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- [#foldr_reject_incomplete]
- [section foldr_reject_incomplete]
- [h1 Synopsis]
- template <class P, class State, class BackwardOp>
- struct foldr_reject_incomplete;
- 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]
- The same as [link foldr `foldr`], but once `P` rejects the input,
- `foldr_reject_incomplete` checks if `P` consumes any characters before rejecting
- the input. If so, `foldr_reject_incomplete` rejects the input with the same
- error message this last application of `P` returned. Otherwise
- `foldr_reject_incomplete` accepts the input and gives the same result as
- [link foldr `foldr`].
- Here is a diagram showing how `foldr_reject_incomplete` works by example:
- using int_token = token<int_>;
- using plus_token = token<lit_c<'+'>>;
- using plus_int = last_of<plus_token, int_token>;
- using sum_op = mpl::lambda<mpl::plus<mpl::_1, mpl::_2>>::type;
- [$images/metaparse/foldr_reject_incomplete_diag1.png [width 70%]]
- Note that `foldr_reject_incomplete` folds from right to left and therefore does
- not start folding until it reaches the end of the sequence. Since at the end of
- the sequence it finds an error, folding does not happen at all.
- [h1 Header]
- #include <boost/metaparse/foldr_reject_incomplete.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_reject_incomplete<p, t, f>::apply<s, pos>
- is equivalent to
- first_of<foldr<p, t, f>, fail_at_first_char_expected<p> >::apply<s, pos>
- [h1 Example]
- #include <boost/metaparse/foldr_reject_incomplete.hpp>
- #include <boost/metaparse/lit_c.hpp>
- #include <boost/metaparse/last_of.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>
- #include <boost/mpl/int.hpp>
-
- using namespace boost::metaparse;
-
- using int_token = token<int_>;
- using plus_token = token<lit_c<'+'>>;
- using plus_int = last_of<plus_token, int_token>;
- using sum_op =
- boost::mpl::lambda<boost::mpl::plus<boost::mpl::_1, boost::mpl::_2>>::type;
-
- using ints = foldr_reject_incomplete<plus_int, boost::mpl::int_<11>, sum_op>;
-
- static_assert(
- get_result<
- ints::apply<BOOST_METAPARSE_STRING("+ 13 + 3 + 21"), start>
- >::type::value == 48,
- "ints should sum the numbers"
- );
-
- static_assert(
- is_error<
- ints::apply<BOOST_METAPARSE_STRING("+ 13 + 3 +"), start>
- >::type::value,
- "when the last number is missing, it should be an error"
- );
- [endsect]
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