text: /*!
  @file
  Defines `boost::hana::unfold_left`.
  
  @copyright Louis Dionne 2013-2016
  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_UNFOLD_LEFT_HPP
  #define BOOST_HANA_UNFOLD_LEFT_HPP
  
  #include <hana/fwd/unfold_left.hpp>
  
  #include <hana/append.hpp>
  #include <hana/concept/sequence.hpp>
  #include <hana/config.hpp>
  #include <hana/core/dispatch.hpp>
  #include <hana/empty.hpp>
  #include <hana/first.hpp>
  #include <hana/functional/partial.hpp>
  #include <hana/optional.hpp>
  #include <hana/second.hpp>
  
  
  BOOST_HANA_NAMESPACE_BEGIN
      //! @cond
      template <typename S>
      struct unfold_left_t {
      #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
          static_assert(hana::Sequence<S>::value,
          "hana::unfold_left<S> requires 'S' to be a Sequence");
      #endif
  
          template <typename State, typename F>
          constexpr auto operator()(State&& state, F&& f) const {
              return unfold_left_impl<S>::apply(
                  static_cast<State&&>(state),
                  static_cast<F&&>(f)
              );
          }
      };
      //! @endcond
  
      template <typename S, bool condition>
      struct unfold_left_impl<S, when<condition>> : default_ {
          struct unfold_left_helper {
              template <typename F, typename P>
              constexpr auto operator()(F&& f, P&& p) const {
                  return hana::append(
                      unfold_left_impl::apply(
                          hana::first(static_cast<P&&>(p)),
                          static_cast<F&&>(f)
                      ),
                      hana::second(static_cast<P&&>(p))
                  );
              }
          };
  
          template <typename Init, typename F>
          static constexpr auto apply(Init&& init, F&& f) {
              decltype(auto) elt = f(static_cast<Init&&>(init));
              return hana::maybe(empty<S>(),
                  hana::partial(unfold_left_helper{}, static_cast<F&&>(f)),
                  static_cast<decltype(elt)&&>(elt)
              );
          }
      };
  BOOST_HANA_NAMESPACE_END
  
  #endif // !BOOST_HANA_UNFOLD_LEFT_HPP
  