1[#foldl]
2[section foldl]
3
4[h1 Synopsis]
5
6  template <class P, class State, class ForwardOp>
7  struct foldl;
8
9This is a [link parser_combinator parser combinator].
10
11[table Arguments
12  [[Name]        [Type]]
13  [[`P`]         [[link parser parser]]]
14  [[`State`]     [[link metaprogramming_value template metaprogramming value]]]
15  [[`ForwardOp`] [[link metafunction_class template metafunction class] taking two arguments]]
16]
17
18[h1 Description]
19
20`foldl` applies `P` on the input string repeatedly as long as `P` accepts the
21input. The result of parsing is equivalent to
22`boost::mpl::fold<Sequence, State, ForwardOp>`, where `Sequence` is the sequence
23of the results of the applications of `P`.
24
25When `P` rejects the input for the first time, `foldl` still accepts the input
26and the result of parsing is `State`.
27
28Here is a diagram showing how `foldl` works by example:
29
30  using int_token = token<int_>;
31  using sum_op = mpl::lambda<mpl::plus<mpl::_1, mpl::_2>>::type;
32
33[$images/metaparse/foldl_diag2.png [width 70%]]
34
35Further details can be found in the [link introducing-foldl Introducing foldl]
36section of the [link manual User Manual].
37
38[h1 Header]
39
40  #include <boost/metaparse/foldl.hpp>
41
42[h1 Expression semantics]
43
44For any `p` parser, `t` class, `f` metafunction class taking two arguments,
45`s` compile-time string and `pos` source position
46
47  foldl<p, t, f>::apply<s, pos>
48
49is equivalent to
50
51  return_<t>::apply<s, pos>
52
53when `p::apply<s, pos>` returns an error. It is
54
55  foldl<p, f::apply<t, get_result<p::apply<s, pos>>::type>::type, f>::apply<
56    get_remaining<p::apply<s, pos>>,
57    get_position<p::apply<s, pos>>
58  >
59
60otherwise.
61
62[h1 Example]
63
64  #include <boost/metaparse/foldl.hpp>
65  #include <boost/metaparse/token.hpp>
66  #include <boost/metaparse/int_.hpp>
67  #include <boost/metaparse/string.hpp>
68  #include <boost/metaparse/start.hpp>
69  #include <boost/metaparse/get_result.hpp>
70
71  #include <boost/mpl/lambda.hpp>
72  #include <boost/mpl/plus.hpp>
73
74  using namespace boost::metaparse;
75
76  using int_token = token<int_>;
77  using sum_op =
78    boost::mpl::lambda<boost::mpl::plus<boost::mpl::_1, boost::mpl::_2>>::type;
79
80  using ints = foldl<int_token, boost::mpl::int_<0>, sum_op>;
81
82  static_assert(
83    get_result<
84      ints::apply<BOOST_METAPARSE_STRING("11 13 3 21"), start>
85    >::type::value == 48,
86    "ints should sum the numbers"
87  );
88
89  static_assert(
90    get_result<
91      ints::apply<BOOST_METAPARSE_STRING(""), start>
92    >::type::value == 0,
93    "the sum of no elements is 0"
94  );
95
96[endsect]
97
98