Re: [Boost-users] [Proto] Using mpl::count as proto::transform

4 Jan 2009


      Joel Falcou wrote:
...
I have a grammar with 3 kind of terminal : A,B and C.
I want to have a transform that returns a mpl::int_ which is equal to 
the number of B terminal that appear
in expression like :
A(B,B,C,B,C ..)
the grammar part for matching this is currently :
when< function< vargarg >
         , mpl::count< _, terminal<B> >()
         >
First, you want to use mpl::count_if with proto::matches so that you get 
the fuzzy terminal matching. Second, it seems that although proto 
expressions are valid Fusion sequences, they are not valid mpl 
sequences. That's a bug and I'll fix it. The following works for me:


     #include <iostream>
     #include 
     #include 
     #include 

     namespace boost { namespace mpl
     {
         template
         struct sequence_tag< proto::expr >
         {
             typedef fusion::fusion_sequence_tag type;
         };
     }}

     namespace mpl = boost::mpl;
     namespace proto = boost::proto;
     namespace fusion = boost::fusion;
     using proto::_;

     struct A {};
     struct B {};
     struct C {};

     struct CountB
       : proto::when<
             proto::function< proto::vararg<_> >
           , mpl::count_if< _, proto::matches
...
...
()
         >
     {};
proto::terminal<A>::type a = {{}};
     proto::terminal<B>::type b = {{}};
     proto::terminal<C>::type c = {{}};

     int main()
     {
         std::cout << CountB()( a(b,c,b,a) ) << std::endl;
     }

HTH,

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com