Re: [boost] [fusion] undocumented sequence requirement: convert

Subject: Re: [boost] [fusion] undocumented sequence requirement: convert
From: Joel de Guzman (joel_at_[hidden])
Date: 2012-08-22 19:45:48

On 8/23/2012 4:38 AM, Eric Niebler wrote:
>
> Say I have a valid fusion sequence. I want to use an mpl algorithm on
> it. I include boost/fusion/mpl.hpp and put a nested tag type in my
> sequence that is an alias for fusion::fusion_sequence_tag. So far, so
> good. Then I try to call mpl::pop_back on this sequence, and I find
> that, despite fulfilling all the documented requirements for Fusion
> ForwardSequence, the code fails to compile. That's because of this in
> boost/fusion/mpl/pop_back.hpp:
>
>      template <>
>      struct pop_back_impl<fusion::fusion_sequence_tag>
>      {
>          template <typename Sequence>
>          struct apply
>          {
>              typedef typename
>                  fusion::result_of::pop_back<Sequence>::type
>              result;
>
>              typedef typename
>                  fusion::result_of::convert<
>                      typename fusion::detail::tag_of<Sequence>::type,
> result>::type
>              type;
>          };
>      };
>
> My code doesn't compile because I haven't implemented the
> fusion::convert customization point. Where is that documented and what
> does it do?

The basic problem is that MPL has this Extensible Sequence concept
that Fusion does not have. Keep in mind that mpl::pop_back requires
a Front Extensible Sequence and Fusion only guarantees adaptation
of an MPL Forward, Bidirectional or Random Access Sequence
(http://tinyurl.com/c7tmyfj). In this regard, we are actually
going beyond what's guaranteed by Fusion.

That being said, I am really considering adding extensible sequences
into Fusion. The Deque, for example is a good example.

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