[#foldl_start_with_parser]
[section foldl_start_with_parser]

[h1 Synopsis]

  template <class P, class StateP, class ForwardOp>
  struct foldl_start_with_parser;

This is a [link parser_combinator parser combinator].

[table Arguments
  [[Name]        [Type]]
  [[`P`]         [[link parser parser]]]
  [[`StateP`]    [[link parser parser]]]
  [[`ForwardOp`] [[link metafunction_class template metafunction class] taking two arguments]]
]

[h1 Description]

The same as [link foldl `foldl`], but before folding it applies a parser,
`StateP` on the input. If it fails, `foldl_start_with_parser` fails. If it
succeeds, `foldl_start_with_parser` works as `foldl` taking the result of
`StateP` as the initial state.

Here is a diagram showing how `foldl_start_with_parser` works by example:

  using int_token = token<int_>;
  using plus_token = token<lit_c<'+'>>;
  using plus_int = last_of<plus_token, int_token>;
  using sum_op = mpl::lambda<mpl::plus<mpl::_1, mpl::_2>>::type;

[$images/metaparse/foldl_start_with_parser_diag1.png [width 70%]]

Further details can be found in the
[link introducing-foldl_start_with_parser Introducing foldl_start_with_parser]
section of the [link manual User Manual].

[h1 Header]

  #include <boost/metaparse/foldl_start_with_parser.hpp>

[h1 Expression semantics]

For any `p` parser, `pt` class, `f` metafunction class taking two arguments,
`s` compile-time string and `pos` source position

  foldl_start_with_parser<p, pt, f>::apply<s, pos>

is equivalent to

  pt::apply<s, pos>

when the above expression returns a parsing error. It is

  foldl<p, get_result<pt::apply<s, pos>>::type, f>::apply<
    get_remaining<pt::apply<s, pos>>::type,
    get_position<pt::apply<s, pos>>::type
  >

otherwise.

[h1 Example]

  #include <boost/metaparse/foldl_start_with_parser.hpp>
  #include <boost/metaparse/lit_c.hpp>
  #include <boost/metaparse/last_of.hpp>
  #include <boost/metaparse/token.hpp>
  #include <boost/metaparse/int_.hpp>
  #include <boost/metaparse/string.hpp>
  #include <boost/metaparse/start.hpp>
  #include <boost/metaparse/get_result.hpp>
  #include <boost/metaparse/is_error.hpp>
  
  #include <boost/mpl/lambda.hpp>
  #include <boost/mpl/plus.hpp>
  
  using namespace boost::metaparse;
  
  using int_token = token<int_>;
  using plus_token = token<lit_c<'+'>>;
  using plus_int = last_of<plus_token, int_token>;
  using sum_op =
    boost::mpl::lambda<boost::mpl::plus<boost::mpl::_1, boost::mpl::_2>>::type;
  
  using ints = foldl_start_with_parser<plus_int, int_token, sum_op>;
  
  static_assert(
    get_result<
      ints::apply<BOOST_METAPARSE_STRING("11 + 13 + 3 + 21"), start>
    >::type::value == 48,
    "ints should sum the numbers"
  );
  
  static_assert(
    is_error<ints::apply<BOOST_METAPARSE_STRING(""), start>>::type::value,
    "when no numbers are provided, it should be an error"
  );

[endsect]

