At the process level there are already plenty of programs that work this way and it's exactly the kind of thing the Clojure ecosystem is geared towards.
That's why the Purely Functional OS is essentially a thought experiment in what it would take to extend this to the entire operating system.
When I said "impossible given the current state of technology" I meant exactly what you said in your last paragraph though: impossible without designing an OS that isolates derived data.
I believe such a system will come in the form of persistent data structures coupled with reactive programming but, while I believe it's inevitable in the long run, extending that concept all the way to the OS will be quite the challenge.
That's why the Purely Functional OS is essentially a thought experiment in what it would take to extend this to the entire operating system.
When I said "impossible given the current state of technology" I meant exactly what you said in your last paragraph though: impossible without designing an OS that isolates derived data.
I believe such a system will come in the form of persistent data structures coupled with reactive programming but, while I believe it's inevitable in the long run, extending that concept all the way to the OS will be quite the challenge.