107 lines
3.2 KiB
Plaintext
107 lines
3.2 KiB
Plaintext
[#foldr_start_with_parser]
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[section foldr_start_with_parser]
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[h1 Synopsis]
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template <class P, class StateP, class BackwardOp>
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struct foldr_start_with_parser;
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This is a [link parser_combinator parser combinator].
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[table Arguments
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[[Name] [Type]]
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[[`P`] [[link parser parser]]]
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[[`StateP`] [[link parser parser]]]
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[[`BackwardOp`] [[link metafunction_class template metafunction class] taking two arguments]]
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]
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[h1 Description]
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The same as [link foldr `foldr`], but after folding it applies a parser,
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`StateP` on the input. If `StateP` fails, `foldr_start_with_parser` fails. If it
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succeeds, the result of parsing is equivalent to
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`boost::reverse_fold<Sequence, State, BackwardOp>`, where `Sequence` is the
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sequence of the results of the applications of `P` and `State` is the result
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`StateP` returned ['after] the repeated application of `P` on the input.
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Here is a diagram showing how `foldr_start_with_parser` works by example:
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using int_token = token<int_>;
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using plus_token = token<lit_c<'+'>>;
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using int_plus = first_of<int_token, plus_token>;
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using sum_op = mpl::lambda<mpl::plus<mpl::_1, mpl::_2>>::type;
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[$images/metaparse/foldr_start_with_parser_diag1.png [width 70%]]
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Further details can be found in the
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[link introducing-foldr_start_with_parser Introducing foldr_start_with_parser]
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section of the [link manual User Manual].
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[h1 Header]
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#include <boost/metaparse/foldr_start_with_parser.hpp>
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[h1 Expression semantics]
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For any `p` parser, `pt` class, `f` metafunction class taking two arguments,
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`s` compile-time string and `pos` source position let `pos_` be the position
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where the repeated application of `p` on `s` fails for the first time. Let
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`s_` be the postfix of `s` starting at that position.
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foldr_start_with_parser<p, pt, f>::apply<s, pos>
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is equivalent to
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pt::apply<s_, pos_>
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when the above expression returns a parsing error. It is
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return_<
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foldr<p, get_result<pt::apply<s_, pos_>>::type, f>::apply<s, pos>
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>::apply<
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get_remaining<pt::apply<s_, pos_>>::type,
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get_position<pt::apply<s_, pos_>>::type
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>
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otherwise.
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[h1 Example]
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#include <boost/metaparse/foldr_start_with_parser.hpp>
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#include <boost/metaparse/lit_c.hpp>
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#include <boost/metaparse/first_of.hpp>
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#include <boost/metaparse/token.hpp>
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#include <boost/metaparse/int_.hpp>
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#include <boost/metaparse/string.hpp>
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#include <boost/metaparse/start.hpp>
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#include <boost/metaparse/get_result.hpp>
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#include <boost/metaparse/is_error.hpp>
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#include <boost/mpl/lambda.hpp>
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#include <boost/mpl/plus.hpp>
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using namespace boost::metaparse;
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using int_token = token<int_>;
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using plus_token = token<lit_c<'+'>>;
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using int_plus = first_of<int_token, plus_token>;
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using sum_op =
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boost::mpl::lambda<boost::mpl::plus<boost::mpl::_1, boost::mpl::_2>>::type;
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using ints = foldr_start_with_parser<int_plus, int_token, sum_op>;
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static_assert(
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get_result<
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ints::apply<BOOST_METAPARSE_STRING("11 + 13 + 3 + 21"), start>
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>::type::value == 48,
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"ints should sum the numbers"
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);
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static_assert(
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is_error<ints::apply<BOOST_METAPARSE_STRING(""), start>>::type::value,
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"when no numbers are provided, it should be an error"
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);
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[endsect]
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