distinct-by.clj

(defn distinct-by
  "Like distinct, but uses f to consider what duplicate values are."
  ([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 coll]
   (let [step (fn step [xs seen]
                (lazy-seq
                  ((fn [[item :as xs] seen]
                     (when-let [s (seq xs)]
                       (let [v (f item)]
                         (if (contains? seen v)
                           (recur (rest s) seen)
                           (cons item (step (rest s) (conj seen v)))))))
                   xs seen)))]
     (step coll #{}))))