instance Alternative ZipList in Haskell?

Question

ZipList comes with a Functor and an Applicative instance (Control.Applicative) but why not Alternative?

• Is

Answer 1

Your instance is OK, but it does something ZipList doesn't by
(a) aiming for the longest list, and
(b) mixing elements between source lists.

Zipping as an operation stops at the length of the shortest list.

That's why I concluded in my answer:

Thus the only sensible Alternative instance is:

instance Alternative Zip where
   empty = Zip []
   Zip [] <|> x = x
   Zip xs <|> _ = Zip xs

This is consistent with the Alternative instances for Maybe and parsers that say you should do a if it doesn't fail and go with b if it does. You can say a shorter list is less successful than a longer one, but I don't think you can say a non-empty list is a complete fail.

empty = Zip [] is chosen because it has to be polymorphic in the element type of the list, and the only such list is []

For balance, I don't think your instance is terrible, I think this is cleaner, but hey ho, roll your own as you need it!