Hello , The following code compiles with GHC but not with Hugs. moreover, if you replace STURef with STRef in instance definition, all will be fine. bug? {-# OPTIONS_GHC -cpp -fglasgow-exts #-} import Control.Monad.ST import Data.STRef -- ----------------------------------------------------------------------------- -- Unboxed references in ST monad type STURef = STRef -- | Create new unboxed reference in ST monad newSTURef :: e -> ST s (STURef s e) newSTURef = newSTRef -- ----------------------------------------------------------------------------- -- Monad-neutral interface for fast unboxed references class (Monad m) => URef m r | m->r, r->m where newURef :: a -> m (r a) instance URef (ST s) (STURef s) where newURef = newSTURef -- ----------------------------------------------------------------------------- -- Main test = runST ( do newURef (0::Int) return '1' ) -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com