Re: [boost] [Proto] Access to member in proto expression tree

From: David A. Greene (greened_at_[hidden])
Date: 2007-07-07 21:01:10

On Sunday 01 July 2007 17:24, Eric Niebler wrote:

> >   template<typename Expr ...>
> >   struct phoenix_expr
> >
> >     : proto::extends<Expr ...>
> >
> >   {
> >      phoenix_expr(...)
> >
> >       : else_(proto::make_expr<phoenix::tag::else_>(*this))
> >
> >      {}
> >
> >      // ...
> >
> >      proto::result_of::make_expr<
> >         pheonix::tag::else_ // else is now an "operator"
> >       , phoenix_expr const
> >
> >      >::type const else_;
> >
> >   };
> >
> > how do I match the new "else_ operator" and transform it?
>
> Well, if you use the BOOST_PROTO_DEFINE_FUNCTION_TEMPLATE macro like I
> show above, the IfElseGrammar might look like this:
>
>    // matches if_(e1)[e2].else_[e3]
>    struct IfElseGrammar
>
>      : subscript<
>
>            unary_expr<
>                phoenix::tag::else_
>              , subscript<
>                    unary_expr<
>                        phoenix::tag::if_
>                      , PhoenixBooleanGrammar
>
>                  , PhoenixGrammar
>
>
>          , PhoenixGrammar
>
>    {};

All right, that makes good sense.  Now the next obvious thing to do is try
do_[e1].while_(e2).  So here's my attempt:

      // A do_ "operator"
      boost::proto::terminal<tag::Do>::type const do_ = {{}};

      // matches do[e1]
      struct DoGrammar
            : boost::proto::subscript<
                 tag::Do,
                 StatementGrammar
              > {};

      // A while_ "operator"
      template<typename Expr>
      struct Expression<
         Expr
         , typename enable_if<proto::matches<Expr, DoGrammar> >::type
      >
      : proto::extends<Expr, Expression<Expr>, Domain>
      {
         Expression()
               : while_(proto::make_expr<tag::While>(*this)) {};

         // Oops, don't know how to do this here
         BOOST_PROTO_DEFINE_FUNCTION_TEMPLATE(
            1
            , while_
            , Domain
            , (tag::While)
            , BOOST_PP_SEQ_NIL
            );
      };

      // matches do[e1].while_(e2)
      struct DoWhileGrammar
            : boost::proto::unary_expr<
                 DoGrammar,
                 tag::While
              > {};

Other than the fact that I don't know how to declare the while_ member
above, does this look correct?

Of course, I want to do while_(e1)[e2] as well, so there already is

      BOOST_PROTO_DEFINE_FUNCTION_TEMPLATE(
         1
         , while_
         , Domain
         , (tag::While)
         , BOOST_PP_SEQ_NIL
         );

at namespace scope.  Is there some way to reuse this in the extension of
expressions that match DoGrammar?

                                         -Dave