ceph/src/boost/libs/metaparse/doc/transform_error.qbk

   1 [#transform_error]
   2 [section transform_error]
   3
   4 [h1 Synopsis]
   5
   6   template <class P, class F>
   7   struct transform_error;
   8
   9 This 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
  19 It parses the input with `P`. When this succeeds, the result of parsing with
  20 `transform_error` will be the result of parsing with `P`. When it fails, `F` is
  21 evaluated with the error `P` returned as argument. Parsing with
  22 `transform_error` will fail and the error will be what `F` returns. Therefore,
  23 `F` is expected to accept and return a [link reject `reject`] value.
  24
  25 [h1 Header]
  26
  27   #include <boost/metaparse/transform_error.hpp>
  28
  29 [h1 Expression semantics]
  30
  31 For any `p` parser and `f` metafunction class accepting one argument
  32
  33   transform_error<p, f>::apply<s, pos>
  34
  35 is equivalent to `p::apply<s, pos>` when `p` accepts the input. It is equivalent
  36 to `f::apply<p::apply<s, pos>::type>` otherwise.
  37
  38 [h1 Example]
  39
  40   #include <boost/metaparse/transform_error.hpp>
  41   #include <boost/metaparse/repeated1.hpp>
  42   #include <boost/metaparse/letter.hpp>
  43   #include <boost/metaparse/keyword.hpp>
  44   #include <boost/metaparse/last_of.hpp>
  45   #include <boost/metaparse/token.hpp>
  46   #include <boost/metaparse/string.hpp>
  47   #include <boost/metaparse/is_error.hpp>
  48   #include <boost/metaparse/start.hpp>
  49   #include <boost/metaparse/get_message.hpp>
  50   #include <boost/metaparse/get_position.hpp>
  51   #include <boost/metaparse/define_error.hpp>
  52   #include <boost/metaparse/reject.hpp>
  53
  54   #include <boost/mpl/lambda.hpp>
  55
  56   #include <type_traits>
  57
  58   using namespace boost::metaparse;
  59
  60   BOOST_METAPARSE_DEFINE_ERROR(name_expected, "Name expected");
  61
  62   using keyword_name = token<keyword<BOOST_METAPARSE_STRING("name")>>;
  63   using name_token = token<repeated1<letter>>;
  64
  65   using name_parser =
  66     last_of<
  67       keyword_name,
  68       transform_error<
  69         name_token,
  70         boost::mpl::lambda<
  71           reject<name_expected, get_position<boost::mpl::_1> >
  72         >::type
  73       >
  74     >;
  75
  76   static_assert(
  77     !is_error<
  78       name_parser::apply<BOOST_METAPARSE_STRING("name Bela"), start>
  79     >::type::value,
  80     "name_parser should accept \"name <a name>\""
  81   );
  82
  83   static_assert(
  84     is_error<
  85       name_parser::apply<BOOST_METAPARSE_STRING("name ?"), start>
  86     >::type::value,
  87     "name_parser should reject input when name is a question mark"
  88   );
  89
  90   static_assert(
  91     std::is_same<
  92       get_message<
  93         name_parser::apply<BOOST_METAPARSE_STRING("name ?"), start>
  94       >::type,
  95       name_expected
  96     >::type::value,
  97     "the error message should be the one specified by change_error_message"
  98   );
  99
 100 [endsect]
 101