Comment by kilgnad

To see immutability on huge state in action see git.

That is essentially what must be going on under the hood to model mutable state. Every mutation made to a variable is a commit. The only commits that are saved are the ones that have existing references.

Now you're thinking it could be a big memory leak. But you pair this with reference counting you can eliminate the leak.

Essentially when the reference of the earliest commit goes out of scope, the next earliest commit is checked out as the initial root commit and the current commit is freed.

The memory model is then like a moving git window/linked list. Mutations are committed at the tail end while memory is freed at the head.