I just had an a-ha! moment. Segmentation in Fusion is *so* much simpler 
than I was making it. Fusion doesn't need segmented iterators, it needs 
segmented sequences! Fusion's algorithms don't take iterators like the 
std algorithms do -- they take sequences. I was trying to shoe-horn Matt 
Austern's formulation into Fusion, but that's the wrong approach.

Consider the joint_view. It should advertise itself as a segmented view. 
Then, the only thing it needs to provide is a way to step through the 
internal segments -- which are Fusion sequences that may or may not be 
segmented. What you get, essentially, is:

void for_each( join_view<First, Second> view, F f )
{
     for_each( view.first, f );
     for_each( view.second, f );
}

or, implemented generically, more like:

// for_each for segmented sequences ...
void for_each( SegmentedSequence & seq, f )
{
     detail::segmented_for_each(
         fusion::begin(seq)
       , fusion::end(seq)
       , f, mpl::false_());
}

where detail::segmented_for_each looks like:

void segmented_for_each(
     SBegin const &sbegin
   , SEnd const &send
   , F f, mpl::false_)
{
     fusion::for_each(
         fusion::deref(sbegin)
       , f);

     fusion::segmented_for_each(
         fusion::next(seq)
       , send
       , f, result_of::equal_to<
               typename result_of::next<SBegin>::type
             , SEnd>());
}

No new Fusion primimtives are need! And we can get better perf by doing 
loop unrolling in segmented_for_each.

Much simpler.

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com

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