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

Subject: Re: [boost] [fusion] undocumented sequence requirement: convert
From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2012-08-22 16:58:04

On 22/08/2012 22:38, 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?

As the name suggests, it converts a Fusion sequence type to a MPL 
sequence type.

For example, it converts a fusion::vector to a mpl::vector, and a 
fusion::list to a mpl::list.

Maybe pop_back shouldn't require that conversion though.