raindrops.clj

(s/def ::phrases (s/map-of number? string?))
(def ^:private phrases {3 "Pling" 5 "Plang" 7 "Plong"})

(s/fdef raindrops
  :args (s/cat :n number?)
  :ret string?)
(defn raindrops
  "Returns number as string unless it is divisible by 3, 5 or 7, in which case it returns
  the concatenation of the words Pling (3), Plang (5) and Plong (7) for the divisible cases."
  [n]
  (as-> phrases $
    (for [[d s] $ :when (= 0 (mod n d))] s)
    (if (empty? $) (str n) (apply str $))))