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Editing: fold_tree.hpp
/////////////////////////////////////////////////////////////////////////////// /// \file fold_tree.hpp /// Contains definition of the fold_tree<> and reverse_fold_tree<> transforms. // // Copyright 2008 Eric Niebler. 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) #ifndef BOOST_PROTO_TRANSFORM_FOLD_TREE_HPP_EAN_11_05_2007 #define BOOST_PROTO_TRANSFORM_FOLD_TREE_HPP_EAN_11_05_2007 #include <boost/type_traits/is_same.hpp> #include <boost/proto/proto_fwd.hpp> #include <boost/proto/traits.hpp> #include <boost/proto/matches.hpp> #include <boost/proto/transform/fold.hpp> #include <boost/proto/transform/impl.hpp> namespace boost { namespace proto { namespace detail { template<typename Tag> struct has_tag { template<typename Expr, typename State, typename Data, typename EnableIf = Tag> struct impl { typedef mpl::false_ result_type; }; template<typename Expr, typename State, typename Data> struct impl<Expr, State, Data, typename Expr::proto_tag> { typedef mpl::true_ result_type; }; template<typename Expr, typename State, typename Data> struct impl<Expr &, State, Data, typename Expr::proto_tag> { typedef mpl::true_ result_type; }; }; template<typename Tag, typename Fun> struct fold_tree_ : if_<has_tag<Tag>, fold<_, _state, fold_tree_<Tag, Fun> >, Fun> {}; template<typename Tag, typename Fun> struct reverse_fold_tree_ : if_<has_tag<Tag>, reverse_fold<_, _state, reverse_fold_tree_<Tag, Fun> >, Fun> {}; } /// \brief A PrimitiveTransform that recursively applies the /// <tt>fold\<\></tt> transform to sub-trees that all share a common /// tag type. /// /// <tt>fold_tree\<\></tt> is useful for flattening trees into lists; /// for example, you might use <tt>fold_tree\<\></tt> to flatten an /// expression tree like <tt>a | b | c</tt> into a Fusion list like /// <tt>cons(c, cons(b, cons(a)))</tt>. /// /// <tt>fold_tree\<\></tt> is easily understood in terms of a /// <tt>recurse_if_\<\></tt> helper, defined as follows: /// /// \code /// template<typename Tag, typename Fun> /// struct recurse_if_ /// : if_< /// // If the current node has type "Tag" ... /// is_same<tag_of<_>, Tag>() /// // ... recurse, otherwise ... /// , fold<_, _state, recurse_if_<Tag, Fun> > /// // ... apply the Fun transform. /// , Fun /// > /// {}; /// \endcode /// /// With <tt>recurse_if_\<\></tt> as defined above, /// <tt>fold_tree\<Sequence, State0, Fun\>()(e, s, d)</tt> is /// equivalent to /// <tt>fold<Sequence, State0, recurse_if_<Expr::proto_tag, Fun> >()(e, s, d).</tt> /// It has the effect of folding a tree front-to-back, recursing into /// child nodes that share a tag type with the parent node. template<typename Sequence, typename State0, typename Fun> struct fold_tree : transform<fold_tree<Sequence, State0, Fun> > { template<typename Expr, typename State, typename Data> struct impl : fold< Sequence , State0 , detail::fold_tree_<typename Expr::proto_tag, Fun> >::template impl<Expr, State, Data> {}; template<typename Expr, typename State, typename Data> struct impl<Expr &, State, Data> : fold< Sequence , State0 , detail::fold_tree_<typename Expr::proto_tag, Fun> >::template impl<Expr &, State, Data> {}; }; /// \brief A PrimitiveTransform that recursively applies the /// <tt>reverse_fold\<\></tt> transform to sub-trees that all share /// a common tag type. /// /// <tt>reverse_fold_tree\<\></tt> is useful for flattening trees into /// lists; for example, you might use <tt>reverse_fold_tree\<\></tt> to /// flatten an expression tree like <tt>a | b | c</tt> into a Fusion list /// like <tt>cons(a, cons(b, cons(c)))</tt>. /// /// <tt>reverse_fold_tree\<\></tt> is easily understood in terms of a /// <tt>recurse_if_\<\></tt> helper, defined as follows: /// /// \code /// template<typename Tag, typename Fun> /// struct recurse_if_ /// : if_< /// // If the current node has type "Tag" ... /// is_same<tag_of<_>, Tag>() /// // ... recurse, otherwise ... /// , reverse_fold<_, _state, recurse_if_<Tag, Fun> > /// // ... apply the Fun transform. /// , Fun /// > /// {}; /// \endcode /// /// With <tt>recurse_if_\<\></tt> as defined above, /// <tt>reverse_fold_tree\<Sequence, State0, Fun\>()(e, s, d)</tt> is /// equivalent to /// <tt>reverse_fold<Sequence, State0, recurse_if_<Expr::proto_tag, Fun> >()(e, s, d).</tt> /// It has the effect of folding a tree back-to-front, recursing into /// child nodes that share a tag type with the parent node. template<typename Sequence, typename State0, typename Fun> struct reverse_fold_tree : transform<reverse_fold_tree<Sequence, State0, Fun> > { template<typename Expr, typename State, typename Data> struct impl : reverse_fold< Sequence , State0 , detail::reverse_fold_tree_<typename Expr::proto_tag, Fun> >::template impl<Expr, State, Data> {}; template<typename Expr, typename State, typename Data> struct impl<Expr &, State, Data> : reverse_fold< Sequence , State0 , detail::reverse_fold_tree_<typename Expr::proto_tag, Fun> >::template impl<Expr &, State, Data> {}; }; /// INTERNAL ONLY /// template<typename Sequence, typename State0, typename Fun> struct is_callable<fold_tree<Sequence, State0, Fun> > : mpl::true_ {}; /// INTERNAL ONLY /// template<typename Sequence, typename State0, typename Fun> struct is_callable<reverse_fold_tree<Sequence, State0, Fun> > : mpl::true_ {}; }} #endif
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