Re: [boost] [guidelines] why template errors suck

Subject: Re: [boost] [guidelines] why template errors suck
From: Doug Gregor (doug.gregor_at_[hidden])
Date: 2010-09-29 10:35:43

On Wed, Sep 29, 2010 at 2:06 AM, Andrzej Krzemienski <akrzemi1_at_[hidden]> wrote:
>> > > Well, I believe I got it wrong. It is concept maps where the
> specializtion
>> > > is to tke plce. It should read:
>> > >
>> > > explicit concept< typename T > IntOrPair
>
>> "explicit concepts" were never part of any accepted proposal ;-)
>
>> > {
>> > int sum_ints_impl( T const& );
>> > }
>> >
>> > concept_map IntOrPair<int>
>> > {
>> > int sum_ints_impl( int const& i ) { return i; }
>> > }
>> >
>> > template< IntOrPair type1, IntOrPair type2 >
>> > concept_map IntOrPair< std::pair<type1, type2> >
>> > {
>> > int sum_ints_impl( std::pair<First, Second> const& p ) {
>> > return sum_ints_impl(p.first) + sum_ints_impl(p.second);
>> > }
>> > }
>> >
>> > template<IntOrPair T>
>> > int sum_ints( T const & t )
>> > {
>> > return IntOrPair::sum_ints_impl( t );
>> > }
>
>> I think what you wrote is equivalent to what I posted in
>> http://permalink.gmane.org/gmane.comp.lib.boost.devel/209012
>
>> However, as I noted in that message, trying to constrain something to
>> be either an Int or a Pair is really against the spirit of GP.
>
> I understand. On the other hand, even though concepts were designed to
> support GP, once they are designed it would be interesting to see how they
> can be used to do other thing, and what other possibilities they enable.

In this thread, we're not even close to the point where we have a
mutual understanding of how concepts work (technically) or how they
can/should be used to describe simple generic algorithms. It seems
rather premature to skip that step and go straight to (ab)using
concepts for something completely different. The danger, of course, is
that we've completely missed the point of concepts, and that our
"other possibilities" are really just poor substitutes for using
concepts well.

  - Doug