I have the following, and it works, but I am trying teach myself Haskell, and I have the suspicion that my solutions is both inefficient and graceless. Any feedback would be appreciated.

Trent.

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{-

8.The Luhn algorithm is used to check bank card numbers for simple errors such as mistyping a digit, and proceeds as follows: 

* consider each digit as a separate number; 

* moving left, double every other number from the second last; 

* subtract 9 from each number that is now greater than 9; 

* add all the resulting numbers together; 

* if the total is divisible by 10, the card number is valid. 

Define a function luhnDouble :: Int -> Int that doubles a digit

and subtracts 9 if the result is greater than 9. 

For example: 

> luhnDouble 3 

6

> luhnDouble 6 

3 

Using luhnDouble and the integer remainder function mod, define a function 

luhn :: Int -> Int -> Int -> Int -> Bool 

that decides if a four-digit bank card number is valid. 

For example: 

> luhn 1 7 8 4 

True

> luhn 4 7 8 3 

False 

In the exercises for chapter 7 we will consider a more general version of this function that accepts card numbers of any length.

Hutton, Graham. Programming in Haskell (pp. 45-46). Cambridge University Press. Kindle Edition. 

-}

luhnDouble :: Int -> Int

luhnDouble x = if (2 * x) > 9

    then (2 * x) - 9

    else 2 * x

luhn :: Int -> Int -> Int -> Int -> Bool

luhn x1 x2 x3 x4 = if 0 == sum[luhnDouble x1, x2, luhnDouble x3, x4] `mod` 10

    then True

    else False

      

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