ref

(ref v) / (ref v :validator validate-fn)

A ref is the same tagged cell as an atom, read with deref/@; only the transaction verbs (alter/commute/ref-set) may write it, and only inside dosync. With a :validator, every write runs it first and a failed one signals (Invalid reference state), leaving the old value behind -- validators run, retries do not (there is a single thread, so nothing ever conflicts). :meta is dropped, like every other metadata; any other option is refused by name.