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- [#transform]
- [section transform]
- [h1 Synopsis]
- template <class P, class T>
- struct transform;
- This is a [link parser_combinator parser combinator].
- [table Arguments
- [[Name] [Type]]
- [[`P`] [[link parser parser]]]
- [[`T`] [[link metafunction_class template metafunction class] taking one argument]]
- ]
- [h1 Description]
- `transform` parses the input using `P` and transforms the result `P` returns
- with `T`. The result of parsing is what `T` returns. When `P` fails, the failure
- is returned unchanged.
- [h1 Header]
- #include <boost/metaparse/transform.hpp>
- [h1 Expression semantics]
- For any `p` parser, `t` metafunction class accepting one argument, `s`
- compile-time string and `pos` source position
- get_result<transform<p, t>::apply<s, pos>>::type
- is equivalent to
- t::apply<get_result<p::apply<s, pos>>::type>::type
- When `p::apply<s, pos>` doesn't return an error. The combinator returns the
- error otherwise.
- [h1 Example]
- #include <boost/metaparse/transform.hpp>
- #include <boost/metaparse/digit.hpp>
- #include <boost/metaparse/start.hpp>
- #include <boost/metaparse/string.hpp>
- #include <boost/metaparse/is_error.hpp>
- #include <boost/metaparse/get_result.hpp>
-
- #include <boost/metaparse/util/digit_to_int.hpp>
-
- using namespace boost::metaparse;
-
- using digit_value = transform<digit, util::digit_to_int<>>;
-
- static_assert(
- !is_error<
- digit_value::apply<BOOST_METAPARSE_STRING("0"), start>
- >::type::value,
- "digit_val should accept a digit"
- );
-
- static_assert(
- is_error<digit_value::apply<BOOST_METAPARSE_STRING("x"), start>>::type::value,
- "digit_val should reject a character"
- );
-
- static_assert(
- get_result<
- digit_value::apply<BOOST_METAPARSE_STRING("0"), start>
- >::type::value == 0,
- "the result of parsing should be the int value"
- );
- [endsect]
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