Re: [boost] [preprocessor] Variadics suggestion

Subject: Re: [boost] [preprocessor] Variadics suggestion
From: Paul Mensonides (pmenso57_at_[hidden])
Date: 2012-09-25 03:57:21

On Mon, 24 Sep 2012 15:09:03 -0700, paul Fultz wrote:

>> #define SEQÂ  Â  (a,b,c)(1,2,3,4)
>> BOOST_PP_VARIADIC_SEQ_TO_SEQ( SEQ )Â  Â  // expands to
>> ((a,b,c))((1,2,3,4))
> 
> In your implementations couldn't you use just two overload macros
> instead of 256? Like this:
> 
> Â Â Â  #Â Â Â  define BOOST_PP_VARIADIC_SEQ_TO_SEQ(seq) BOOST_PP_CAT(
> Â Â Â  BOOST_PP_VARIADIC_SEQ_TO_SEQ_1_ seq, BOOST_PP_NIL )() #
> Â Â Â  #Â Â Â  define BOOST_PP_VARIADIC_SEQ_TO_SEQ_1_(...) (( __VA_ARGS__
> Â Â Â  )) BOOST_PP_VARIADIC_SEQ_TO_SEQ_2_ #Â Â Â  define
> Â Â Â  BOOST_PP_VARIADIC_SEQ_TO_SEQ_2_(...) (( __VA_ARGS__ ))
> Â Â Â  BOOST_PP_VARIADIC_SEQ_TO_SEQ_1_
> 
> Â Â Â  #Â Â Â  define BOOST_PP_VARIADIC_SEQ_TO_SEQ_1_BOOST_PP_NIL() #Â Â Â 
> Â Â Â  define BOOST_PP_VARIADIC_SEQ_TO_SEQ_2_BOOST_PP_NIL()
> 
> Also on a side note, when vardiacs are enabled it would be nice to add
> an `IS_PAREN` macro like this:

Following the other similar macros that are already there, IS_NULLARY, 
IS_UNARY, and IS_BINARY, this would be named IS_N_ARY or IS_VARIADIC.

However, what many people fail to realize is just how enormously difficult 
(and in some cases impossible) it is to modernize Boost.Preprocessor when 
support for VC++ (in particular) is required.  It is usually easy to get a 
trivial case working.  It is amazingly difficult to get non-trivial cases 
working because the nature of VC++'s macro replacement algorithm leads to 
combinatorial difficulties.  E.g. It isn't that difficult to write macros 
A and B, test them individually to confirm that they work, but then some 
other macro C (possibly even written by a user) uses both and everything 
breaks.  The point is that there is no black box testing.  It is a 
*horrible* preprocessor.  Edward has had some exposure to this when we 
worked on the limited variadic support that we did add.

Besides the above, adding significant work to Boost.Preprocessor is 
iterating a dead horse.  It is *way* outdated in terms of modern 
preprocessor metaprogramming technique.  It needs a complete rewrite (i.e. 
it needs to be Chaos or similar to it) to support variadics properly.  To 
modernize it, one has to abandon VC++'s preprocessor or somehow get MS to 
fix it (which is also beating a dead horse).  There have been claims (on 
this list) that you can modernize it with current VC++, but, sorry, you 
can't.  Not without throwing out a predictable, self-terminating recursion 
model.  You could maybe get about a third of the way from 
Boost.Preprocessor to Chaos.

Regards,
Paul Mensonides