Atoms and swap! in Clojure

Tracks a visitor count and a front desk register in atoms, updating both through plain functions.

;; An atom holds one value that is allowed to change over time.
;; swap! applies a function to the current value and stores the
;; result, so the update stays a plain function call.
(def visitors (atom 0))

(swap! visitors inc)
(swap! visitors + 10)
(println (format "visitors %d" @visitors))

;; @ is deref: it reads the value in place without changing it.
(def register (atom {:desk "Ida" :guests []}))
(swap! register update :guests conj "Perez")
(swap! register update :guests conj "Okoye")
(println (format "%s logged %d" (:desk @register)
                 (count (:guests @register))))

;; reset! replaces the value outright instead of deriving a new one.
(reset! visitors 0)
(println (format "visitors %d" @visitors))

How it works

1. (swap! visitors inc) and (swap! visitors + 10) show that swap! hands the current value to a function with any extra arguments after it.
2. (swap! register update :guests conj "Perez") rebuilds a nested map, since the update function may return any value.
3. @visitors derefs the atom to read it, while (reset! visitors 0) replaces the value instead of deriving one.

Keywords and builtins used here

• atom
• conj
• count
• def
• format
• println

The run, in numbers

Lines

19

Characters to type

707

Tokens

97

Three-star pace

70 tpm

At the three-star pace of 70 tokens a minute, this run takes about 83 seconds.

Type this snippet

Step 1 of 3 in State & records, step 22 of 27 in Language basics.

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