It's only a test case. The real thing is for a game and will be something like:
class EntityT e where update :: e -> e render :: e -> IO () handleEvent :: e -> Event -> e getBound :: e -> Maybe Bound
data Entity = forall e. (EntityT e) => Entity e
data Level = Level { entities = [Entity], ... }
I suspected the real case would look like that. It is also covered on the same web page on Eliminating existentials. Here is your example without existentials, in pure Haskell98 (I took a liberty to fill in missing parts to make the example running) data Event = Event Int -- Stubs type Bound = Pos type Pos = (Float,Float) data EntityO = EntityO{ update :: EntityO, render :: IO (), handleEvent :: Event -> EntityO, getBound :: Maybe Bound } type Name = String new_entity :: Name -> Pos -> EntityO new_entity name pos@(posx,posy) = EntityO{update = update, render = render, handleEvent = handleEvent, getBound = getBound} where update = new_entity name (posx+1,posy+1) render = putStrLn $ name ++ " at " ++ show pos handleEvent (Event x) = new_entity name (posx + fromIntegral x,posy) getBound = if abs posx + abs posy < 1.0 then Just pos else Nothing data Level = Level { entities :: [EntityO] } levels = Level { entities = [new_entity "e1" (0,0), new_entity "e2" (1,1)] } evolve :: Level -> Level evolve l = l{entities = map update (entities l)} renderl :: Level -> IO () renderl l = mapM_ render (entities l) main = renderl . evolve $ levels I think the overall pattern should look quite familiar. The code shows off the encoding of objects as records of closures. See http://okmij.org/ftp/Scheme/oop-in-fp.txt for the references and modern reconstruction of Ken Dickey's ``Scheming with Objects''. It is this encoding that gave birth to Scheme -- after Steele and Sussman realized that closures emulate actors (reactive objects). Incidentally, existentials do enter the picture: the emerge upon closure conversion: Yasuhiko Minamide, J. Gregory Morrisett and Robert Harper Typed Closure Conversion. POPL1996 http://www.cs.cmu.edu/~rwh/papers/closures/popl96.ps This message demonstrates the inverse of the closure conversion, eliminating existentials and introducing closures.