Re: Proposal: Add missing Monoid for ZipList

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.