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. ------------------------------------ {- 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