Luhn Aglorithm in F#

luhn.fsx

// C# example on Wikipedia here https://en.wikipedia.org/wiki/Luhn_algorithm#C#
//Find exmaple credit card numbers here https://www.freeformatter.com/credit-card-number-generator-validator.html
let ccNbr = "5181789570841591" //Master Card
let digits = (string ccNbr).ToCharArray() |> Array.rev
printfn "%A" digits
let mutable sum = 0
//let mutable n = 
let mutable alternate = false
for d in digits do //i in [0 .. digits.Length - 1] do
    let mutable tmp = int (d.ToString())
    printfn "%A" tmp
    if (alternate) then
        tmp <- tmp * 2
        if tmp > 9 then
            tmp <- (tmp % 10) + 1
    sum <- sum + tmp
    alternate <- not alternate
sum % 10 = 0    

//How to make this more functional?

//from this gist https://github.com/ChrisPritchard/My-Exercism-Solutions/blob/master/fsharp/luhn/Luhn.fs
open System
let number = "11111111"
if Seq.exists (fun c -> not (Char.IsNumber c || c = ' ')) number then false
else
    printfn "Start check"
    let clean = number |> Seq.filter Char.IsNumber |> Seq.map (string >> int)
    if Seq.length clean = 1 then false
    else
        let multiply i n = 
            match i % 2 with 
            | 1 when n < 5 -> n * 2 
            | 1 -> (n * 2) - 9 
            | _ -> n
        let sum = clean |> Seq.rev |> Seq.mapi multiply |> Seq.sum
        sum % 10 = 0