Re: Proposal: Add missing Monoid for ZipList

26 Jul 2015

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On 26 Jul 2015, at 06:43, M Farkas-Dyck  wrote:
On 26/07/2015 at 12:23:13 +0900, Fumiaki Kinoshita wrote:
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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