Accounts concurrency example in Clojure

accounts.clj

;; An example of the "accounts" program for Venkat Subramaniam's
;; Programming Concurrency Workshop, part 1
;; 
;; Original Java code by Venkat Subramaniam (@venkat_s)
;; available at http://www.agiledeveloper.com/downloads.html
;; under "Workshop: Programming Concurrency"
;;
;; This code example by Stuart Sierra (@stuartsierra)
;; 
;; Überconf 2011, Denver, Colorado
;; July 13, 2011
;;
;; To run this code:
;; Download Clojure from http://clojure.org/
;; Run `java -jar clojure.jar accounts.clj`


(defn account [balance]
  (ref {:balance balance}))

(defn deposit [account amount]
  {:pre [(pos? amount)]}
  (alter account update-in [:balance] + amount))

(defn withdraw [account amount]
  {:pre [(pos? amount)]}
  (alter account update-in [:balance]
         (fn [bal]
           (if (> amount bal)
             (throw (Exception. "Insufficient funds"))
             (- bal amount)))))

(defn balance [account]
  (:balance @account))

(defn transfer [from to amount]
  (dosync
   (withdraw from amount)
   (deposit to amount)))

(def transfer-service
  (agent nil :error-handler (fn [agnt err] (println (.getMessage err)))))


;;; Main script begins here

(def account1 (account 1000))
(def account2 (account 1000))

(println "Account1: " (balance account1))
(println "Account2: " (balance account2))

(send transfer-service
      (fn [_] (transfer account1 account2 100)))

(send transfer-service
      (fn [_] (transfer account2 account1 200)))

(await transfer-service)

(println "Account1: " (balance account1))
(println "Account2: " (balance account2))

(send transfer-service
      (fn [_] (transfer account1 account2 2000)))

(await transfer-service)

(println "Account1: " (balance account1))
(println "Account2: " (balance account2))

(shutdown-agents)