boost-hana

I have a template function that has no definition by default but it specialised by some types: template <typename T> auto foo(bar &, const T &) -> void; template <> auto foo<std::string>(bar &, const std::string &) -> void {} How do I write a constexpr function that tells me if type T has a specialisation for the above function? My best effort: namespace detail { auto has_foo(hana::is_valid([](auto &b, const auto &t) -> decltype(foo(b, t)) {})); } // namespace detail template <typename T> constexpr auto has_foo() -> bool { using hana::type_c; return detail::has_foo(type_c<bar>, type_c<T>); }

I have a hana::tuple_t<int, char, double, float> , and I want to use this to create a hana::tuple<int, char, double, float> . I thought that using hana::to<hana::tuple_tag> would transform the hana::tuple_t<int, char, double, float> into a hana::tuple<int, char, double, float> ; but that is not the case since the following always fails: auto oType = hana::tuple_t<int, char, double, float>; BOOST_HANA_CONSTANT_ASSERT( hana::to<hana::tuple_tag>(oType) == hana::make_tuple(1, 'C', 1.0, 1.0f) ); I've also tried using hana::transform , but with no luck (although I suspect I'm doing it wrong): auto

So I am trying to make a library using boost::hana that requires the functionality to get the index of a element based on the value: constexpr auto tup = boost::hana::make_tuple(3_c, boost::hana::type_c<bool>); auto index = get_index_of_first_matching(tup, boost::hana::type_c<bool>); // ^^^^^ would be a boost::hana::int_<1> Is there a possible way to do this? Better yet, is it already in hana and I don't know about it? Thanks for the support! Hana does not provide an algorithm to do this out-of-the-box. If it seems like a much desired feature, I could add such an algorithm fairly easily. It

I'm trying to access a hana::type from a pair using hana::second ... namespace hana = boost::hana; using namespace hana::literals; struct Key {}; struct Foo {}; int main() { auto test = hana::make_tuple( hana::make_pair( hana::type_c<Key>, hana::type_c<Foo>)); typename decltype(hana::type_c<Foo>)::type finalTest; //Ok typename decltype(hana::second(test[0_c]))::type finalTest2; //Error } But I am getting the following compiler error: stacktest.cpp: In function ‘int main()’: stacktest.cpp:17:12: error: decltype evaluates to ‘boost::hana::type_impl<Foo>::_&’, which is not a class or enumeration