ceph/src/boost/boost/hana/fwd/chain.hpp

   1 /*!
   2 @file
   3 Forward declares `boost::hana::chain`.
   4
   5 @copyright Louis Dionne 2013-2017
   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_CHAIN_HPP
  11 #define BOOST_HANA_FWD_CHAIN_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/core/when.hpp>
  15
  16
  17 BOOST_HANA_NAMESPACE_BEGIN
  18     //! Feed a monadic value into a monadic computation.
  19     //! @ingroup group-Monad
  20     //!
  21     //! Given a monadic value and a monadic function, `chain` feeds the
  22     //! monadic value into the function, thus performing some Monad-specific
  23     //! effects, and returns the result. An implementation of `chain` must
  24     //! satisfy
  25     //! @code
  26     //!     chain(xs, f) == flatten(transform(xs, f))
  27     //! @endcode
  28     //!
  29     //!
  30     //! Signature
  31     //! ---------
  32     //! For a monad `M`, given a monadic value of type `M(A)` and a monadic
  33     //! function @f$ f : A \to M(B) @f$, `chain` has the signature
  34     //! @f$
  35     //!     \mathtt{chain} : M(A) \times (A \to M(B)) \to M(B)
  36     //! @f$.
  37     //!
  38     //! @param xs
  39     //! A monadic value to be fed to the function `f`.
  40     //!
  41     //! @param f
  42     //! A function taking a normal value in the `xs` structure, and returning
  43     //! a monadic value. This function is called as  `f(x)`, where `x` is an
  44     //! element of the structure `xs`.
  45     //!
  46     //!
  47     //! Example
  48     //! -------
  49     //! @include example/chain.cpp
  50 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  51     constexpr auto chain = [](auto&& xs, auto&& f) -> decltype(auto) {
  52         return tag-dispatched;
  53     };
  54 #else
  55     template <typename M, typename = void>
  56     struct chain_impl : chain_impl<M, when<true>> { };
  57
  58     struct chain_t {
  59         template <typename Xs, typename F>
  60         constexpr decltype(auto) operator()(Xs&& xs, F&& f) const;
  61     };
  62
  63     constexpr chain_t chain{};
  64 #endif
  65 BOOST_HANA_NAMESPACE_END
  66
  67 #endif // !BOOST_HANA_FWD_CHAIN_HPP