/*=============================================================================

    Copyright (c) 2001-2006 Joel de Guzman



    Distributed under 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)

==============================================================================*/

#if !defined(FUSION_AT_IMPL_07172005_0726)

#define FUSION_AT_IMPL_07172005_0726



#include <boost/fusion/support/detail/access.hpp>

#include <boost/type_traits/is_const.hpp>

#include <boost/type_traits/add_const.hpp>

#include <boost/mpl/eval_if.hpp>

#include <boost/mpl/bool.hpp>



namespace boost { namespace fusion

{

    struct cons_tag;



    namespace extension

    {

        template <typename Tag>

        struct at_impl;



        template <>

        struct at_impl<cons_tag>

        {

            template <typename Sequence, typename N>

            struct apply 

            {

                typedef typename

                    mpl::eval_if<

                        is_const<Sequence>

                      , add_const<typename Sequence::cdr_type>

                      , mpl::identity<typename Sequence::cdr_type>

                    >::type

                cdr_type;



                typedef typename 

                    mpl::eval_if<

                        mpl::bool_<N::value == 0>

                      , mpl::identity<typename Sequence::car_type>

                      , apply<cdr_type, mpl::int_<N::value-1> >

                    >

                element;



                typedef typename

                    mpl::eval_if<

                        is_const<Sequence>

                      , detail::cref_result<element>

                      , detail::ref_result<element>

                    >::type

                type;



                template <typename Cons, int N2>

                static type

                call(Cons& s, mpl::int_<N2>)

                {

                    return call(s.cdr, mpl::int_<N2-1>());

                }



                template <typename Cons>

                static type

                call(Cons& s, mpl::int_<0>)

                {

                    return s.car;

                }



                static type

                call(Sequence& s)

                {

                    return call(s, mpl::int_<N::value>());

                }

            };

        };

    }

}}



#endif

