///////////////////////////////////////////////////////////////////////////////

 /// \file conditional.hpp

 /// A special-purpose proto compiler for compiling an expression either one

 /// way or another depending on the properties of the expression.

 //

 //  Copyright 2004 Eric Niebler. Distributed under 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_PROTO_COMPILER_CONDITIONAL_HPP_EAN_04_01_2005

 #define BOOST_PROTO_COMPILER_CONDITIONAL_HPP_EAN_04_01_2005

 #include <boost/mpl/if.hpp>

 #include <boost/mpl/bool.hpp>

 #include <boost/xpressive/proto/proto_fwd.hpp>

 namespace boost { namespace proto

 {

     ///////////////////////////////////////////////////////////////////////////////

     // conditional_compiler

     template<typename Predicate, typename IfCompiler, typename ElseCompiler>

     struct conditional_compiler

     {

         template<typename Op, typename State, typename Visitor>

         struct apply

         {

             typedef typename boost::mpl::if_

             <

                 typename Predicate::BOOST_NESTED_TEMPLATE apply<Op, State, Visitor>::type

               , IfCompiler

               , ElseCompiler

             >::type compiler_type;

             typedef typename compiler_type::BOOST_NESTED_TEMPLATE apply

             <

                 Op

               , State

               , Visitor

             >::type type;

         };

         template<typename Op, typename State, typename Visitor>

         static typename apply<Op, State, Visitor>::type

         call(Op const &op, State const &state, Visitor &visitor)

         {

             typedef typename apply<Op, State, Visitor>::compiler_type compiler_type;

             return compiler_type::call(op, state, visitor);

         }

     };

 }}

 #endif