text: /// \file
  // Range v3 library
  //
  //  Copyright Eric Niebler 2014-present
  //
  //  Use, modification and distribution is subject to the
  //  Boost Software License, Version 1.0. (See accompanying
  //  file LICENSE_1_0.txt or copy at
  //  http://www.boost.org/LICENSE_1_0.txt)
  //
  // Project home: https://github.com/ericniebler/range-v3
  //
  
  #ifndef RANGES_V3_VIEW_FOR_EACH_HPP
  #define RANGES_V3_VIEW_FOR_EACH_HPP
  
  #include <utility>
  
  #include <meta/meta.hpp>
  
  #include <range/v3/range_fwd.hpp>
  
  #include <range/v3/functional/bind_back.hpp>
  #include <range/v3/functional/invoke.hpp>
  #include <range/v3/utility/static_const.hpp>
  #include <range/v3/view/all.hpp>
  #include <range/v3/view/generate_n.hpp>
  #include <range/v3/view/join.hpp>
  #include <range/v3/view/repeat_n.hpp>
  #include <range/v3/view/single.hpp>
  #include <range/v3/view/transform.hpp>
  #include <range/v3/view/view.hpp>
  
  #include <range/v3/detail/prologue.hpp>
  
  namespace ranges
  {
      /// \addtogroup group-views
      /// @{
  
      namespace views
      {
          /// Lazily applies an unary function to each element in the source
          /// range that returns another range (possibly empty), flattening
          /// the result.
          struct for_each_base_fn
          {
              template(typename Rng, typename Fun)(
                  /// \pre
                  requires viewable_range<Rng> AND transformable_range<Rng, Fun> AND
                      joinable_range<transform_view<all_t<Rng>, Fun>>)
              constexpr auto operator()(Rng && rng, Fun fun) const
              {
                  return join(transform(static_cast<Rng &&>(rng), std::move(fun)));
              }
          };
  
          struct for_each_fn : for_each_base_fn
          {
              using for_each_base_fn::operator();
  
              template<typename Fun>
              constexpr auto operator()(Fun fun) const
              {
                  return make_view_closure(bind_back(for_each_base_fn{}, std::move(fun)));
              }
          };
  
          /// \relates for_each_fn
          RANGES_INLINE_VARIABLE(for_each_fn, for_each)
      } // namespace views
  
      struct yield_fn
      {
          template(typename V)(
              /// \pre
              requires copy_constructible<V>)
          single_view<V> operator()(V v) const
          {
              return views::single(std::move(v));
          }
      };
  
      /// \relates yield_fn
      RANGES_INLINE_VARIABLE(yield_fn, yield)
  
      struct yield_from_fn
      {
          template(typename Rng)(
              /// \pre
              requires view_<Rng>)
          Rng operator()(Rng rng) const
          {
              return rng;
          }
      };
  
      /// \relates yield_from_fn
      RANGES_INLINE_VARIABLE(yield_from_fn, yield_from)
  
      struct yield_if_fn
      {
          template<typename V>
          repeat_n_view<V> operator()(bool b, V v) const
          {
              return views::repeat_n(std::move(v), b ? 1 : 0);
          }
      };
  
      /// \relates yield_if_fn
      RANGES_INLINE_VARIABLE(yield_if_fn, yield_if)
  
      struct lazy_yield_if_fn
      {
          template(typename F)(
              /// \pre
              requires invocable<F &>)
          generate_n_view<F> operator()(bool b, F f) const
          {
              return views::generate_n(std::move(f), b ? 1 : 0);
          }
      };
  
      /// \relates lazy_yield_if_fn
      RANGES_INLINE_VARIABLE(lazy_yield_if_fn, lazy_yield_if)
      /// @}
  
      /// \cond
      template(typename Rng, typename Fun)(
          /// \pre
          requires viewable_range<Rng> AND views::transformable_range<Rng, Fun> AND
              input_range<invoke_result_t<Fun &, range_reference_t<Rng>>>)
          auto
          operator>>=(Rng && rng, Fun fun)
      {
          return views::for_each(static_cast<Rng &&>(rng), std::move(fun));
      }
      /// \endcond
  } // namespace ranges
  
  #include <range/v3/detail/epilogue.hpp>
  
  #endif
  