| 1 | /*! |
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| 2 | @file |
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| 3 | Forward declares `boost::hana::minus`. |
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| 4 | |
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| 5 | Copyright Louis Dionne 2013-2022 |
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| 6 | Distributed under the Boost Software License, Version 1.0. |
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| 7 | (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) |
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| 8 | */ |
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| 9 | |
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| 10 | #ifndef BOOST_HANA_FWD_MINUS_HPP |
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| 11 | #define BOOST_HANA_FWD_MINUS_HPP |
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| 12 | |
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| 13 | #include <boost/hana/config.hpp> |
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| 14 | #include <boost/hana/core/when.hpp> |
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| 15 | |
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| 16 | |
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| 17 | namespace boost { namespace hana { |
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| 18 | //! Subtract two elements of a group. |
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| 19 | //! @ingroup group-Group |
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| 20 | //! |
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| 21 | //! Specifically, this performs the `Monoid` operation on the first |
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| 22 | //! argument and on the inverse of the second argument, thus being |
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| 23 | //! equivalent to: |
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| 24 | //! @code |
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| 25 | //! minus(x, y) == plus(x, negate(y)) |
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| 26 | //! @endcode |
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| 27 | //! |
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| 28 | //! |
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| 29 | //! Cross-type version of the method |
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| 30 | //! -------------------------------- |
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| 31 | //! The `minus` method is "overloaded" to handle distinct data types |
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| 32 | //! with certain properties. Specifically, `minus` is defined for |
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| 33 | //! _distinct_ data types `A` and `B` such that |
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| 34 | //! 1. `A` and `B` share a common data type `C`, as determined by the |
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| 35 | //! `common` metafunction |
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| 36 | //! 2. `A`, `B` and `C` are all `Group`s when taken individually |
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| 37 | //! 3. `to<C> : A -> B` and `to<C> : B -> C` are `Group`-embeddings, as |
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| 38 | //! determined by the `is_embedding` metafunction. |
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| 39 | //! |
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| 40 | //! The definition of `minus` for data types satisfying the above |
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| 41 | //! properties is obtained by setting |
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| 42 | //! @code |
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| 43 | //! minus(x, y) = minus(to<C>(x), to<C>(y)) |
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| 44 | //! @endcode |
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| 45 | //! |
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| 46 | //! |
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| 47 | //! Example |
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| 48 | //! ------- |
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| 49 | //! @include example/minus.cpp |
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| 50 | #ifdef BOOST_HANA_DOXYGEN_INVOKED |
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| 51 | constexpr auto minus = [](auto&& x, auto&& y) -> decltype(auto) { |
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| 52 | return tag-dispatched; |
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| 53 | }; |
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| 54 | #else |
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| 55 | template <typename T, typename U, typename = void> |
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| 56 | struct minus_impl : minus_impl<T, U, when<true>> { }; |
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| 57 | |
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| 58 | struct minus_t { |
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| 59 | template <typename X, typename Y> |
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| 60 | constexpr decltype(auto) operator()(X&& x, Y&& y) const; |
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| 61 | }; |
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| 62 | |
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| 63 | BOOST_HANA_INLINE_VARIABLE constexpr minus_t minus{}; |
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| 64 | #endif |
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| 65 | }} // end namespace boost::hana |
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| 66 | |
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| 67 | #endif // !BOOST_HANA_FWD_MINUS_HPP |
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| 68 | |
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