Hello Boost.Proto, firstly, thank you very much for your great work for providing such a powerful framework. My Setup: I defined a grammar, like grammar * grammar, a domain and so on. Based on the domain two evaluator for different contexts are provided. For standard operators that works great. For expression evaluation I have a function defined as follow: template<typename T, typename Expr, typename Arg0, typename Arg1, ... > generate (T& t, Expr const& expr, Arg0 const& arg0, ... ) { call the first or the second evaluator switched by T and passes the value arg0, ... } Therewith, I can write for example, generate (foo, _1 * _2, a, b); Coming to my problem. I would like to write, generate (foo, _1 * func (_2, _3), a ,b, c); Overall I would like to embedd function calls into the expression and the pass them to the generate function. I tried to implement a make_expr based on func, but that does not work for me, i.e., compiler error for fmi::ite (_0, _1, _2) (a,b,c); error: invalid use of incomplete type 'struct fmi::ite_::result<fmi::ite_(boost::fusion::vector1<const fmi::bv&>, boost::fusion::vector1<const fmi::bv&>, boost::fusion::vector1<mpl_::void_&>)>' error: declaration of 'struct fmi::ite_::result<fmi::ite_(boost::fusion::vector1<const fmi::bv&>, boost::fusion::vector1<const fmi::bv&>, boost::fusion::vector1<mpl_::void_&>)>' cpp:7: Can you imaging what the problem is? Otherwise I will attach further cleaned code. struct ite_ { template<typename Sig> struct result; template<typename This, typename If, typename Then, typename Else> struct result<This(If const&, Then const&, Else const&)> : fusion::result_of::make_vector<bv> {}; template<typename If, typename Then, typename Else> fusion::result_of::make_vector<bv>::type operator() (If const& _if, Then const& _then, Else const& _else) const { bv test; /// some more code return fusion::make_vector (test); } }; template<typename If, typename Then, typename Else> typename proto::result_of::make_expr<proto::tag::function , constraint_domain , ite_ const , If const& , Then const& , Else const&>::type ite (If const& _if, Then const& _then, Else const& _else) { return proto::make_expr<proto::tag::function, constraint_domain>( ite_() , boost::ref(_if) , boost::ref(_then) , boost::ref(_else)); } Examplarily, the placeholder definition: typedef constraint<proto::terminal<placeholder<mpl::int_<0> > >::type> var0_type Best wishes, sfrehse