[Haskell-cafe] Usage of | in classes

Hi, Gracjan's question led me to another question:

class Monad m => Ref m r | m -> r where newRef :: a -> m (r a) readRef :: r a -> m a writeRef :: r a -> a -> m ()

What's the meaning of the bar and the function type in this declaration. I've seen something like that before (with state monads, I think), but couldn't find an explanation in the Haskell grammar (or anywhere else on haskell.org). Thanks in advance, Frank-Andre