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Subject: Re: [boost] [Concepts] Definition. Was [GSoC] [Boost.Hana] Formal review request
From: Mostafa (mostafa_working_away_at_[hidden])
Date: 2014-08-05 07:48:30

On Tue, 05 Aug 2014 04:27:31 -0700, Roland Bock <rbock_at_[hidden]> wrote:

> On 2014-08-05 12:14, Mostafa wrote:
>> On Tue, 05 Aug 2014 01:59:34 -0700, Roland Bock <rbock_at_[hidden]>
>> wrote:
>>> On 2014-08-05 10:30, Krzysztof Czainski wrote:
>>>> 2014-08-05 6:33 GMT+02:00 Michael Shepanski <mps_at_[hidden]>:
>>>>> 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 ...
>>>> Please don't forget, that not all requirements can be checked at
>>>> compile
>>>> time. For example: TotallyOrdered. Just by checking, that an
>>>> expression
>>>> (a<b) is valid and yields something boolable, you can't distinguish
>>>> between
>>>> TotallyOrdered, PartiallyOrdered, or maybe neiter.
>>>> So, while I think it doesn't matter, whether we view concepts as
>>>> sets of
>>>> types or requirements
>>> Well, if you consider TotallyOrdered a concept, then by this reasoning,
>>> it cannot be a set of types. But it could be a set of requirements :-)
>> Yes it can. TotallyOrdered := { T | if a,b,c are objects of T then ... }
> As Kris pointed out: Whether an object is TotallyOrdered or not cannot
> be checked at compile time because it does not depend on the type alone,
> it also depends on runtime information. Hence, not a set of types.

That's irrelevant. My original definition of concepts can easily be
expanded to remove the compile time determinability restriction. Then,
TotallyOrdered as a set of types does conceptually exist, and can be
effectively used as a documentation tool. The concept TotallyOrdered may
not depend on the *static* information of the type, but it sure does
depend on objects of the type, and, ergo, it depends on the type.

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