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1 | /*! |
2 | @file | |
3 | Forward declares `boost::hana::flatten`. | |
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_FLATTEN_HPP | |
11 | #define BOOST_HANA_FWD_FLATTEN_HPP | |
12 | ||
13 | #include <boost/hana/config.hpp> | |
14 | #include <boost/hana/core/when.hpp> | |
15 | ||
16 | ||
17 | BOOST_HANA_NAMESPACE_BEGIN | |
18 | //! Collapse two levels of monadic structure into a single level. | |
19 | //! @ingroup group-Monad | |
20 | //! | |
21 | //! Given a monadic value wrapped into two levels of monad, `flatten` | |
22 | //! removes one such level. An implementation of `flatten` must satisfy | |
23 | //! @code | |
24 | //! flatten(xs) == chain(xs, id) | |
25 | //! @endcode | |
26 | //! | |
27 | //! For `Sequence`s, this simply takes a `Sequence` of `Sequence`s, and | |
28 | //! returns a (non-recursively) flattened `Sequence`. | |
29 | //! | |
30 | //! | |
31 | //! Signature | |
32 | //! --------- | |
33 | //! For a `Monad` `M`, the signature of `flatten` is | |
34 | //! @f$ | |
35 | //! \mathtt{flatten} : M(M(T)) \to M(T) | |
36 | //! @f$ | |
37 | //! | |
38 | //! @param xs | |
39 | //! A value with two levels of monadic structure, which should be | |
40 | //! collapsed into a single level of structure. | |
41 | //! | |
42 | //! | |
43 | //! Example | |
44 | //! ------- | |
45 | //! @include example/flatten.cpp | |
46 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
47 | constexpr auto flatten = [](auto&& xs) { | |
48 | return tag-dispatched; | |
49 | }; | |
50 | #else | |
51 | template <typename M, typename = void> | |
52 | struct flatten_impl : flatten_impl<M, when<true>> { }; | |
53 | ||
54 | struct flatten_t { | |
55 | template <typename Xs> | |
56 | constexpr auto operator()(Xs&& xs) const; | |
57 | }; | |
58 | ||
59 | constexpr flatten_t flatten{}; | |
60 | #endif | |
61 | BOOST_HANA_NAMESPACE_END | |
62 | ||
63 | #endif // !BOOST_HANA_FWD_FLATTEN_HPP |