101 lines
2.8 KiB
Plaintext
101 lines
2.8 KiB
Plaintext
[#foldl_start_with_parser]
|
|
[section foldl_start_with_parser]
|
|
|
|
[h1 Synopsis]
|
|
|
|
template <class P, class StateP, class ForwardOp>
|
|
struct foldl_start_with_parser;
|
|
|
|
This is a [link parser_combinator parser combinator].
|
|
|
|
[table Arguments
|
|
[[Name] [Type]]
|
|
[[`P`] [[link parser parser]]]
|
|
[[`StateP`] [[link parser parser]]]
|
|
[[`ForwardOp`] [[link metafunction_class template metafunction class] taking two arguments]]
|
|
]
|
|
|
|
[h1 Description]
|
|
|
|
The same as [link foldl `foldl`], but before folding it applies a parser,
|
|
`StateP` on the input. If it fails, `foldl_start_with_parser` fails. If it
|
|
succeeds, `foldl_start_with_parser` works as `foldl` taking the result of
|
|
`StateP` as the initial state.
|
|
|
|
Here is a diagram showing how `foldl_start_with_parser` works by example:
|
|
|
|
using int_token = token<int_>;
|
|
using plus_token = token<lit_c<'+'>>;
|
|
using plus_int = last_of<plus_token, int_token>;
|
|
using sum_op = mpl::lambda<mpl::plus<mpl::_1, mpl::_2>>::type;
|
|
|
|
[$images/metaparse/foldl_start_with_parser_diag1.png [width 70%]]
|
|
|
|
Further details can be found in the
|
|
[link introducing-foldl_start_with_parser Introducing foldl_start_with_parser]
|
|
section of the [link manual User Manual].
|
|
|
|
[h1 Header]
|
|
|
|
#include <boost/metaparse/foldl_start_with_parser.hpp>
|
|
|
|
[h1 Expression semantics]
|
|
|
|
For any `p` parser, `pt` class, `f` metafunction class taking two arguments,
|
|
`s` compile-time string and `pos` source position
|
|
|
|
foldl_start_with_parser<p, pt, f>::apply<s, pos>
|
|
|
|
is equivalent to
|
|
|
|
pt::apply<s, pos>
|
|
|
|
when the above expression returns a parsing error. It is
|
|
|
|
foldl<p, get_result<pt::apply<s, pos>>::type, f>::apply<
|
|
get_remaining<pt::apply<s, pos>>::type,
|
|
get_position<pt::apply<s, pos>>::type
|
|
>
|
|
|
|
otherwise.
|
|
|
|
[h1 Example]
|
|
|
|
#include <boost/metaparse/foldl_start_with_parser.hpp>
|
|
#include <boost/metaparse/lit_c.hpp>
|
|
#include <boost/metaparse/last_of.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/metaparse/is_error.hpp>
|
|
|
|
#include <boost/mpl/lambda.hpp>
|
|
#include <boost/mpl/plus.hpp>
|
|
|
|
using namespace boost::metaparse;
|
|
|
|
using int_token = token<int_>;
|
|
using plus_token = token<lit_c<'+'>>;
|
|
using plus_int = last_of<plus_token, int_token>;
|
|
using sum_op =
|
|
boost::mpl::lambda<boost::mpl::plus<boost::mpl::_1, boost::mpl::_2>>::type;
|
|
|
|
using ints = foldl_start_with_parser<plus_int, int_token, 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(
|
|
is_error<ints::apply<BOOST_METAPARSE_STRING(""), start>>::type::value,
|
|
"when no numbers are provided, it should be an error"
|
|
);
|
|
|
|
[endsect]
|
|
|