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

   1 /*!
   2 @file
   3 Forward declares `boost::hana::duplicate`.
   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_DUPLICATE_HPP
  11 #define BOOST_HANA_FWD_DUPLICATE_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/core/when.hpp>
  15
  16
  17 BOOST_HANA_NAMESPACE_BEGIN
  18     //! Add an extra layer of comonadic context to a comonadic value.
  19     //! @ingroup group-Comonad
  20     //!
  21     //! Given a value already in a comonadic context, `duplicate` wraps this
  22     //! value with an additional layer of comonadic context. This can be seen
  23     //! as the dual operation to `flatten` from the Monad concept.
  24     //!
  25     //!
  26     //! Signature
  27     //! ---------
  28     //! Given a Comonad `W`, the signature is
  29     //! \f$
  30     //!     \mathtt{duplicate} : W(T) \to W(W(T))
  31     //! \f$
  32     //!
  33     //! @param w
  34     //! The value to wrap in an additional level of comonadic context.
  35     //!
  36     //!
  37     //! Example
  38     //! -------
  39     //! @include example/duplicate.cpp
  40 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  41     constexpr auto duplicate = [](auto&& w) -> decltype(auto) {
  42         return tag-dispatched;
  43     };
  44 #else
  45     template <typename W, typename = void>
  46     struct duplicate_impl : duplicate_impl<W, when<true>> { };
  47
  48     struct duplicate_t {
  49         template <typename W_>
  50         constexpr decltype(auto) operator()(W_&& w) const;
  51     };
  52
  53     constexpr duplicate_t duplicate{};
  54 #endif
  55 BOOST_HANA_NAMESPACE_END
  56
  57 #endif // !BOOST_HANA_FWD_DUPLICATE_HPP