Proposal: instance Alternative ZipList

[Edward Kmett]

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
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