(defn fact [x]
  (loop [n x prod 1] ;; this works just like a 'let' binding.
    (if (= 1 n)  ;; this is the base case.
      prod
      (recur (dec n) (* prod n)))))
 
(println (fact 5)) ;; change 5 to whatever number you like!
 
;; ...but not more than 20, or you'll get an integer overflow error.
;; to fix this, you can add 'N' to the end of any number to make it a bigint.
;; 21 -> 21N
 
(defn fact-no-loop [n]
  (if (= 1 n)
    1
    (* n (fact-no-loop (dec n)))))
 
(println (fact-no-loop 5)) ;; still works!
 
(time (fact 20))
 
(time (fact-no-loop 20))
 

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