Proto for Boost mailing list page: Re: [proto] Proto Transform Questions

Subject: Re: [proto] Proto Transform Questions
From: Nate Knight (Nate.Knight_at_[hidden])
Date: 2011-01-14 18:41:44

On Jan 14, 2011, at 12:29 PM, Nate Knight wrote:

> I've pasted some code below where I am trying to transform expressions of the form
> 
> (a op b op c)[i] 
> 
> to 
> 
> (a[i] op b[i] op c[i])
> 
> I managed to get this to work for the simple case
> 
> (a+b)[i]
> 
> but I'm curious about how to generalize this to include other operators (without explicitly handling them all).  Also, as written the transform doesn't recurse properly, and I'm having some trouble seeing how to correct this.

I figured out the recursion problem.  This should have been obvious to me.  Generalizing the transformation to other subexpressions still eludes me due to an issue illustrated below.

struct vec 
    : or_<
          when<
              subscript< binary_expr<_,vec,vec>, terminal<_> >
            , functional::make_expr< ??? >(
                  vec(
                    functional::make_expr<tag::subscript>(
                      _left(_left), _right
                    )   
                  ),  
                  vec(
                    functional::make_expr<tag::subscript>(
                      _right(_left), _right
                    )   
                  )   
              )   
          >   
        , plus< vec, vec >
        , minus< vec, vec >
        , terminal< _ > 
      >   
{};

How do I (or can I at all) get the tag of the matched binary expression to pass back to make_expr?  Perhaps I'm still going about this the wrong way?

Thanks
Nate