Re: [boost] [contract] toward N3351 concepts

Subject: Re: [boost] [contract] toward N3351 concepts
From: Lorenzo Caminiti (lorcaminiti_at_[hidden])
Date: 2012-10-02 12:32:08

Sorry, I sent my previous email before completing it by mistake--I'm
re-sending it.

On Tue, Oct 2, 2012 at 9:27 AM, Lorenzo Caminiti <lorcaminiti_at_[hidden]> wrote:
> On Tue, Oct 2, 2012 at 8:07 AM, Andrew Sutton <asutton.list_at_[hidden]> wrote:
>>> However, syntactically you use it similarly to a template:
>>>
>>> MyConcept<T1, T2, T3>
>>>
>>> my_template<T1, T2, T3>
>>
>> Syntactically yes, but they are allowed to mean very different things.
>>
>>
>>>> concept <typename T> EqualityComparable = ... // (3)
>>>
>>> Actually, this would make the most sense to me, why wouldn't we use
>>> this syntax (3)?
>>
>> Alex wanted the syntax to reflect, to some extent, the style that he
>> used to define concepts in Elements of Programming.

In any case, why is (2) better than (3)? What was the rationale for
such a syntax in Elements of Programming? (At the end I can adopt
whatever syntax but I will need to justify the choice in a "rationale"
section.)

    template< typename T >
    concept EqualityComparable = ... // (1) no good because concepts
are not templates

    concept EqualityComparable< typename T > = ... // (2)

    concept <typename T> EqualityComparable = ... // (3)

> A concept is a
>> type predicate (function) defined by a conjunction of syntactic and
>> semantic requirements. Compare:
>>
>> // EoP (with pseudo-C++ syntax)
>> Relation(Op) =
>>        Predicate(Op)
>>   && HomogeneousFunction(Op)
>>   && Arity(Op) == 2
>>
>> // N3351
>> concept Relation<typename Op, typename T> =
>>   Predicate<Op, T, T>;
>>
>> In the writing, we wanted to emphasize the idea that a concept was
>> "just" a predicate on template arguments. That made it easier to focus
>> on the algorithm's requirements rather than to figure out the
>> extensive set of language rules that would make it work.
>
> But already the fact that the first argument of a concept is
> automatically replaced with the type when concepts are used in
> template parameter declarations:
>
> template< typename T, Relation<T> Op >
> void func ( ... )
>
> Already requires to know something special about the language that
> makes concept work.

Thanks,
--Lorenzo