Returns a lazy sequence of the elements of coll, removing any elements that return duplicate values when passed to a function f

clojure distinct by

(defn distinct-by
  "Returns a lazy sequence of the elements of coll, removing any elements that
  return duplicate values when passed to a function f."
  [f coll]
  (let [step (fn step [xs seen]
               (lazy-seq
                 ((fn [[x :as xs] seen]
                    (when-let [s (seq xs)]
                      (let [fx (f x)]
                        (if (contains? seen fx)
                          (recur (rest s) seen)
                          (cons x (step (rest s) (conj seen fx)))))))
                   xs seen)))]
    (step coll #{})))