/*! 
@file
Forward declares `boost::hana::chain`. 

@copyright Louis Dionne 2013-2016
Distributed under the Boost Software License, Version 1.0. 
(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
 */

#ifndef BOOST_HANA_FWD_CHAIN_HPP
#define BOOST_HANA_FWD_CHAIN_HPP

#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>


BOOST_HANA_NAMESPACE_BEGIN
    //!  Feed a monadic value into a monadic computation. 
    //!  @ingroup group-Monad
    //! 
    //!  Given a monadic value and a monadic function, `chain` feeds the
    //!  monadic value into the function, thus performing some Monad-specific
    //!  effects, and returns the result.  An implementation of `chain` must
    //!  satisfy
    //!  @code
    //!      chain(xs, f) == flatten(transform(xs, f))
    //!  @endcode
    //! 
    //! 
    //!  Signature
    //! --------- 
    //!  For a monad `M`, given a monadic value of type `M(A)` and a monadic
    //!  function @f$ f : A \to M(B) @f$, `chain` has the signature
    //!  @f$
    //!      \mathtt{chain} : M(A) \times (A \to M(B)) \to M(B)
    //!  @f$.
    //! 
    //!  @param xs
    //!  A monadic value to be fed to the function `f`. 
    //! 
    //!  @param f
    //!  A function taking a normal value in the `xs` structure, and returning
    //!  a monadic value.  This function is called as  `f(x)`, where `x` is an
    //!  element of the structure `xs`. 
    //! 
    //! 
    //!  Example
    //! ------- 
    //!  @include example/chain.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
    constexpr auto chain = [](auto&& xs, auto&& f) -> decltype(auto) {
        return tag-dispatched;
    };
#else
    template <typename M, typename = void>
    struct chain_impl : chain_impl<M, when<true>> { };

    struct chain_t {
        template <typename Xs, typename F>
        constexpr decltype(auto) operator()(Xs&& xs, F&& f) const;
    };

    constexpr chain_t chain{};
#endif
BOOST_HANA_NAMESPACE_END

#endif // ! BOOST_HANA_FWD_CHAIN_HPP
