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Subject: [boost] [Concepts] Definition. Was [GSoC] [Boost.Hana] Formal review request
From: Mostafa (mostafa_working_away_at_[hidden])
Date: 2014-08-04 07:32:10

On Thu, 31 Jul 2014 10:54:59 -0700, Robert Ramey <ramey_at_[hidden]> wrote:

> Eric Niebler-4 wrote
>> On 07/29/2014 05:14 PM, Niall Douglas wrote:
>>> I'm all for Concepts as in compiler enforced ones, and I'll add them
>>> to AFIO when and only when C++ gets them. But for documentation they
>>> don't help.
>> Wow, I couldn't disagree more. I can't imagine how the standard
>> algorithms would be specified without the use of concepts like
>> "RandomAccessIterator", for instance. Clustering requirements into
>> meaningful abstractions and assigning them names makes it possible to
>> document library interfaces without an explosion of verbosity and
>> repetition.
> +10
> Usage of concepts is greatly:
> a) misunderstood
> b) misunderestimated as to their value in design AND documentation
> d) The word "concepts" is a big contributor to the problem - substitute
> "type requirements" or "type constraints" for concepts.

-1 to the above term substitutions for concepts. Type requirements/type
constrains are not concepts. The reason concepts are misunderstood is
because they have not been well defined. FWIW, here's my take on how they
should be defined:

Concepts are sets of types whose membership are compile-time determinable.
Precisely, a concept is any set expressible in the following form:

         { T | T is a type and p(T ...) }

where p is a (n+1)-ary compile time computable boolean function (a function
in the mathematical sense, not an actual C++ function). p is said
to be (or to express) the type requirements or type constrains for its
associated concept. (Note: just as a set membership predicate is not its
a type requirement is not its concept. For example,
the predicate "there exists a,b and a,b are integers and b != 0 and c =
a/b" does not denote the set of rational numbers, the symbol Q does.)

A type T is said to model a concept C if T is a member of C. In such a
case we also say that "T is a C".
A concept B is said to be a refinement of a concept A if B is a subset of

Why use concepts? Because they are helpful in documenting generic code,
especially template type parameter requirements.


1) DefaultConstructible :=
          { T | T has the method "T()" defined and accessible. }

       // Requirements:
       // T is DefaultConstructible.
       template <typename T>
       struct FooBar { ... };

       std::tuple<int> is DefaultConstructible, therefore
       FooBar< std::tuple<int> > is well formed.


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