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
