Looks fine to me. Maybe drop the if-then, and simply return the result of
the == ? (Maybe not possible in Haskell (I'm just a duffer myself) but
extraneous trues and falses always drive me nuts.)
--
Sent from my tablet, which has a funny keyboard. Makes me sound more curt
and muted than normal.
On Dec 30, 2017 11:03 PM, "trent shipley"
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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