Hello, Perhaps it's just me being inattentive or Boost.Lambda lacks some feature I would consider quite useful. Rummaging through documentation and source code did not turn out to be constructive. Well, the problem is: struct foo { template<typename ExprT> typename ExprT::result_type operator[](ExprT expr) { return expr(/*params here*/); } }; Umm... We can write as follows then: foo bar; bar[ some_stl_conformant_unary_functor() // or binary, or whatever... compiles OK ]; It would be nice to write little lambda expressions too, bar[ std::cout << _1 // Fails miserably ]; for example. The problem is that lambda expressions do not have result_type defined although technically they are functors too (like STL functors). It would be trivial to write little metafunction wrapper though. After all, boost::lambda::is_lambda_functor metafunction exists but I haven't found lambda_result_type traits or something like that. Documentation conveniently skips through that part of implementation and code is a bit obscure at times. Is there a way to implement such a metafunction boost::lambda::functor_result<T>::type that it would return result type of lambda expression if T is lambda expression or T::result_type if T is ordinary STL functor, or just null_type if neither of them? that its implementation would not turn out to be a pain in the... emm... neck? Or perhaps this could be a part of Boost.Lambda library? Regards, Justinas V.D.