Haskell

what is | in this code?

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

It is most likely an OR in one way or another... I'm afraid my grasp of the Haskell syntax is not that great, many things are still out of my reach.

By the way, I've always wondered what the forall thingy does in type signatures!