distinct-by.cljc

(defn distinct-by
  ([f]
   (fn [rf]
     (let [seen (volatile! #{})]
       (fn
         ([] (rf))
         ([result] (rf result))
         ([result x]
          (let [v (f x)]
            (if (contains? @seen v)
             result
             (do (vswap! seen conj v)
                 (rf result x)))))))))
  ([f coll]
   (let [step (fn step [xs seen]
                (lazy-seq
                  ((fn [[x :as xs] seen]
                     (when-let [s (seq xs)]
                       (let [v (f x)]
                         (if (contains? seen v)
                           (recur (rest s) seen)
                           (cons x (step (rest s) (conj seen v)))))))
                   xs seen)))]
     (step coll #{}))))


(->> [[1 1] [2 1] [1 2] [2 2] [1 3] [2 3]]
  (distinct-by first))

;=> ([1 1] [2 1])


(->> [[1 1] [2 1] [1 2] [2 2] [1 3] [2 3]]
  (into []
    (comp
      (filter #(-> % second odd?))
      (distinct-by first))))

;=> [[1 1] [2 1]]