montyhall.clj

(defn gen-doors []
  (update (vec (repeat 3 0)) (rand-int 3) inc))

(defn stay [doors chosen]
  (doors chosen))

(defn switch [doors chosen]
  (let [reveal (rand-nth 
                 (keep 
                   (fn [[idx v]]
                     (when (and (not= idx chosen) (= 0 v))
                       idx))
                   (map-indexed vector doors)))]
    (doors (some 
            #(when ((complement #{chosen reveal}) %)
               %)
            (range 3)))))

(defn simulate [trials]
  (reduce
    (fn [acc [doors chosen]]
      (-> acc
          (update :stay + (stay doors chosen))
          (update :switch + (switch doors chosen))))
    {:stay 0
     :switch 0}
    (take trials (repeatedly (fn [] [(gen-doors) (rand-int 3)])))))

(defn score [trials]
  (let [{:keys [stay switch]} (simulate trials)]
    {:stay (float (/ stay trials))
     :switch (float (/ switch trials))}))