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/*!
@file
Forward declares `boost::hana::comparing`.
@copyright Louis Dionne 2013-2016
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
*/
#ifndef BOOST_HANA_FWD_COMPARING_HPP
#define BOOST_HANA_FWD_COMPARING_HPP
#include <boost/hana/config.hpp>
BOOST_HANA_NAMESPACE_BEGIN
//! Returns a function performing `equal` after applying a transformation
//! to both arguments.
//! @ingroup group-Comparable
//!
//! `comparing` creates an equivalence relation based on the result of
//! applying a function to some objects, which is especially useful in
//! conjunction with algorithms that accept a custom predicate that must
//! represent an equivalence relation.
//!
//! Specifically, `comparing` is such that
//! @code
//! comparing(f) == equal ^on^ f
//! @endcode
//! or, equivalently,
//! @code
//! comparing(f)(x, y) == equal(f(x), f(y))
//! @endcode
//!
//! @note
//! This is not a tag-dispatched method (hence it can't be customized),
//! but just a convenience function provided with the `Comparable` concept.
//!
//!
//! Signature
//! ---------
//! Given a Logical `Bool` and a Comparable `B`, the signature is
//! @f$ \mathtt{comparing} : (A \to B) \to (A \times A \to Bool) @f$.
//!
//!
//! Example
//! -------
//! @include example/comparing.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto comparing = [](auto&& f) {
return [perfect-capture](auto&& x, auto&& y) {
return equal(f(forwarded(x)), f(forwarded(y)));
};
};
#else
struct comparing_t {
template <typename F>
constexpr auto operator()(F&& f) const;
};
constexpr comparing_t comparing{};
#endif
BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_FWD_COMPARING_HPP
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