LUHN validation with F#

I'm on vacation, and since I'm a nerd I take that amount of spare time and use it to learn a new language. This year I decided to learn F#. I've been working with Lisp before, so the concept of functional oriented langauges is not new to me. The syntax however is. After a couple of hours with the book Foundations of F# (Robert Pickering). I've already written my first program. It is a LUHN10 validator. Every Swedish citizen has a personal identity number (almost like social security number) that uniquely identifies that person. The format of this number looks like this

y y M M d d x x x z
6 9 0 1 0 1 1 2 3 6

y is year, M is month and d'is day. x is some number and z is a control digit. From this number you could tell when I was born (birth date), what region of Sweden I was born and my gender. The last digit is a control number used to validate the rest of the number. The LUHN10 validation works like this.

  6 9 0 1 0 1 1 2 3 6
* 2 1 2 1 2 1 2 1 2 1
---------------------
 12 9 0 1 0 1 2 2 6 6

You calculate the products as I shown above, and then you add every digit each of their own.

1 + 2 + 9 + 0 + 1 + 0 + 1 + 2 + 2 + 6 + 6 = 30

If the result is evenly divisible by 10, then this personal identity number is valid. That is where the LUHN10 name comes from. You can also use this to calculate the last digit of the personal identity number.

  6 9 0 1 0 1 1 2 3 x
* 2 1 2 1 2 1 2 1 2 1
---------------------
1 + 2 + 9 + 0 + 1 + 0 + 1 + 2 + 2 + 6 + x = y
24 + x = y

At this point you set x some a number that will make y evenly divisible with 10, and you have your control digit. I wrote this validation algorithm in F# as practice. This piece of code probably looks like shit to those of you that are already fluent with F#.

// Product may be 1 or 2, largest possible double is 18
// If result of product is greater than 9 add the parts of
// the product together.
// Example 9 * 2 = 18 => 1 + 8 = 9
// Example 6 * 2 = 12 => 1 + 2 = 3
let calculateProduct x product =
    let double = x * product
    match double with
    | double when (double > 9) -> 1 + (double - 10)
    | double -> double


// 2, 1, 2, 1, 2, 1, 2, 1....
let indexProduct x = ((x + 1) % 2) + 1

// For each and every number in pidList add result of calcAtom
let rec luhnAlgorithm numberList index =
    match numberList with
    | head :: tail -> calculateProduct head (indexProduct index) + luhnAlgorithm tail (index + 1)
    | [] -> 0

// PID is valid if the result from luhnAlgorithm divides evenly with 10
let luhn10 pid = ((luhnAlgorithm pid 0) % 10) = 0

And the usage may look like this

let pid = [6; 9; 0; 1; 0; 1; 1; 2; 3; 6]
if luhn10 pid then
    printfn "Your personal identity number is valid"
else
    printfn "Your personal identity number is invalid"