Er.. I obviously meant

instance Alternative ZipList where
   empty = ZipList []
   ZipList xs <|> ZipList ys = ZipList (xs ++ drop (length xs) ys)

sorry for any confusion.

On Fri, Jul 17, 2015 at 11:37 AM, Edward Kmett <ekmett@gmail.com> wrote:
The ZipList data type admits one legal Alternative instance

instance Alternative ZipList where
   pure = ZipList []
   ZipList xs <|> ZipList ys = ZipList (xs ++ drop (length xs) ys)

The legality of this instance was first noted in http://people.cs.kuleuven.be/~tom.schrijvers/Research/papers/ppdp2015.pdf

I propose adding this instance, which acts like a generalized version of the Alternative for Maybe, choosing with a left bias.

Discussion Period: 2 weeks

-Edward