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1 | /////////////////////////////////////////////////////////////////////////////// |
2 | /// \file fold_tree.hpp | |
3 | /// Contains definition of the fold_tree<> and reverse_fold_tree<> transforms. | |
4 | // | |
5 | // Copyright 2008 Eric Niebler. Distributed under the Boost | |
6 | // Software License, Version 1.0. (See accompanying file | |
7 | // LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) | |
8 | ||
9 | #ifndef BOOST_PROTO_TRANSFORM_FOLD_TREE_HPP_EAN_11_05_2007 | |
10 | #define BOOST_PROTO_TRANSFORM_FOLD_TREE_HPP_EAN_11_05_2007 | |
11 | ||
12 | #include <boost/type_traits/is_same.hpp> | |
13 | #include <boost/proto/proto_fwd.hpp> | |
14 | #include <boost/proto/traits.hpp> | |
15 | #include <boost/proto/matches.hpp> | |
16 | #include <boost/proto/transform/fold.hpp> | |
17 | #include <boost/proto/transform/impl.hpp> | |
18 | ||
19 | namespace boost { namespace proto | |
20 | { | |
21 | namespace detail | |
22 | { | |
23 | template<typename Tag> | |
24 | struct has_tag | |
25 | { | |
26 | template<typename Expr, typename State, typename Data, typename EnableIf = Tag> | |
27 | struct impl | |
28 | { | |
29 | typedef mpl::false_ result_type; | |
30 | }; | |
31 | ||
32 | template<typename Expr, typename State, typename Data> | |
33 | struct impl<Expr, State, Data, typename Expr::proto_tag> | |
34 | { | |
35 | typedef mpl::true_ result_type; | |
36 | }; | |
37 | ||
38 | template<typename Expr, typename State, typename Data> | |
39 | struct impl<Expr &, State, Data, typename Expr::proto_tag> | |
40 | { | |
41 | typedef mpl::true_ result_type; | |
42 | }; | |
43 | }; | |
44 | ||
45 | template<typename Tag, typename Fun> | |
46 | struct fold_tree_ | |
47 | : if_<has_tag<Tag>, fold<_, _state, fold_tree_<Tag, Fun> >, Fun> | |
48 | {}; | |
49 | ||
50 | template<typename Tag, typename Fun> | |
51 | struct reverse_fold_tree_ | |
52 | : if_<has_tag<Tag>, reverse_fold<_, _state, reverse_fold_tree_<Tag, Fun> >, Fun> | |
53 | {}; | |
54 | } | |
55 | ||
56 | /// \brief A PrimitiveTransform that recursively applies the | |
57 | /// <tt>fold\<\></tt> transform to sub-trees that all share a common | |
58 | /// tag type. | |
59 | /// | |
60 | /// <tt>fold_tree\<\></tt> is useful for flattening trees into lists; | |
61 | /// for example, you might use <tt>fold_tree\<\></tt> to flatten an | |
62 | /// expression tree like <tt>a | b | c</tt> into a Fusion list like | |
63 | /// <tt>cons(c, cons(b, cons(a)))</tt>. | |
64 | /// | |
65 | /// <tt>fold_tree\<\></tt> is easily understood in terms of a | |
66 | /// <tt>recurse_if_\<\></tt> helper, defined as follows: | |
67 | /// | |
68 | /// \code | |
69 | /// template<typename Tag, typename Fun> | |
70 | /// struct recurse_if_ | |
71 | /// : if_< | |
72 | /// // If the current node has type "Tag" ... | |
73 | /// is_same<tag_of<_>, Tag>() | |
74 | /// // ... recurse, otherwise ... | |
75 | /// , fold<_, _state, recurse_if_<Tag, Fun> > | |
76 | /// // ... apply the Fun transform. | |
77 | /// , Fun | |
78 | /// > | |
79 | /// {}; | |
80 | /// \endcode | |
81 | /// | |
82 | /// With <tt>recurse_if_\<\></tt> as defined above, | |
83 | /// <tt>fold_tree\<Sequence, State0, Fun\>()(e, s, d)</tt> is | |
84 | /// equivalent to | |
85 | /// <tt>fold<Sequence, State0, recurse_if_<Expr::proto_tag, Fun> >()(e, s, d).</tt> | |
86 | /// It has the effect of folding a tree front-to-back, recursing into | |
87 | /// child nodes that share a tag type with the parent node. | |
88 | template<typename Sequence, typename State0, typename Fun> | |
89 | struct fold_tree | |
90 | : transform<fold_tree<Sequence, State0, Fun> > | |
91 | { | |
92 | template<typename Expr, typename State, typename Data> | |
93 | struct impl | |
94 | : fold< | |
95 | Sequence | |
96 | , State0 | |
97 | , detail::fold_tree_<typename Expr::proto_tag, Fun> | |
98 | >::template impl<Expr, State, Data> | |
99 | {}; | |
100 | ||
101 | template<typename Expr, typename State, typename Data> | |
102 | struct impl<Expr &, State, Data> | |
103 | : fold< | |
104 | Sequence | |
105 | , State0 | |
106 | , detail::fold_tree_<typename Expr::proto_tag, Fun> | |
107 | >::template impl<Expr &, State, Data> | |
108 | {}; | |
109 | }; | |
110 | ||
111 | /// \brief A PrimitiveTransform that recursively applies the | |
112 | /// <tt>reverse_fold\<\></tt> transform to sub-trees that all share | |
113 | /// a common tag type. | |
114 | /// | |
115 | /// <tt>reverse_fold_tree\<\></tt> is useful for flattening trees into | |
116 | /// lists; for example, you might use <tt>reverse_fold_tree\<\></tt> to | |
117 | /// flatten an expression tree like <tt>a | b | c</tt> into a Fusion list | |
118 | /// like <tt>cons(a, cons(b, cons(c)))</tt>. | |
119 | /// | |
120 | /// <tt>reverse_fold_tree\<\></tt> is easily understood in terms of a | |
121 | /// <tt>recurse_if_\<\></tt> helper, defined as follows: | |
122 | /// | |
123 | /// \code | |
124 | /// template<typename Tag, typename Fun> | |
125 | /// struct recurse_if_ | |
126 | /// : if_< | |
127 | /// // If the current node has type "Tag" ... | |
128 | /// is_same<tag_of<_>, Tag>() | |
129 | /// // ... recurse, otherwise ... | |
130 | /// , reverse_fold<_, _state, recurse_if_<Tag, Fun> > | |
131 | /// // ... apply the Fun transform. | |
132 | /// , Fun | |
133 | /// > | |
134 | /// {}; | |
135 | /// \endcode | |
136 | /// | |
137 | /// With <tt>recurse_if_\<\></tt> as defined above, | |
138 | /// <tt>reverse_fold_tree\<Sequence, State0, Fun\>()(e, s, d)</tt> is | |
139 | /// equivalent to | |
140 | /// <tt>reverse_fold<Sequence, State0, recurse_if_<Expr::proto_tag, Fun> >()(e, s, d).</tt> | |
141 | /// It has the effect of folding a tree back-to-front, recursing into | |
142 | /// child nodes that share a tag type with the parent node. | |
143 | template<typename Sequence, typename State0, typename Fun> | |
144 | struct reverse_fold_tree | |
145 | : transform<reverse_fold_tree<Sequence, State0, Fun> > | |
146 | { | |
147 | template<typename Expr, typename State, typename Data> | |
148 | struct impl | |
149 | : reverse_fold< | |
150 | Sequence | |
151 | , State0 | |
152 | , detail::reverse_fold_tree_<typename Expr::proto_tag, Fun> | |
153 | >::template impl<Expr, State, Data> | |
154 | {}; | |
155 | ||
156 | template<typename Expr, typename State, typename Data> | |
157 | struct impl<Expr &, State, Data> | |
158 | : reverse_fold< | |
159 | Sequence | |
160 | , State0 | |
161 | , detail::reverse_fold_tree_<typename Expr::proto_tag, Fun> | |
162 | >::template impl<Expr &, State, Data> | |
163 | {}; | |
164 | }; | |
165 | ||
166 | /// INTERNAL ONLY | |
167 | /// | |
168 | template<typename Sequence, typename State0, typename Fun> | |
169 | struct is_callable<fold_tree<Sequence, State0, Fun> > | |
170 | : mpl::true_ | |
171 | {}; | |
172 | ||
173 | /// INTERNAL ONLY | |
174 | /// | |
175 | template<typename Sequence, typename State0, typename Fun> | |
176 | struct is_callable<reverse_fold_tree<Sequence, State0, Fun> > | |
177 | : mpl::true_ | |
178 | {}; | |
179 | ||
180 | }} | |
181 | ||
182 | #endif |