Yes true. You also want the data which is passed between the different layers to be as simple as possible to avoid leakiness. I try to avoid accepting complex i

Yes true. You also want the data which is passed between the different layers to be as simple as possible to avoid leakiness. I try to avoid accepting complex instances as function arguments and try to avoid returning complex instances that will allow the user to manipulate shared state. It takes a lot of effort and thinking to figure out an abstraction which fulfils all of its requirements while at the same time exposes a simple communication interface.