Re: [boost] [Hana] Informal review request

Subject: Re: [boost] [Hana] Informal review request
From: Louis Dionne (ldionne.2_at_[hidden])
Date: 2015-03-07 13:09:26

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.

Louis