Re: new function

[William ML Leslie <william.leslie.ttg@gmail.com>]

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On Thu, 23 Sep 2021, 4:51 am Taylan Kammer, <taylan.kammer@gmail.com> wrote:

> On 22.09.2021 11:53, Damien Mattei wrote:
> > i already do it this way for internal defines ,using a recursive macro
> that build a list of variable using an accumulator. It can works but macro
> expansion seems slow, it was not immediate at compilation on a little
> example (oh nothing more that 2 seconds) but i'm not sure it is easily
> maintainable, it is at the limit what macro can do i think ,for speed
> reasons. In fact i can not really understand in Guile as it is based on C
> and compiled when macro expansion happens,what is the time cost... so for
> all those ,perhaps not objective reason ,i prefer to avoid.
>
> I don't think there's any other way to achieve what you want, especially
> using portable Scheme code.  The lexical scoping semantics of Scheme are
> a very fundamental part of the language, and cannot be worked around in
> portable Scheme code without using a macro that rewrites whole bodies of
> lambda expressions.
>
> Even using implementation-specific hacks, you won't get very far.  Any
> compiled Scheme implementation, and even most interpreted ones, won't
> allow you to modify an outer scope's set of variable definitions from
> within an inner scope.
>
> So if you really want to have Python's scoping semantics in Scheme, you
> will probably have to write a complex 'def' macro that walks through the
> body and "hoists" variable definitions to the outermost scope.
>

Python is lexically scoped, and the assignment here is supposed to be local.


> If you're targeting R6RS implementations, you can use syntax-case to
> write such a macro, but it won't be easy.
>
> If you're targeting R5RS or R7RS-small implementations, you will have to
> rely on syntax-rules, which will probably be extremely difficult for this
> kind of complex macro.
>
> Personally I don't even know how I would approach the problem using the
> more capable syntax-case, let alone pure syntax-rules.
>
> --
> Taylan
>

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