102 lines
2.7 KiB
Plaintext
102 lines
2.7 KiB
Plaintext
[#transform_error]
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[section transform_error]
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[h1 Synopsis]
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template <class P, class F>
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struct transform_error;
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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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[[`F`] [[link metafunction_class template metafunction class] taking one argument]]
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]
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[h1 Description]
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It parses the input with `P`. When this succeeds, the result of parsing with
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`transform_error` will be the result of parsing with `P`. When it fails, `F` is
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evaluated with the error `P` returned as argument. Parsing with
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`transform_error` will fail and the error will be what `F` returns. Therefore,
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`F` is expected to accept and return a [link reject `reject`] value.
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[h1 Header]
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#include <boost/metaparse/transform_error.hpp>
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[h1 Expression semantics]
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For any `p` parser and `f` metafunction class accepting one argument
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transform_error<p, f>::apply<s, pos>
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is equivalent to `p::apply<s, pos>` when `p` accepts the input. It is equivalent
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to `f::apply<p::apply<s, pos>::type>` otherwise.
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[h1 Example]
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#include <boost/metaparse/transform_error.hpp>
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#include <boost/metaparse/repeated1.hpp>
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#include <boost/metaparse/letter.hpp>
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#include <boost/metaparse/keyword.hpp>
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#include <boost/metaparse/last_of.hpp>
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#include <boost/metaparse/token.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/start.hpp>
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#include <boost/metaparse/get_message.hpp>
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#include <boost/metaparse/get_position.hpp>
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#include <boost/metaparse/define_error.hpp>
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#include <boost/metaparse/reject.hpp>
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#include <boost/mpl/lambda.hpp>
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#include <type_traits>
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using namespace boost::metaparse;
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BOOST_METAPARSE_DEFINE_ERROR(name_expected, "Name expected");
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using keyword_name = token<keyword<BOOST_METAPARSE_STRING("name")>>;
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using name_token = token<repeated1<letter>>;
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using name_parser =
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last_of<
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keyword_name,
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transform_error<
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name_token,
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boost::mpl::lambda<
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reject<name_expected, get_position<boost::mpl::_1> >
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>::type
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>
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>;
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static_assert(
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!is_error<
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name_parser::apply<BOOST_METAPARSE_STRING("name Bela"), start>
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>::type::value,
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"name_parser should accept \"name <a name>\""
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);
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static_assert(
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is_error<
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name_parser::apply<BOOST_METAPARSE_STRING("name ?"), start>
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>::type::value,
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"name_parser should reject input when name is a question mark"
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);
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static_assert(
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std::is_same<
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get_message<
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name_parser::apply<BOOST_METAPARSE_STRING("name ?"), start>
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>::type,
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name_expected
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>::type::value,
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"the error message should be the one specified by change_error_message"
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);
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[endsect]
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