Vars (both static and dynamic), atoms, refs, and agents provide a way to validate the value being set as state—a validator function that accepts new value as argument, and returns the logical as true if it succeeds, or throws exception/returns logical as false (the false and nil values) if there's an error. They all honor what the validator function returns. If it is a success, the update goes through, and if an error, an exception is thrown instead. Here is the syntax on how the validators can be declared and associated with the reference types:

(def t (atom 1 :validator pos?))
(def g (agent 1 :validator pos?))
(def r (ref 1 :validator pos?))
(swap! t inc)  ; goes through, because value after increment (2) is positive
(swap! t (constantly -3))  ; throws exception
(def v 10)
(set-validator! (var v) pos?)
(set-validator! t (partial < 10)) ; throws exception
(set-validator! g (partial < 10)) ; throws exception
(set-validator! r #(< % 10)...