66 lines
1.9 KiB
C++
66 lines
1.9 KiB
C++
/*!
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@file
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Forward declares `boost::hana::comparing`.
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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_COMPARING_HPP
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#define BOOST_HANA_FWD_COMPARING_HPP
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#include <boost/hana/config.hpp>
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BOOST_HANA_NAMESPACE_BEGIN
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//! Returns a function performing `equal` after applying a transformation
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//! to both arguments.
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//! @ingroup group-Comparable
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//!
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//! `comparing` creates an equivalence relation based on the result of
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//! applying a function to some objects, which is especially useful in
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//! conjunction with algorithms that accept a custom predicate that must
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//! represent an equivalence relation.
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//!
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//! Specifically, `comparing` is such that
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//! @code
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//! comparing(f) == equal ^on^ f
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//! @endcode
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//! or, equivalently,
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//! @code
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//! comparing(f)(x, y) == equal(f(x), f(y))
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//! @endcode
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//!
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//! @note
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//! This is not a tag-dispatched method (hence it can't be customized),
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//! but just a convenience function provided with the `Comparable` concept.
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//!
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//!
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//! Signature
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//! ---------
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//! Given a Logical `Bool` and a Comparable `B`, the signature is
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//! @f$ \mathtt{comparing} : (A \to B) \to (A \times A \to Bool) @f$.
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//!
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//!
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//! Example
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//! -------
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//! @include example/comparing.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto comparing = [](auto&& f) {
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return [perfect-capture](auto&& x, auto&& y) {
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return equal(f(forwarded(x)), f(forwarded(y)));
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};
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};
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#else
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struct comparing_t {
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template <typename F>
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constexpr auto operator()(F&& f) const;
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};
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constexpr comparing_t comparing{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_COMPARING_HPP
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