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/*!
@file
Forward declares `boost::hana::maximum`.
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_MAXIMUM_HPP
#define BOOST_HANA_FWD_MAXIMUM_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
#include <boost/hana/detail/nested_by_fwd.hpp>
namespace boost { namespace hana {
//! Return the greatest element of a non-empty structure with respect to
//! a `predicate`, by default `less`.
//! @ingroup group-Foldable
//!
//! Given a non-empty structure and an optional binary predicate
//! (`less` by default), `maximum` returns the greatest element of
//! the structure, i.e. an element which is greater than or equal to
//! every other element in the structure, according to the predicate.
//!
//! If the structure contains heterogeneous objects, then the predicate
//! must return a compile-time `Logical`. If no predicate is provided,
//! the elements in the structure must be Orderable, or compile-time
//! Orderable if the structure is heterogeneous.
//!
//!
//! Signature
//! ---------
//! Given a Foldable `F`, a Logical `Bool` and a predicate
//! \f$ \mathtt{pred} : T \times T \to Bool \f$, `maximum` has the
//! following signatures. For the variant with a provided predicate,
//! \f[
//! \mathtt{maximum} : F(T) \times (T \times T \to Bool) \to T
//! \f]
//!
//! for the variant without a custom predicate, `T` is required to be
//! Orderable. The signature is then
//! \f[
//! \mathtt{maximum} : F(T) \to T
//! \f]
//!
//! @param xs
//! The structure to find the greatest element of.
//!
//! @param predicate
//! A function called as `predicate(x, y)`, where `x` and `y` are elements
//! of the structure. `predicate` should be a strict weak ordering on the
//! elements of the structure and its return value should be a Logical,
//! or a compile-time Logical if the structure is heterogeneous.
//!
//! ### Example
//! @include example/maximum.cpp
//!
//!
//! Syntactic sugar (`maximum.by`)
//! ------------------------------
//! `maximum` can be called in a third way, which provides a nice syntax
//! especially when working with the `ordering` combinator:
//! @code
//! maximum.by(predicate, xs) == maximum(xs, predicate)
//! maximum.by(predicate) == maximum(-, predicate)
//! @endcode
//!
//! where `maximum(-, predicate)` denotes the partial application of
//! `maximum` to `predicate`.
//!
//! ### Example
//! @include example/maximum_by.cpp
//!
//!
//! Tag dispatching
//! ---------------
//! Both the non-predicated version and the predicated versions of
//! `maximum` are tag-dispatched methods, and hence they can be
//! customized independently. One reason for this is that some
//! structures are able to provide a much more efficient implementation
//! of `maximum` when the `less` predicate is used. Here is how the
//! different versions of `maximum` are dispatched:
//! @code
//! maximum(xs) -> maximum_impl<tag of xs>::apply(xs)
//! maximum(xs, pred) -> maximum_pred_impl<tag of xs>::apply(xs, pred)
//! @endcode
//!
//! Also note that `maximum.by` is not tag-dispatched on its own, since it
//! is just syntactic sugar for calling the corresponding `maximum`.
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto maximum = [](auto&& xs[, auto&& predicate]) -> decltype(auto) {
return tag-dispatched;
};
#else
template <typename T, typename = void>
struct maximum_impl : maximum_impl<T, when<true>> { };
template <typename T, typename = void>
struct maximum_pred_impl : maximum_pred_impl<T, when<true>> { };
struct maximum_t : detail::nested_by<maximum_t> {
template <typename Xs>
constexpr decltype(auto) operator()(Xs&& xs) const;
template <typename Xs, typename Predicate>
constexpr decltype(auto) operator()(Xs&& xs, Predicate&& pred) const;
};
BOOST_HANA_INLINE_VARIABLE constexpr maximum_t maximum{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_MAXIMUM_HPP
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