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

From: David A. Greene (greened_at_[hidden])
Date: 2007-07-01 16:25:25

On Sunday 01 July 2007 10:13, Eric Niebler wrote:

> Oh wait, that's not quite what you want, because then if_[e1].else_[e2]
> will chop off the if_ part of the tree. What you need instead is a
> member like (untested):
>
> 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_;
>
> };
>
> Then, if_/else_ statements would conform to the following grammar:
>
> subscript<
>    unary_expr<
>      phoenix::tag::else_
>     ,subscript<terminal<phoenix::tag::if_>,_>
>
>   ,_

Ok, that makes sense to me, modulo the current lack of make_expr
documentation.  :)

I'm looking to do something similar to what Dan is doing so I'm glad he
asked the question.  I was just about to.

One thing that's not clear to me is how he would handle the
if_(expr)[...] syntax.  In your first message you have:

  // Here is the grammar for if_ expressions
  struct IfGrammar
    : proto::subscript<
        proto::terminal<phoenix::tag::if_>
        proto::_
      >
  {};

I don't understand the derivation from proto::subscript.  What about the
call operator?

From your second message:

  Then, if_/else_ statements would conform to the following grammar:

  subscript<
     unary_expr<
       phoenix::tag::else_
      ,subscript<terminal<phoenix::tag::if_>,_>
     >
    ,_
   >

Again what about the call operator?

Dan has two uses of operator() here.  The first is a proto expression that
needs to be part of the grammar to match the if expression.  The other is
the operator() provided through proto::extends to do the runtime evaluation.

I'm going to be doing something different.  Rather than evaluate the 
expression, I'm going to want to run a transformation on it.  Given the
example from your second message:

  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?

Thanks.

                                            -Dave