Yes, I think the great lesson we can take from functional programming is not of being functional 100% of the time, but of not having mutable global state. Your inner loops, local variables, etc. can all be mutable and nobody will care, as long as your interface is functional.
Yes and no. It is true that what matters most of the time is a functional interface, not how that is achieved. That being said, it is much harder to successfully implement a functional interface if you are using side-effects willy-nilly inside.
