Re: Proposal: Add missing Monoid for ZipList

26 Jul 2015


      On 26/07/2015 at 08:30:33 +0200, Henning Thielemann wrote:
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On Sun, 26 Jul 2015, Oleg Grenrus wrote:
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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.
I don't see, how it breaks the identity law. :-(
Derp. I misread the code, sorry.