boost/hana/fold_right.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::fold_right`.
   4
   5 @copyright Louis Dionne 2013-2017
   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_FOLD_RIGHT_HPP
  11 #define BOOST_HANA_FOLD_RIGHT_HPP
  12
  13 #include <boost/hana/fwd/fold_right.hpp>
  14
  15 #include <boost/hana/concept/foldable.hpp>
  16 #include <boost/hana/config.hpp>
  17 #include <boost/hana/core/dispatch.hpp>
  18 #include <boost/hana/detail/variadic/foldr1.hpp>
  19 #include <boost/hana/functional/partial.hpp>
  20 #include <boost/hana/fwd/unpack.hpp>
  21
  22
  23 BOOST_HANA_NAMESPACE_BEGIN
  24     //! @cond
  25     template <typename Xs, typename State, typename F>
  26     constexpr decltype(auto) fold_right_t::operator()(Xs&& xs, State&& state, F&& f) const {
  27         using S = typename hana::tag_of<Xs>::type;
  28         using FoldRight = BOOST_HANA_DISPATCH_IF(fold_right_impl<S>,
  29             hana::Foldable<S>::value
  30         );
  31
  32     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  33         static_assert(hana::Foldable<S>::value,
  34         "hana::fold_right(xs, state, f) requires 'xs' to be Foldable");
  35     #endif
  36
  37         return FoldRight::apply(static_cast<Xs&&>(xs),
  38                                 static_cast<State&&>(state),
  39                                 static_cast<F&&>(f));
  40     }
  41
  42     template <typename Xs, typename F>
  43     constexpr decltype(auto) fold_right_t::operator()(Xs&& xs, F&& f) const {
  44         using S = typename hana::tag_of<Xs>::type;
  45         using FoldRight = BOOST_HANA_DISPATCH_IF(fold_right_impl<S>,
  46             hana::Foldable<S>::value
  47         );
  48
  49     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  50         static_assert(hana::Foldable<S>::value,
  51         "hana::fold_right(xs, f) requires 'xs' to be Foldable");
  52     #endif
  53
  54         return FoldRight::apply(static_cast<Xs&&>(xs), static_cast<F&&>(f));
  55     }
  56     //! @endcond
  57
  58     namespace detail {
  59         template <typename F, typename State>
  60         struct variadic_foldr {
  61             F& f;
  62             State& state;
  63             template <typename ...T>
  64             constexpr decltype(auto) operator()(T&& ...t) const {
  65                 return detail::variadic::foldr(
  66                     static_cast<F&&>(f),
  67                     static_cast<State&&>(state),
  68                     static_cast<T&&>(t)...
  69                 );
  70             }
  71         };
  72     }
  73
  74     template <typename T, bool condition>
  75     struct fold_right_impl<T, when<condition>> : default_ {
  76         // with state
  77         template <typename Xs, typename S, typename F>
  78         static constexpr decltype(auto) apply(Xs&& xs, S&& s, F&& f) {
  79             return hana::unpack(static_cast<Xs&&>(xs),
  80                 detail::variadic_foldr<F, S>{f, s}
  81             );
  82         }
  83
  84         // without state
  85         template <typename Xs, typename F>
  86         static constexpr decltype(auto) apply(Xs&& xs, F&& f) {
  87             return hana::unpack(static_cast<Xs&&>(xs),
  88                 hana::partial(
  89                     detail::variadic::foldr1,
  90                     static_cast<F&&>(f)
  91                 )
  92             );
  93         }
  94     };
  95 BOOST_HANA_NAMESPACE_END
  96
  97 #endif // !BOOST_HANA_FOLD_RIGHT_HPP