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 m determines the type w. (This may be a better link.)