ceph/src/boost/libs/hana/include/boost/hana/fwd/flatten.hpp

   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