F# command line arguments parsing, factorial, lambdas, and pipe operator fun

ztmr.fs

// Build:
//   $ fsharpc ztmr.fs
// Use:
//   $ mono ztmr.exe 5

open System

let rec factorial n =
    match n with
    | 0 -> 1
    | _ -> n * factorial (n - 1);

[<EntryPoint>]
let main(args : string[]) =
    if args.Length <> 1 then
        failwith "Usage: ztmr.exe <number>"
    let n = int args.[0]
    n
        |> factorial
        |> (fun i -> (n+1 |> factorial)/i)
        |> printfn "%d = (%d+1)!/%d! = %d" (n+1) n n
    0