gistfile1.sml

(* Iterative approach; O(1) memory *)
fun factorial n =
  let
    val tally = ref 1
    val i = ref 1
  in
    while !i < n do
      tally := !tally * i
      i := !i + 1;
  end;

(* Naive recursion; O(n) memory *)
fun factorial 0 = 1
  | factorial n = n * factorial(n - 1)
  
(* Tail recursion; O(1) memory *)
fun factorial n =
  let
    fun factorial_iter i tally =
      if i > n then
        tally
      else
        factorial_iter(i + 1)(tally * i)
  in
    factorial_iter 1 1
  end;