1[/==============================================================================
2    Copyright (C) 2001-2010 Joel de Guzman
3    Copyright (C) 2001-2005 Dan Marsden
4    Copyright (C) 2001-2010 Thomas Heller
5
6    Distributed under the Boost Software License, Version 1.0. (See accompanying
7    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
8===============================================================================/]
9
10[section Transforming the Expression Tree]
11
12This example will show how to write __phoenix_actions__ that transform the
13Phoenix AST.
14
15[:
16"/Lisp macros transform the program structure itself, with the full language
17available to express such transformations./"
18
19[@http://en.wikipedia.org/wiki/Lisp_macro#Lisp_macros Wikipedia]
20]
21
22What we want to do is to invert some arithmetic operators, i.e. plus will be
23transformed to minus, minus to plus, multiplication to division and division to
24multiplication.
25
26Let's start with defining our default action:
27
28    struct invert_actions
29    {
30        template <typename Rule>
31        struct when
32            : proto::_ // the default is proto::_
33        {};
34    };
35
36By default, we don't want to do anything, well, not exactly nothing, but just
37return the expression. This is done by
38[@http://www.boost.org/doc/libs/release/doc/html/boost/proto/_.html proto::_]
39which, used as a transform, just passes the current expression along. Making this
40action an identity transform.
41
42[def __proto_make_expr__     [@http://www.boost.org/doc/libs/release/doc/html/boost/proto/functional/make_expr.html `proto::functional::make_expr`]]
43
44So, after the basics are set up, we can start by writing the transformations we
45want to have on our tree:
46
47    // Transform plus to minus
48    template <>
49    struct invert_actions::when<phoenix::rule::plus>
50        : __proto_call__<
51            __proto_make_expr__<proto::tag::minus>(
52                phoenix::evaluator(proto::_left, phoenix::_context)
53              , phoenix::evaluator(proto::_right, phoenix::_context)
54            )
55        >
56    {};
57
58Wow, this looks complicated! Granted you need to know a little bit about __proto__
59(For a good introduction read through the
60 [@http://cpp-next.com/archive/2010/08/expressive-c-introduction/ Expressive C++] series).
61
62What is done is the following:
63
64* The left expression is passed to evaluator (with the current context, that contains our invert_actions)
65* The right expression is passed to evaluator (with the current context, that contains our invert_actions)
66* The result of these two __proto_transforms__ is passed to __proto_make_expr__ which returns the freshly created expression
67
68After you know what is going on, maybe the rest doesn't look so scary anymore:
69
70    // Transform minus to plus
71    template <>
72    struct invert_actions::when<phoenix::rule::minus>
73        : __proto_call__<
74            __proto_make_expr__<proto::tag::plus>(
75                phoenix::evaluator(proto::_left, phoenix::_context)
76              , phoenix::evaluator(proto::_right, phoenix::_context)
77            )
78        >
79    {};
80
81    // Transform multiplies to divides
82    template <>
83    struct invert_actions::when<phoenix::rule::multiplies>
84        : __proto_call__<
85            __proto_make_expr__<proto::tag::divides>(
86                phoenix::evaluator(proto::_left, phoenix::_context)
87              , phoenix::evaluator(proto::_right, phoenix::_context)
88            )
89        >
90    {};
91
92    // Transform divides to multiplies
93    template <>
94    struct invert_actions::when<phoenix::rule::divides>
95        : __proto_call__<
96            __proto_make_expr__<proto::tag::multiplies>(
97                phoenix::evaluator(proto::_left, phoenix::_context)
98              , phoenix::evaluator(proto::_right, phoenix::_context)
99            )
100        >
101    {};
102
103That's it! Now that we have our actions defined, we want to evaluate some of our expressions with them:
104
105    template <typename Expr>
106    // Calculate the result type: our transformed AST
107    typename boost::result_of<
108        phoenix::evaluator(
109            Expr const&
110          , phoenix::result_of::context<int, invert_actions>::type
111        )
112    >::type
113    invert(Expr const & expr)
114    {
115        return
116            // Evaluate it with our actions
117            phoenix::eval(
118                expr
119              , phoenix::context(
120                    int()
121                  , invert_actions()
122                )
123            );
124    }
125
126Run some tests to see if it is working:
127
128    invert(_1);                    // --> _1
129    invert(_1 + _2);               // --> _1 - _2
130    invert(_1 + _2 - _3);          // --> _1 - _2 + _3
131    invert(_1 * _2);               // --> _1 / _2
132    invert(_1 * _2 / _3);          // --> _1 / _2 * _3
133    invert(_1 * _2 + _3);          // --> _1 / _2 - _3
134    invert(_1 * _2 - _3);          // --> _1 / _2 + _2
135    invert(if_(_1 * _4)[_2 - _3]); // --> if_(_1 / _4)[_2 + _3]
136    _1 * invert(_2 - _3));         // --> _1 * _2 + _3
137
138__note__ The complete example can be found here: [@../../example/invert.cpp example/invert.cpp]
139
140/Pretty simple .../
141
142[endsect]
143