[Haskell-cafe] Usage of | in classes

[Frank-Andre Riess]

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