Getting Clojure

Swapping Maps

Atoms are not limited to storing just numbers: in fact, you can wrap an atom around any Clojure value. Thus a more practical example might look like this:

(ns inventory)

(def by-title (atom {}))

(defn add-book [{title :title :as book}]
  (swap! by-title #(assoc % title book)))

(defn del-book [title]
  (swap! by-title #(dissoc % title )))

(defn find-book [title]
  (get @by-title title))

That gives us a simple but complete stateful inventory manager:

(find-book "Emma")    ; Nope

(add-book {:title "1984", :copies 1948})
(add-book {:title "Emma", :copies 100})
(del-book "1984")

(find-book "Emma")    ; Yup
(find-book "1984")    ; Nope

There really is not much to an atom. An atom is a container ...