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1[#transform_error_message]
2[section transform_error_message]
3
4[h1 Synopsis]
5
6  template <class P, class F>
7  struct transform_error_message;
8
9This is a [link parser_combinator parser combinator].
10
11[table Arguments
12  [[Name] [Type]]
13  [[`P`]  [[link parser parser]]]
14  [[`F`]  [[link metafunction_class template metafunction class] taking one argument]]
15]
16
17[h1 Description]
18
19It parses the input with `P`. When this succeeds, the result of parsing with
20`transform_error_message` will be the result of parsing with `P`. When it fails,
21the error `P` returns is forwarded to the caller of `transform_error_message`,
22but the message of it is transformed with `F`.
23
24[h1 Header]
25
26  #include <boost/metaparse/transform_error_message.hpp>
27
28[h1 Expression semantics]
29
30For any `p` parser and `f` metafunction class accepting one argument
31
32  transform_error_message<p, f>::apply<s, pos>
33
34is equivalent to `p::apply<s, pos>` when `p` accepts the input.
35It is equivalent to
36`reject<f::apply<get_message<p::apply<s, pos>>::type>, get_position<p::apply<s, pos>>>`
37otherwise.
38
39[h1 Example]
40
41  #include <boost/metaparse/transform_error_message.hpp>
42  #include <boost/metaparse/repeated1.hpp>
43  #include <boost/metaparse/letter.hpp>
44  #include <boost/metaparse/keyword.hpp>
45  #include <boost/metaparse/last_of.hpp>
46  #include <boost/metaparse/token.hpp>
47  #include <boost/metaparse/string.hpp>
48  #include <boost/metaparse/is_error.hpp>
49  #include <boost/metaparse/start.hpp>
50  #include <boost/metaparse/get_message.hpp>
51  #include <boost/metaparse/define_error.hpp>
52  #include <boost/metaparse/reject.hpp>
53
54  #include <type_traits>
55
56  using namespace boost::metaparse;
57
58  BOOST_METAPARSE_DEFINE_ERROR(name_expected, "Name expected");
59
60  struct return_name_expected
61  {
62    typedef return_name_expected type;
63
64    template <class>
65    struct apply : name_expected {};
66  };
67
68  using keyword_name = token<keyword<BOOST_METAPARSE_STRING("name")>>;
69  using name_token = token<repeated1<letter>>;
70
71  using name_parser =
72    last_of<
73      keyword_name,
74      transform_error_message<name_token, return_name_expected>
75    >;
76
77  static_assert(
78    !is_error<
79      name_parser::apply<BOOST_METAPARSE_STRING("name Bela"), start>
80    >::type::value,
81    "name_parser should accept \"name <a name>\""
82  );
83
84  static_assert(
85    is_error<
86      name_parser::apply<BOOST_METAPARSE_STRING("name ?"), start>
87    >::type::value,
88    "name_parser should reject input when name is a question mark"
89  );
90
91  static_assert(
92    std::is_same<
93      get_message<
94        name_parser::apply<BOOST_METAPARSE_STRING("name ?"), start>
95      >::type,
96      name_expected
97    >::type::value,
98    "the error message should be the one specified by change_error_message"
99  );
100
101[endsect]
102
103