foldr1.qbk
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[#foldr1]
[section foldr1]
[h1 Synopsis]
template <class P, class State, class BackwardOp>
struct foldr1;
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]
`foldr1` 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, `foldr1` rejects it as well. At
least one successful application of `P` is required for `foldr1` to accept the
input.
[h1 Header]
#include <boost/metaparse/foldr1.hpp>
[h1 Expression semantics]
For any `p` parser, `t` class, `f` metafunction class taking two arguments the
following are equivalent:
foldr1<p, t, f>
last_of<look_ahead<p>, foldr<p, t, f>>
[h1 Example]
#include <boost/metaparse/foldr1.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 sum_op =
boost::mpl::lambda<boost::mpl::plus<boost::mpl::_1, boost::mpl::_2>>::type;
using ints = foldr1<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(
is_error<ints::apply<BOOST_METAPARSE_STRING(""), start>>::type::value,
"when no numbers are provided, it should be an error"
);
[endsect]