Also, I think that class CoArbitrary can help to generate random *functions *(as opposed to values; but tell me if I'm wrong) but I don't see how this could be useful.
How would you test that a function with the type
endo :: Functor f => (a -> a) -> f a -> f a
satisfies some property? You pass in "random" functions which satisfy "some" property, and pass in "random" (f a)'s to try it on.