Approximating pi with the Leibniz formula

leibniz.clj

(use 'clojure.contrib.math)

(defn leibniz-pi [accuracy]
  (let [target-accuracy (expt 10 accuracy)]
    (loop [acc 1.0 sub 3 sign -1]
      (let [attempt (* 4.0 acc)]
        (if (> (abs (- attempt Math/PI)) target-accuracy)
          (recur (+ acc (* sign (/ 1.0 sub))) (+ 2 sub) (- sign))
          attempt)))))

; test
(println (leibniz-pi 0))
(println (leibniz-pi -1))
(println (leibniz-pi -4))