Re: [Boost-users] [Proto] Forwarding evaluation via tag

Subject: Re: [Boost-users] [Proto] Forwarding evaluation via tag
From: Eric Niebler (eric_at_[hidden])
Date: 2009-09-29 10:08:01

Joel Falcou wrote:
> I have the following grammar that handles the evaluation of various 
> matrix classes:
> 
>   struct eval_xpr
>         : bp::or_< bp::when< bp::terminal< block<bp::_,bp::_> >
>                            , _load(bp::_)
>                            >
>                  , bp::when< bp::plus< eval_xpr,eval_xpr >
>                            , bp::call< 
> functor<bp::tag::plus>(eval_xpr(bp::_left ), eval_xpr(bp::_right) )
>                                      >
>                            >
>                  > {};
> 
> As you can see, the functor<T> class is a Callable object but not a 
> primitive Transform so I use call to be able to forward the proper 
> things to it. Currently I only play with + but I wanted to know how i 
> can use proto::tag_of and a clever way to conserve the recursive look so 
> it works for any operator of any tag. Basically what should I do to have :
> 
> bp::when< bp::nary_expr< bp::_, bp::vararg< eval_xpr > >
>         , bp::call< functor< ???? >( eval_xpr(????), ???, ??? ) >
>         >
> 
> to work for any tag and any number of parameters while typiong the less 
> code possible. I *think* I should make a new Primitive Transform that 
> extarct tags and arguments from bp::_expr and then perform the call to 
> functor but doesn't this mean I have to explicitly write the code for 
> 1,2,3 etc arguments ?

I don't have a good solution at my fingertips or the time to come up 
with something really whiz-bang, but you can get part of the way with 
the proto::lazy transform. For instance, you can handle all binary 
expressions with:

   bp::when< bp::nary_expr< bp::_, eval_xpr, eval_xpr >
           , bp::lazy< functor< bp::tag_of< bp::_ > >(
                 eval_xpr(bp::_left)
               , eval_xpr(bp::_right)
             )>
           >

This uses proto::make to turn functor< bp::tag_of< bp::_ > > into, e.g., 
function< proto::tag::plus > and then uses proto::call to invoke it.

Hope that moves you in the right direction,

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