Clojure implementation of Project Euler problem 12

gistfile1.clj

;; Utility functions that will be used later

(def naturals (iterate inc 1))

(defn all-factors
  [n]
  (filter #(= (mod n %) 0)
          (take-while #(<= % (sqrt n))
                      (rest naturals))))

(defn first-matching
  "Given a function and a sequence, return the first element of the sequence for
  which the function returns true."
  [f s]
  (if (f (first s))
    (first s)
    (if (seq (rest s))
      (recur f (rest s))
      nil)))

;; Project Euler problem 12

(def triangle-numbers (map #(reduce + (take % naturals)) naturals))

(def problem12
  (delay (first-matching #(> (count (all-factors #)) 500) triangle-numbers)))