[boost] [mpl] ForwardIterator concept recursion

Subject: [boost] [mpl] ForwardIterator concept recursion
From: Marcin Zalewski (marcin.zalewski_at_[hidden])
Date: 2009-09-08 23:53:04

I have been trying to implement a concept for mpl ForwardSequence, and
besides the lack of const expr requirements in the latest version of
concepts I have stumbled across the problem of concept recursion. One
example of such recursion is the ForwardIterator [1] concept. The
requirement on the next<i>::type type is that it must be a
ForwardIterator. One could try to write a concept like that:

concept ForwardIteratorStatic<typename I> {
  typename deref;
  typename next;

  requires ForwardIteratorStatic<next>;
}

But for the above concept, the ConceptGCC compiler gives an error.
Also, this should not compile according to the latest concepts
proposal that states that a concept is not defined until the closing
brace of the concept body. As for now, I think that one can try to
apply technique described by Doug [2]:

concept IteratorStatic<typename I> { }

concept ForwardIteratorStatic<typename I> : IteratorStatic<I> {
  typename deref;
  typename next;

  requires IteratorStatic<next>;
}

But to get to my question finally, I wonder if the recursion in mpl is
intentional and thought out, or is it an accident that should be
re-examined. If the decision was indeed thought out, I really wonder
what would be a reasonable semantics of concept recursion.

Cheers,
Marcin

[1] http://www.boost.org/doc/libs/1_40_0/libs/mpl/doc/refmanual/forward-iterator.html
[2] http://www.osl.iu.edu/MailArchives/conceptgcc/2007/03/0084.php