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 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, boost::fusion::vector1, boost::fusion::vector1)>' error: declaration of 'struct fmi::ite_::result, boost::fusion::vector1, boost::fusion::vector1)>' cpp:7: Can you imaging what the problem is? Otherwise I will attach further cleaned code. struct ite_ { template<typename Sig> struct result; template struct result : fusion::result_of::make_vector<bv> {}; template 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 proto::result_of::make_expr::type ite (If const& _if, Then const& _then, Else const& _else) { return proto::make_expr( ite_() , boost::ref(_if) , boost::ref(_then) , boost::ref(_else)); } Examplarily, the placeholder definition: typedef constraint > >::type> var0_type Best wishes, sfrehse