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