1[#foldr_start_with_parser] 2[section foldr_start_with_parser] 3 4[h1 Synopsis] 5 6 template <class P, class StateP, class BackwardOp> 7 struct foldr_start_with_parser; 8 9This is a [link parser_combinator parser combinator]. 10 11[table Arguments 12 [[Name] [Type]] 13 [[`P`] [[link parser parser]]] 14 [[`StateP`] [[link parser parser]]] 15 [[`BackwardOp`] [[link metafunction_class template metafunction class] taking two arguments]] 16] 17 18[h1 Description] 19 20The same as [link foldr `foldr`], but after folding it applies a parser, 21`StateP` on the input. If `StateP` fails, `foldr_start_with_parser` fails. If it 22succeeds, the result of parsing is equivalent to 23`boost::reverse_fold<Sequence, State, BackwardOp>`, where `Sequence` is the 24sequence of the results of the applications of `P` and `State` is the result 25`StateP` returned ['after] the repeated application of `P` on the input. 26 27Here is a diagram showing how `foldr_start_with_parser` works by example: 28 29 using int_token = token<int_>; 30 using plus_token = token<lit_c<'+'>>; 31 using int_plus = first_of<int_token, plus_token>; 32 using sum_op = mpl::lambda<mpl::plus<mpl::_1, mpl::_2>>::type; 33 34[$images/metaparse/foldr_start_with_parser_diag1.png [width 70%]] 35 36Further details can be found in the 37[link introducing-foldr_start_with_parser Introducing foldr_start_with_parser] 38section of the [link manual User Manual]. 39 40[h1 Header] 41 42 #include <boost/metaparse/foldr_start_with_parser.hpp> 43 44[h1 Expression semantics] 45 46For any `p` parser, `pt` class, `f` metafunction class taking two arguments, 47`s` compile-time string and `pos` source position let `pos_` be the position 48where the repeated application of `p` on `s` fails for the first time. Let 49`s_` be the postfix of `s` starting at that position. 50 51 foldr_start_with_parser<p, pt, f>::apply<s, pos> 52 53is equivalent to 54 55 pt::apply<s_, pos_> 56 57when the above expression returns a parsing error. It is 58 59 return_< 60 foldr<p, get_result<pt::apply<s_, pos_>>::type, f>::apply<s, pos> 61 >::apply< 62 get_remaining<pt::apply<s_, pos_>>::type, 63 get_position<pt::apply<s_, pos_>>::type 64 > 65 66otherwise. 67 68[h1 Example] 69 70 #include <boost/metaparse/foldr_start_with_parser.hpp> 71 #include <boost/metaparse/lit_c.hpp> 72 #include <boost/metaparse/first_of.hpp> 73 #include <boost/metaparse/token.hpp> 74 #include <boost/metaparse/int_.hpp> 75 #include <boost/metaparse/string.hpp> 76 #include <boost/metaparse/start.hpp> 77 #include <boost/metaparse/get_result.hpp> 78 #include <boost/metaparse/is_error.hpp> 79 80 #include <boost/mpl/lambda.hpp> 81 #include <boost/mpl/plus.hpp> 82 83 using namespace boost::metaparse; 84 85 using int_token = token<int_>; 86 using plus_token = token<lit_c<'+'>>; 87 using int_plus = first_of<int_token, plus_token>; 88 using sum_op = 89 boost::mpl::lambda<boost::mpl::plus<boost::mpl::_1, boost::mpl::_2>>::type; 90 91 using ints = foldr_start_with_parser<int_plus, int_token, sum_op>; 92 93 static_assert( 94 get_result< 95 ints::apply<BOOST_METAPARSE_STRING("11 + 13 + 3 + 21"), start> 96 >::type::value == 48, 97 "ints should sum the numbers" 98 ); 99 100 static_assert( 101 is_error<ints::apply<BOOST_METAPARSE_STRING(""), start>>::type::value, 102 "when no numbers are provided, it should be an error" 103 ); 104 105[endsect] 106 107
