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 /*!
 @file
 Forward declares `boost::hana::minimum`.
 
 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_MINIMUM_HPP
 #define BOOST_HANA_FWD_MINIMUM_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 least 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), `minimum` returns the least element of
     //! the structure, i.e. an element which is less 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$, `minimum` has the
     //! following signatures. For the variant with a provided predicate,
     //! \f[
     //!     \mathtt{minimum} : 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{minimum} : F(T) \to T
     //! \f]
     //!
     //! @param xs
     //! The structure to find the least 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/minimum.cpp
     //!
     //!
     //! Syntactic sugar (`minimum.by`)
     //! ------------------------------
     //! `minimum` can be called in a third way, which provides a nice syntax
     //! especially when working with the `ordering` combinator:
     //! @code
     //!     minimum.by(predicate, xs) == minimum(xs, predicate)
     //!     minimum.by(predicate) == minimum(-, predicate)
     //! @endcode
     //!
     //! where `minimum(-, predicate)` denotes the partial application of
     //! `minimum` to `predicate`.
     //!
     //! ### Example
     //! @include example/minimum_by.cpp
     //!
     //!
     //! Tag dispatching
     //! ---------------
     //! Both the non-predicated version and the predicated versions of
     //! `minimum` 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 `minimum` when the `less` predicate is used. Here is how the
     //! different versions of `minimum` are dispatched:
     //! @code
     //!     minimum(xs) -> minimum_impl<tag of xs>::apply(xs)
     //!     minimum(xs, pred) -> minimum_pred_impl<tag of xs>::apply(xs, pred)
     //! @endcode
     //!
     //! Also note that `minimum.by` is not tag-dispatched on its own, since it
     //! is just syntactic sugar for calling the corresponding `minimum`.
 #ifdef BOOST_HANA_DOXYGEN_INVOKED
     constexpr auto minimum = [](auto&& xs[, auto&& predicate]) -> decltype(auto) {
         return tag-dispatched;
     };
 #else
     template <typename T, typename = void>
     struct minimum_impl : minimum_impl<T, when<true>> { };
 
     template <typename T, typename = void>
     struct minimum_pred_impl : minimum_pred_impl<T, when<true>> { };
 
     struct minimum_t : detail::nested_by<minimum_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 minimum_t minimum{};
 #endif
 }} // end namespace boost::hana
 
 #endif // !BOOST_HANA_FWD_MINIMUM_HPP

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