Mutable references exposed in `PrimIO`. This includes the ability to mutate via a CAS-loop to avoid locking with a mutex in certain occasions.
import public IO.Async.Looponce : IORef Bool -> IO1 () -> IO1 ()Guaranteed to run the given cleanup function exactly once:
The boolean flag is atomically read and set to false before running the
cleanup hook, and `act` is only run if the flag has been `True`.