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/*!
@file
Forward declares `boost::hana::chain`.
@copyright Louis Dionne 2013-2017
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
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