ceph/src/boost/libs/metaparse/doc/foldr_reject_incomplete.qbk

   1 [#foldr_reject_incomplete]
   2 [section foldr_reject_incomplete]
   3
   4 [h1 Synopsis]
   5
   6   template <class P, class State, class BackwardOp>
   7   struct foldr_reject_incomplete;
   8
   9 This is a [link parser_combinator parser combinator].
  10
  11 [table Arguments
  12   [[Name]         [Type]]
  13   [[`P`]          [[link parser parser]]]
  14   [[`State`]      [[link metaprogramming_value template metaprogramming value]]]
  15   [[`BackwardOp`] [[link metafunction_class template metafunction class] taking two arguments]]
  16 ]
  17
  18 [h1 Description]
  19
  20 The same as [link foldr `foldr`], but once `P` rejects the input,
  21 `foldr_reject_incomplete` checks if `P` consumes any characters before rejecting
  22 the input. If so, `foldr_reject_incomplete` rejects the input with the same
  23 error message this last application of `P` returned. Otherwise
  24 `foldr_reject_incomplete` accepts the input and gives the same result as
  25 [link foldr `foldr`].
  26
  27 Here is a diagram showing how `foldr_reject_incomplete` works by example:
  28
  29   using int_token = token<int_>;
  30   using plus_token = token<lit_c<'+'>>;
  31   using plus_int = last_of<plus_token, int_token>;
  32   using sum_op = mpl::lambda<mpl::plus<mpl::_1, mpl::_2>>::type;
  33
  34 [$images/metaparse/foldr_reject_incomplete_diag1.png [width 70%]]
  35
  36 Note that `foldr_reject_incomplete` folds from right to left and therefore does
  37 not start folding until it reaches the end of the sequence. Since at the end of
  38 the sequence it finds an error, folding does not happen at all.
  39
  40 [h1 Header]
  41
  42   #include <boost/metaparse/foldr_reject_incomplete.hpp>
  43
  44 [h1 Expression semantics]
  45
  46 For any `p` parser, `t` class, `f` metafunction class taking two arguments,
  47 `s` compile-time string and `pos` source position
  48
  49   foldr_reject_incomplete<p, t, f>::apply<s, pos>
  50
  51 is equivalent to
  52
  53   first_of<foldr<p, t, f>, fail_at_first_char_expected<p> >::apply<s, pos>
  54
  55 [h1 Example]
  56
  57   #include <boost/metaparse/foldr_reject_incomplete.hpp>
  58   #include <boost/metaparse/lit_c.hpp>
  59   #include <boost/metaparse/last_of.hpp>
  60   #include <boost/metaparse/token.hpp>
  61   #include <boost/metaparse/int_.hpp>
  62   #include <boost/metaparse/string.hpp>
  63   #include <boost/metaparse/start.hpp>
  64   #include <boost/metaparse/get_result.hpp>
  65   #include <boost/metaparse/is_error.hpp>
  66
  67   #include <boost/mpl/lambda.hpp>
  68   #include <boost/mpl/plus.hpp>
  69   #include <boost/mpl/int.hpp>
  70
  71   using namespace boost::metaparse;
  72
  73   using int_token = token<int_>;
  74   using plus_token = token<lit_c<'+'>>;
  75   using plus_int = last_of<plus_token, int_token>;
  76   using sum_op =
  77     boost::mpl::lambda<boost::mpl::plus<boost::mpl::_1, boost::mpl::_2>>::type;
  78
  79   using ints = foldr_reject_incomplete<plus_int, boost::mpl::int_<11>, sum_op>;
  80
  81   static_assert(
  82     get_result<
  83       ints::apply<BOOST_METAPARSE_STRING("+ 13 + 3 + 21"), start>
  84     >::type::value == 48,
  85     "ints should sum the numbers"
  86   );
  87
  88   static_assert(
  89     is_error<
  90       ints::apply<BOOST_METAPARSE_STRING("+ 13 + 3 +"), start>
  91     >::type::value,
  92     "when the last number is missing, it should be an error"
  93   );
  94
  95 [endsect]
  96