103 lines
2.5 KiB
Plaintext
103 lines
2.5 KiB
Plaintext
[#foldr]
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[section foldr]
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[h1 Synopsis]
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template <class P, class State, class BackwardOp>
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struct foldr;
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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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[[`State`] [[link metaprogramming_value template metaprogramming value]]]
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[[`BackwardOp`] [[link metafunction_class template metafunction class] taking two arguments]]
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]
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[h1 Description]
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`foldr` applies `P` on the input string repeatedly as long as `P` accepts the
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input. The result of parsing is equivalent to
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`boost::reverse_fold<Sequence, State, BackwardOp>`, where `Sequence` is the
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sequence of the results of the applications of `P`.
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When `P` rejects the input for the first time, `foldr` still accepts the input
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and the result of parsing is `State`.
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Here is a diagram showing how `foldr` works by example:
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using int_token = token<int_>;
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using sum_op = mpl::lambda<mpl::plus<mpl::_1, mpl::_2>>::type;
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[$images/metaparse/foldr_diag1.png [width 70%]]
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Further details can be found in the [link introducing-foldr Introducing foldr]
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section of the [link manual User Manual].
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[h1 Header]
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#include <boost/metaparse/foldr.hpp>
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[h1 Expression semantics]
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For any `p` parser, `t` class, `f` metafunction class taking two arguments,
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`s` compile-time string and `pos` source position
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foldr<p, t, f>::apply<s, pos>
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is equivalent to
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return_<t>::apply<s, pos>
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when `p::apply<s, pos>` returns an error. It is
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f::apply<
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get_result<
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foldr<p, t, f>::apply<
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get_remaining<p::apply<s, pos>>,
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get_position<p::apply<s, pos>>
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>
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>::type,
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get_result<p::apply<s, pos>>::type
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>
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otherwise.
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[h1 Example]
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#include <boost/metaparse/foldr.hpp>
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#include <boost/metaparse/token.hpp>
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#include <boost/metaparse/int_.hpp>
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#include <boost/metaparse/string.hpp>
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#include <boost/metaparse/start.hpp>
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#include <boost/metaparse/get_result.hpp>
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#include <boost/mpl/lambda.hpp>
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#include <boost/mpl/plus.hpp>
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using namespace boost::metaparse;
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using int_token = token<int_>;
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using sum_op =
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boost::mpl::lambda<boost::mpl::plus<boost::mpl::_1, boost::mpl::_2>>::type;
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using ints = foldr<int_token, boost::mpl::int_<0>, sum_op>;
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static_assert(
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get_result<
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ints::apply<BOOST_METAPARSE_STRING("11 13 3 21"), start>
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>::type::value == 48,
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"ints should sum the numbers"
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);
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static_assert(
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get_result<
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ints::apply<BOOST_METAPARSE_STRING(""), start>
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>::type::value == 0,
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"the sum of no elements is 0"
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);
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[endsect]
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