17 PM, Aaron W. Hsu <[email protected]> wrote:
> On Fri, 2013-01-04 at 13:41 +0900, Alex Shinn wrote:
>
> > Although Aaron is correct that in the general case we can't single out
> > the specific macros used, it is not impossible in all cases, and would
> > therefore be wrong for the standard to forbid such optimizations.
>
> [...]
>
> > Assuming we don't want to add a long discussion of the definition of
> > visitation for something that is fairly simple in the small language,
> > I would suggest:
> >
> > Similarly, during the expansion of a library {\cf foo}, if any
> > syntax keywords imported from another library {\cf (bar)} are
> > needed to expand the library, then the corresponding syntax
> > definitions of {\cf (bar)} must be expanded before the expansion
> > of {\cf (foo)}.
>
> The problem with this definition and the one that John gave previously
> is that you are not just expanding those definitions. The right hand
> sides of syntax definitions must be expanded *and* evaluated, though we
> only allow for syntax-rules at the moment.
>
Well, generally expansion of a top-level define-syntax is equivalent to
its evaluation. I'm fine with explicitly adding that they are evaluated,
as you suggest in your change below. I dislike using the term "loaded"
here.
> Also, while you are correct that there are some cases where it is not
> necessary to expand the whole library in order to obtain the definition
> of one keyword, I believe it is incorrect to say that this is an
> optimization that can be implemented in practice. Namely, I submit that
> the check to determine whether this optimization is safe is precisely
> the process of expansion.
I disagree. There's the trivially simple optimization of lazy
expansion - not expanding lambda bodies until they are
applied.
> To illustrate the point, consider two possibilities, one where there is
> a syntax error that is generated, either intentionally or
> unintentionally by expanding the library.
Then "it an error" and implementations are free to perform whatever
optimizations they want.
> So, I would propose the following language:
>
> Similarly, during the expansion of a library {\cf foo}, if
> any syntax keywords imported from another library {\cf (bar)}
> are needed to expand the library, then the library {\cf (bar)}
> must be expanded and its syntax definitions evaluated before the
> expansion of {\cf (foo)}.
>
> I have thought of all of this rather quickly, and it is rather late
> here, so I hope someone will try to figure out where I have gone wrong
> here.
>
This looks fine to me.
--
Alex
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