(* iterative (tail-recursive) version of factorial *)

let fact n =
      let (* if n >= 0, then fct(n, m) returns n factorial * m *)

          rec fct(n, m) =
            if n = 0
            then m
            else fct(n - 1, n * m)
      in if n < 0
         then -1
         else fct(n, 1)

(* try: fact 6;; *)