Proposal: Add missing Monoid for ZipList

[Oleg Grenrus]

> On 26 Jul 2015, at 06:43, M Farkas-Dyck <strake888 at gmail.com> wrote:
> 
> On 26/07/2015 at 12:23:13 +0900, Fumiaki Kinoshita wrote:
>> There is another possible instance:
>> 
>> instance Semigroup a => Monoid (ZipList a) where
>>  mempty = ZipList []
>>  mappend (ZipList xs0) (ZipList ys0) = ZipList (mappend xs0 ys0) where
>>    go (x:xs) (y:ys) = x <> y : go xs ys
>>    go xs [] = xs
>>    go [] ys = ys
> 
> This breaks the identity law.

    instance Monoid a a => Monoid (ZipList a) where
      mempty = ZipList (repeat mempty)
      mappend (ZipList xs) (ZipList ys) = ZipList (zipWith (<>) xs ys)

No preference on actual proposal or the instance. I have no strong opinion whether Alternative and Monoid1 should be the same.

Cheers,
Oleg Grenrus
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