merge-with-mapper.clj

(defn- merge-entry [mapper m e]
  (let [k (key e) v (val e)]
    (if (contains? m k)
      (assoc m k ((safe-get mapper k) (get m k) v))
      (assoc m k v))))

(defn merge-with-mapper
  "Returns a map that consists of the rest of the maps conj-ed onto
   the first. If a key occurs in more that one map, the mapping(s)
   from the latter (left-to-right) will be combined with the mapping in
   the result by calling ((mapper key) val-in-result val-in-latter)."
  [mapper & maps]
  (when (some identity maps)
    (let [merge2 (fn [m1 m2]
                   (reduce (partial merge-entry mapper) (or m1 {}) (seq m2)))]
    (reduce merge2 (map #(select-keys % (keys mapper)) maps)))))