[Haskell-beginners] Simplified Luhn Algorithm

[Alex Rozenshteyn]

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 <johnlusk4 at gmail.com> 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" <trent.shipley at gmail.com> 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.
>>
>> ------------------------------------
>>
>> {-
>> 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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