Re: [boost] [mpl, fusion] minimal conforming sequence

From: Joel de Guzman (joel_at_[hidden])
Date: 2008-08-22 22:40:55

David Abrahams wrote:
> on Fri Aug 22 2008, "Peter Dimov" <pdimov-AT-pdimov.com> wrote:
> 
>> David Abrahams:
>>> on Wed Aug 20 2008, "Peter Dimov" <pdimov-AT-pdimov.com> wrote:
>>>
>>>> Hi,
>>>>
>>>> Assume that I have a custom type sequence S that has two operations,
>>>> size<S>::value and element<S,I>::type (with whatever spelling).
>>>>
>>>> What needs to be done to adapt S into a conforming MPL or Fusion
>>>> sequence?
>>>>
>>>> In a perfect world, the answer would not require the inclusion any MPL
>>>> or Fusion headers and would consist of three specializations. ;-)
>>> Well, if it's an MPL sequence it's a fusion sequence, IIUC.  So you need
>>> to decide which sequence to model
>>> http://www.boost.org/doc/libs/1_36_0/libs/mpl/doc/refmanual/concepts.html
>> My original S is a random access sequence, so I'd probably want to
>> model that.
>>
>>> According to
>>> http://www.boost.org/doc/libs/1_36_0/libs/mpl/doc/refmanual/forward-sequence.html
>>> you can make a forward sequence with 5 specializations.
>> That's manageable, but I'd need a random access iterator first. An
>> integral constant, too, if I really don't want to include an MPL
>> header. It would've been nice if I could just point MPL to size<> and
>> element<> and tell it to build the rest of the required
>> boilerplate. Patches welcome, you say? :-) 
> 
> Always :-)

Ditto for Fusion, if you want to have values too. The "ideal" minimal,
AFAICT, would be something like a specialization of:

     namespace boost { namespace fusion { namespace extension
     {
       template<typename S>
       struct random_access_sequence_adaptor;
     }}}

with these minimal requirements:

    sequence::template size<Seq>::type
    sequence::template at_c<Seq, N>::type
    sequence::template at_c<Seq, N>::call(seq)

(akin (and in addition to) http://tinyurl.com/6grjy9)

I don't see any need for including any fusion headers with the interface
above; just the forward declaration of random_access_sequence_adaptor.

I can add this if time permits.

Regards,
-- 
Joel de Guzman
http://www.boostpro.com
http://spirit.sf.net