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

   1 [#foldl]
   2 [section foldl]
   3
   4 [h1 Synopsis]
   5
   6   template <class P, class State, class ForwardOp>
   7   struct foldl;
   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   [[`ForwardOp`] [[link metafunction_class template metafunction class] taking two arguments]]
  16 ]
  17
  18 [h1 Description]
  19
  20 `foldl` applies `P` on the input string repeatedly as long as `P` accepts the
  21 input. The result of parsing is equivalent to
  22 `boost::mpl::fold<Sequence, State, ForwardOp>`, where `Sequence` is the sequence
  23 of the results of the applications of `P`.
  24
  25 When `P` rejects the input for the first time, `foldl` still accepts the input
  26 and the result of parsing is `State`.
  27
  28 Here is a diagram showing how `foldl` works by example:
  29
  30   using int_token = token<int_>;
  31   using sum_op = mpl::lambda<mpl::plus<mpl::_1, mpl::_2>>::type;
  32
  33 [$images/metaparse/foldl_diag2.png [width 70%]]
  34
  35 Further details can be found in the [link introducing-foldl Introducing foldl]
  36 section of the [link manual User Manual].
  37
  38 [h1 Header]
  39
  40   #include <boost/metaparse/foldl.hpp>
  41
  42 [h1 Expression semantics]
  43
  44 For any `p` parser, `t` class, `f` metafunction class taking two arguments,
  45 `s` compile-time string and `pos` source position
  46
  47   foldl<p, t, f>::apply<s, pos>
  48
  49 is equivalent to
  50
  51   return_<t>::apply<s, pos>
  52
  53 when `p::apply<s, pos>` returns an error. It is
  54
  55   foldl<p, f::apply<t, get_result<p::apply<s, pos>>::type>::type, f>::apply<
  56     get_remaining<p::apply<s, pos>>,
  57     get_position<p::apply<s, pos>>
  58   >
  59
  60 otherwise.
  61
  62 [h1 Example]
  63
  64   #include <boost/metaparse/foldl.hpp>
  65   #include <boost/metaparse/token.hpp>
  66   #include <boost/metaparse/int_.hpp>
  67   #include <boost/metaparse/string.hpp>
  68   #include <boost/metaparse/start.hpp>
  69   #include <boost/metaparse/get_result.hpp>
  70
  71   #include <boost/mpl/lambda.hpp>
  72   #include <boost/mpl/plus.hpp>
  73
  74   using namespace boost::metaparse;
  75
  76   using int_token = token<int_>;
  77   using sum_op =
  78     boost::mpl::lambda<boost::mpl::plus<boost::mpl::_1, boost::mpl::_2>>::type;
  79
  80   using ints = foldl<int_token, boost::mpl::int_<0>, sum_op>;
  81
  82   static_assert(
  83     get_result<
  84       ints::apply<BOOST_METAPARSE_STRING("11 13 3 21"), start>
  85     >::type::value == 48,
  86     "ints should sum the numbers"
  87   );
  88
  89   static_assert(
  90     get_result<
  91       ints::apply<BOOST_METAPARSE_STRING(""), start>
  92     >::type::value == 0,
  93     "the sum of no elements is 0"
  94   );
  95
  96 [endsect]
  97