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1 | /*! |
2 | @file | |
3 | Defines `boost::hana::empty`. | |
4 | ||
5 | @copyright Louis Dionne 2013-2016 | |
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_EMPTY_HPP | |
11 | #define BOOST_HANA_EMPTY_HPP | |
12 | ||
13 | #include <boost/hana/fwd/empty.hpp> | |
14 | ||
15 | #include <boost/hana/concept/monad_plus.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 M> | |
24 | struct empty_t { | |
25 | #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS | |
26 | static_assert(hana::MonadPlus<M>::value, | |
27 | "hana::empty<M>() requires 'M' to be a MonadPlus"); | |
28 | #endif | |
29 | ||
30 | constexpr auto operator()() const { | |
31 | using Empty = BOOST_HANA_DISPATCH_IF(empty_impl<M>, | |
32 | hana::MonadPlus<M>::value | |
33 | ); | |
34 | ||
35 | return Empty::apply(); | |
36 | } | |
37 | }; | |
38 | ||
39 | template <typename M, bool condition> | |
40 | struct empty_impl<M, when<condition>> : default_ { | |
41 | template <typename ...Args> | |
42 | static constexpr auto apply(Args&& ...) = delete; | |
43 | }; | |
44 | ||
45 | template <typename S> | |
46 | struct empty_impl<S, when<Sequence<S>::value>> { | |
47 | static constexpr auto apply() { | |
48 | return hana::make<S>(); | |
49 | } | |
50 | }; | |
51 | BOOST_HANA_NAMESPACE_END | |
52 | ||
53 | #endif // !BOOST_HANA_EMPTY_HPP |