https://en.m.wikibooks.org/wiki/Haskell/Category_theory says

join :: Monad m => m (m a) -> m a
join x = x >>= id

Is id the identity functor on category Hask and does id have type a-> a?

>>= requires its second argument to have type a->ma, and if id has type a-> a, how can id be used as the second argument of >>=?

return has type a -> ma. Are join and return the inverse of each other?