You'll have to post your code ... I can't tell what problem you might be running into. Well, I decided to be a coward for today. I'll shelf this functional solution for later. I just added my own ternary_expr & quaternary_expr class for the moment as it seems it's all the client wants. I'll come back when I've digested those infos as I htink I'm just doing a small error.
For reference here is the code I wrote: namespace bp = boost::proto; struct push_front_ { template<class Sig> struct result; template<class This,class Seq,class Elem> struct result<This(Seq,Elem)> { typedef typename boost::fusion::result_of::push_front<Seq,Elem>::type type; }; template<class Seq,class Elem> typename result<push_front_(Seq,Elem)>::type operator()(Elem const& e, Seq& s ) const { return boost::fusion::push_front(s,e); } }; struct _load : bp::transform< _load > { template<typename Expr, typename State, typename Data> struct impl : bp::transform_impl<Expr, State, Data> { typedef typename meta::strip<Expr>::type::value_type result_type; result_type operator()( typename impl::expr_param expr , typename impl::state_param , typename impl::data_param data ) const { return expr(data); } }; }; struct eval_xpr : bp::or_< bp::when< bp::terminal< block<bp::_,bp::_> > , _load(bp::_) > , bp::when< bp::nary_expr< bp::_, bp::vararg<eval_xpr> > , bp::functional::unpack_expr<bp::tag::function>( push_front_(bp::_, bp::terminal<functor<bp::tag_of<bp::_> > >() ) ) ) > > {}; -- ___________________________________________ Joel Falcou - Assistant Professor PARALL Team - LRI - Universite Paris Sud XI Tel : (+33)1 69 15 66 35