luhnDouble = (2*x) `mod` 9 almost works, but produces 0 for 2*9 when the answer should be 9. So. luhnDouble :: Int -> Int luhnDouble x = let y = (2 * x) in if y > 9 then y - 9 else y (Note, at this point in the book, the "let" trick has not been introduced.) luhn :: Int -> Int -> Int -> Int -> Bool luhn x1 x2 x3 x4 = 0 == sum[luhnDouble x1, x2, luhnDouble x3, x4] `mod` 10 (This suggested modification works wonderfully.) A solution for a list of arbitrary length awaits chapter 7, this question was from chapter 4. Note, Wikipedia.en implies that originally the digits of the multiplication result were to be added, as in 8 * 2 = 16 = 1 + 6 = 7 OR 16 - 9 = 7. On Sun, Dec 31, 2017 at 8:09 AM Alex Rozenshteyn wrote:

Haskell can totally return the result of the "==", and that would be one of my suggestions as well. The other suggestion is for "luhnDouble": I would just compute `rem (2 * x) 9`, but if you need to explicitly subtract, you can do `let d = 2 * x in if d > 9 then d - 9 else d`, which does the computation just once.

On Sun, Dec 31, 2017 at 9:12 AM John Lusk wrote:

...

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" wrote:

...

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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