[#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]

