Re: [boost] Re: boos::mpl::for_each: call function with return value

From: Joel de Guzman (joel_at_[hidden])
Date: 2003-11-04 01:45:53

Aleksey Gurtovoy <agurtovoy_at_[hidden]> wrote:

>> Very nice! And it is doable now. Apart from identity_view, we have
>> all the necessary ingredients now. identity_view is quite simple.
>> It is just mpl::transform<s, mpl::identity<_> >, I'm not sure if
>> it's worthy to be included into the library.
> 
> I was thinking more of
> 
>     fusion::identity_view<s> ==
>         fusion::type_sequence< mpl::transform_view<s, mpl::identity<_> > >

I see.

> which is a little too verbose than I would be confortable to spell out
> every time :) - but please see below.

Yes.

>> I think it is possible to work directly on mpl sequences.
> 
> That would be really cool, although AFAICS for that to work reliably we
> would need something like 'fusion::is_sequence'.

Although I wish to have is_sequence<S>, it might not be necessary. Fusion
uses MPL style tag mechanism. I can just use MPL's sequence tags to 
specialize fusion::begin(s) and fusion::end(s) inside fusion's namespace.
In the same manner, I can also specialize mpl::begin<s> and mpl::end<t>
for fusion sequences, thus giving us full round-trip interoperability. Very
cool, IMO :-)

>> you don't need the identity, so you can further simplify:
>> 
>>     typedef mpl::vector<vehicle*,car*,bicycle*,train*> vec;
>>     vehicle* result = fusion::fold(vec(), static_cast<vehicle*>(0),
> make_vehicle);
> 
> IMO, it's beautiful. Or at least as beautiful as it can get within the
> current
> language. I would love to see things work this way.

How about putting identity_view inside MPL instead? Example:

namespace mpl
{
    template <class s>
    struct identity_view : transform_view<s, mpl::identity<_> >  {};
};

Or how about pointer_view:

namespace mpl
{
    template <class s>
    struct pointer_view : transform_view<s, add_pointer<_> >  {};
};

Views are one honking great idea -- let's do more of those! :-)

Then we can write:

    typedef mpl::pointer_view<mpl::vector<vehicle, car, bicycle, train> > vec;
    vehicle* result = fusion::fold(vec(), static_cast<vehicle*>(0), make_vehicle);

I'm not sure, however, if that's any better than:

    mpl::transform_view<
        mpl::vector<vehicle, car, bicycle, train>
      , add_pointer<_> 
    > vec;

    vehicle* result = fusion::fold(vec, static_cast<vehicle*>(0), make_vehicle);

Or simply:

    mpl::vector<vehicle*,car*,bicycle*,train*> vec;
    vehicle* result = fusion::fold(vec, static_cast<vehicle*>(0), make_vehicle);

Jeez, it's all connected somehow! I can imagine an Phoenix/LL extension
that will allow:

    mpl::identity_view<vehicle, car, bicycle, train> vec;
    vehicle* result = fusion::fold(
        vec
      , static_cast<vehicle*>(0)
      , new_<identity_of<mpl::_1> >(ll::_2)
    );

Exciting!

Cheers,
-- 
Joel de Guzman
http://www.boost-consulting.com
http://spirit.sf.net