[Tainnor]

> except throwing OOP into something that doesn't need it

I feel like you're jumping to wild conclusions. All I'm suggesting is to change your code to

  private object EUR : Currency
  private object USD : Currency

  data class Money<C : Currency>(...) { ... }

That's just an empty tag interface and that's not particularly hardcore OOP, you can do the same thing with Haskell typeclasses.