Constraints & Concepts [Was: Re: C++0x news]

[Andrei Alexandrescu]

On 03/22/2010 02:09 PM, bearophile wrote:
> Andrei Alexandrescu:
>
>> you'd have to understand the reasons that made concepts unsuitable
>> for C++0x and then work out ways to make concepts work for D.
>
> I don't know exactly why they have refused Concepts, but to refuse
> them after so many years of developments and discussions (and
> ConceptG++, many nice papers written about their usage, etc) then
> they have good reasons. You are right, better to not go there, even
> if it's seducing stuff.

It's a long story. I think I'd summarize it by saying again: just like 
in chess, in PL design ignoring the whole for the beauty of the part is 
a capital mistake.

>> Detecting mistaken use of concepts is practically very easy - just
>> write a type that implements the concept vacuously and then
>> instantiate the tested template against it in a unittest. I
>> sometimes do that to test my range-based code.
>
> Thank you for this interesting suggestion, I'll try it. In the
> meantime I have found a Boost docs page about the same topic, I think
> their 'archetype class' is the same thing you are saying me:
> http://www.boost.org/doc/libs/1_42_0/libs/concept_check/concept_covering.htm

Yah, archetype-based checking was endorsed by Bjarne Stroustrup and 
myself (though my participation was negligible, so really I'm the only 
one claiming credit :o)). That proposal "lost" and concepts as 
ultimately proposed "won". If asked to speculate, I'd say that the other 
approach, if chosen, would have change the state of affairs dramatically.


Andrei