hana/fwd/flatten.hpp

0001 /*!
0002 @file
0003 Forward declares `boost::hana::flatten`.
0004
0005 Copyright Louis Dionne 2013-2022
0006 Distributed under the Boost Software License, Version 1.0.
0007 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
0008  */
0009
0010 #ifndef BOOST_HANA_FWD_FLATTEN_HPP
0011 #define BOOST_HANA_FWD_FLATTEN_HPP
0012
0013 #include <boost/hana/config.hpp>
0014 #include <boost/hana/core/when.hpp>
0015
0016
0017 namespace boost { namespace hana {
0018     //! Collapse two levels of monadic structure into a single level.
0019     //! @ingroup group-Monad
0020     //!
0021     //! Given a monadic value wrapped into two levels of monad, `flatten`
0022     //! removes one such level. An implementation of `flatten` must satisfy
0023     //! @code
0024     //!     flatten(xs) == chain(xs, id)
0025     //! @endcode
0026     //!
0027     //! For `Sequence`s, this simply takes a `Sequence` of `Sequence`s, and
0028     //! returns a (non-recursively) flattened `Sequence`.
0029     //!
0030     //!
0031     //! Signature
0032     //! ---------
0033     //! For a `Monad` `M`, the signature of `flatten` is
0034     //! @f$
0035     //!     \mathtt{flatten} : M(M(T)) \to M(T)
0036     //! @f$
0037     //!
0038     //! @param xs
0039     //! A value with two levels of monadic structure, which should be
0040     //! collapsed into a single level of structure.
0041     //!
0042     //!
0043     //! Example
0044     //! -------
0045     //! @include example/flatten.cpp
0046 #ifdef BOOST_HANA_DOXYGEN_INVOKED
0047     constexpr auto flatten = [](auto&& xs) {
0048         return tag-dispatched;
0049     };
0050 #else
0051     template <typename M, typename = void>
0052     struct flatten_impl : flatten_impl<M, when<true>> { };
0053
0054     struct flatten_t {
0055         template <typename Xs>
0056         constexpr auto operator()(Xs&& xs) const;
0057     };
0058
0059     BOOST_HANA_INLINE_VARIABLE constexpr flatten_t flatten{};
0060 #endif
0061 }} // end namespace boost::hana
0062
0063 #endif // !BOOST_HANA_FWD_FLATTEN_HPP