distinct-by clojure transducer

distinct-by.clj

(defn distinct-by
  "Returns a stateful transducer that removes elements by calling f on each step as a uniqueness key.
   Returns a lazy sequence when provided with a collection."
  ([f]
   (fn [rf]
     (let [seen (volatile! #{})]
       (fn
         ([] (rf))
         ([result] (rf result))
         ([result input]
          (let [v (f input)]
            (if (contains? @seen v)
              result
              (do (vswap! seen conj v)
                  (rf result input)))))))))
  ([f xs]
   (sequence (distinct-by f) xs)))