Simple example to illustrate the use of refs in Clojure

refex.clj

;;
;; A simple banking system as an example to illustrate refs in Clojure
;;

(def account-a (ref 100))
(def account-b (ref 100))

(defn transfer! [amount from to]
  (dosync
   (if (>= (- @from amount) 0)
     (do
       (alter from - amount)
       (alter to + amount)))))

(defn deposit! [amount account]
  (dosync
   (alter account + amount)))

(defn withdraw! [amount account]
  (dosync
   (if (>= (- @account amount) 0)
     (alter account - amount))))

(defn show-balance [account]
  (str "Balance: " @account))