Re: [boost] [contract] concepts: pseudo-signatures vs. usage patterns

Subject: Re: [boost] [contract] concepts: pseudo-signatures vs. usage patterns
From: Dave Abrahams (dave_at_[hidden])
Date: 2012-10-11 19:57:58

on Thu Oct 11 2012, Lorenzo Caminiti <lorcaminiti-AT-gmail.com> wrote:

> On Thu, Oct 11, 2012 at 5:25 AM, Andrew Sutton <asutton.list_at_[hidden]> wrote:
>>> Look for example at
>>> http://www.sgi.com/tech/stl/find_if.html.  I can easily satisfy all the
>>> stated requirements and produce an example that won't compile.  I can
>>> also easily fail to satisfy the stated requirements and produce an
>>> example that *will* compile.  That's because there's a great deal of
>>> potential mischief hiding in expressions such as
>>>
>>>   f(*p)
>>
>>
>> Can you produce those examples? You've made me curious.
>
> Andrew, what were the reasons in N3351 to move away from
> pseudo-signatures and toward usage-patterns?
>
> I red N3351 but if I had to summarize the rationale for the
> usage-pattern approach instead of C++0x concept's pseudo-signature
> then I'd say that N3351's argument is that its concept design for STL
> algorithms is "simpler" using usage-pattern than using C++0x
> pseudo-signatures. However, "simpler" is a subjective metric... I'm
> sure I'm missing something maybe reading N3351 one more time will
> clarify my thinking.

One other thing about this rationale: it doesn't appear to consider
what's simple for someone trying to write a generic algorithm.  It's
easy to maintain that rationale when you don't have an implementation of
algorithm type-checking to force rigor on you.  When you start working
with an actual implementation, and it refuses to compile "obviously
right" things like

          f(*p)

because in fact, the requirements as stated do not guarantee that it
*should* compile, then your idea of what's simple may change.  A usable
system has to be simple both for programmers modeling concepts and for
programmers using those concepts in generic code.

-- 
Dave Abrahams
BoostPro Computing                  Software Development        Training
http://www.boostpro.com             Clang/LLVM/EDG Compilers  C++  Boost