1/*!
2@file
3Forward declares `boost::hana::flatten`.
4
5Copyright Louis Dionne 2013-2022
6Distributed 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
17namespace boost { namespace hana {
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 BOOST_HANA_INLINE_VARIABLE constexpr flatten_t flatten{};
60#endif
61}} // end namespace boost::hana
62
63#endif // !BOOST_HANA_FWD_FLATTEN_HPP
64

source code of boost/libs/hana/include/boost/hana/fwd/flatten.hpp