fold.hpp

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                    typedef
                        typename fusion::BOOST_PROTO_FUSION_RESULT_OF::fold<
                            BOOST_PROTO_AS_FUSION_SEQUENCE_TYPE(sequence)
                          , state0
                          , fun
                        >::type
                    type;
                };

                /// Let \c seq be <tt>when\<_, Sequence\>()(expr, state, visitor)</tt>, let
                /// \c state0 be <tt>when\<_, State0\>()(expr, state, visitor)</tt>, and
                /// let \c fun(visitor) be an object such that <tt>fun(visitor)(expr, state)</tt>
                /// is equivalent to <tt>when\<_, Fun\>()(expr, state, visitor)</tt>. Then, this
                /// function returns <tt>fusion::fold(seq, state0, fun(visitor))</tt>.
                ///
                /// \param expr The current expression
                /// \param state The current state
                /// \param visitor An arbitrary visitor
                template<typename Expr, typename State, typename Visitor>
                typename result<void(Expr, State, Visitor)>::type
                operator ()(Expr const &expr, State const &state, Visitor &visitor) const
                {
                    when<_, Sequence> sequence;
                    detail::as_callable<Fun, Visitor> fun(visitor);
                    return fusion::fold(
                        BOOST_PROTO_AS_FUSION_SEQUENCE(sequence(expr, state, visitor))
                      , when<_, State0>()(expr, state, visitor)
                      , fun
                    );
                }
            };

            /// \brief A PrimitiveTransform that is the same as the
            /// <tt>fold\<\></tt> transform, except that it folds
            /// back-to-front instead of front-to-back. It uses
            /// the \c _reverse callable PolymorphicFunctionObject
            /// to create a <tt>fusion::reverse_view\<\></tt> of the
            /// sequence before invoking <tt>fusion::fold\<\></tt>.
            template<typename Sequence, typename State0, typename Fun>
            struct reverse_fold
              : fold<call<_reverse(Sequence)>, State0, Fun>
            {};

            // This specialization is only for improved compile-time performance
            // in the commom case when the Sequence transform is \c proto::_.
            //
            /// INTERNAL ONLY
            ///
            template<typename State0, typename Fun>
            struct fold<_, State0, Fun> : proto::callable
            {
                template<typename Sig>
                struct result;

                template<typename This, typename Expr, typename State, typename Visitor>
                struct result<This(Expr, State, Visitor)>
                {
                    typedef
                        typename detail::fold_impl<
                            Fun
                          , typename Expr::proto_base_expr
                          , typename when<_, State0>::template result<void(Expr, State, Visitor)>::type
                          , Visitor
                        >::type
                    type;
                };

                template<typename Expr, typename State, typename Visitor>
                typename result<void(Expr, State, Visitor)>::type
                operator ()(Expr const &expr, State const &state, Visitor &visitor) const
                {
                    typedef
                        detail::fold_impl<
                            Fun
                          , typename Expr::proto_base_expr
                          , typename when<_, State0>::template result<void(Expr, State, Visitor)>::type
                          , Visitor
                        >
                    impl;

                    return impl::call(
                        expr.proto_base()
                      , when<_, State0>()(expr, state, visitor)
                      , visitor
                    );
                }
            };

            // This specialization is only for improved compile-time performance
            // in the commom case when the Sequence transform is \c proto::_.
            //
            /// INTERNAL ONLY
            ///
            template<typename State0, typename Fun>
            struct reverse_fold<_, State0, Fun> : proto::callable
            {
                template<typename Sig>
                struct result;

                template<typename This, typename Expr, typename State, typename Visitor>
                struct result<This(Expr, State, Visitor)>
                {
                    typedef
                        typename detail::reverse_fold_impl<
                            Fun
                          , typename Expr::proto_base_expr
                          , typename when<_, State0>::template result<void(Expr, State, Visitor)>::type
                          , Visitor
                        >::type
                    type;
                };

                template<typename Expr, typename State, typename Visitor>
                typename result<void(Expr, State, Visitor)>::type
                operator ()(Expr const &expr, State const &state, Visitor &visitor) const
                {
                    typedef
                        detail::reverse_fold_impl<
                            Fun
                          , typename Expr::proto_base_expr
                          , typename when<_, State0>::template result<void(Expr, State, Visitor)>::type
                          , Visitor
                        >
                    impl;

                    return impl::call(
                        expr.proto_base()
                      , when<_, State0>()(expr, state, visitor)
                      , visitor
                    );
                }
            };
        }

        /// INTERNAL ONLY
        ///
        template<typename Sequence, typename State, typename Fun>
        struct is_callable<transform::fold<Sequence, State, Fun> >
          : mpl::true_
        {};

        /// INTERNAL ONLY
        ///
        template<typename Sequence, typename State, typename Fun>
        struct is_callable<transform::reverse_fold<Sequence, State, Fun> >
          : mpl::true_
        {};

    }}

    #endif

#else

    #define N BOOST_PP_ITERATION()

            template<typename Fun, typename Expr, typename state0, typename Visitor>
            struct fold_impl<Fun, Expr, state0, Visitor, N>
            {
                BOOST_PP_REPEAT(N, BOOST_PROTO_FOLD_STATE_TYPE, N)
                typedef BOOST_PP_CAT(state, N) type;

                static type call(Expr const &expr, state0 const &s0, Visitor &visitor)
                {
                    BOOST_PP_REPEAT(N, BOOST_PROTO_FOLD_STATE, N)
                    return BOOST_PP_CAT(s, N);
                }
            };

            template<typename Fun, typename Expr, typename BOOST_PP_CAT(state, N), typename Visitor>
            struct reverse_fold_impl<Fun, Expr, BOOST_PP_CAT(state, N), Visitor, N>
            {
                BOOST_PP_REPEAT(N, BOOST_PROTO_REVERSE_FOLD_STATE_TYPE, N)
                typedef state0 type;

                static type call(Expr const &expr, BOOST_PP_CAT(state, N) const &BOOST_PP_CAT(s, N), Visitor &visitor)
                {
                    BOOST_PP_REPEAT(N, BOOST_PROTO_REVERSE_FOLD_STATE, N)
                    return s0;
                }
            };

    #undef N

#endif