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64 lines
2.0 KiB
C++
64 lines
2.0 KiB
C++
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/*!
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@file
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Forward declares `boost::hana::plus`.
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@copyright Louis Dionne 2013-2016
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_FWD_PLUS_HPP
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#define BOOST_HANA_FWD_PLUS_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/when.hpp>
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BOOST_HANA_NAMESPACE_BEGIN
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//! Associative binary operation on a `Monoid`.
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//! @ingroup group-Monoid
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//!
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//! @param x, y
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//! Two objects to combine with the `Monoid`'s binary operation.
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//!
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//!
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//! Cross-type version of the method
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//! --------------------------------
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//! The `plus` method is "overloaded" to handle distinct data types
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//! with certain properties. Specifically, `plus` is defined for
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//! _distinct_ data types `A` and `B` such that
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//! 1. `A` and `B` share a common data type `C`, as determined by the
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//! `common` metafunction
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//! 2. `A`, `B` and `C` are all `Monoid`s when taken individually
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//! 3. `to<C> : A -> B` and `to<C> : B -> C` are `Monoid`-embeddings, as
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//! determined by the `is_embedding` metafunction.
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//!
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//! The definition of `plus` for data types satisfying the above
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//! properties is obtained by setting
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//! @code
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//! plus(x, y) = plus(to<C>(x), to<C>(y))
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//! @endcode
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//!
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//!
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//! Example
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//! -------
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//! @include example/plus.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto plus = [](auto&& x, auto&& y) -> decltype(auto) {
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return tag-dispatched;
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};
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#else
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template <typename T, typename U, typename = void>
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struct plus_impl : plus_impl<T, U, when<true>> { };
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struct plus_t {
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template <typename X, typename Y>
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constexpr decltype(auto) operator()(X&& x, Y&& y) const;
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};
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constexpr plus_t plus{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_PLUS_HPP
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