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/*!
@file
Forward declares `boost::hana::plus`.
Copyright Louis Dionne 2013-2022
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_PLUS_HPP
#define BOOST_HANA_FWD_PLUS_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
namespace boost { namespace hana {
//! Associative binary operation on a `Monoid`.
//! @ingroup group-Monoid
//!
//! @param x, y
//! Two objects to combine with the `Monoid`'s binary operation.
//!
//!
//! Cross-type version of the method
//! --------------------------------
//! The `plus` method is "overloaded" to handle distinct data types
//! with certain properties. Specifically, `plus` is defined for
//! _distinct_ data types `A` and `B` such that
//! 1. `A` and `B` share a common data type `C`, as determined by the
//! `common` metafunction
//! 2. `A`, `B` and `C` are all `Monoid`s when taken individually
//! 3. `to<C> : A -> B` and `to<C> : B -> C` are `Monoid`-embeddings, as
//! determined by the `is_embedding` metafunction.
//!
//! The definition of `plus` for data types satisfying the above
//! properties is obtained by setting
//! @code
//! plus(x, y) = plus(to<C>(x), to<C>(y))
//! @endcode
//!
//!
//! Example
//! -------
//! @include example/plus.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto plus = [](auto&& x, auto&& y) -> decltype(auto) {
return tag-dispatched;
};
#else
template <typename T, typename U, typename = void>
struct plus_impl : plus_impl<T, U, when<true>> { };
struct plus_t {
template <typename X, typename Y>
constexpr decltype(auto) operator()(X&& x, Y&& y) const;
};
BOOST_HANA_INLINE_VARIABLE constexpr plus_t plus{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_PLUS_HPP
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