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 9This 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 18It tries to parse the input using `P`. When `P` rejects the input without 19consuming any character, `fail_at_first_char_expected` accepts the input. 20Otherwise (when `P` accepts the input or when it consumes characters before 21rejecting 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 29For any `p` parser, `s` compile-time string and `pos` source position: 30 31When `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 36When `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 38true, the following are equivalent: 39 40 get_remaining<except<p, c, msg>, s, pos>::type 41 accept</* unspecified */, s, pos> 42 43Otherwise 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 94