88 lines
2.1 KiB
Plaintext
88 lines
2.1 KiB
Plaintext
[#nth_of]
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[section nth_of]
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[h1 Synopsis]
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template <class N, class... Ps>
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struct nth_of;
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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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[[`N`] [[link boxed_value boxed] integer value in the range `[0..sizeof...(Ps)]`]]
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[[`Ps`] [[link parser parser]s]]
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]
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[h1 Description]
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`nth_of` applies the `Ps...` parsers in sequence. It accepts an input when all
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of these parsers accept it. The result of parsing is the result of the `N`.
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parser.
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On compilers, which are not C++11-compliant, the maximum number of parsers
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`nth_of` accepts can be specified with the `BOOST_METAPARSE_LIMIT_SEQUENCE_SIZE`
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macro. Its default value is `5`.
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[h1 Header]
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#include <boost/metaparse/nth_of.hpp>
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[h1 Expression semantics]
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For any `p0`, ..., `pn` parsers and `k` boxed integer value the following are
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equivalent
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nth_of<k, p0, ..., pn>
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nth_of_c<k::type::value, p0, ..., pn>
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[h1 Example]
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#include <boost/metaparse/nth_of.hpp>
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#include <boost/metaparse/int_.hpp>
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#include <boost/metaparse/lit_c.hpp>
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#include <boost/metaparse/token.hpp>
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#include <boost/metaparse/start.hpp>
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#include <boost/metaparse/string.hpp>
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#include <boost/metaparse/is_error.hpp>
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#include <boost/metaparse/get_result.hpp>
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#include <type_traits>
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using namespace boost::metaparse;
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using int_token = token<int_>;
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using left_paren_token = token<lit_c<'('>>;
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using right_paren_token = token<lit_c<')'>>;
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using int_in_parens =
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nth_of<
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std::integral_constant<int, 1>,
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left_paren_token, int_token, right_paren_token
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>;
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static_assert(
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get_result<
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int_in_parens::apply<BOOST_METAPARSE_STRING("(13)"), start>
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>::type::value == 13,
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"it should return the result of the second parser"
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);
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static_assert(
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is_error<
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int_in_parens::apply<BOOST_METAPARSE_STRING("13"), start>
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>::type::value,
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"it should reject the input when there are no parens"
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);
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static_assert(
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is_error<
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int_in_parens::apply<BOOST_METAPARSE_STRING("(13"), start>
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>::type::value,
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"it should reject the input when there is no closing paren"
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);
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[endsect]
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