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 10:59:42

joel wrote:
> Eric Niebler wrote:
>> 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,
>
> Aaaah bp::lazy ! I was indeed trying to find such thing like that.Maybe 
> I can combine this with some fusion::fused or unfused somehow.
> I'll give this a try.

Also look into proto::functional::unpack_expr, which turns a Fusion 
sequence into a proto expression node. For instance, you might turn an 
N-ary expression into an (N+1)-ary expression representing a function 
invocation like (untested) ...

    when< nary_expr< _, vararg<eval_expr> >
        , function< vararg<eval_expr> >(                        // #1
            functional::unpack_expr<tag::function>(             // #2
              push_front(_, terminal<functor<tag_of<_> > >())   // #3
            )
          )
        >

The idea here is to create an expression that, when evaluated by 
proto::function's pass-through transform (#1), does the Right Thing. You 
can accomplish that by first building a new expression from the old that 
(a) has an extra argument in the 0th position that is a terminal 
containing your function object, and (b) has a tag type of 
tag::function. First you build a new sequence with push_front (#3), and 
then you unpack that Fusion sequence into a new expression (#2).

push_front doesn't exist yet; you'll have to write it. Just create a 
callable function object that does what fusion::push_front does.

HTH,

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