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/*!
@file
Forward declares `boost::hana::equal`.
Copyright Louis Dionne 2013-2022
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_EQUAL_HPP
#define BOOST_HANA_FWD_EQUAL_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
#include <boost/hana/detail/nested_to_fwd.hpp>
namespace boost { namespace hana {
//! Returns a `Logical` representing whether `x` is equal to `y`.
//! @ingroup group-Comparable
//!
//! The `equal` function can be called in two different ways. First, it
//! can be called like a normal function:
//! @code
//! equal(x, y)
//! @endcode
//!
//! However, it may also be partially applied to an argument by using
//! `equal.to`:
//! @code
//! equal.to(x)(y) == equal(x, y)
//! @endcode
//!
//! In other words, `equal.to(x)` is a function object that is equivalent
//! to `partial(equal, x)`. This is provided to enhance the readability of
//! some constructs, especially when using higher order algorithms.
//!
//!
//! Signature
//! ---------
//! Given a Logical `Bool` and two Comparables `A` and `B` that
//! share a common embedding, the signature is
//! @f$ \mathtt{equal} : A \times B \to Bool @f$.
//!
//! @param x, y
//! Two objects to compare for equality.
//!
//!
//! Example
//! -------
//! @include example/equal.cpp
//!
//!
//! > #### Rationale for the arity of `equal`
//! > It is a valid question whether `equal` should accept more than 2
//! > arguments and have semantics matching those of Python's `==`. This
//! > is not supported right now for the following reasons:
//! > - It was implemented in the MPL11, but it was not shown to be useful
//! > so far.
//! > - It does not make sense for `not_equal` to have an arity of more
//! > than 2, only `equal` could maybe have those semantics, which would
//! > break symmetry.
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto equal = [](auto&& x, auto&& y) {
return tag-dispatched;
};
#else
template <typename T, typename U, typename = void>
struct equal_impl : equal_impl<T, U, when<true>> { };
struct equal_t : detail::nested_to<equal_t> {
template <typename X, typename Y>
constexpr auto operator()(X&& x, Y&& y) const;
};
BOOST_HANA_INLINE_VARIABLE constexpr equal_t equal{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_EQUAL_HPP
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