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

Subject: Re: [Boost-users] [Proto] Forwarding evaluation via tag
From: joel (joel.falcou_at_[hidden])
Date: 2009-09-29 12:47:20

Eric Niebler wrote:
> 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.
OK, I like the idea. I tried to get it working but I stumple against the 
fatc that proto make_expr takes push_front() as a Sequence and don't try 
to evaulate it before hand, leading to a "no size for psuh_front" erro 
in fusion. I'll try to get this sorted though.

My main question is that, when and when not a transform call get 
evaluated ? Like if i do
foo( bar ( _) )

is bar(_) evaluated before being evaluated by foo ?
Sometimes it seems it does and sometimes no :/

-- 
___________________________________________
Joel Falcou - Assistant Professor
PARALL Team - LRI - Universite Paris Sud XI
Tel : [phone]