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

Subject: Re: [boost] [fusion] undocumented sequence requirement: convert
From: Eric Niebler (eric_at_[hidden])
Date: 2012-08-22 23:04:06

On 8/22/2012 7:45 PM, Joel de Guzman wrote:
> 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.

The point remains: there is a required Fusion customization point that
is not documented. (Or is it?)

So, should convert generate an mpl sequence? Or a Fusion sequence? Is
there any post-condition regarding the tag type (or fusion_tag type?) of
the resulting sequence?

TIA,

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com