F# Luhny Bin

gistfile1.txt

let luhn chars =
    let rec luhn even sum digits =
        match digits, even with
        | [], _ -> sum % 10 = 0
        | head :: tail, false when head > 4 -> luhn true (sum + head*2-9) tail
        | head :: tail, false -> luhn true (sum + head*2) tail
        | head :: tail, true -> luhn false (sum + head) tail         
    chars
        |> List.rev 
        |> List.map(fun (c:char) -> int c - int '0')
        |> luhn true 0