"Steve Downey" wrote:

I'm not sure, though, what would happen if I tried to add state to the types. With the previous example, using existentials to create a reference type that holds elements of a type class that matches the interface, I think that it would be natural to hold state by having that element stored in a mutable state variable, and replacing the held values.

State is often best avoided, but if it's really what you want to do, you can still capture a mutable variable in the closures captured inside the record of functions. Tim import Data.IORef data Component = Component { draw :: IO (), move :: (Int,Int) -> IO () } newCircle :: IO Component newCircle = do pref <- newIORef (0,0) return Component{draw=draw' pref, move=move' pref} where draw' pref = do p <- readIORef pref putStrLn ("circle at " ++ show p) move' pref p = writeIORef pref p