| And to | be honest, I'm not sure we need arbitrary data types in Unlifted; | Force (which would be primitive) might be enough. That's an interesting thought. But presumably you'd have to use 'suspend' (a terrible name) a lot: type StrictList a = Force (StrictList' a) data StrictList' a = Nil | Cons !a (StrictList a) mapStrict :: (a -> b) -> StrictList a -> StrictList b mapStrict f xs = mapStrict' f (suspend xs) mapStrict' :: (a -> b) -> StrictList' a -> StrictList' b mapStrict' f Nil = Nil mapStrict' f (Cons x xs) = Cons (f x) (mapStrict f xs) That doesn't look terribly convenient. | ensure that threads don't simply | pass thunks between each other. But, if you have unlifted types, then | you can have: | | data UMVar (a :: Unlifted) | | and then the type rules out the possibility of passing thunks through | a reference (at least at the top level). Really? Presumably UMVar is a new primitive? With a family of operations like MVar? If so can't we just define newtype UMVar a = UMV (MVar a) putUMVar :: UMVar a -> a -> IO () putUMVar (UMVar v) x = x `seq` putMVar v x I don't see Force helping here. Simon