Re: [boost] [mpl] is there a or_seq like logical metafunction?

Subject: Re: [boost] [mpl] is there a or_seq like logical metafunction?
From: Larry Evans (cppljevans_at_[hidden])
Date: 2009-02-07 14:24:36

On 04/27/08 20:50, David Abrahams wrote:
> on Fri Mar 07 2008, Steven Watanabe <watanabesj-AT-gmail.com> wrote:
> 
>> AMDG
>>
>> Giovanni Piero Deretta wrote:
>>> It seems a perfect application for fold:
>>>
>>> template<typename Seq>
>>> struct or_seq
>>>     : mpl::apply<
>>>           mpl::fold<mpl::_1, mpl::false_,  mpl::quote2<mpl::or_> >
>>>         , Seq
>>>       > {};
>>>
>>> template<typename Seq>
>>> struct and_seq
>>>     : mpl::apply<
>>>           mpl::fold<mpl::_1, mpl::true_,  mpl::quote2<mpl::and_> >
>>>         , Seq
>>>       > {};
>>>   
>> Not quite, because we want it to short circuit.
>> Not to mention that quote2 will not work on and_ and or_.
> 
> Yeah, these should be rewritten in terms of mpl::find.
> Not sure why mpl::apply was used here either, though maybe I'm missign
> something.
> 

The curren mpl::find uses and_ indirectly (in iter_fold_if.hpp:46).
Would such a recursive implementation still work?

Anyway, J.Fletcher is now attempting a variadic template rewrite of
mpl:;find:

http://sourceforge.net/mailarchive/message.php?msg_name=F0DEE5AE9F6027488C064CD31A5B26AC07BA6EDC78%40EXCHANGEMB.campus.aston.ac.uk

which doesn't use and_; however, another alternative which does 
short-circuit is attached.  It uses eval_if to short-circuit the
evaluation.  Fletcher's find, OTOH, used if_; however, I think
that's just because he's not interested in optimization yet.

A very similar or_ is done by simply exchanging the Zero and One
parameters.  Interestingly enough, the current and_  uses
a similar pattern in that it recursively calls a template (and.hpp:23).

The attached (with the similar or.hpp) passes the logical.cpp test.