boost/hana/lift.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::lift`.
   4
   5 @copyright Louis Dionne 2013-2017
   6 Distributed under the Boost Software License, Version 1.0.
   7 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   8  */
   9
  10 #ifndef BOOST_HANA_LIFT_HPP
  11 #define BOOST_HANA_LIFT_HPP
  12
  13 #include <boost/hana/fwd/lift.hpp>
  14
  15 #include <boost/hana/concept/applicative.hpp>
  16 #include <boost/hana/concept/sequence.hpp>
  17 #include <boost/hana/config.hpp>
  18 #include <boost/hana/core/dispatch.hpp>
  19 #include <boost/hana/core/make.hpp>
  20
  21
  22 BOOST_HANA_NAMESPACE_BEGIN
  23     template <typename A>
  24     struct lift_t {
  25     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  26         static_assert(hana::Applicative<A>::value,
  27         "hana::lift<A> requires 'A' to be an Applicative");
  28     #endif
  29
  30         template <typename X>
  31         constexpr auto operator()(X&& x) const {
  32             using Lift = BOOST_HANA_DISPATCH_IF(lift_impl<A>,
  33                 hana::Applicative<A>::value
  34             );
  35
  36             return Lift::apply(static_cast<X&&>(x));
  37         }
  38     };
  39
  40     template <typename A, bool condition>
  41     struct lift_impl<A, when<condition>> : default_ {
  42         template <typename ...Args>
  43         static constexpr auto apply(Args&& ...args) = delete;
  44     };
  45
  46     template <typename S>
  47     struct lift_impl<S, when<Sequence<S>::value>> {
  48         template <typename X>
  49         static constexpr decltype(auto) apply(X&& x)
  50         { return hana::make<S>(static_cast<X&&>(x)); }
  51     };
  52 BOOST_HANA_NAMESPACE_END
  53
  54 #endif // !BOOST_HANA_LIFT_HPP