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7c673cae FG |
1 | [#transform] |
2 | [section transform] | |
3 | ||
4 | [h1 Synopsis] | |
5 | ||
6 | template <class P, class T> | |
7 | struct transform; | |
8 | ||
9 | This is a [link parser_combinator parser combinator]. | |
10 | ||
11 | [table Arguments | |
12 | [[Name] [Type]] | |
13 | [[`P`] [[link parser parser]]] | |
14 | [[`T`] [[link metafunction_class template metafunction class] taking one argument]] | |
15 | ] | |
16 | ||
17 | [h1 Description] | |
18 | ||
19 | `transform` parses the input using `P` and transforms the result `P` returns | |
20 | with `T`. The result of parsing is what `T` returns. When `P` fails, the failure | |
21 | is returned unchanged. | |
22 | ||
23 | [h1 Header] | |
24 | ||
25 | #include <boost/metaparse/transform.hpp> | |
26 | ||
27 | [h1 Expression semantics] | |
28 | ||
29 | For any `p` parser, `t` metafunction class accepting one argument, `s` | |
30 | compile-time string and `pos` source position | |
31 | ||
32 | get_result<transform<p, t>::apply<s, pos>>::type | |
33 | ||
34 | is equivalent to | |
35 | ||
36 | t::apply<get_result<p::apply<s, pos>>::type>::type | |
37 | ||
38 | When `p::apply<s, pos>` doesn't return an error. The combinator returns the | |
39 | error otherwise. | |
40 | ||
41 | [h1 Example] | |
42 | ||
43 | #include <boost/metaparse/transform.hpp> | |
44 | #include <boost/metaparse/digit.hpp> | |
45 | #include <boost/metaparse/start.hpp> | |
46 | #include <boost/metaparse/string.hpp> | |
47 | #include <boost/metaparse/is_error.hpp> | |
48 | #include <boost/metaparse/get_result.hpp> | |
49 | ||
50 | #include <boost/metaparse/util/digit_to_int.hpp> | |
51 | ||
52 | using namespace boost::metaparse; | |
53 | ||
54 | using digit_value = transform<digit, util::digit_to_int<>>; | |
55 | ||
56 | static_assert( | |
57 | !is_error< | |
58 | digit_value::apply<BOOST_METAPARSE_STRING("0"), start> | |
59 | >::type::value, | |
60 | "digit_val should accept a digit" | |
61 | ); | |
62 | ||
63 | static_assert( | |
64 | is_error<digit_value::apply<BOOST_METAPARSE_STRING("x"), start>>::type::value, | |
65 | "digit_val should reject a character" | |
66 | ); | |
67 | ||
68 | static_assert( | |
69 | get_result< | |
70 | digit_value::apply<BOOST_METAPARSE_STRING("0"), start> | |
71 | >::type::value == 0, | |
72 | "the result of parsing should be the int value" | |
73 | ); | |
74 | ||
75 | [endsect] | |
76 |