WTF transients?

gistfile1.clj

;; a big list of urls

(def furls ["support.google.com" "docs.google.com" "wwwapps.ups.com" "www.twitter.com" "careers.stackoverflow.com" "careers.stackoverflow.com" "careers.stackoverflow.com" "support.google.com" "docs.google.com" "support.office.com" "salesforce.com" "tryscratch.com" "gist.github.com" "gist.github.com" "gist.github.com" "www.zensight.co" "gist.github.com" "gist.github.com" "www.linkedin.com" "gist.github.com" "www.amazon.com" "gist.github.com" "gist.github.com" "www.joyofclojure.com" "gist.github.com" "gist.github.com" "www.linkedin.com" "www.joyofclojure.com" "gist.github.com" "gist.github.com" "gist.github.com" "gist.github.com" "www.linkedin.com" "www.linkedin.com" "www.linkedin.com" "gist.github.com" "clojure.org" "clojure.org" "clojure.org" "clojure.org" "gist.github.com" "gist.github.com" "gist.github.com" "www.google.com" "gist.github.com" "www.google.com" "www.google.com" "gist.github.com" "gist.github.com" "www.google.com" "gist.github.com" "careers.stackoverflow.com" "meta.stackexchange.com" "careers.stackoverflow.com" "careers.stackoverflow.com" "gist.github.com" "www.google.com" "www.google.com" "gist.github.com" "www.zensight.co" "isaaccambron.com" "www.google.com" "www.google.com" "www.google.com" "www.linkedin.com" "www.linkedin.com" "www.linkedin.com" "www.linkedin.com" "www.linkedin.com" "docs.google.com" "support.google.com" "docs.google.com" "support.google.com" "support.office.com" "salesforce.com" "tryscratch.com" "wwwapps.ups.com" "www.twitter.com" "gist.github.com" "gist.github.com" "gist.github.com" "www.linkedin.com" "gist.github.com" "gist.github.com" "gist.github.com" "gist.github.com" "gist.github.com" "gist.github.com" "gist.github.com" "gist.github.com" "gist.github.com" "gist.github.com" "www.google.com" "gist.github.com" "www.google.com" "www.google.com" "www.google.com" "www.google.com" "gist.github.com" "gist.github.com" "www.zensight.co" "gist.github.com" "gist.github.com" "gist.github.com" "www.linkedin.com" "www.amazon.com" "www.joyofclojure.com" "www.linkedin.com" "www.linkedin.com" "www.linkedin.com" "gist.github.com" "clojure.org" "clojure.org" "www.google.com" "www.google.com" "gist.github.com" "gist.github.com" "www.google.com" "gist.github.com" "careers.stackoverflow.com" "meta.stackexchange.com" "careers.stackoverflow.com" "careers.stackoverflow.com" "www.google.com" "gist.github.com" "www.zensight.co" "isaaccambron.com" "www.linkedin.com" "www.linkedin.com" "www.linkedin.com" "www.linkedin.com" "www.linkedin.com"])

;;a stupid function for uniq'ing them (ignore that this is not the best way to do this

(defn found [strs]
  (let [c (transient {})]
    (doseq [s strs] (assoc! c s true))
    (persistent! c)))

;;calling
(keys (found furls))

;;in my repl, i get: ("support.google.com" "docs.google.com" "wwwapps.ups.com" "www.twitter.com" "careers.stackoverflow.com" "support.office.com" "salesforce.com" "tryscratch.com")
;;which is missing, say, linkedin