What does a pipe in a class definition mean?

Question

class (Monoid w, Monad m) => MonadWriter w m | m -> w where 
   pass   :: m (a,w -> w) -> m a 
   listen :: m a -> m (a,w) 
   tell   :: w -> m ()

What is the meaning of the pipe above? The snippet comes from here.

Answer 1

Actually, it's a "functional dependency". In this case that means that m uniquely identifies w -- the type of m determines the type of w. (This may be a better link.)