fun fact n =
  if n = 0 then 1
  else n*fact(n-1);

fun fact' 0 = 1              (* pattern matching, "by cases" *)
  | fact' n = n*fact'(n-1);

val rec fact'' =             (* as a val declaration *)
  fn n => if n = 0 then 1    (* note "rec" because fact'' is *)
          else n*fact''(n-1);

(* three factorial functions; CSSE, Monash U., 7/2005 *)