| 1 | /*! |
|---|
| 2 | @file |
|---|
| 3 | Forward declares `boost::hana::equal`. |
|---|
| 4 | |
|---|
| 5 | Copyright Louis Dionne 2013-2022 |
|---|
| 6 | Distributed under the Boost Software License, Version 1.0. |
|---|
| 7 | (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) |
|---|
| 8 | */ |
|---|
| 9 | |
|---|
| 10 | #ifndef BOOST_HANA_FWD_EQUAL_HPP |
|---|
| 11 | #define BOOST_HANA_FWD_EQUAL_HPP |
|---|
| 12 | |
|---|
| 13 | #include <boost/hana/config.hpp> |
|---|
| 14 | #include <boost/hana/core/when.hpp> |
|---|
| 15 | #include <boost/hana/detail/nested_to_fwd.hpp> |
|---|
| 16 | |
|---|
| 17 | |
|---|
| 18 | namespace boost { namespace hana { |
|---|
| 19 | //! Returns a `Logical` representing whether `x` is equal to `y`. |
|---|
| 20 | //! @ingroup group-Comparable |
|---|
| 21 | //! |
|---|
| 22 | //! The `equal` function can be called in two different ways. First, it |
|---|
| 23 | //! can be called like a normal function: |
|---|
| 24 | //! @code |
|---|
| 25 | //! equal(x, y) |
|---|
| 26 | //! @endcode |
|---|
| 27 | //! |
|---|
| 28 | //! However, it may also be partially applied to an argument by using |
|---|
| 29 | //! `equal.to`: |
|---|
| 30 | //! @code |
|---|
| 31 | //! equal.to(x)(y) == equal(x, y) |
|---|
| 32 | //! @endcode |
|---|
| 33 | //! |
|---|
| 34 | //! In other words, `equal.to(x)` is a function object that is equivalent |
|---|
| 35 | //! to `partial(equal, x)`. This is provided to enhance the readability of |
|---|
| 36 | //! some constructs, especially when using higher order algorithms. |
|---|
| 37 | //! |
|---|
| 38 | //! |
|---|
| 39 | //! Signature |
|---|
| 40 | //! --------- |
|---|
| 41 | //! Given a Logical `Bool` and two Comparables `A` and `B` that |
|---|
| 42 | //! share a common embedding, the signature is |
|---|
| 43 | //! @f$ \mathtt{equal} : A \times B \to Bool @f$. |
|---|
| 44 | //! |
|---|
| 45 | //! @param x, y |
|---|
| 46 | //! Two objects to compare for equality. |
|---|
| 47 | //! |
|---|
| 48 | //! |
|---|
| 49 | //! Example |
|---|
| 50 | //! ------- |
|---|
| 51 | //! @include example/equal.cpp |
|---|
| 52 | //! |
|---|
| 53 | //! |
|---|
| 54 | //! > #### Rationale for the arity of `equal` |
|---|
| 55 | //! > It is a valid question whether `equal` should accept more than 2 |
|---|
| 56 | //! > arguments and have semantics matching those of Python's `==`. This |
|---|
| 57 | //! > is not supported right now for the following reasons: |
|---|
| 58 | //! > - It was implemented in the MPL11, but it was not shown to be useful |
|---|
| 59 | //! > so far. |
|---|
| 60 | //! > - It does not make sense for `not_equal` to have an arity of more |
|---|
| 61 | //! > than 2, only `equal` could maybe have those semantics, which would |
|---|
| 62 | //! > break symmetry. |
|---|
| 63 | #ifdef BOOST_HANA_DOXYGEN_INVOKED |
|---|
| 64 | constexpr auto equal = [](auto&& x, auto&& y) { |
|---|
| 65 | return tag-dispatched; |
|---|
| 66 | }; |
|---|
| 67 | #else |
|---|
| 68 | template <typename T, typename U, typename = void> |
|---|
| 69 | struct equal_impl : equal_impl<T, U, when<true>> { }; |
|---|
| 70 | |
|---|
| 71 | struct equal_t : detail::nested_to<equal_t> { |
|---|
| 72 | template <typename X, typename Y> |
|---|
| 73 | constexpr auto operator()(X&& x, Y&& y) const; |
|---|
| 74 | }; |
|---|
| 75 | |
|---|
| 76 | BOOST_HANA_INLINE_VARIABLE constexpr equal_t equal{}; |
|---|
| 77 | #endif |
|---|
| 78 | }} // end namespace boost::hana |
|---|
| 79 | |
|---|
| 80 | #endif // !BOOST_HANA_FWD_EQUAL_HPP |
|---|
| 81 | |
|---|
