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1 | [#foldr] |
2 | [section foldr] | |
3 | ||
4 | [h1 Synopsis] | |
5 | ||
6 | template <class P, class State, class BackwardOp> | |
7 | struct foldr; | |
8 | ||
9 | This is a [link parser_combinator parser combinator]. | |
10 | ||
11 | [table Arguments | |
12 | [[Name] [Type]] | |
13 | [[`P`] [[link parser parser]]] | |
14 | [[`State`] [[link metaprogramming_value template metaprogramming value]]] | |
15 | [[`BackwardOp`] [[link metafunction_class template metafunction class] taking two arguments]] | |
16 | ] | |
17 | ||
18 | [h1 Description] | |
19 | ||
20 | `foldr` applies `P` on the input string repeatedly as long as `P` accepts the | |
21 | input. The result of parsing is equivalent to | |
22 | `boost::reverse_fold<Sequence, State, BackwardOp>`, where `Sequence` is the | |
23 | sequence of the results of the applications of `P`. | |
24 | ||
25 | When `P` rejects the input for the first time, `foldr` still accepts the input | |
26 | and the result of parsing is `State`. | |
27 | ||
28 | Here is a diagram showing how `foldr` works by example: | |
29 | ||
30 | using int_token = token<int_>; | |
31 | using sum_op = mpl::lambda<mpl::plus<mpl::_1, mpl::_2>>::type; | |
32 | ||
33 | [$images/metaparse/foldr_diag1.png [width 70%]] | |
34 | ||
35 | Further details can be found in the [link introducing-foldr Introducing foldr] | |
36 | section of the [link manual User Manual]. | |
37 | ||
38 | [h1 Header] | |
39 | ||
40 | #include <boost/metaparse/foldr.hpp> | |
41 | ||
42 | [h1 Expression semantics] | |
43 | ||
44 | For any `p` parser, `t` class, `f` metafunction class taking two arguments, | |
45 | `s` compile-time string and `pos` source position | |
46 | ||
47 | foldr<p, t, f>::apply<s, pos> | |
48 | ||
49 | is equivalent to | |
50 | ||
51 | return_<t>::apply<s, pos> | |
52 | ||
53 | when `p::apply<s, pos>` returns an error. It is | |
54 | ||
55 | f::apply< | |
56 | get_result< | |
57 | foldr<p, t, f>::apply< | |
58 | get_remaining<p::apply<s, pos>>, | |
59 | get_position<p::apply<s, pos>> | |
60 | > | |
61 | >::type, | |
62 | get_result<p::apply<s, pos>>::type | |
63 | > | |
64 | ||
65 | otherwise. | |
66 | ||
67 | [h1 Example] | |
68 | ||
69 | #include <boost/metaparse/foldr.hpp> | |
70 | #include <boost/metaparse/token.hpp> | |
71 | #include <boost/metaparse/int_.hpp> | |
72 | #include <boost/metaparse/string.hpp> | |
73 | #include <boost/metaparse/start.hpp> | |
74 | #include <boost/metaparse/get_result.hpp> | |
75 | ||
76 | #include <boost/mpl/lambda.hpp> | |
77 | #include <boost/mpl/plus.hpp> | |
78 | ||
79 | using namespace boost::metaparse; | |
80 | ||
81 | using int_token = token<int_>; | |
82 | using sum_op = | |
83 | boost::mpl::lambda<boost::mpl::plus<boost::mpl::_1, boost::mpl::_2>>::type; | |
84 | ||
85 | using ints = foldr<int_token, boost::mpl::int_<0>, sum_op>; | |
86 | ||
87 | static_assert( | |
88 | get_result< | |
89 | ints::apply<BOOST_METAPARSE_STRING("11 13 3 21"), start> | |
90 | >::type::value == 48, | |
91 | "ints should sum the numbers" | |
92 | ); | |
93 | ||
94 | static_assert( | |
95 | get_result< | |
96 | ints::apply<BOOST_METAPARSE_STRING(""), start> | |
97 | >::type::value == 0, | |
98 | "the sum of no elements is 0" | |
99 | ); | |
100 | ||
101 | [endsect] | |
102 |