Re: [boost] [Preprocessor] Adding variadic macros support

Subject: Re: [boost] [Preprocessor] Adding variadic macros support
From: Jeffrey Lee Hellrung, Jr. (jhellrung_at_[hidden])
Date: 2010-11-29 00:09:34

On 11/27/2010 6:36 AM, Paul Mensonides wrote:
[...]
> ===== Lambdas/Binding =====
>
> Chaos has a pretty heavyweight lambda engine.  For example (contrived),
>
> #include<chaos/preprocessor.h>
>
> CHAOS_PP_EXPR(CHAOS_PP_ENUM(
>      5,
>      CHAOS_PP_PRIMITIVE_CAT_(class T, CHAOS_PP_ARG(1))
>          CHAOS_PP_WHEN_(CHAOS_PP_ARG(1))(
>              CHAOS_PP_CAT_(= T, CHAOS_PP_DEC_(CHAOS_PP_ARG(1)))
>          )
> ))
>
> ...results in:
>
> class T0, class T1 = T0, class T2 = T1, class T3 = T2, class T4 = T3
>
> However, the problem with lambdas (with the preprocessor) is that they
> are considerably more expensive.  No matter how you do it, it requires
> some sort of interpretation--which is slower--and simply having a macro
> is usually clearer.  I've actually been considering their removal from
> Chaos for several years.
>
> #define _(s, n) \
>      class T ## n \
>          CHAOS_PP_WHEN(n)(= CHAOS_PP_CAT(T, CHAOS_PP_DEC(n))) \
>      /**/
>
> CHAOS_PP_EXPR(CHAOS_PP_ENUM(5, _))
>
> #undef _

I believe the now-impossible-to-find "Avalanche Preprocessor library" 
that that a user posted a link to some time back used a somewhat 
different syntax for lambdas.  Recalling strictly from memory, it would 
look something like

CHAOS_PP_EXPR( CHAOS_PP_ENUM(
     5,
     CHAOS_PP_BIND( CHAOS_PP_PRIMITIVE_CAT_, ( class T ), 1 )
     CHAOS_PP_BIND( CHAOS_PP_WHEN_, 1, CHAOS_PP_BIND( CHAOS_PP_CAT_, ( = 
T ), CHAOS_PP_BIND( CHAOS_PP_DEC_, 1 ) )
)

In other words, there was some very heavily-used CHAOS_PP_BIND-like 
macro that essentially initiated a lambda sequence, which accepted the 
macro to bind followed by the arguments to bind.  Arguments appearing as 
numerals (1, 2, etc.) were placeholders, while arguments appearing in 
parantheses were literals.  I don't remember much of the syntax, 
semantics, or mechanics past that.  What I wrote above is probably not 
considered preferable to using the existing Chaos lambda syntax, but 
maybe some combination of these ideas could be massaged into a better 
syntax with faster compile times, given willing investigative souls.

- Jeff