I am a haskell-beginner and I wish to write a Forth-like interpreter. (Only for practice, no usefulness.) I would like use a list (as stack) that can contain several kinds of values. data Element = Int Int | Float Float | Func : Machine -> Machine | ... Now I would like to have this type be an instance of the class Show, so that I can see what the stack contains in ghci. "deriving Show" is impossible as Func is not instance of Show. Can I make it instance of Show? I just want to define something like show (Func _) = "Function, cannot show" and I am not interested in actually displaying any information about the function, but I am interested in getting information about elements of other kinds. So far I have just guessed, but failed to produce anything that does not give an error stating that my function is not of the form (T a b c) where T is not an alias and a b c are simple type variables (or so). Any help appreciated! Johan