Mimic Java's Map::computeIfAbsent in Clojure

compute-if-absent.clj

(def store (atom {}))

(defn nil-protect [f]
  (fn [m k]
    (if-let [v' (f k)]
      (assoc m k v')
      m)))

(defn compute-if-absent [a k f]
  (let [m (swap! a (nil-protect f) k)] ;   in essence: (f m k)
    (get m k)))

(defn do-something [k]
  (str k k))

(compute-if-absent store :asaf do-something)
=> ":asaf:asaf"

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