• Home
  • Line#
  • Scopes#
  • Navigate#
  • Raw
  • Download
1[#foldl_start_with_parser]
2[section foldl_start_with_parser]
3
4[h1 Synopsis]
5
6  template <class P, class StateP, class ForwardOp>
7  struct foldl_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  [[`ForwardOp`] [[link metafunction_class template metafunction class] taking two arguments]]
16]
17
18[h1 Description]
19
20The same as [link foldl `foldl`], but before folding it applies a parser,
21`StateP` on the input. If it fails, `foldl_start_with_parser` fails. If it
22succeeds, `foldl_start_with_parser` works as `foldl` taking the result of
23`StateP` as the initial state.
24
25Here is a diagram showing how `foldl_start_with_parser` works by example:
26
27  using int_token = token<int_>;
28  using plus_token = token<lit_c<'+'>>;
29  using plus_int = last_of<plus_token, int_token>;
30  using sum_op = mpl::lambda<mpl::plus<mpl::_1, mpl::_2>>::type;
31
32[$images/metaparse/foldl_start_with_parser_diag1.png [width 70%]]
33
34Further details can be found in the
35[link introducing-foldl_start_with_parser Introducing foldl_start_with_parser]
36section of the [link manual User Manual].
37
38[h1 Header]
39
40  #include <boost/metaparse/foldl_start_with_parser.hpp>
41
42[h1 Expression semantics]
43
44For any `p` parser, `pt` class, `f` metafunction class taking two arguments,
45`s` compile-time string and `pos` source position
46
47  foldl_start_with_parser<p, pt, f>::apply<s, pos>
48
49is equivalent to
50
51  pt::apply<s, pos>
52
53when the above expression returns a parsing error. It is
54
55  foldl<p, get_result<pt::apply<s, pos>>::type, f>::apply<
56    get_remaining<pt::apply<s, pos>>::type,
57    get_position<pt::apply<s, pos>>::type
58  >
59
60otherwise.
61
62[h1 Example]
63
64  #include <boost/metaparse/foldl_start_with_parser.hpp>
65  #include <boost/metaparse/lit_c.hpp>
66  #include <boost/metaparse/last_of.hpp>
67  #include <boost/metaparse/token.hpp>
68  #include <boost/metaparse/int_.hpp>
69  #include <boost/metaparse/string.hpp>
70  #include <boost/metaparse/start.hpp>
71  #include <boost/metaparse/get_result.hpp>
72  #include <boost/metaparse/is_error.hpp>
73
74  #include <boost/mpl/lambda.hpp>
75  #include <boost/mpl/plus.hpp>
76
77  using namespace boost::metaparse;
78
79  using int_token = token<int_>;
80  using plus_token = token<lit_c<'+'>>;
81  using plus_int = last_of<plus_token, int_token>;
82  using sum_op =
83    boost::mpl::lambda<boost::mpl::plus<boost::mpl::_1, boost::mpl::_2>>::type;
84
85  using ints = foldl_start_with_parser<plus_int, int_token, sum_op>;
86
87  static_assert(
88    get_result<
89      ints::apply<BOOST_METAPARSE_STRING("11 + 13 + 3 + 21"), start>
90    >::type::value == 48,
91    "ints should sum the numbers"
92  );
93
94  static_assert(
95    is_error<ints::apply<BOOST_METAPARSE_STRING(""), start>>::type::value,
96    "when no numbers are provided, it should be an error"
97  );
98
99[endsect]
100
101