69 lines
1.5 KiB
Plaintext
69 lines
1.5 KiB
Plaintext
[#always]
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[section always]
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[h1 Synopsis]
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template <class P, class T>
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struct always;
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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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[[`T`] [[link metaprogramming_value template metaprogramming value]]]
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]
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[h1 Description]
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It accepts an input if and only if `P` accepts it, but the result of parsing
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will be `T` instead of the result `P` returned.
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[h1 Header]
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#include <boost/metaparse/always.hpp>
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[h1 Expression semantics]
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For any `p` parser and `t` class the following are equivalent:
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always<p, t>
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last_of<p, return_<t>>
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[h1 Example]
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#include <boost/metaparse/always.hpp>
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#include <boost/metaparse/lit_c.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 <type_traits>
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using namespace boost::metaparse;
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using always13 = always<lit_c<'x'>, std::integral_constant<int, 13>>;
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static_assert(
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!is_error<always13::apply<BOOST_METAPARSE_STRING("x"), start>>::type::value,
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"always13 should accept x"
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);
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static_assert(
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std::is_same<
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get_result<always13::apply<BOOST_METAPARSE_STRING("x"), start>>::type,
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std::integral_constant<int, 13>
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>::type::value,
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"the result of parsing should be the integral_constant type"
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);
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static_assert(
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is_error<always13::apply<BOOST_METAPARSE_STRING("y"), start>>::type::value,
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"always13 should reject characters other than x"
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);
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[endsect]
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