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Subject: Re: [boost] [Concepts] Definition. Was [GSoC] [Boost.Hana] Formal review request
From: Roland Bock (rbock_at_[hidden])
Date: 2014-08-05 07:58:35

On 2014-08-05 12:13, Mostafa wrote:
> On Tue, 05 Aug 2014 00:07:42 -0700, Roland Bock <rbock_at_[hidden]> wrote:
>> On 2014-08-05 02:08, Mostafa wrote:
>>> On Mon, 04 Aug 2014 08:39:36 -0700, Roland Bock <rbock_at_[hidden]>
>>> wrote:
> [snip]
>>> Additionally, if concepts were sets of type requirements than the
>>> phrase "T is DefaultConstructible" reads awkwardly, it almost sounds
>>> as if T were a type requirement itself, when we actually mean to say
>>> that T satisfies the requirements of default constructibility. And the
>>> language "x satisfies abc" usually connotates set membership.
>> Sure, that type is a member of T(DefaultConstructible).
>> I don't get your argument at all. If you say "this car is green", you
>> say both "this car is a member of the set of green things" and "this car
>> meets the requirements of being green". It does neither imply that
>> "green is the set of green things" (endless recursion) nor does it say
>> that "green is a set of being-green-requirements" (also endless
>> recursion). Still, everyone knows what is meant without feeling awkward
>> about it.
> Unfortunately English can be ambiguous, so let's be more precise. When
> one says "this car is green" one actually means "this car is a green
> thing" (green is being used as a noun, not as an adjective like "green
> balloon"). That means "this car is a member of GreenThings". If
> GreenThings were a set of requirements then that means "this car" is
> also a requirement, which obviously it's not.

You're only ever seeing your view :-)
Another option is: When one says "this car is green", one actually means
"this car emits light with a wavelength around 510nm". So the concept
"green" is not a set of green things, it is a set of requirements (in
this case with one element, namely "emits light with a wavelength around

Since both views are equally possible, I'd stick with the one closer to

> Therefore GreenThings, the concept in this discussion, is a set of
> objects that satisfy the requirement of being green.
>> So why not use the term concept in a way that matches what we actually
>> do in code?
> Because that's a narrow and implementation-dependent point of view.

I did not write "concepts are pieces of code"...

> When we introduce people to classes, hopefully we don't first define
> them to be some set of rules for determining the layout of a region of
> memory, rather we define them to be nouns; that is it's a
> communication tool for expressing programmer intent. By thinking about
> classes abstractly, we are able to more easily reason with them, like
> how UML robustness diagrams can be used to discover classes and the
> relationships between them.
> FWIW, it might interest you to read this blog on concepts by Bartosz
> Milewski:
> (Disclaimer: he holds my position and terms your definition
> "constrained templates", but his posting is informative in comparing
> our approaches.)
I read that one before, but thanks for the reminder :-)

Afaict, your view is in line with what he calls concept maps.

Anyway, this has gone far too long already, I am out of here.



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