Eric Niebler wrote:
> On 11/17/2010 2:18 PM, joel falcou wrote:
>> On 17/11/10 19:46, Eric Niebler wrote:
>>> See the attached code. I wish I had a better answer. It sure would be
>>> nice to generalize this for other times when new state needs to bubble
>>> up and back down.
>>
>> Just chiming in. We had the exact same problem in quaff where needed to
>> carry on a process ID over the trasnform of parallel statement. If it can
>> make you worry less Eric, we ended with the exact same workaround.
>
> There's another issue. Look here:
>
> // don't evaluate T at runtime, but default-construct
> // an object of T's result type.
> template<typename T>
> struct type_of
> : proto::make<proto::call<T> >
> {};
>
> struct RenumberFun
> : proto::fold<
> _
> , make_pair(fusion::vector0<>(), proto::_state)
> , make_pair(
> push_back(
> first(proto::_state)
> //----------------------1
> , first(Renumber(_, second(proto::_state)))
> )
> //---------------------------2
> , type_of<second(Renumber(_, second(proto::_state))) >
> )
> >
> {};
>
> Notice that the Renumber algorithm needs to be invoked twice with the
> same arguments. In this case, we can avoid the runtime overhead of the
> second invocation by just using the type information, but that's not
> always going to be the case. There doesn't seem to be a way around it,
> either.
>
> I think Proto transforms need a "let" statement for storing intermediate
> results. Maybe something like this:
>
> struct RenumberFun
> : proto::fold<
> _
> , make_pair(fusion::vector0<>(), proto::_state)
> , let<
> _a( Renumber(_, second(proto::_state))> )
> , make_pair(
> push_back(
> first(proto::_state)
> , first(_a)
> )
> , type_of<second(_a) >
> )
> >
> >
> {};
>
> I haven't a clue how this would be implemented.
>
> It's fun to think about this stuff, but I wish it actually payed the
> bills.
>
WOW! A let statement this would indeed make proto even more awesome!
Let's see what we can do ;)
<hijacking the thread>
Btw, i just finished implementing the unpack feature we were talking about
...
Short description:
Calling some_callable with the expression unpacked:
proto::when<proto::_, some_callable(unpack)>
Calling some_callable with an arbitrary sequence unpacked:
proto::when<proto::_, some_callable(unpack(some_fusion_seq())>
Calling some_callable with an arbitrary sequence unpacked, and apply a proto
transform before:
proto::when<proto::_, some_callable(unpack(some_fusion_seq(),
some_transform)>
Additionally it is possible to have arbitrary parameters before or after
unpack. The implementation is located at:
http://svn.boost.org/svn/boost/sandbox/SOC/2010/phoenix3/boost/phoenix/core/unpack.hpp
Just a whole mess of preprocessor generation ... this is not really fast to
compile at the moment, PROTO_MAX_ARITY of 5 is fine, everything above will
just blow the compiler :(
_______________________________________________
proto mailing list
proto@lists.boost.org
http://lists.boost.org/mailman/listinfo.cgi/proto
