Proto for Boost mailing list page: [proto] Inter-domain interaction - Question and bug ?

Subject: [proto] Inter-domain interaction - Question and bug ?
From: Joel Falcou (joel.falcou_at_[hidden])
Date: 2011-02-23 15:54:20

Hi,

while adding function to my SIMD library built on proto. I stumbled into 
this problem:

http://codepad.org/dd5WB8Xu

I have a template grammar/generator/domain that helps me not mixing 
vector of different type/cardinal.
However some functions are designed to take vector of float and turn 
them into vector of int.
I wrote one there (exponenbits) by just forcing the domain to be the one 
from int. When I call it alone, it works.
But if I do exponentbits( some_float ) + some_int doesnt work.

The error is just :

/Users/joelfalcou/nt2/sandbox/proto.cpp:161:46: error: no match for 
'operator+' in 'nt2::simd::exponentbits(((const nt2::simd::pack<float, 
4ul>&)((const nt2::simd::pack<float, 4ul>*)(& x)))) + y'

With none of the usual 'no proto_grammar in not_a_domain' error I got 
when I mix element of different grammar with each other.

Now, if i turn the grammar into:

   
////////////////////////////////////////////////////////////////////////////////
   // the template grammar
   
////////////////////////////////////////////////////////////////////////////////
   template<class T, class C>
   struct grammar
     : boost::proto
         ::or_ < boost::proto::terminal< data<T,C> >
               , boost::proto::unary_expr<exponentbits_, 
grammar<boost::proto::_,C> >
               , boost::proto::
                 and_< boost::proto::
                       nary_expr < boost::proto::_
                                 , boost::proto::vararg< grammar<T,C> >
 >
                     , boost::proto::
                       not_< boost::proto::or_ < boost::proto::
                                                 address_of< grammar<T,C> >
                                               , boost::proto::
                                                 dereference< grammar<T,C> >
                                               , boost::proto::
                                                 comma < grammar<T,C>
                                                       , grammar<T,C>
 >
 >
 >
 >
 >
     {};

It works. Alas, I have a potentially high and extensible set of such 
function (some being binary or ternary or w/e) and so I want
some way to accept such constructs without an explicit list of supported 
function. nt2 currently has something like 17 such cross domain function.

My question is why building such an expression:
  - make an error. Once I wrapped a terminal of domain Foo in domain 
Bar, it should be composable with other element of Bar domain
  - if this is an error, why dont I get the usual error of mixing wrong 
element with each other ?

Mixing element from unrelated domain through such domain wrapping is a 
very valauble techniaues in EDSL in my opinion as it allows efficient 
operations to be done.
nt2 has a vector and covector class in different domain and transposing 
a vector yields a covector, and vice versa. Being able to just have 
somehting that wraps the vector terminal and change its semantic this 
way is very valuable.

Thansk for any pointers or discussion on this.