deep_merge_with_key.clj

(defn merge-with-key
  [f & maps]
  (when (some identity maps)
    (reduce (fn [acc x]
              (reduce (fn [m [k v]]
                        (if (contains? m k)
                          (assoc m k (f k (get m k) v))
                          (assoc m k v)))
                      acc
                      x))
            maps)))

(defn deep-merge-with-key
  [f & maps]
  (apply
    (fn m [& maps]
      (let [[k & args] maps]
        (cond (every? map? maps) (apply merge-with-key m maps)
              (every? map? args) (apply merge-with-key m args)
              :else (apply f k args))))
    maps))