As a bit of a digression:
The ML languages, as with most things, get this (mostly) right, in that by convention types are encapsulated in modules that know how to operate on them - although I can't help but think there ought to be more than convention enforcing that, at the language level.
There is the problem that it's unclear - if you can Frobnicate a Foo and a Baz together to make a Bar, is that an operation on Foos, on Bazes, or on Bars? Or maybe you want a separate Frobnicator to do it? (Pure) OOP languages force you to make an arbitrary choice, Lisp and co. just kind of shrug, the ML languages let you take your take your pick, for better or worse.