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

   1 [#fail_at_first_char_expected]
   2 [section fail_at_first_char_expected]
   3
   4 [h1 Synopsis]
   5
   6   template <class P>
   7   struct fail_at_first_char_expected;
   8
   9 This is a [link parser_combinator parser combinator].
  10
  11 [table Arguments
  12   [[Name] [Type]]
  13   [[`P`]  [[link parser parser]]]
  14 ]
  15
  16 [h1 Description]
  17
  18 It tries to parse the input using `P`. When `P` rejects the input without
  19 consuming any character, `fail_at_first_char_expected` accepts the input.
  20 Otherwise (when `P` accepts the input or when it consumes characters before
  21 rejecting the input) `fail_at_first_char_expected` rejects the input.
  22
  23 [h1 Header]
  24
  25   #include <boost/metaparse/fail_at_first_char_expected.hpp>
  26
  27 [h1 Expression semantics]
  28
  29 For any `p` parser, `s` compile-time string and `pos` source position:
  30
  31 When `is_error<p::apply<s, pos>>::type` is false, the following are equivalent:
  32
  33   fail_at_first_char_expected<p>::apply<s, pos>::type
  34   reject<error::expected_to_fail, pos>
  35
  36 When `is_error<p::apply<s, pos>>::type` is true and
  37 `boost::mpl::equal_to<pos, get_position<p::apply<s, pos>>::type>::type` is also
  38 true, the following are equivalent:
  39
  40   get_remaining<except<p, c, msg>, s, pos>::type
  41   accept</* unspecified */, s, pos>
  42
  43 Otherwise the following are equivalent:
  44
  45   get_remaining<except<p, c, msg>, s, pos>::type
  46   p::apply<s, pos>::type
  47
  48 [h1 Example]
  49
  50   #include <boost/metaparse/fail_at_first_char_expected.hpp>
  51   #include <boost/metaparse/int_.hpp>
  52   #include <boost/metaparse/lit_c.hpp>
  53   #include <boost/metaparse/foldl_start_with_parser.hpp>
  54   #include <boost/metaparse/first_of.hpp>
  55   #include <boost/metaparse/last_of.hpp>
  56   #include <boost/metaparse/string.hpp>
  57   #include <boost/metaparse/start.hpp>
  58   #include <boost/metaparse/is_error.hpp>
  59   #include <boost/metaparse/get_result.hpp>
  60
  61   #include <boost/mpl/lambda.hpp>
  62   #include <boost/mpl/plus.hpp>
  63
  64   using namespace boost::metaparse;
  65
  66   using plus_int = last_of<lit_c<'+'>, int_>;
  67
  68   using plus_exp =
  69     first_of<
  70       foldl_start_with_parser<
  71         plus_int,
  72         int_,
  73         boost::mpl::lambda<boost::mpl::plus<boost::mpl::_1, boost::mpl::_2>>::type
  74       >,
  75       fail_at_first_char_expected<plus_int>
  76     >;
  77
  78   static_assert(
  79     get_result<
  80       plus_exp::apply<BOOST_METAPARSE_STRING("1+2+3"), start>
  81     >::type::value == 6,
  82     "it should accept the input when it is a list of '+' separated ints"
  83   );
  84
  85   static_assert(
  86     is_error<
  87       plus_exp::apply<BOOST_METAPARSE_STRING("1+2+3+"), start>
  88     >::type::value,
  89     "it should reject the input when it has an extra +"
  90   );
  91
  92 [endsect]
  93