Feature Request: SFINAE-friendliness

[apmccartney]

Firstly, thank you for hana. It's simply fantastic.

Currently, unpack, transform, along with the callable objects returned from partial, reverse_partial are not SFINAE-friendly. Is this behavior intentional?

If not, would a PR to provide this functionality be welcome?

[pfultz2]

I am not sure how possible it is to do everywhere in hana since its common to use static_assert to report errors.

Boost.HOF does provide SFINAE-friendly versions for some of those already(such as unpack and partial).

[ricejasonf]

@pfultz2 hof::unpack does not appear to support hana::Foldable AFAICT.

If Boost.Hof doesn't have it already, perhaps a function to create a SFINAE friendly wrapper would be useful. It could take predicates to apply to each argument.

constexpr auto unpack = make_friendly(hana::unpack,
                                      requires_<Foldable>,
                                      always(hana::true_));

... or something like that.

[apmccartney]

I was unaware of Boost.HoF. It looks like an impressive piece of work!

At the moment, the code base I have in mind is pretty heavily invested in hana. Given HoF seems to be oriented towards the standard library, it appears there would be a bit of lifting required to implement hana interoperabilty. At this point, it's not clear to me that the necessary level of effort would be signicantly less than reimplementing the hana higher order functions in question.

That said, I very much intend to give the library a spin on my next project.

I am not sure how possible it is to do everywhere in hana since its common to use static_assert to report errors.

It very well may not be applicable in general, but it's a nice feature to have where it could be reasonably implemented. For instance, in the case of partial and reverse_partial, SFINAE could be implemented trivially using a trailing return type. Borrowing from the Niebler/Carter ranges library

#define HANA_DECLTYPE_AUTO_RETURN(...)   \
    -> decltype(__VA_ARGS__)             \
{ return (__VA_ARGS__); } 

The partial closure's call operator could be rewritten as

template <typename ...Y>
constexpr auto operator()(Y&& ...y) const&
HANA_DECLTYPE_AUTO_RETURN(
  hana::at_c<0>(storage_)(
    hana::at_c<n+1>(storage_)...,
    static_cast<Y&&>(y)...
  )
)

[pfultz2]

hof::unpack does not appear to support hana::Foldable AFAICT.

To support Foldable, hana would need to specialize unpack_sequence so it can be unpacked by boost::hof::unpack.