Gisted version of jsa's comment on What is Clojure

largest-size.clj

(defn largest-size [kfn l item]
  (if (empty? l)
    item
    (if (nil? item)
      (recur kfn (next l) (first l))
      (let [f (first l)]
        (recur kfn
               (next l)
               (if (> (kfn f) (kfn item)) f item))))))

(largest-size identity '(1 3 8 5 77 33 3 99 22) nil)
=> 99
(largest-size identity [1 3 8 5 77 33 3 99 22] nil)
=> 99
(largest-size val {:one 1 :two 3 :three 8 :four 5 :five 77 :six 33 :seven 3 :eight 99 :nine 22} nil)
=> [:eight 99]