As you can see from other posts. It's been quite painful to me to debug programs with expression templates. In order to debug some code I implemented some functions to print the expression trees, this would be a nice addition to Boost.Lambda and also to Boost.Phoenix (although Boost.Phoenix will probably have it through Boost.Proto which apparently already can print its expression trees). Here it is a *minimal* code for printing some Boost.Lambda expression (involving only [binary +,-,/,*],bind,constants), (let me know if I am reinventing the wheel) namespace boost{ namespace lambda{ using namespace boost::tuples; template<int N> //print placeholder std::string to_string(const lambda_functor<boost::lambda::placeholder<N> >&){ return "_"+boost::lexical_cast<std::string>(N); } //template<class T> //std::string to_string(const T& t){ // return boost::lexical_cast<std::string>(t); //} //binary operators #define BOOST_LAMBDA_TO_STRING_GEN(action_name, symbol_string)\ template<class LExp1, class LExp2> \ std::string to_string(const \ lambda_functor<lambda_functor_base<arithmetic_action<action_name>, tuple< \ LExp1, \ LExp2 \ > > >& action_name##_expr){ \ return to_string(get<0>(action_name##_expr.args)) + #symbol_string +to_string(get<1>(action_name##_expr.args)); \ } BOOST_LAMBDA_TO_STRING_GEN(plus_action, +); BOOST_LAMBDA_TO_STRING_GEN(divide_action, /); BOOST_LAMBDA_TO_STRING_GEN(minus_action, -); BOOST_LAMBDA_TO_STRING_GEN(multiply_action, *); //bind template<class FunctionType, class LExpr> std::string to_string(const lambda_functor<lambda_functor_base<action<2, function_action<2> >, tuple< FunctionType, //double (* const)(double), LExpr > > >& bind_expr){ return std::string("bind(") +boost::units::detail::demangle(typeid(FunctionType).name()) +", "+to_string(get<1>(bind_expr.args)) + ")"; } }} Cheers, Alfredo