97 lines
2.9 KiB
Plaintext
97 lines
2.9 KiB
Plaintext
[#foldr_reject_incomplete]
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[section foldr_reject_incomplete]
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[h1 Synopsis]
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template <class P, class State, class BackwardOp>
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struct foldr_reject_incomplete;
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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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The same as [link foldr `foldr`], but once `P` rejects the input,
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`foldr_reject_incomplete` checks if `P` consumes any characters before rejecting
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the input. If so, `foldr_reject_incomplete` rejects the input with the same
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error message this last application of `P` returned. Otherwise
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`foldr_reject_incomplete` accepts the input and gives the same result as
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[link foldr `foldr`].
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Here is a diagram showing how `foldr_reject_incomplete` works by example:
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using int_token = token<int_>;
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using plus_token = token<lit_c<'+'>>;
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using plus_int = last_of<plus_token, int_token>;
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using sum_op = mpl::lambda<mpl::plus<mpl::_1, mpl::_2>>::type;
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[$images/metaparse/foldr_reject_incomplete_diag1.png [width 70%]]
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Note that `foldr_reject_incomplete` folds from right to left and therefore does
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not start folding until it reaches the end of the sequence. Since at the end of
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the sequence it finds an error, folding does not happen at all.
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[h1 Header]
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#include <boost/metaparse/foldr_reject_incomplete.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_reject_incomplete<p, t, f>::apply<s, pos>
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is equivalent to
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first_of<foldr<p, t, f>, fail_at_first_char_expected<p> >::apply<s, pos>
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[h1 Example]
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#include <boost/metaparse/foldr_reject_incomplete.hpp>
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#include <boost/metaparse/lit_c.hpp>
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#include <boost/metaparse/last_of.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/metaparse/is_error.hpp>
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#include <boost/mpl/lambda.hpp>
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#include <boost/mpl/plus.hpp>
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#include <boost/mpl/int.hpp>
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using namespace boost::metaparse;
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using int_token = token<int_>;
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using plus_token = token<lit_c<'+'>>;
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using plus_int = last_of<plus_token, int_token>;
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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_reject_incomplete<plus_int, boost::mpl::int_<11>, sum_op>;
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static_assert(
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get_result<
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ints::apply<BOOST_METAPARSE_STRING("+ 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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is_error<
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ints::apply<BOOST_METAPARSE_STRING("+ 13 + 3 +"), start>
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>::type::value,
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"when the last number is missing, it should be an error"
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);
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[endsect]
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