(def throws 10)

(defn r-squared [x y]
 (+ (* (- 0.5 x) (- 0.5 x))
    (* (- 0.5 y) (- 0.5 y))))

(loop [hits 0]
  (let [x (rand)
        y (rand)]
    ; still inside the let
    (if (< (r-squared x y) 0.25)
        (recur (inc hits)) ; then expression, restarts the loop
        (* 4 (/ hits throws))))) ; else-expression, the return value of the loop

; compare
(defn calculate-pi [hits]
  (let [x (rand)
        y (rand)]
    ; still inside the let
    (if (< (r-squared x y) 0.25)
        (recur (inc hits)) ; then expression, restarts the function
        (* 4 (/ hits throws))))) ; else-expression, the return value of the function

(println (calculate-pi 0)) ; probably not working :P