Re: [boost] [Hana] Informal review request

Subject: Re: [boost] [Hana] Informal review request
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2015-03-07 13:32:23

Le 07/03/15 19:09, Louis Dionne a écrit :
> Louis Dionne <ldionne.2 <at> gmail.com> writes:
>
>> Vicente J. Botet Escriba <vicente.botet <at> wanadoo.fr> writes:
>>
>>> [...]
>>>
>>> Compile-time error reporting
>>> --------------------
>>>
>>> I would expect that the library provide some king of "TypeClass" check
>>> use (As Eric's Range library does with Concepts) so that the compile
>>> time errors are more explicit.
>> Doing this for all parameters of all functions is intractable
>> due to the heterogeneous context, but basic support could
>> be added easily. I'm promoting this on my todo list.
> Before I go an change the code, I'd like to make sure I understand
> your comment properly. Currently, if you use a method with a parameter
> that does not model the required concept, you'll get an assertion.
> For example,
>
>      hana::head(1);
>
>
> Produces
>
> [...] error: static_assert failed
>      "no definition of boost::hana::head for the given data types"
>              static_assert(wrong<head_impl<It>, Xs>{},
>              ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~
> [...] note: in instantiation of function template specialization
>      'boost::hana::head_impl<int, boost::hana::when<true> >::apply<int>'
>      requested here
>              return head_impl<typename datatype<Xs>::type>::apply(
>                                                             ^
> [...] note: in instantiation of function template specialization
>      'boost::hana::_head::operator()<int>' requested here
>          hana::head(1);
>
>
> The check is currently being done in the base case of the
> tag-dispatched method, `head_impl`. It could also be done
> higher up, in the "interface method", i.e. `head` itself.
> For example, here's what I experimented with locally:
>
>      hana::head(1);
>
> Produces
>
>      [...] error: static_assert failed
>          "hana::head(xs) requires xs to be an Iterable"
>                  static_assert(models<Iterable(typename datatype<Xs>::type)>{},
>                  ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>      [...] note: in instantiation of function template specialization
>          'boost::hana::_head::operator()<int>' requested here
>              hana::head(1);
>
>
> Is this the kind of improvement you were talking about, or
> was it something else entirely? Regardless, I like this
> better than my current system and I'll probably do it,
> but I'd like to make sure I understand your point.
>
Yes, this is exactly what I would expect. Report errors at the interface 
level, not the implementation.

Vicente