David Abrahams wrote:
Joel <joel@boost-consulting.com> writes:
I think the problem is the way mpl::is_sequence is implemented. It tries to detect the presence of a "tag" typedef and a "begin" typedef and concludes that a type is an mpl::sequence if it has both. I've complained a long time ago about this. I think this behavior is error prone. It's quite easy to come up with a situation where I have a type T with a "tag" and a "begin" but is not an MPL sequence. There must be a better way. MPL should not monopolize on the usage of "tag" and a "begin" in a type.
I never liked is_sequence in the first place.
1. Anything that makes structural checks is prone to masquerades like the one you cite.
2. Whenever you check if something is a sequence and do something different based on that fact, genericity is broken. We went through this discussion with boost::variant.
Are you sure you want to be using even a perfect implementation of is_sequence?
I'll have to think about this question some more. Right off the bat, type categorization plus enable_if allows constrained genericity, which IMO, is a good thing. We don't really break genericity, we constrain it. That said, both you and Peter have good points. Points to ponder on some more... I wonder what Aleksey thinks about this. Regards, -- Joel de Guzman http://www.boost-consulting.com http://spirit.sf.net