73ef4ff3
 Hu Chunming
提交三方库
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[#foldr]
[section foldr]
[h1 Synopsis]
template <class P, class State, class BackwardOp>
struct foldr;
This is a [link parser_combinator parser combinator].
[table Arguments
[[Name] [Type]]
[[`P`] [[link parser parser]]]
[[`State`] [[link metaprogramming_value template metaprogramming value]]]
[[`BackwardOp`] [[link metafunction_class template metafunction class] taking two arguments]]
]
[h1 Description]
`foldr` applies `P` on the input string repeatedly as long as `P` accepts the
input. The result of parsing is equivalent to
`boost::reverse_fold<Sequence, State, BackwardOp>`, where `Sequence` is the
sequence of the results of the applications of `P`.
When `P` rejects the input for the first time, `foldr` still accepts the input
and the result of parsing is `State`.
Here is a diagram showing how `foldr` works by example:
using int_token = token<int_>;
using sum_op = mpl::lambda<mpl::plus<mpl::_1, mpl::_2>>::type;
[$images/metaparse/foldr_diag1.png [width 70%]]
Further details can be found in the [link introducing-foldr Introducing foldr]
section of the [link manual User Manual].
[h1 Header]
#include <boost/metaparse/foldr.hpp>
[h1 Expression semantics]
For any `p` parser, `t` class, `f` metafunction class taking two arguments,
`s` compile-time string and `pos` source position
foldr<p, t, f>::apply<s, pos>
is equivalent to
return_<t>::apply<s, pos>
when `p::apply<s, pos>` returns an error. It is
f::apply<
get_result<
foldr<p, t, f>::apply<
get_remaining<p::apply<s, pos>>,
get_position<p::apply<s, pos>>
>
>::type,
get_result<p::apply<s, pos>>::type
>
otherwise.
[h1 Example]
#include <boost/metaparse/foldr.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 = foldr<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]
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