[boost] [mpl]if_recur vs. iter_fold_if (was Re: [mpl]iter_fold_if Forward Backward rationale?

Subject: [boost] [mpl]if_recur vs. iter_fold_if (was Re: [mpl]iter_fold_if Forward Backward rationale?
From: Larry Evans (cppljevans_at_[hidden])
Date: 2009-04-07 20:15:25

On 04/05/09 08:39, Larry Evans wrote:
[snip]
 >
 > The previous attachment should have had:
 >
 >   push_front<arg<1>,deref<arg<2> > >
 >
 > instead of:
 >
 >   push_front<arg<1>,arg<2> >
 >
 > as both the ForwardOp and BackwardOp.  Also, the
 > BackwardPredicate should have been:
 >
 >    protect
 >    < aux::iter_fold_if_pred
 >      < always<true_>
 >      , typename end<numbers>::type
 >      >
 >    >
 >
 > With those changes, the correct palindrome is produced.
 >
 > Just as the ForwardPredicate is "augmented" with protection
 > against dereferencing the end<numbers>::type in this code:
 >
 > 
https://svn.boost.org/trac/boost/browser/trunk/boost/mpl/iter_fold_if.hpp#L42 

 >
 >
 > shouldn't the BackwardPredicate also be augmented with the same
 > protection here:
 >
 > 
https://svn.boost.org/trac/boost/browser/trunk/boost/mpl/iter_fold_if.hpp#L77 

 >
 >
 > ?

The attached code has the template, if_recur, which, AFAICT, is more
general than iter_fold_if.  It's more general because it's not "tied"
to sequences.  IOW, it's more like a while loop only instead of just
tail recursion, it implements a more general type of "linear"recursion
as described in the reference shown in the comments.  The test shows
both the equivalent of the fold and the reverse_fold (the
result_up_push and the result_down_push_).  It also shows the
equivalent to the palindrome example of the previous post.  Of cousse
it maybe not as efficient as just using fold or reverse_fole; however,
that's just the cost of more generality.

It would be interesting to figure out how to code a sequence of linear
recursive equations. The if_recur is just a single equation.  Anybody
care to try?

-regards,
Larry