I've the following template function: template<typename T> T exp2(const T T0_d) { return exp(0.5*T0_d) + 2*exp(T0_d); } I want to define this as a lambda function in place (but don't know how). It will be used with T=double and T=MyAutomaticDerivativeClass. I've already defined the return type of arithmetic operators applied to MyAutomaticDerivativeClass: namespace boost { namespace lambda { template<class Act, typename X, typename Y> struct plain_return_type_2<arithmetic_action<Act>, X, Y> { typedef ... type; }; } } What else is missing? Thanks Peter