68 lines
2.1 KiB
C++
68 lines
2.1 KiB
C++
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/*!
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@file
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Forward declares `boost::hana::minus`.
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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_MINUS_HPP
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#define BOOST_HANA_FWD_MINUS_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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//! Subtract two elements of a group.
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//! @ingroup group-Group
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//!
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//! Specifically, this performs the `Monoid` operation on the first
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//! argument and on the inverse of the second argument, thus being
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//! equivalent to:
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//! @code
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//! minus(x, y) == plus(x, negate(y))
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//! @endcode
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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 `minus` method is "overloaded" to handle distinct data types
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//! with certain properties. Specifically, `minus` 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 `Group`s when taken individually
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//! 3. `to<C> : A -> B` and `to<C> : B -> C` are `Group`-embeddings, as
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//! determined by the `is_embedding` metafunction.
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//!
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//! The definition of `minus` for data types satisfying the above
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//! properties is obtained by setting
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//! @code
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//! minus(x, y) = minus(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/minus.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto minus = [](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 minus_impl : minus_impl<T, U, when<true>> { };
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struct minus_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 minus_t minus{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_MINUS_HPP
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