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/*!
@file
Forward declares `boost::hana::minus`.
@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_MINUS_HPP
#define BOOST_HANA_FWD_MINUS_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
BOOST_HANA_NAMESPACE_BEGIN
//! Subtract two elements of a group.
//! @ingroup group-Group
//!
//! Specifically, this performs the `Monoid` operation on the first
//! argument and on the inverse of the second argument, thus being
//! equivalent to:
//! @code
//! minus(x, y) == plus(x, negate(y))
//! @endcode
//!
//!
//! Cross-type version of the method
//! --------------------------------
//! The `minus` method is "overloaded" to handle distinct data types
//! with certain properties. Specifically, `minus` 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 `Group`s when taken individually
//! 3. `to<C> : A -> B` and `to<C> : B -> C` are `Group`-embeddings, as
//! determined by the `is_embedding` metafunction.
//!
//! The definition of `minus` for data types satisfying the above
//! properties is obtained by setting
//! @code
//! minus(x, y) = minus(to<C>(x), to<C>(y))
//! @endcode
//!
//!
//! Example
//! -------
//! @include example/minus.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto minus = [](auto&& x, auto&& y) -> decltype(auto) {
return tag-dispatched;
};
#else
template <typename T, typename U, typename = void>
struct minus_impl : minus_impl<T, U, when<true>> { };
struct minus_t {
template <typename X, typename Y>
constexpr decltype(auto) operator()(X&& x, Y&& y) const;
};
constexpr minus_t minus{};
#endif
BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_FWD_MINUS_HPP
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