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

   1 /*!
   2 @file
   3 Forward declares `boost::hana::empty`.
   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_EMPTY_HPP
  11 #define BOOST_HANA_FWD_EMPTY_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/core/when.hpp>
  15
  16
  17 BOOST_HANA_NAMESPACE_BEGIN
  18     //! Identity of the monadic combination `concat`.
  19     //! @ingroup group-MonadPlus
  20     //!
  21     //! Signature
  22     //! ---------
  23     //! Given a MonadPlus `M`, the signature is
  24     //! @f$ \mathtt{empty}_M : \emptyset \to M(T) @f$.
  25     //!
  26     //! @tparam M
  27     //! The tag of the monadic structure to return. This must be
  28     //! a model of the MonadPlus concept.
  29     //!
  30     //!
  31     //! Example
  32     //! -------
  33     //! @include example/empty.cpp
  34 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  35     template <typename M>
  36     constexpr auto empty = []() {
  37         return tag-dispatched;
  38     };
  39 #else
  40     template <typename M, typename = void>
  41     struct empty_impl : empty_impl<M, when<true>> { };
  42
  43     template <typename M>
  44     struct empty_t;
  45
  46     template <typename M>
  47     constexpr empty_t<M> empty{};
  48 #endif
  49 BOOST_HANA_NAMESPACE_END
  50
  51 #endif // !BOOST_HANA_FWD_EMPTY_HPP