Factorial implemented in F#

factorial.fs

// Get the factorial of an integer.
let factorial n =
    // Actual recursive factorial implementation.
    let rec f n =
        match n with
        | 0 -> 1
        | _ -> n * f (n - 1)

    // Validate that n isn't negative
    match n with
    | _ when n < 0 -> None
    | _ -> Some (f n)