84 lines
2.0 KiB
Plaintext
84 lines
2.0 KiB
Plaintext
[#except]
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[section except]
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[h1 Synopsis]
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template <class P, class Result, class ErrorMsg>
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struct except;
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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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[[`Result`] [[link metaprogramming_value template metaprogramming value]]]
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[[`ErrorMsg`] [[link parsing_error_message parsing error message]]]
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]
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[h1 Description]
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`except` accepts the input when `P` rejects it and the result of parsing is the
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`Result` argument. When `P` accepts the input, `except` rejects it and the
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reason is `ErrorMsg`.
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[h1 Header]
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#include <boost/metaparse/except.hpp>
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[h1 Expression semantics]
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For any `p` parser, `c` class, `msg` parsing error message, `s` compile-time
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string and `pos` source position the following are equivalent
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get_result<except<p, c, msg>, s, pos>::type
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c
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get_remaining<except<p, c, msg>, s, pos>::type
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s
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get_position<except<p, c, msg>, s, pos>::type
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pos
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when `p` rejects the input. The result of the parser is an error with the error
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message `msg` otherwise.
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[h1 Example]
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#include <boost/metaparse/except.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/get_message.hpp>
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#include <boost/metaparse/define_error.hpp>
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#include <type_traits>
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using namespace boost::metaparse;
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BOOST_METAPARSE_DEFINE_ERROR(
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number_is_not_allowed,
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"numbers are not allowed here"
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);
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using except_int =
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except<int_, std::integral_constant<int, 1>, number_is_not_allowed>;
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static_assert(
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get_result<
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except_int::apply<BOOST_METAPARSE_STRING("foo"), start>
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>::type::value == 1,
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"it should accept the input when it is not an integer"
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);
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static_assert(
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std::is_same<
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number_is_not_allowed,
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get_message<except_int::apply<BOOST_METAPARSE_STRING("13"), start>>::type
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>::type::value,
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"it should reject the input when it is an integer"
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);
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[endsect]
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