Ok, so I'm inching closer to understanding monads, and this question popped up today. Consider the following 2 declarations: data Foo a = Bar a data (Ord a) => Baz a = Bah a Note that both of these have kind * -> *. However, Baz could never be an instance of monad, because there is a restriction on the types it can operate on. Foo, however, is completely polymorphic, without limitation. It seems to me that there ought to be a way to express the difference between the two in the type/kind system. For example, you can almost, but not quite, say that in the declaration "class Monad m where..", m must be of kind *->*, but that's not quite enough to say, because of this example. Am I just missing something, or is there a reason the kind of Baz shouldn't be something other than *->*?