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Subject: Re: Interest in a "concepts lite" library?
From: Juan Pedro Bolívar Puente (raskolnikov_at_[hidden])
Date: 2016-04-20 10:16:15

Hi Paul!

Thanks for taking a look! Your library makes a very interesting
proposition. Comments follow:

> It supports gcc, clang, and MSVC, and has several already defined concept
> traits in the library(such as `is_container` or `is_iterator`). There is some
> more work that needs to be finished with the documentation. Once I finish work
> with the Fit library, I plan to finish up that library and submit it for
> formal review as well.

How did you get MSVC to work? Last time I tried to port my
implementation I got stuck into a network of compiler crashes that I
eventually assumed were due to the lack of proper expression SFINAE
support and fundamentally unavoidable for such a library.

>> - It wants to be future-proof. Concepts defined with the
>> `ABL_CONCEPT_SPEC` macro can be automatically upgraded to a Concepts
>> Lite concept by redefining `ABL_CONCEPT` as `constexpr concept`.
> Integral constants are more powerful and expressive than raw constexpr values.

An older version of my library was using integral constants too, but
eventually I decided against it. Rationale:

1. this library wants to bring concepts to casual users of templates,
not just metaprogrammers. constexpr functions might be easier to

2. constexpr functions would users to the library get the benefits of
real concepts straight away, once they are there. Again, this is
specially important for the intended audience, that would benefit a lot
from better error messages, etc.

3. something similar to the standard might help the standard committee
in getting examples of practical use-cases.

3. integral-constant returning metafunctions can still be provided. In
the library, the macro defines both `MyConcept` and `MyConcept_spec`,
the later being usable with various metafunctions, giving that extra
functionality to "power users".

> Just some notes, you will get better error reporting from the compiler using
> `typename std::enable_if<(...)>::type` instead of using template aliases such
> as `enable_if_t`. Most compilers don't have the infrastucture to trace
> template aliases(and I don't think they will anytime soon). This is why
> libraries have a `REQUIRES` macro instead. In addition for the Tick library,
> it proved benifecial as it allowed to handle boolean expression that are not
> dependent on the deduced template parameter, workaround constexpr bugs on MSVC
> and better handle function overloading.

That's interesting! Is that what allowed you to get rid of the MSVC

While in principle I am not fan of macros, specially when mixed in funny
places of type declarations and signatures, a carefully designed
REQUIRES macro may also provide a smooth C++-1N transition for concept

> Also, the Tick library doesn't use the `valid_expr(f(x))` to check valid
> expressions, because if `f` returns void then it will fail substitution which
> means the trait is false even though it is a valid expression. Instead, it
> follows the suggestion by Jamboree of using a template `valid<...>` instead.

Atria concepts use the sintax `(f(x), may_be_void)` when `f` is allowed
to return void, `may_be_void` being an arbitrary int constexpr. There
are trade-offs to each and can't judge now what is better.



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