group_by_with_transducer.clj

(defn group-by-with-transducer [key-f xf data]
  (let [ff (memoize (fn [k] (xf conj)))
        r  (reduce (fn [m d]
                     (let [k (key-f d)
                           v (get m k [])]
                       (if (reduced? v)
                         m
                         (assoc m k ((ff k) v d)))))
                   {}
                   data)]
    (into {}
          (map (fn [[k v]] [k ((ff k) (unreduced v))]))
          r)))