boost/spirit/core/composite/epsilon.hpp
/*=============================================================================
Copyright (c) 1998-2003 Joel de Guzman
Copyright (c) 2002-2003 Martin Wille
http://spirit.sourceforge.net/
Use, modification and distribution is subject to the Boost Software
License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
http://www.boost.org/LICENSE_1_0.txt)
=============================================================================*/
#ifndef BOOST_SPIRIT_EPSILON_HPP
#define BOOST_SPIRIT_EPSILON_HPP
////////////////////////////////////////////////////////////////////////////////
#include <boost/spirit/core/parser.hpp>
#include <boost/spirit/meta/parser_traits.hpp>
#include <boost/spirit/core/composite/composite.hpp>
#include <boost/spirit/core/composite/no_actions.hpp>
////////////////////////////////////////////////////////////////////////////////
namespace boost { namespace spirit {
///////////////////////////////////////////////////////////////////////////////
//
// condition_parser class
//
// handles expresions of the form
//
// epsilon_p(cond)
//
// where cond is a function or a functor that returns a value suitable
// to be used in boolean context. The expression returns a parser that
// returns an empty match when the condition evaluates to true.
//
///////////////////////////////////////////////////////////////////////////////
template <typename CondT, bool positive_ = true>
struct condition_parser : parser<condition_parser<CondT, positive_> >
{
typedef condition_parser<CondT, positive_> self_t;
// not explicit! (needed for implementation of if_p et al.)
condition_parser(CondT const& cond_) : cond(cond_) {}
template <typename ScannerT>
typename parser_result<self_t, ScannerT>::type
parse(ScannerT const& scan) const
{
if (positive_ == bool(cond())) // allow cond to return int
return scan.empty_match();
else
return scan.no_match();
}
condition_parser<CondT, !positive_>
negate() const
{ return condition_parser<CondT, !positive_>(cond); }
private:
CondT cond;
};
#if BOOST_WORKAROUND(BOOST_MSVC, == 1310) || \
BOOST_WORKAROUND(BOOST_MSVC, == 1400)
// VC 7.1 and VC8
template <typename CondT>
inline condition_parser<CondT, false>
operator~(condition_parser<CondT, true> const& p)
{ return p.negate(); }
template <typename CondT>
inline condition_parser<CondT, true>
operator~(condition_parser<CondT, false> const& p)
{ return p.negate(); }
#else // BOOST_WORKAROUND(BOOST_MSVC, == 1310) || == 1400
template <typename CondT, bool positive>
inline condition_parser<CondT, !positive>
operator~(condition_parser<CondT, positive> const& p)
{ return p.negate(); }
#endif // BOOST_WORKAROUND(BOOST_MSVC, == 1310) || == 1400
///////////////////////////////////////////////////////////////////////////////
//
// empty_match_parser class
//
// handles expressions of the form
// epsilon_p(subject)
// where subject is a parser. The expresion returns a composite
// parser that returns an empty match if the subject parser matches.
//
///////////////////////////////////////////////////////////////////////////////
struct empty_match_parser_gen;
struct negated_empty_match_parser_gen;
template <typename SubjectT>
struct negated_empty_match_parser; // Forward declaration
template<typename SubjectT>
struct empty_match_parser
: unary<SubjectT, parser<empty_match_parser<SubjectT> > >
{
typedef empty_match_parser<SubjectT> self_t;
typedef unary<SubjectT, parser<self_t> > base_t;
typedef unary_parser_category parser_category_t;
typedef empty_match_parser_gen parser_genererator_t;
typedef self_t embed_t;
explicit empty_match_parser(SubjectT const& p) : base_t(p) {}
template <typename ScannerT>
struct result
{ typedef typename match_result<ScannerT, nil_t>::type type; };
template <typename ScannerT>
typename parser_result<self_t, ScannerT>::type
parse(ScannerT const& scan) const
{
typename ScannerT::iterator_t save(scan.first);
typedef typename no_actions_scanner<ScannerT>::policies_t
policies_t;
bool matches = this->subject().parse(
scan.change_policies(policies_t(scan)));
if (matches)
{
scan.first = save; // reset the position
return scan.empty_match();
}
else
{
return scan.no_match();
}
}
negated_empty_match_parser<SubjectT>
negate() const
{ return negated_empty_match_parser<SubjectT>(this->subject()); }
};
template<typename SubjectT>
struct negated_empty_match_parser
: public unary<SubjectT, parser<negated_empty_match_parser<SubjectT> > >
{
typedef negated_empty_match_parser<SubjectT> self_t;
typedef unary<SubjectT, parser<self_t> > base_t;
typedef unary_parser_category parser_category_t;
typedef negated_empty_match_parser_gen parser_genererator_t;
explicit negated_empty_match_parser(SubjectT const& p) : base_t(p) {}
template <typename ScannerT>
struct result
{ typedef typename match_result<ScannerT, nil_t>::type type; };
template <typename ScannerT>
typename parser_result<self_t, ScannerT>::type
parse(ScannerT const& scan) const
{
typename ScannerT::iterator_t save(scan.first);
typedef typename no_actions_scanner<ScannerT>::policies_t
policies_t;
bool matches = this->subject().parse(
scan.change_policies(policies_t(scan)));
if (!matches)
{
scan.first = save; // reset the position
return scan.empty_match();
}
else
{
return scan.no_match();
}
}
empty_match_parser<SubjectT>
negate() const
{ return empty_match_parser<SubjectT>(this->subject()); }
};
struct empty_match_parser_gen
{
template <typename SubjectT>
struct result
{ typedef empty_match_parser<SubjectT> type; };
template <typename SubjectT>
static empty_match_parser<SubjectT>
generate(parser<SubjectT> const& subject)
{ return empty_match_parser<SubjectT>(subject.derived()); }
};
struct negated_empty_match_parser_gen
{
template <typename SubjectT>
struct result
{ typedef negated_empty_match_parser<SubjectT> type; };
template <typename SubjectT>
static negated_empty_match_parser<SubjectT>
generate(parser<SubjectT> const& subject)
{ return negated_empty_match_parser<SubjectT>(subject.derived()); }
};
//////////////////////////////
template <typename SubjectT>
inline negated_empty_match_parser<SubjectT>
operator~(empty_match_parser<SubjectT> const& p)
{ return p.negate(); }
template <typename SubjectT>
inline empty_match_parser<SubjectT>
operator~(negated_empty_match_parser<SubjectT> const& p)
{ return p.negate(); }
///////////////////////////////////////////////////////////////////////////////
//
// epsilon_ parser and parser generator class
//
// Operates as primitive parser that always matches an empty sequence.
//
// Also operates as a parser generator. According to the type of the
// argument an instance of empty_match_parser<> (when the argument is
// a parser) or condition_parser<> (when the argument is not a parser)
// is returned by operator().
//
///////////////////////////////////////////////////////////////////////////////
namespace impl
{
template <typename SubjectT>
struct epsilon_selector
{
typedef typename as_parser<SubjectT>::type subject_t;
typedef typename
mpl::if_<
is_parser<subject_t>
,empty_match_parser<subject_t>
,condition_parser<subject_t>
>::type type;
};
}
struct epsilon_parser : public parser<epsilon_parser>
{
typedef epsilon_parser self_t;
epsilon_parser() {}
template <typename ScannerT>
typename parser_result<self_t, ScannerT>::type
parse(ScannerT const& scan) const
{ return scan.empty_match(); }
template <typename SubjectT>
typename impl::epsilon_selector<SubjectT>::type
operator()(SubjectT const& subject) const
{
typedef typename impl::epsilon_selector<SubjectT>::type result_t;
return result_t(subject);
}
};
epsilon_parser const epsilon_p = epsilon_parser();
epsilon_parser const eps_p = epsilon_parser();
///////////////////////////////////////////////////////////////////////////////
}} // namespace boost::spirit
#endif
