Exchange atom value in Clojure. This function seems like a common enough pattern in Clojure it should find its place in the standard library.

exchange.clj

(defn exchange!
  "Atomically set the value of `atom` to `(apply f @atom args)`,
  and return the value of `@atom` just before the assignment.
  (See also: `clojure.core/swap!`)"
  [atom f & args]
  {:pre [(instance? clojure.lang.Atom atom)]}
  (loop [oldval @atom]
    (if (compare-and-set! atom oldval (apply f oldval args))
      oldval
      (recur @atom))))

;; Example:

(def tokens (atom #{10 20 30 40 50}))

(defn pop-token!
  "Atomically `disj` an element in the set-valued atom,
  returning the removed element."
  [tokens-atom]
  (first (exchange! tokens-atom
                    #(disj % (first %)))))

(future (pop-token! tokens))
(future (pop-token! tokens))
(future (pop-token! tokens))
(future (pop-token! tokens))
(pop-token! tokens)