Try F# Generics

CatalanNumber.fs

let rec fact n =
    if n = 0I then 1I
    else n * fact (n-1I)

let comb n r = 
    let a = fact n
    let b = fact r
    let c = fact (n-r)
    (a / (b*c))

let catalan_number n =
    match n with
    | n when n = 0I -> 1I
    | _             -> comb (2I*n) n - comb (2I*n) (n-1I)

[<EntryPoint>]
let main argv = 
    let ans = [|1; 1; 2; 5; 14; 42; 132; 429; 1430; 4862; 16796; 58786; 208012; 742900; 2674440; 9694845|]
    for i in 0..(ans.Length-1) do
        printfn "catalan %A = %A ; %b" i (catalan_number (bigint i)) (catalan_number (bigint i) = bigint(ans.[i]))
    0