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

Hi! On Sun, Jul 27, 2014 at 12:55 AM, Henk-Jan van Tuyl wrote:

On Sat, 26 Jul 2014 21:39:01 +0200, Niklas Hambüchen 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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