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template <class P, class F> struct transform_error_message;
This is a parser combinator.
It parses the input with P
.
When this succeeds, the result of parsing with transform_error_message
will be the result of parsing with P
.
When it fails, the error P
returns is forwarded to the caller of transform_error_message
,
but the message of it is transformed with F
.
#include <boost/metaparse/transform_error_message.hpp>
For any p
parser and f
metafunction class accepting one argument
transform_error_message<p, f>::apply<s, pos>
is equivalent to p::apply<s, pos>
when p
accepts the input. It is equivalent to reject<f::apply<get_message<p::apply<s, pos>>::type>, get_position<p::apply<s, pos>>>
otherwise.
#include <boost/metaparse/transform_error_message.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/define_error.hpp> #include <boost/metaparse/reject.hpp> #include <type_traits> using namespace boost::metaparse; BOOST_METAPARSE_DEFINE_ERROR(name_expected, "Name expected"); struct return_name_expected { typedef return_name_expected type; template <class> struct apply : 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_message<name_token, return_name_expected> >; 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" );