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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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