hana/fwd/chain.hpp

0001 /*!
0002 @file
0003 Forward declares `boost::hana::chain`.
0004
0005 Copyright Louis Dionne 2013-2022
0006 Distributed under the Boost Software License, Version 1.0.
0007 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
0008  */
0009
0010 #ifndef BOOST_HANA_FWD_CHAIN_HPP
0011 #define BOOST_HANA_FWD_CHAIN_HPP
0012
0013 #include <boost/hana/config.hpp>
0014 #include <boost/hana/core/when.hpp>
0015
0016
0017 namespace boost { namespace hana {
0018     //! Feed a monadic value into a monadic computation.
0019     //! @ingroup group-Monad
0020     //!
0021     //! Given a monadic value and a monadic function, `chain` feeds the
0022     //! monadic value into the function, thus performing some Monad-specific
0023     //! effects, and returns the result. An implementation of `chain` must
0024     //! satisfy
0025     //! @code
0026     //!     chain(xs, f) == flatten(transform(xs, f))
0027     //! @endcode
0028     //!
0029     //!
0030     //! Signature
0031     //! ---------
0032     //! For a monad `M`, given a monadic value of type `M(A)` and a monadic
0033     //! function @f$ f : A \to M(B) @f$, `chain` has the signature
0034     //! @f$
0035     //!     \mathtt{chain} : M(A) \times (A \to M(B)) \to M(B)
0036     //! @f$.
0037     //!
0038     //! @param xs
0039     //! A monadic value to be fed to the function `f`.
0040     //!
0041     //! @param f
0042     //! A function taking a normal value in the `xs` structure, and returning
0043     //! a monadic value. This function is called as  `f(x)`, where `x` is an
0044     //! element of the structure `xs`.
0045     //!
0046     //!
0047     //! Example
0048     //! -------
0049     //! @include example/chain.cpp
0050 #ifdef BOOST_HANA_DOXYGEN_INVOKED
0051     constexpr auto chain = [](auto&& xs, auto&& f) -> decltype(auto) {
0052         return tag-dispatched;
0053     };
0054 #else
0055     template <typename M, typename = void>
0056     struct chain_impl : chain_impl<M, when<true>> { };
0057
0058     struct chain_t {
0059         template <typename Xs, typename F>
0060         constexpr decltype(auto) operator()(Xs&& xs, F&& f) const;
0061     };
0062
0063     BOOST_HANA_INLINE_VARIABLE constexpr chain_t chain{};
0064 #endif
0065 }} // end namespace boost::hana
0066
0067 #endif // !BOOST_HANA_FWD_CHAIN_HPP