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.
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