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