is-prime.clj

(defn prime-candidates
  ([] (lazy-seq (list* 2 3 (prime-candidates 1))))
  ([x] (lazy-seq (list* (dec (* x 6)) (inc (* x 6)) (prime-candidates (inc x))))))

(defn prime-candidates-less-than
  [threshold]
  (take-while (partial >= threshold) (prime-candidates)))

(defn is-prime?
  [x]
  (let [threshold (closest-integer-sqrt x)
        x-divisible-by? (partial can-divide? x)]
    (none x-divisible-by? (prime-candidates-less-than threshold))))