94 lines
2.5 KiB
Plaintext
94 lines
2.5 KiB
Plaintext
[#fail_at_first_char_expected]
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[section fail_at_first_char_expected]
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[h1 Synopsis]
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template <class P>
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struct fail_at_first_char_expected;
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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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]
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[h1 Description]
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It tries to parse the input using `P`. When `P` rejects the input without
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consuming any character, `fail_at_first_char_expected` accepts the input.
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Otherwise (when `P` accepts the input or when it consumes characters before
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rejecting the input) `fail_at_first_char_expected` rejects the input.
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[h1 Header]
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#include <boost/metaparse/fail_at_first_char_expected.hpp>
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[h1 Expression semantics]
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For any `p` parser, `s` compile-time string and `pos` source position:
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When `is_error<p::apply<s, pos>>::type` is false, the following are equivalent:
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fail_at_first_char_expected<p>::apply<s, pos>::type
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reject<error::expected_to_fail, pos>
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When `is_error<p::apply<s, pos>>::type` is true and
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`boost::mpl::equal_to<pos, get_position<p::apply<s, pos>>::type>::type` is also
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true, the following are equivalent:
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get_remaining<except<p, c, msg>, s, pos>::type
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accept</* unspecified */, s, pos>
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Otherwise the following are equivalent:
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get_remaining<except<p, c, msg>, s, pos>::type
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p::apply<s, pos>::type
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[h1 Example]
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#include <boost/metaparse/fail_at_first_char_expected.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/foldl_start_with_parser.hpp>
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#include <boost/metaparse/first_of.hpp>
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#include <boost/metaparse/last_of.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/is_error.hpp>
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#include <boost/metaparse/get_result.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 plus_int = last_of<lit_c<'+'>, int_>;
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using plus_exp =
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first_of<
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foldl_start_with_parser<
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plus_int,
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int_,
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boost::mpl::lambda<boost::mpl::plus<boost::mpl::_1, boost::mpl::_2>>::type
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>,
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fail_at_first_char_expected<plus_int>
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>;
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static_assert(
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get_result<
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plus_exp::apply<BOOST_METAPARSE_STRING("1+2+3"), start>
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>::type::value == 6,
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"it should accept the input when it is a list of '+' separated ints"
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);
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static_assert(
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is_error<
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plus_exp::apply<BOOST_METAPARSE_STRING("1+2+3+"), start>
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>::type::value,
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"it should reject the input when it has an extra +"
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);
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[endsect]
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