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 /*!
 @file
 Forward declares `boost::hana::sort`.
 
 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_SORT_HPP
 #define BOOST_HANA_FWD_SORT_HPP
 
 #include <boost/hana/config.hpp>
 #include <boost/hana/core/when.hpp>
 #include <boost/hana/detail/nested_by_fwd.hpp>
 
 
 namespace boost { namespace hana {
     //! Sort a sequence, optionally based on a custom `predicate`.
     //! @ingroup group-Sequence
     //!
     //! Given a Sequence and an optional predicate (by default `less`), `sort`
     //! returns a new sequence containing the same elements as the original,
     //! except they are ordered in such a way that if `x` comes before `y` in
     //! the sequence, then either `predicate(x, y)` is true, or both
     //! `predicate(x, y)` and `predicate(y, x)` are false.
     //!
     //! Also note that the sort is guaranteed to be stable. Hence, if `x`
     //! comes before `y` in the original sequence and both `predicate(x, y)`
     //! and `predicate(y, x)` are false, then `x` will come before `y` in the
     //! resulting sequence.
     //!
     //! If no predicate is provided, the elements in the sequence must all be
     //! compile-time `Orderable`.
     //!
     //! Signature
     //! ---------
     //! Given a `Sequence` `S(T)`, a boolean `IntegralConstant` `Bool` and a
     //! binary predicate \f$ T \times T \to Bool \f$, `sort` has the following
     //! signatures. For the variant with a provided predicate,
     //! \f[
     //!     \mathtt{sort} : S(T) \times (T \times T \to Bool) \to S(T)
     //! \f]
     //!
     //! for the variant without a custom predicate, `T` is required to be
     //! `Orderable`. The signature is then
     //! \f[
     //!     \mathtt{sort} : S(T) \to S(T)
     //! \f]
     //!
     //! @param xs
     //! The sequence to sort.
     //!
     //! @param predicate
     //! A function called as `predicate(x, y)` for two elements `x` and `y` of
     //! the sequence, and returning a boolean `IntegralConstant` representing
     //! whether `x` is to be considered _less_ than `y`, i.e. whether `x` should
     //! appear _before_ `y` in the resulting sequence. More specifically,
     //! `predicate` must define a [strict weak ordering][1] on the elements
     //! of the sequence. When the predicate is not specified, this defaults
     //! to `less`. In the current version of the library, the predicate has
     //! to return an `IntegralConstant` holding a value convertible to a `bool`.
     //!
     //!
     //! Syntactic sugar (`sort.by`)
     //! ---------------------------
     //! `sort` can be called in a third way, which provides a nice syntax
     //! especially when working with the `ordering` combinator:
     //! @code
     //!     sort.by(predicate, xs) == sort(xs, predicate)
     //!     sort.by(predicate) == sort(-, predicate)
     //! @endcode
     //!
     //! where `sort(-, predicate)` denotes the partial application of
     //! `sort` to `predicate`.
     //!
     //!
     //! Example
     //! -------
     //! @include example/sort.cpp
     //!
     //! [1]: http://en.wikipedia.org/wiki/Strict_weak_ordering
 #ifdef BOOST_HANA_DOXYGEN_INVOKED
     constexpr auto sort = [](auto&& xs[, auto&& predicate]) {
         return tag-dispatched;
     };
 #else
     template <typename S, typename = void>
     struct sort_impl : sort_impl<S, when<true>> { };
 
     struct sort_t : detail::nested_by<sort_t> {
         template <typename Xs>
         constexpr auto operator()(Xs&& xs) const;
 
         template <typename Xs, typename Predicate>
         constexpr auto operator()(Xs&& xs, Predicate&& pred) const;
     };
 
     BOOST_HANA_INLINE_VARIABLE constexpr sort_t sort{};
 #endif
 }} // end namespace boost::hana
 
 #endif // !BOOST_HANA_FWD_SORT_HPP

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