The "categorical" definition of monads uses return and, instead of bind, join :: m(m a) -> m a You don't need to specify it if you already have bind/>>= and

The "categorical" definition of monads uses return and, instead of bind, join :: m(m a) -> m a You don't need to specify it if you already have bind/>>= and return/pure, since they can be derived from each other. For instance, purely by "following the types", f :: a -> m b fmap f :: m a -> m (m b) join (fmap f) :: m a -> m b In other words, (>>=) :: m a -> (a -> m b) -> m b ma >>= f = join (fmap f ma) is a perfectly good definition of bind I you have join.