87 lines
2.3 KiB
Plaintext
87 lines
2.3 KiB
Plaintext
[#accept_when]
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[section accept_when]
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[h1 Synopsis]
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template <class P, class Pred, class Msg>
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struct accept_when;
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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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[[`Pred`] [[link predicate predicate]]]
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[[`Msg`] [[link parsing_error_message parsing error message]]]
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]
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[h1 Description]
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It parses the input with `P`. When `P` rejects the input, `accept_when` rejects
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it as well. When `P` accepts it, `accept_when` evaluates `Pred` with the result
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of parsing the input with `P`. When `Pred` returns `true`, `accept_when` accepts
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the input and the result of parsing will be what `P` returned. Otherwise
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`accept_when` rejects the input and `Msg` is used as the error reason.
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[h1 Header]
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#include <boost/metaparse/accept_when.hpp>
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[h1 Expression semantics]
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For any `p` parser, `pred` predicate, `msg` parsing error message, `s`
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compile-time string and `pos` source position
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accept_when<p, pred, msg>i::apply<s, pos>::type
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is equivalent to
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p::apply<s, pos>::type
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when `p::apply<s, pos>` doesn't return an error and
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`pred::apply<get_result<p::apply<s, pos>>>::type` is `true`. Otherwise it is
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equivalent to
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fail<msg>
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[h1 Example]
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#include <boost/metaparse/accept_when.hpp>
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#include <boost/metaparse/one_char.hpp>
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#include <boost/metaparse/util/is_digit.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 <boost/metaparse/define_error.hpp>
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using namespace boost::metaparse;
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BOOST_METAPARSE_DEFINE_ERROR(digit_expected, "Digit expected");
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using accept_digit = accept_when<one_char, util::is_digit<>, digit_expected>;
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static_assert(
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!is_error<
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accept_digit::apply<BOOST_METAPARSE_STRING("0"), start>
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>::type::value,
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"accept_digit should accept a digit"
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);
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static_assert(
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get_result<
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accept_digit::apply<BOOST_METAPARSE_STRING("0"), start>
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>::type::value == '0',
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"the result of parsing should be the character value"
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);
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static_assert(
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is_error<
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accept_digit::apply<BOOST_METAPARSE_STRING("x"), start>
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>::type::value,
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"accept_digit should reject a character that is not a digit"
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);
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[endsect]
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