unit :: x -> M x join :: M (M x) -> M x For example, unit 3 = [3] and join [[1,2], [3]] = [1,2,3] (iii) map f . unit = unit . f (iv) map f . join = join . map (map f) Laws (iii) and (iv) may be derived by systematic transformations of the polymorphic types of unit and join.