ceph/src/boost/libs/hana/include/boost/hana/fwd/minimum.hpp

   1 /*!
   2 @file
   3 Forward declares `boost::hana::minimum`.
   4
   5 @copyright Louis Dionne 2013-2016
   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_MINIMUM_HPP
  11 #define BOOST_HANA_FWD_MINIMUM_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/core/when.hpp>
  15 #include <boost/hana/detail/nested_by_fwd.hpp>
  16
  17
  18 BOOST_HANA_NAMESPACE_BEGIN
  19     //! Return the least element of a non-empty structure with respect to
  20     //! a `predicate`, by default `less`.
  21     //! @ingroup group-Foldable
  22     //!
  23     //! Given a non-empty structure and an optional binary predicate
  24     //! (`less` by default), `minimum` returns the least element of
  25     //! the structure, i.e. an element which is less than or equal to
  26     //! every other element in the structure, according to the predicate.
  27     //!
  28     //! If the structure contains heterogeneous objects, then the predicate
  29     //! must return a compile-time `Logical`. If no predicate is provided,
  30     //! the elements in the structure must be Orderable, or compile-time
  31     //! Orderable if the structure is heterogeneous.
  32     //!
  33     //!
  34     //! Signature
  35     //! ---------
  36     //! Given a `Foldable` `F`, a Logical `Bool` and a predicate
  37     //! \f$ \mathtt{pred} : T \times T \to Bool \f$, `minimum` has the
  38     //! following signatures. For the variant with a provided predicate,
  39     //! \f[
  40     //!     \mathtt{minimum} : F(T) \times (T \times T \to Bool) \to T
  41     //! \f]
  42     //!
  43     //! for the variant without a custom predicate, `T` is required to be
  44     //! Orderable. The signature is then
  45     //! \f[
  46     //!     \mathtt{minimum} : F(T) \to T
  47     //! \f]
  48     //!
  49     //! @param xs
  50     //! The structure to find the least element of.
  51     //!
  52     //! @param predicate
  53     //! A function called as `predicate(x, y)`, where `x` and `y` are elements
  54     //! of the structure. `predicate` should be a strict weak ordering on the
  55     //! elements of the structure and its return value should be a Logical,
  56     //! or a compile-time Logical if the structure is heterogeneous.
  57     //!
  58     //! ### Example
  59     //! @include example/minimum.cpp
  60     //!
  61     //!
  62     //! Syntactic sugar (`minimum.by`)
  63     //! ------------------------------
  64     //! `minimum` can be called in a third way, which provides a nice syntax
  65     //! especially when working with the `ordering` combinator:
  66     //! @code
  67     //!     minimum.by(predicate, xs) == minimum(xs, predicate)
  68     //!     minimum.by(predicate) == minimum(-, predicate)
  69     //! @endcode
  70     //!
  71     //! where `minimum(-, predicate)` denotes the partial application of
  72     //! `minimum` to `predicate`.
  73     //!
  74     //! ### Example
  75     //! @include example/minimum_by.cpp
  76     //!
  77     //!
  78     //! Tag dispatching
  79     //! ---------------
  80     //! Both the non-predicated version and the predicated versions of
  81     //! `minimum` are tag-dispatched methods, and hence they can be
  82     //! customized independently. One reason for this is that some
  83     //! structures are able to provide a much more efficient implementation
  84     //! of `minimum` when the `less` predicate is used. Here is how the
  85     //! different versions of `minimum` are dispatched:
  86     //! @code
  87     //!     minimum(xs) -> minimum_impl<tag of xs>::apply(xs)
  88     //!     minimum(xs, pred) -> minimum_pred_impl<tag of xs>::apply(xs, pred)
  89     //! @endcode
  90     //!
  91     //! Also note that `minimum.by` is not tag-dispatched on its own, since it
  92     //! is just syntactic sugar for calling the corresponding `minimum`.
  93 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  94     constexpr auto minimum = [](auto&& xs[, auto&& predicate]) -> decltype(auto) {
  95         return tag-dispatched;
  96     };
  97 #else
  98     template <typename T, typename = void>
  99     struct minimum_impl : minimum_impl<T, when<true>> { };
 100
 101     template <typename T, typename = void>
 102     struct minimum_pred_impl : minimum_pred_impl<T, when<true>> { };
 103
 104     struct minimum_t : detail::nested_by<minimum_t> {
 105         template <typename Xs>
 106         constexpr decltype(auto) operator()(Xs&& xs) const;
 107
 108         template <typename Xs, typename Predicate>
 109         constexpr decltype(auto) operator()(Xs&& xs, Predicate&& pred) const;
 110     };
 111
 112     constexpr minimum_t minimum{};
 113 #endif
 114 BOOST_HANA_NAMESPACE_END
 115
 116 #endif // !BOOST_HANA_FWD_MINIMUM_HPP