Re: [boost] [mpl] bug: mpl::is_sequence< mpl::int_<1> > fails to compile

From: Joel de Guzman (joel_at_[hidden])
Date: 2006-05-08 10:22:31

David Abrahams wrote:
> Joel de Guzman <joel_at_[hidden]> writes:
> 
>> David Abrahams wrote:
>>> Tobias Schwinger <tschwinger_at_[hidden]> writes:
>>>
>>>> Why not make is_sequence work reliably? 
>>> Because it's impossible.  It's always going to look for some features
>>> of the type being tested that can be present even if the type is not,
>>> in fact, a sequence.
>> I don't see why it is impossible. MPL sequences can always
>> specialize is_sequence<T>. is_sequence *can* work reliably
>> 100% of the time:
>>
>>      template <class S>
>>      struct is_sequence : mpl::false_ {}; // default
>>
>>      // your specializations here
> 
> The problem isn't false negatives, it's false positives.  In practice
> you can probably make it vanishingly unlikely that some random
> third-party type will pass all the default is_sequence tests, but you
> can never make it 100% impossible as long as:
> 
>   a. sequences can be arbitrary types, such as void(int,long,char)
> 
>   b. is_sequence<S> reporting correctly is not part of the sequence
>      requirements

Is that a or b? or a and b?

I believe that the default is_sequence should strictly be
mpl::false_: no guessing games.

I think:

a) if you want to make specific types (e.g. RT(A0,A1,...AN)),
a sequence, then specialize is_sequence for those. Sure you can
do it for only a limited number of args, but, hey, so do other
libraries that provide traits over function types/ member function
pointer types, function pointers. Are there other arbitrary types
you have in mind that can be sequences? If there are, specialize
for them. Explicit is better than implicit, right? I think the
guessing game where MPL is trying some heuristics as to how an
MPL sequence looks like (e.g. it has a tag typedef, etc), is
what's causing us problems. I think it is the wrong approach.

b) If is_sequence is part of MPL's public interface, then it
should give reliable results. If that requires b, then so be it.

With Fusion, I had to go through hoops to get around this MPL
problem. I really hope this issue is resolved once and for all.
I believe it is possible for MPL to have a 100% correct is_sequence
(no false positives/negatives). I believe that specialization is
the correct approach, not some guessing game over how an MPL
sequence might look like.

Regards,
-- 
Joel de Guzman
http://www.boost-consulting.com
http://spirit.sf.net