Synopsis

template <class P, class State, class BackwardOp>
struct foldr;

This is a parser combinator.

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;

Further details can be found in the Introducing foldr section of the User Manual.

Header

#include <boost/metaparse/foldr.hpp>

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.

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"
);