81 lines
2.6 KiB
C++
81 lines
2.6 KiB
C++
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/*!
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@file
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Forward declares `boost::hana::equal`.
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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_EQUAL_HPP
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#define BOOST_HANA_FWD_EQUAL_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/when.hpp>
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#include <boost/hana/detail/nested_to_fwd.hpp>
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BOOST_HANA_NAMESPACE_BEGIN
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//! Returns a `Logical` representing whether `x` is equal to `y`.
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//! @ingroup group-Comparable
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//!
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//! The `equal` function can be called in two different ways. First, it
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//! can be called like a normal function:
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//! @code
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//! equal(x, y)
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//! @endcode
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//!
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//! However, it may also be partially applied to an argument by using
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//! `equal.to`:
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//! @code
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//! equal.to(x)(y) == equal(x, y)
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//! @endcode
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//!
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//! In other words, `equal.to(x)` is a function object that is equivalent
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//! to `partial(equal, x)`. This is provided to enhance the readability of
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//! some constructs, especially when using higher order algorithms.
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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 two Comparables `A` and `B` that
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//! share a common embedding, the signature is
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//! @f$ \mathtt{equal} : A \times B \to Bool @f$.
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//!
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//! @param x, y
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//! Two objects to compare for equality.
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//!
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//!
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//! Example
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//! -------
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//! @include example/equal.cpp
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//!
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//!
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//! > #### Rationale for the arity of `equal`
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//! > It is a valid question whether `equal` should accept more than 2
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//! > arguments and have semantics matching those of Python's `==`. This
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//! > is not supported right now for the following reasons:
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//! > - It was implemented in the MPL11, but it was not shown to be useful
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//! > so far.
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//! > - It does not make sense for `not_equal` to have an arity of more
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//! > than 2, only `equal` could maybe have those semantics, which would
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//! > break symmetry.
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto equal = [](auto&& x, auto&& y) {
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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 equal_impl : equal_impl<T, U, when<true>> { };
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struct equal_t : detail::nested_to<equal_t> {
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template <typename X, typename Y>
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constexpr auto operator()(X&& x, Y&& y) const;
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};
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constexpr equal_t equal{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_EQUAL_HPP
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