boost/hana/monadic_fold_left.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::monadic_fold_left`.
   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_MONADIC_FOLD_LEFT_HPP
  11 #define BOOST_HANA_MONADIC_FOLD_LEFT_HPP
  12
  13 #include <boost/hana/fwd/monadic_fold_left.hpp>
  14
  15 #include <boost/hana/chain.hpp>
  16 #include <boost/hana/concept/foldable.hpp>
  17 #include <boost/hana/concept/monad.hpp>
  18 #include <boost/hana/config.hpp>
  19 #include <boost/hana/core/dispatch.hpp>
  20 #include <boost/hana/detail/decay.hpp>
  21 #include <boost/hana/fold_right.hpp>
  22 #include <boost/hana/functional/curry.hpp>
  23 #include <boost/hana/functional/partial.hpp>
  24 #include <boost/hana/lift.hpp>
  25
  26 #include <type_traits>
  27
  28
  29 BOOST_HANA_NAMESPACE_BEGIN
  30     template <typename M>
  31     struct monadic_fold_left_t {
  32     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  33         static_assert(hana::Monad<M>::value,
  34         "hana::monadic_fold_left<M> requires 'M' to be a Monad");
  35     #endif
  36
  37         template <typename Xs, typename State, typename F>
  38         constexpr decltype(auto) operator()(Xs&& xs, State&& state, F&& f) const {
  39             using S = typename hana::tag_of<Xs>::type;
  40             using MonadicFoldLeft = BOOST_HANA_DISPATCH_IF(monadic_fold_left_impl<S>,
  41                 hana::Foldable<S>::value
  42             );
  43
  44         #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  45             static_assert(hana::Foldable<S>::value,
  46             "hana::monadic_fold_left<M>(xs, state, f) requires 'xs' to be Foldable");
  47         #endif
  48
  49             return MonadicFoldLeft::template apply<M>(static_cast<Xs&&>(xs),
  50                                                       static_cast<State&&>(state),
  51                                                       static_cast<F&&>(f));
  52         }
  53
  54         template <typename Xs, typename F>
  55         constexpr decltype(auto) operator()(Xs&& xs, F&& f) const {
  56             using S = typename hana::tag_of<Xs>::type;
  57             using MonadicFoldLeft = BOOST_HANA_DISPATCH_IF(monadic_fold_left_impl<S>,
  58                 hana::Foldable<S>::value
  59             );
  60
  61         #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  62             static_assert(hana::Foldable<S>::value,
  63             "hana::monadic_fold_left<M>(xs, f) requires 'xs' to be Foldable");
  64         #endif
  65
  66             return MonadicFoldLeft::template apply<M>(static_cast<Xs&&>(xs),
  67                                                       static_cast<F&&>(f));
  68         }
  69     };
  70
  71     namespace detail {
  72         struct foldlM_helper {
  73             template <typename F, typename X, typename K, typename Z>
  74             constexpr decltype(auto) operator()(F&& f, X&& x, K&& k, Z&& z) const {
  75                 return hana::chain(
  76                     static_cast<F&&>(f)(
  77                         static_cast<Z&&>(z),
  78                         static_cast<X&&>(x)
  79                     ),
  80                     static_cast<K&&>(k)
  81                 );
  82             }
  83         };
  84
  85         template <typename End, typename M, typename F>
  86         struct monadic_foldl1_helper {
  87             F f;
  88             template <typename X, typename Y>
  89             constexpr decltype(auto) operator()(X&& x, Y&& y) const
  90             { return f(static_cast<X&&>(x), static_cast<Y&&>(y)); }
  91             template <typename Y>
  92             constexpr decltype(auto) operator()(End, Y&& y) const
  93             { return hana::lift<M>(static_cast<Y&&>(y)); }
  94         };
  95     }
  96
  97     template <typename T, bool condition>
  98     struct monadic_fold_left_impl<T, when<condition>> : default_ {
  99         // with state
 100         template <typename M, typename Xs, typename S, typename F>
 101         static constexpr decltype(auto) apply(Xs&& xs, S&& s, F&& f) {
 102             return hana::fold_right(
 103                 static_cast<Xs&&>(xs),
 104                 hana::lift<M>,
 105                 hana::curry<3>(hana::partial(
 106                     detail::foldlM_helper{}, static_cast<F&&>(f)
 107                 ))
 108             )(static_cast<S&&>(s));
 109         }
 110
 111         // without state
 112         template <typename M, typename Xs, typename F>
 113         static constexpr decltype(auto) apply(Xs&& xs, F&& f) {
 114             struct end { };
 115             using G = detail::monadic_foldl1_helper<end, M, typename detail::decay<F>::type>;
 116             decltype(auto) result = hana::monadic_fold_left<M>(
 117                 static_cast<Xs&&>(xs),
 118                 end{},
 119                 G{static_cast<F&&>(f)}
 120             );
 121
 122             static_assert(!std::is_same<
 123                 std::remove_reference_t<decltype(result)>,
 124                 decltype(hana::lift<M>(end{}))
 125             >{},
 126             "hana::monadic_fold_left<M>(xs, f) requires 'xs' to be non-empty");
 127             return result;
 128         }
 129     };
 130 BOOST_HANA_NAMESPACE_END
 131
 132 #endif // !BOOST_HANA_MONADIC_FOLD_LEFT_HPP