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Subject: Re: [boost] [GSoC] [Boost.Hana] Formal review request
From: Roland Bock (rbock_at_[hidden])
Date: 2014-08-01 05:39:31

On 2014-07-31 19:54, Robert Ramey 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

Can't wait till Concepts Lite are there!

> Usage of concepts is greatly:
> a) misunderstood
> b) misunderestimated as to their value in design AND documentation
Right, last year, I heard several people basically asking "why on earth
would anyone want this?" after listening to a talk about Concepts Lite.
It took me a about an hour at dinner to convince some of them that it
might be worth looking into the topic a bit more...

> d) The word "concepts" is a big contributor to the problem - substitute
> "type requirements" or "type constraints" for concepts.
I like "type constraints" best.
> c) usage of concepts is much confused with implementation of concepts.
> Usage of "type constraints"
> doesn't require any special support from C++. static_assert with type
> traits is usually all that is
> necessary.
> e) recent papers using examples such as "Sortable" add more bad advice and
> confusion.
> I've included a page in the Boost Incubator to promote my views on the
> subject - if anyone cares.
Very nice compilation!

I am impressed by Boost Concept Check Library (the second link to it is
broken in your document, btw). But I feel rather helpless when stumbling
over problems (for example, your code does not compile with the
admittedly ancient boost 1.46 on my machine and I have no idea what to
do with the compile errors).

I am therefore rather sticking with static_assert to enforce constraints
with friendly error messages for the time being until Concepts Lite are
available as TS or part of the standard.



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