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: Eric Niebler (eric_at_[hidden])
Date: 2010-09-21 09:37:26

On 9/21/2010 3:15 AM, Roland Bock wrote:
> That did not make a difference:
> 
> // -----------------------------------------------------------------
> #include<complex>
> #include<boost/proto/proto.hpp>
> 
> using namespace boost;
> 
> struct addition :
>    proto::or_
>    <
>       proto::terminal<proto::_>
>     , proto::plus<addition,addition>
>    >
> {};
> 
> struct equation:
>    proto::or_
>    <
>       proto::equal_to<addition, addition>
>    >
> {};
> 
> template<class Expr>
> struct extension;
> 
> struct my_domain
>   : proto::domain<
>         proto::pod_generator<  extension>
>       , equation
>     >
> {};
> 
> 
> template<class Expr>
> struct extension
> {
>     BOOST_PROTO_EXTENDS(
>         Expr
>       , extension<Expr>
>       , my_domain
>     )
> };
> 
> int main()
> {
>     extension<proto::terminal<int>::type>  i;

Trouble here. The "equation" grammar only matches things of the form "x
== y". The terminal "i" *by itself* is not of that form. You won't be
able to use this terminal at all.

>     i == i; // ERROR

As expected. You need to change your grammar to allow lone terminals.

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