hoeck

(defn map-map [from-map func] (reduce merge (for [[key value] from-map] (hash-map key (func value)))))
(map-map {:a 2 :b 4} #(* 3 %))

;; other possible solutions:
(defn map-map [f m]
  (into (empty m) (map (fn [[k v]] [k (f v)]) m)))

(let [m {:a 2 :b 4}]
  (merge-with * m (zipmap (keys m) (repeat 3))))