[boost] Re: [MPL] is_sequence problem

From: Joel de Guzman (joel_at_[hidden])
Date: 2005-05-06 12:42:57

David Abrahams wrote:
> Joel <joel_at_[hidden]> 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