#include<boost/proto/proto.hpp>

using namespace boost;

// Terminals
struct A_ {};
typedef proto::terminal<A_> A_terminal;

struct B_ {};
typedef proto::terminal<B_> B_terminal;

struct C_ {};
typedef proto::terminal<C_> C_terminal;

struct D_ {};
typedef proto::terminal<D_> D_terminal;

struct fun_ {};
typedef proto::terminal<fun_> fun_terminal;

// Functions
struct A:
   proto::function<A_terminal, proto::_>
{};

struct B:
   proto::function<B_terminal, proto::_>
{};

struct C:
   proto::function<C_terminal, proto::_>
{};

struct D:
   proto::function<D_terminal, proto::_>
{};


// Comparator
template<class Left, class Right>
struct grammar_less: mpl::false_ {};

template<>
struct grammar_less<A, B>: mpl::true_ {};

template<class Right>
struct grammar_less<A, Right>: grammar_less<B, Right> {};

template<>
struct grammar_less<B, C>: mpl::true_ {};

template<class Right>
struct grammar_less<B, Right>: grammar_less<C, Right> {};

template<>
struct grammar_less<C, D>: mpl::true_ {};

template<class Right>
struct grammar_less<C, Right>: grammar_less<D, Right> {};

BOOST_MPL_ASSERT_NOT((grammar_less<A, A>));
BOOST_MPL_ASSERT    ((grammar_less<A, B>));
BOOST_MPL_ASSERT    ((grammar_less<A, C>));
BOOST_MPL_ASSERT    ((grammar_less<A, D>));

// How to do this correctly?
struct sorted_argument_function:
   proto::and_
   <
      proto::function<fun_terminal, proto::_, proto::_>,
      proto::if_
      < 
         grammar_less<proto::_child1, proto::_child2>()
      >
   >
{};

int main()
{
   fun_terminal::type fun;
   A_terminal::type a;
   B_terminal::type b;
   C_terminal::type c;
   D_terminal::type d;
   BOOST_PROTO_ASSERT_MATCHES(a(0), A);
   BOOST_PROTO_ASSERT_MATCHES(b(0), B);
   BOOST_PROTO_ASSERT_MATCHES(c(0), C);
   BOOST_PROTO_ASSERT_MATCHES(d(0), D);

   // These should compile
   BOOST_PROTO_ASSERT_MATCHES((fun(a(0), d(0))), sorted_argument_function);
   BOOST_PROTO_ASSERT_MATCHES((fun(b(0), c(0))), sorted_argument_function);
   BOOST_PROTO_ASSERT_MATCHES_NOT((fun(b(0), a(0))), sorted_argument_function);
}