| 1 | /*! |
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| 2 | @file |
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| 3 | Forward declares `boost::hana::ordering`. |
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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_ORDERING_HPP |
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| 11 | #define BOOST_HANA_FWD_ORDERING_HPP |
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| 12 | |
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| 13 | #include <boost/hana/config.hpp> |
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| 14 | |
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| 15 | |
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| 16 | namespace boost { namespace hana { |
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| 17 | //! Returns a function performing `less` after applying a transformation |
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| 18 | //! to both arguments. |
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| 19 | //! @ingroup group-Orderable |
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| 20 | //! |
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| 21 | //! `ordering` creates a total order based on the result of applying a |
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| 22 | //! function to some objects, which is especially useful in conjunction |
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| 23 | //! with algorithms that accept a custom predicate that must represent |
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| 24 | //! a total order. |
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| 25 | //! |
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| 26 | //! Specifically, `ordering` is such that |
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| 27 | //! @code |
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| 28 | //! ordering(f) == less ^on^ f |
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| 29 | //! @endcode |
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| 30 | //! or, equivalently, |
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| 31 | //! @code |
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| 32 | //! ordering(f)(x, y) == less(f(x), f(y)) |
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| 33 | //! @endcode |
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| 34 | //! |
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| 35 | //! @note |
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| 36 | //! This is not a tag-dispatched method (hence it can't be customized), |
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| 37 | //! but just a convenience function provided with the `Orderable` concept. |
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| 38 | //! |
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| 39 | //! |
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| 40 | //! Signature |
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| 41 | //! --------- |
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| 42 | //! Given a Logical `Bool` and an Orderable `B`, the signature is |
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| 43 | //! @f$ \mathrm{ordering} : (A \to B) \to (A \times A \to Bool) @f$. |
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| 44 | //! |
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| 45 | //! |
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| 46 | //! Example |
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| 47 | //! ------- |
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| 48 | //! @include example/ordering.cpp |
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| 49 | #ifdef BOOST_HANA_DOXYGEN_INVOKED |
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| 50 | constexpr auto ordering = [](auto&& f) { |
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| 51 | return [perfect-capture](auto&& x, auto&& y) -> decltype(auto) { |
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| 52 | return less(f(forwarded(x)), f(forwarded(y))); |
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| 53 | }; |
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| 54 | }; |
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| 55 | #else |
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| 56 | struct ordering_t { |
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| 57 | template <typename F> |
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| 58 | constexpr auto operator()(F&& f) const; |
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| 59 | }; |
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| 60 | |
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| 61 | BOOST_HANA_INLINE_VARIABLE constexpr ordering_t ordering{}; |
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| 62 | #endif |
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| 63 | }} // end namespace boost::hana |
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| 64 | |
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| 65 | #endif // !BOOST_HANA_FWD_ORDERING_HPP |
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| 66 | |
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