Re: [boost] [proto] design of expr<>

From: Eric Niebler (eric_at_[hidden])
Date: 2008-03-31 02:11:30

Larry Evans wrote:
> 
> The file, expk_test.zip here:
> 
> http://www.boost-consulting.com/vault/index.php?&directory=Strings%20-%20Text%20Processing
> 
> Contains a prototype where tags contain the arity.  This prototype
> enables initialization like:
> <===== cut here =======
> 
> using namespace boost::proto;
> 
> typedef 
> tags::tag_kind_arity<tags::tag2<tags::tag2_shift_right>,tags::tag_valu,2> 
> tag_shift_right_2;

What's that?

> expk<tag_shift_right_2,args2<int,int> > shift_right_int_int={1,2};

Why? What is there works very well and nobody is complaining about it.

> I tried to figure out if putting arity in tags would prevent some
> specialization on arity which proto currently uses to do it's work.  I
> guessed that maybe this specialization was used in or_ or and_;

Just about every part of Proto is specialized on expression arity. See 
the file iteration in deep_copy.hpp for an example.

> however, when I looked in matches.hpp, all I found (that was relevant,
> AFAICT) was:
<snip>
> and I couldn't figure out how it works.  I would have guessed there
> would be some specialization on the bool B template parameter, but
> apparently not.
> 
> Eric, could you mention the code where specialization on arity is used
> and how it works.  I might could figure it out eventually, but I'm
> guessing others, in the future, might like to understand this also,
> and this information, documented somewhere in an implementation or
> design guide, would be *real* handy.
> 
> Of course, I realize you're in the middle of analyzing all the proto
> reviews,  but eventually, getting this information about how it works
> would be quite helpful.

I probably won't ever document Proto's internal implementation details, 
sorry. Use the source. Preprocess it first, if it makes it easier.

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com