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

Subject: Re: [boost] [Concepts] Definition. Was [GSoC] [Boost.Hana] Formal review request
From: Robert Ramey (ramey_at_[hidden])
Date: 2014-08-14 02:00:52

Niall Douglas wrote
> So tl;dr; I am really saying the time for concepts - whatever they 
> are to whoever's definition - isn't here yet.

I would disagree.  The whole STL library is based on "concepts".  That
is, all template parameter have explicit requirements.  This is true
now and has always been so.

It's true that compiler  support for verification of all the type
requirements
which someone might come up with falls short of perfection.  But
as a practical matter, most type requirements are pretty simple and
easily supported with current type traits.  For example, type requirements
for 

template<class T>
quick_sort(iterator<T> begin, iterator<T> end)

are easily stated AND enforced:
BOOST_CONCEPT_REQUIRES((Swappable<T>);
BOOST_CONCEPT_REQUIRES((StrictWeakOrdering<T>));

This is useful right now.  And few library authors even explicitly state
their type requirements - much less include them in code.

This could change in a month - and it will if I have anything to say 
about it.  There is absolutely no advantage to not doing this.

> Let's get a few concept 
> programming libraries based on a Concepts Lite compiler around first. 
> Let's not dig ourselves now into a straightjacket we later regret.

I've included a table which maps boost concept library macros
to the concept lite syntax. see
http://rrsd.com/blincubator.com/advice_concepts/
"Future Compatibility". Concepts lite is not really relevant here.  
It won't change the landscape in any way.  Library authors
aren't failing to explicitly define and enforce their type requirements
because they lack compiler support for this.  They're doing it because
we don't demand that we do so.

Robert Ramey

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