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1 | /*! |
2 | @file | |
3 | Forward declares `boost::hana::concat`. | |
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_FWD_CONCAT_HPP | |
11 | #define BOOST_HANA_FWD_CONCAT_HPP | |
12 | ||
13 | #include <boost/hana/config.hpp> | |
14 | #include <boost/hana/core/when.hpp> | |
15 | ||
16 | ||
17 | BOOST_HANA_NAMESPACE_BEGIN | |
18 | //! Combine two monadic structures together. | |
19 | //! @ingroup group-MonadPlus | |
20 | //! | |
21 | //! Given two monadic structures, `concat` combines them together and | |
22 | //! returns a new monadic structure. The exact definition of `concat` | |
23 | //! will depend on the exact model of MonadPlus at hand, but for | |
24 | //! sequences it corresponds intuitively to simple concatenation. | |
25 | //! | |
26 | //! Also note that combination is not required to be commutative. | |
27 | //! In other words, there is no requirement that | |
28 | //! @code | |
29 | //! concat(xs, ys) == concat(ys, xs) | |
30 | //! @endcode | |
31 | //! and indeed it does not hold in general. | |
32 | //! | |
33 | //! | |
34 | //! Signature | |
35 | //! --------- | |
36 | //! Given a `MonadPlus` `M`, the signature of `concat` is | |
37 | //! @f$ \mathtt{concat} : M(T) \times M(T) \to M(T) @f$. | |
38 | //! | |
39 | //! @param xs, ys | |
40 | //! Two monadic structures to combine together. | |
41 | //! | |
42 | //! | |
43 | //! Example | |
44 | //! ------- | |
45 | //! @include example/concat.cpp | |
46 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
47 | constexpr auto concat = [](auto&& xs, auto&& ys) { | |
48 | return tag-dispatched; | |
49 | }; | |
50 | #else | |
51 | template <typename M, typename = void> | |
52 | struct concat_impl : concat_impl<M, when<true>> { }; | |
53 | ||
54 | struct concat_t { | |
55 | template <typename Xs, typename Ys> | |
56 | constexpr auto operator()(Xs&& xs, Ys&& ys) const; | |
57 | }; | |
58 | ||
59 | constexpr concat_t concat{}; | |
60 | #endif | |
61 | BOOST_HANA_NAMESPACE_END | |
62 | ||
63 | #endif // !BOOST_HANA_FWD_CONCAT_HPP |