Function to detect isograms.

isogram.clj

(defn isogram?
  "An isogram is a word in which no letters occur more than once.
   Empty string is considered an isogram in this implementation."
  [^String s]
  (boolean
   (when (string? s)
     (loop [letters (sort (seq s))]
       (let [[current next & _] letters]
         (cond
           (nil? next) true
           (= current next) false
           :else (recur (rest letters))))))))

(assert
 (every? isogram? [""
                   " "
                   "c"
                   "ca"
                   "cat"
                   "cats"]))

(assert
 (not-any? isogram? [nil
                     "cc"
                     "cct"
                     "caa"
                     "catt"]))

;; TODO solve in another way or two
;; Can isograms have only 1, 2 or more?

```clojure
#(= (count (set (vals (frequencies %)))) 1 )
#(= (count %) (count (into #{} %))
```