Re: [Boost-users] [Proto]: How to use complex grammars in a domain/extension

Subject: Re: [Boost-users] [Proto]: How to use complex grammars in a domain/extension
From: Roland Bock (rbock_at_[hidden])
Date: 2010-09-21 09:19:58

On 09/21/2010 11:55 AM, Thomas Heller wrote:
> <snip>
> Solved the mistery. here is the code, explanation comes afterward:
>
> <snip>
> So, everything works as expected!
>    
Thomas,

wow, this was driving me crazy, thanks for solving and explaining in all 
the detail! :-)

I am still not sure if this isn't a conceptual problem, though:

The "equation" grammar is perfectly OK. But without or-ing it with the 
"addition" grammar, the extension does not allow ANY valid expression to 
be created. I wonder if that is a bug or a feature?

Thanks and regards,

Roland

> #include<boost/proto/proto.hpp>
>
> using namespace boost;
>
> typedef proto::terminal<int>::type terminal;
>
> struct addition:
>      proto::or_
>      <
>         proto::plus<addition, addition>,
>         proto::terminal<proto::_>
>      >
> {};
>
> struct equation:
>      proto::or_
>      <
>          proto::equal_to<addition, addition>
>      >
> {};
>
> template<class Expr>
> struct extension;
>
> struct my_domain:
>      proto::domain
>      <
>           proto::pod_generator<   extension>,
>           // we need both grammars in our domain grammar
>           proto::or_<equation, addition>,
>           proto::default_domain
>      >
> {};
>
>
> template<class Expr>
> struct extension
> {
>       BOOST_PROTO_EXTENDS(
>           Expr
>         , extension<Expr>
>         , my_domain
>       )
> };
>
> template<typename Grammar, typename Expr>
> void matches(Expr const&)
> {
>       std::cout<<  std::boolalpha
>           <<  proto::matches<Expr, Grammar>::value<<  "\n";
> }
>
> int main()
> {
>       extension<terminal>   i;
>       extension<terminal>   j;
>
>
>       matches<equation>(i);              // 1) false
>       matches<equation>(i == j);         // 2) true
>       matches<equation>(i == i + i);     // 3) true
>       matches<equation>(i + i == i);     // 4) true
>       matches<equation>(i + i == i + i); // 5) true
>       matches<equation>(i + i);          // 6) false
> }
>
>