At a datastructure level, perhaps, although functional datastructures are pretty fringe. (cf. Okasaki) In general, they can still be implemented by traditional

At a datastructure level, perhaps, although functional datastructures are pretty fringe. (cf. Okasaki) In general, they can still be implemented by traditional imperative languages (obviously). At the type/variable level, it's the opposite. We see it with const. Once it's in here, it's in there.