[Haskell-beginners] class (Monad m) => MonadState s m | m -> s where - What does m | m -> s construct do?

11 Apr 2011

      I can across the following construct in ghc source

class (Monad m) => MonadState s m | m -> s where
    get :: m s
    put :: s -> m ()

I am not aware of the syntactic construct  | m -> s just before the where?
Can someone please explain it?

Thanks & Regards,
Amitava Shee