Re: [boost] [contract] toward N3351 concepts

Subject: Re: [boost] [contract] toward N3351 concepts
From: Lorenzo Caminiti (lorcaminiti_at_[hidden])
Date: 2012-10-02 12:27:09

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. 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. In any case, why is

    concept EqualityComparable = ... // (1)

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

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

--Lorenzo