stdin/stdout/stderr may be redirected during the lifetime of a program. The OS manages them and they are "reference pointers" (file handles) for multiple reasons.
Also, "state" does not necessarily imply "mutable". (Redux encourages an immutable state approach where good reducers don't mutate state in place, but instead build new state from old, though Redux is not strict in enforcing this best practice. For Redux the "current state" is "file handle" that simply updates from one object to the next, not all that different from stdin/stdout/stderr or even somewhat to static configuration that is somewhat updateable without restarting the application.)
