Yeah there are no hard and fast rules but a lot of time transformations can be in the object as well. If I need a function to transform Foo to Bar I could just as easily send a toBar() message to an instance of Foo.

I think c# really got the best of both worlds with extensions methods, where you can actually define functions that act on a object but are separated from the actual class definition. I still think that pure functions and especially higher kinded types are better probably, although I have no direct experience with Haskell type classes, scala implicits and ocaml modules..