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

   1 /*!
   2 @file
   3 Forward declares `boost::hana::then`.
   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_THEN_HPP
  11 #define BOOST_HANA_FWD_THEN_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/core/when.hpp>
  15
  16
  17 BOOST_HANA_NAMESPACE_BEGIN
  18     //! Sequentially compose two monadic actions, discarding any value
  19     //! produced by the first but not its effects.
  20     //! @ingroup group-Monad
  21     //!
  22     //!
  23     //! @param before
  24     //! The first `Monad` in the monadic composition chain. The result of
  25     //! this monad is ignored, but its effects are combined with that of the
  26     //! second monad.
  27     //!
  28     //! @param xs
  29     //! The second `Monad` in the monadic composition chain.
  30     //!
  31     //!
  32     //! Example
  33     //! -------
  34     //! @include example/then.cpp
  35 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  36     constexpr auto then = [](auto&& before, auto&& xs) -> decltype(auto) {
  37         return tag-dispatched;
  38     };
  39 #else
  40     template <typename M, typename = void>
  41     struct then_impl : then_impl<M, when<true>> { };
  42
  43     struct then_t {
  44         template <typename Before, typename Xs>
  45         constexpr decltype(auto) operator()(Before&& before, Xs&& xs) const;
  46     };
  47
  48     constexpr then_t then{};
  49 #endif
  50 BOOST_HANA_NAMESPACE_END
  51
  52 #endif // !BOOST_HANA_FWD_THEN_HPP