As a way to make sure that a ref you don’t replace is however unchanged after a transaction for coordination purposes, use determine on it in the transaction. Examples directory 6-1 illustrates the vintage instance of transactional habit formerly pointed out, moving funds from one checking account to a different. this can be a situation within which coordination among the 2 items of state—the accounts—is extremely important. If the values weren't coordinated, it'd be attainable, in spite of the fact that in brief, to be in a nation within which the cash was once further to at least one account yet now not but subtracted from the opposite (or vice versa). utilizing refs and transactions guarantees that the account addition and subtraction take place atomically. directory 6-1. financial institution bills in STM (def account1 (ref 1000)) (def account2 (ref 1500)) (defn move "transfers sum of money from a to b" [a b quantity] (dosync (alter a - quantity) (alter b + amount))) (transfer account1 account2 three hundred) (transfer account2 account1 50) one hundred and one CHAPTER 6 „ nation administration (println “Account #1:” @account1) (println “Account #2:” @account2) operating this code yields the predicted output after the 2 transactions. as the transaction is assured via Clojure’s STM, the implications often is the constant irrespective of what percentage threads have been simultaneously updating the bills. as a result, the output is: Account number one: 750 Account #2: 1750 the next instance is far extra complicated, and demonstrates how refs might be saved in any facts constitution (not simply def’d on the best level), how they could have any information constitution as their worth, not only integers, and the way even refs may be a part of the worth of one other ref. it's only a simple instance of utilizing refs: you'll likely are looking to strategy the ref constitution in a precise application with greatly extra inspiration. generally, it’s higher to be sensible and use as few refs as will meet your wishes. this system represents a rudimentary deal with publication. the most facts constitution is a vector of contacts. it's contained in a ref, because you have to be capable of replace it and it starts off out empty. every one touch is a map containing first identify and final identify. instead of storing the entries without delay, although, they're each one saved as a ref themselves, given that every one is an separately updateable piece of country (see directory 6-2). Lilsting 6-2. An tackle ebook in STM (def my-contacts (ref [])) (defn add-contact "adds a touch to the supplied touch checklist" [contacts touch] (dosync (alter contacts conj (ref contact)))) (defn print-contacts "prints an inventory of contacts" [contacts] (doseq [c @contacts] (println (str "Name: " (@c :lname) ", " (@c :fname))) )) (add-contact my-contacts {:fname "Luke" :lname "VanderHart"}) (add-contact my-contacts {:fname "Stuart" :lname "Sierra"}) (add-contact my-contacts {:fname "John" :lname "Doe"}) (print-contacts my-contacts) working the scripts creates a listing of contacts, provides a number of contacts to it (as refs), after which prints the record, yielding: identify: VanderHart , Luke identify: Sierra, Stuart identify: Doe, John 102 CHAPTER 6 „ country administration word how the print-contacts functionality must dereference the contacts record and in addition every one touch earlier than it could actually use it, on the grounds that either are references.