clojure.core/swap!

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Type: function Added: Clojure 1.0 Examples: 6 Runnable: 6

([atom f] [atom f x] [atom f x y] [atom f x y & args])

Atomically swaps the value of atom to be: (apply f current-value-of-atom args). Note that f may be called multiple times, and thus should be free of side effects. Returns the value that was swapped in.

Examples

by Kototama on 2010-10-07 ▶

;; make an atomic list (def players (atom ())) ;; #'user/players ;; conjoin a keyword into that list (swap! players conj :player1) ;;=> (:player1) ;; conjoin a second keyword into the list (swap! players conj :player2) ;;=> (:player2 :player1) ;; take a look at what is in the list (deref players) ;;=> (:player2 :player1)

by Kototama on 2010-10-07 ▶

;; how to: atomic counter (def counter (atom 0)) ;; #'user/counter (swap! counter inc) ;;=> 1 (swap! counter inc) ;;=> 2

by esumitra on 2014-08-20 ▶

;; swap map values (def m1 (atom {:a "A" :b "B"})) ;; atom ;; dereference the atom @m1 ;;=> {:a "A", :b "B"} ;; notice that the value swapped in, is part of the returned value (swap! m1 assoc :a "Aaay") ;;=> {:a "Aaay", :b "B"}

by ghiden on 2015-11-17 ▶

;; increment a map value (def m1 (atom {:a "A" :b "B" :n 0})) ;; atom ;; dereference the atom @m1 ;;=> {:a "A", :b "B", :n 0} ;; notice that the value swapped in, is part of the returned value (swap! m1 update-in [:n] inc) ;;=> {:a "A", :b "B", :n 1}

by ibercode on 2018-02-13 ▶

(def car (atom {:make "Audi" :model "Q3"})) @car ;;{:make "Audi", :model "Q3"} (swap! car assoc :model "Q5") ;;{:make "Audi", :model "Q5"} (swap! car update-in [:model] str " Sport") ;;{:make "Audi", :model "Q5 Sport"}

by Kaspazza on 2022-04-02 ▶

;; Update vector inside map (def data (atom {:name "hello" :nums []})) @data ;; {:name "hello" :nums []} (swap! data update :nums conj {:first 1 :second 2}) ;; {:name "hello" :nums [{:first 1 :second 2}]}

Note by amirrajan

If you want to swap the entire atom, user reset!.

Note by KyleErhabor

When using swap!, it's important you're only taking into consideration the value of the atom inside f. See https://ericnormand.me/mini-guide/atom-code-explanation for more information.

See also

• clojure.core/atom (2)
• clojure.core/reset!
• clojure.core/compare-and-set! (1)
• clojure.core/swap-vals!

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