Supporting code for a talk on STM in Clojure

stm-talk.clj

(ns stm-talk.core)

;;This creates our ledger
(def ledger (ref {}))

(defn journal
  "Apply a debit and credit simultaneously to a given account."
  [debit credit amount]
  (dosync
   (when (nil? (@ledger debit)) (alter ledger assoc debit 0))
   (when (nil? (@ledger credit)) (alter ledger assoc credit 0))
   (alter ledger update-in [debit] + amount)
   (alter ledger update-in [credit] - amount)))

(defn accountant []
  (let [debits [:cash :accts-recv :salesrev :otherassets]
        credits [:accts-payable :loans-payable :otherliabilities :sales]]
    (journal (rand-nth debits) (rand-nth credits) (rand-int 4000))))

(dorun (apply pcalls (repeat 1000000 accountant)))

(defn balance [] (reduce #(+ %1 (second %2)) 0 @ledger))