Interesting definition of immutable values, and a valid one. My perspective is different. An immutable value has a value add more than that of avoiding defensiv

Interesting definition of immutable values, and a valid one. My perspective is different. An immutable value has a value add more than that of avoiding defensive copying, such as structural sharing. When you add a key to an immutable map, the new map can point to the old. It has become more and more common to model systems with append-only as the only form of write (Event Store, Datomic), so readwrite can probably be avoided in some cases. And things like IP packets certainly makes sense to represent immutably - anything else is a bit of a lie :) But there is certainly a risk that the trafeoff of immutabulity becomes too costly for an OS.. Also, the system language will not be pure, like Haskell. Just immutable values, like Clojure :)