Hi fellow beginners (and everyone else), As an exercise, I'm implementing a simple, untyped lambda calculus: -- a term is a variable, an application, or abstraction (lambda) data T = V String | A (T) (T) | L String (T) deriving (Eq) So I'm writing a function that returns a list of all the free variables in a term and descendants. I can only get it to compile with type: freev :: T -> [T] It'd be nice for the type of that function to be restricted to just variables like: freev :: T -> [V String] -- compile error: "Not in scope: type constructor or class `V'" Is there some way to express that? The error seems to suggest maybe haskell could do it if I'd just say it correctly. I mean, isn't "V String" a type constructor? Thanks, kov