[Boost-users] [proto] need explanation in implementing distributive law

22 Mar 2010

      Hi all,

i would like to implement a grammar that transform a*(b+c) to a*b+a*c 
where a,b and c are terminals. The implementation of this grammar is 
straightforward and should compile. However i got errors that i dont 
understand.
Below is the code that reproduces the errors.

Thanks for any help,
Kim

# include <boost/cstdlib.hpp>
# include <boost/proto/proto.hpp>
# include <boost/fusion/container/list/cons.hpp>

# include <boost/proto/tags.hpp>
# include <boost/proto/make_expr.hpp>

# define PRECISION_MAKE_tag(name)                                        
        \
struct name                                                            
                                   \
{                                                                        
                                       \
    friend std::ostream& operator<< (std::ostream& s, name const&)    
       \
    {                                                                    
                                       \
    return s<< #name;                                                    
                             \
    }                                                                    
                                       \
};

# define PRECISION_MAKE_terminal(name)                                
            \
proto::terminal< tag::name >::type const name = {{}}

namespace proto = boost::proto;

namespace tag{   
    PRECISION_MAKE_tag(a);
    PRECISION_MAKE_tag(b);
    PRECISION_MAKE_tag(c);
}

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

struct DistributiveLaw
    : proto::or_<
        proto::when<
              proto::multiplies< proto::terminal< _ > , proto::plus< _, 
_ > >
            , proto::plus<

proto::multiplies<proto::_left,proto::_left(proto::_right)>
                , 
proto::multiplies<proto::_left,proto::_right(proto::_right)>
                //, proto::_right(proto::_right)
              >(

proto::multiplies<proto::_left,proto::_left(proto::_right)>
                  (proto::_left,proto::_left(proto::_right))
                , 
proto::multiplies<proto::_left,proto::_right(proto::_right)>
                  (proto::_left,proto::_right(proto::_right))
                //, proto::_right(proto::_right)
              )
        >
        , proto::otherwise<proto::nary_expr<_, 
proto::vararg<DistributiveLaw> > >       
    >
{};

PRECISION_MAKE_terminal(a);
PRECISION_MAKE_terminal(b);
PRECISION_MAKE_terminal(c);

# undef PRECISION_MAKE_tag
# undef PRECISION_MAKE_terminal

int main()
{
    proto::display_expr(a*(b+c));

    proto::display_expr(DistributiveLaw()( a*(b+c)  ));
    return boost::exit_success;
}

[Boost-users] [proto] need explanation in implementing distributive law

Kim Kuen Tang