boost/hana/sum.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::sum`.
   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_SUM_HPP
  11 #define BOOST_HANA_SUM_HPP
  12
  13 #include <boost/hana/fwd/sum.hpp>
  14
  15 #include <boost/hana/concept/foldable.hpp>
  16 #include <boost/hana/concept/monoid.hpp>
  17 #include <boost/hana/config.hpp>
  18 #include <boost/hana/core/dispatch.hpp>
  19 #include <boost/hana/fold_left.hpp>
  20 #include <boost/hana/integral_constant.hpp> // required by fwd decl
  21 #include <boost/hana/plus.hpp>
  22 #include <boost/hana/zero.hpp>
  23
  24
  25 BOOST_HANA_NAMESPACE_BEGIN
  26     template <typename M>
  27     struct sum_t {
  28     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  29         static_assert(hana::Monoid<M>::value,
  30         "hana::sum<M> requires 'M' to be a Monoid");
  31     #endif
  32
  33         template <typename Xs>
  34         constexpr decltype(auto) operator()(Xs&& xs) const {
  35             using S = typename hana::tag_of<Xs>::type;
  36             using Sum = BOOST_HANA_DISPATCH_IF(sum_impl<S>,
  37                 hana::Foldable<S>::value
  38             );
  39
  40         #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  41             static_assert(hana::Foldable<S>::value,
  42             "hana::sum<M>(xs) requires 'xs' to be Foldable");
  43         #endif
  44
  45             return Sum::template apply<M>(static_cast<Xs&&>(xs));
  46         }
  47     };
  48
  49     template <typename T, bool condition>
  50     struct sum_impl<T, when<condition>> : default_ {
  51         template <typename M, typename Xs>
  52         static constexpr decltype(auto) apply(Xs&& xs) {
  53             return hana::fold_left(static_cast<Xs&&>(xs), hana::zero<M>(), hana::plus);
  54         }
  55     };
  56 BOOST_HANA_NAMESPACE_END
  57
  58 #endif // !BOOST_HANA_SUM_HPP