ceph/src/boost/libs/hana/example/monadic_fold_left.cpp

   1 // Copyright Louis Dionne 2013-2017
   2 // Distributed under the Boost Software License, Version 1.0.
   3 // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   4
   5 #include <boost/hana/assert.hpp>
   6 #include <boost/hana/equal.hpp>
   7 #include <boost/hana/monadic_fold_left.hpp>
   8 #include <boost/hana/optional.hpp>
   9 #include <boost/hana/traits.hpp>
  10 #include <boost/hana/tuple.hpp>
  11 #include <boost/hana/type.hpp>
  12
  13 #include <type_traits>
  14 namespace hana = boost::hana;
  15
  16
  17 auto builtin_common_t = hana::sfinae([](auto&& t, auto&& u) -> hana::type<
  18     std::decay_t<decltype(true ? hana::traits::declval(t) : hana::traits::declval(u))>
  19 > { return {}; });
  20
  21 template <typename ...T>
  22 struct common_type { };
  23
  24 template <typename T, typename U>
  25 struct common_type<T, U>
  26     : std::conditional_t<std::is_same<std::decay_t<T>, T>{} &&
  27                          std::is_same<std::decay_t<U>, U>{},
  28         decltype(builtin_common_t(hana::type_c<T>, hana::type_c<U>)),
  29         common_type<std::decay_t<T>, std::decay_t<U>>
  30     >
  31 { };
  32
  33 template <typename T1, typename ...Tn>
  34 struct common_type<T1, Tn...>
  35     : decltype(hana::monadic_fold_left<hana::optional_tag>(
  36         hana::tuple_t<Tn...>,
  37         hana::type_c<std::decay_t<T1>>,
  38         hana::sfinae(hana::metafunction<common_type>)
  39     ))
  40 { };
  41
  42 template <typename ...Ts>
  43 using common_type_t = typename common_type<Ts...>::type;
  44
  45 BOOST_HANA_CONSTANT_CHECK(
  46   builtin_common_t(hana::type_c<int>, hana::type_c<float>)
  47     ==
  48   hana::just(hana::type_c<float>)
  49 );
  50
  51 static_assert(std::is_same<
  52     common_type_t<char, short, char, short>,
  53     int
  54 >{}, "");
  55
  56 static_assert(std::is_same<
  57     common_type_t<char, double, short, char, short, double>,
  58     double
  59 >{}, "");
  60
  61 static_assert(std::is_same<
  62     common_type_t<char, short, float, short>,
  63     float
  64 >{}, "");
  65
  66 static_assert(
  67     hana::sfinae(hana::metafunction<common_type>)(
  68         hana::type_c<int>, hana::type_c<int>, hana::type_c<int*>
  69     ) == hana::nothing
  70 , "");
  71
  72 int main() { }