I was playing with the idea of having the user declare functions (polymorphic, possibly recursive) in a lambda like language, and the capability to later call that function with different types of arguments. Here is an example of what I am thinking about. function fib = if_(_1<2) [ _1 ].else_[fib(_1-1) + (_1-2)]; int x = fib.eval(10); How should I define the "function" class? My first attempt at a solution was to use "boost::any" like class for function, I overloaded operator() in that class so that it returns a proto expression. I am attaching some source code that has a function class and a phoenix like language. Now, I am not able to come up with a solution for defining a polymorphic eval() for the function class. Since "function" has to be able to store any type inside it, it has to do type erasure. All methods in "function" has to be delegated to value_base class but that cannot delegate a polymorphic eval() function down to value_type. I can templatize "function" class and get monomorphic functions defined in this language. But that is less interesting. I would greatly appreciate any advice and suggestions on other solutions to this problem. I realize that this is not entirely proto specific and maybe it is a general C++ language question, but I am hoping proto users might have faced similar problems and provide guidance. Thanks, Manjunath