Lazily distinct list

gistfile1.clj

(defn lazily-distinct [coll]
  "Returns a lazy distinct list of possibly infinite collection.
   Does not realize the whole collection."
  (letfn [(lazystep [seen l]
            (lazy-seq 
              (when (seq l)
                (loop [next-item (first l) lazy-list (rest l)]
                  (if (get seen next-item)
                    (recur (first lazy-list) (rest lazy-list))
                    (cons next-item (lazystep (conj seen next-item)
                                              lazy-list)))))))]
    (lazystep #{} coll)))

;; (count (distinct (take 1000 (repeatedly #(rand-int 5000)))))  ->  probably less than 1000
;; (count (distinct (take 1000 (lazily-distinct (repeatedly #(rand-int 5000)))))) -> exactly 1000