group-by as a transducer

group_by.clj

(defn group-by
  ([f]
    (fn [rf]
      (let [grouped-value (volatile! (transient {}))]
        (fn
          ([] (rf))
          ([result]
           (rf result (persistent! @grouped-value)))
          ([result input]
           (let [key (f input)]
             (vswap! grouped-value assoc! key (conj (get @grouped-value key []) input))
             result))))))
  ([f coll]
   (into {} (group-by f) coll)))