Catalan Numbers in SML

catalan.sml

(* Catalan Numbers *)
open LargeInt;
fun fact 0 = 1
 |  fact n = n * (fact (n-1));

fun catalan n = let val factofn = fact n 
                in (fact (2 * n)) div (((n+1) * factofn) * (factofn)) end;

fun firstn f 0 = [f 0]
 |  firstn f n = (f n)::(firstn f (n-1));

val catalanfifteen = rev (firstn catalan 15);