[Haskell-cafe] [] \\ [1..] diverges - intended?

[Daniil Frumin]

Hi!

On Sun, Jul 27, 2014 at 12:55 AM, Henk-Jan van Tuyl <hjgtuyl at chello.nl> wrote:
> On Sat, 26 Jul 2014 21:39:01 +0200, Niklas Hambüchen <mail at nh2.me> wrote:
>
>> Hi,
>>
>> I just noticed that
>>
>>   import Data.List
>>   [] \\ [1..]
>>
>> diverges, although technically it doesn't have to (the docs suggest to
>> me that it could just be [], and a non-diverging implementation is
>> possible).
>>
>> Same for [1,2] \\ [1..] of course.
>>
>
> You can define (\\) as follows, terminating in case of the samples you gave:
>


>   (\\)  :: (Eq a) => [a] -> [a] -> [a]
>   (\\) [] _ = []
>   (\\) _ [] = []
>   (\\) xs (y : ys) = delete y xs \\ ys
>

Is this actually correct? Shouldn't the second line be

(\\) x [] = x

?

> but this will not terminate in cases like:
>   [0] \\ [1..]
>
> I don't think there is a way to get this terminating for all cases.
>
> Regards,
> Henk-Jan van Tuyl
>
>
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-- 
Sincerely yours,
-- Daniil