Is it possible to evaluate terminal and custom_terminal expressions in the some custom actions class? For instance how to implement "terminal_eval" to print something in the following code? #include <boost/phoenix/phoenix.hpp> #include <iostream> using namespace boost; struct actions { template <typename Rule, typename Dummy = void> struct when; }; template <> struct actions::when<phoenix::rule::terminal> : phoenix::call<terminal_eval> {}; int main() { int var = 11; phoenix::eval(phoenix::val(var), phoenix::context(int(), actions())); return 0; } I've tried to use the following form, but encountered some compiler errors: (GCC-4.2.1[FreeBSD], boost and phoenix were taken from SVN, latest versions) struct terminal_eval { typedef void result_type; template <typename Context, typename Expr> result_type operator ()(Context const &ctx, Expr const &expr) { std::cout << "terminal" << std::endl; } }; ../../../../boost.phoenix/boost/phoenix/core/call.hpp:32: error: invalid use of incomplete type 'struct boost::phoenix::detail::call_impl<terminal_eval, const boost::phoenix::actor<boost::proto::exprns_::expr<boost::proto::tag::terminal, boost::proto::argsns_::term<int>, 0l> >&, const int&, const actions&, 0l>' ../../../../boost.phoenix/boost/phoenix/core/call.hpp:22: error: declaration of 'struct boost::phoenix::detail::call_impl<terminal_eval, const boost::phoenix::actor<boost::proto::exprns_::expr<boost::proto::tag::terminal, boost::proto::argsns_::term<int>, 0l> >&, const int&, const actions&, 0l>' ../../../../boost/boost/proto/transform/call.hpp:315: error: no type named 'result_type' in 'struct boost::proto::switch_<boost::phoenix::meta_grammar>::impl<const boost::phoenix::actor<boost::proto::exprns_::expr<boost::proto::tag::terminal, boost::proto::argsns_::term<int>, 0l> >&, const int&, const actions&>' ../../../../boost/boost/proto/transform/call.hpp:320: error: no type named 'result_type' in 'struct boost::proto::switch_<boost::phoenix::meta_grammar>::impl<const boost::phoenix::actor<boost::proto::exprns_::expr<boost::proto::tag::terminal, boost::proto::argsns_::term<int>, 0l> >&, const int&, const actions&>' The same situation with the phoenix::ref and custom_terminal rule.