Re: [boost] [Concepts] Definition. Was [GSoC] [Boost.Hana] Formal review request

Subject: Re: [boost] [Concepts] Definition. Was [GSoC] [Boost.Hana] Formal review request
From: Michael Shepanski (mps_at_[hidden])
Date: 2014-08-05 00:33:39

On 5/08/2014 1:39 AM, Roland Bock wrote:
> On 2014-08-04 13:32, Mostafa wrote:
>> Concepts are sets of types
> I think you are mistaken. A concept is a set of requirements,

I am under a New Year's resolution not to start philosophical arguments, 
but this one has already begun, so ...

It may be helpful to consider this principle about sets: set A and set B 
are the same set whenever all the elements of A are elements of B and 
all the elements of B are elements of A. Or in symbols:

     (A = B) iff (forall x)(x in A iff x in B)

(Standard example: The set of equilateral triangles and the set of 
equiangular triangles are the same set, because all equilateral 
triangles are equiangular and vice versa.)

So Mostafa's statement (above) commits him to this:
     Concept A and concept B are the same concept whenever all the types 
in A are in B, and all the types in B are in A.

And Roland's statement commits him to this:
     Concept A and concept B are the same concept whenever all the 
requirements in A are in B, and all the requirements in B are in A.

I think this means that Roland will be able to discriminate concepts 
more finely than Mostafa can. Now, whether that is an advantage or a 
disadvantage, I leave for you gentlemen to discuss.  :)

--- Michael