instance Alternative ZipList in Haskell?

Question

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

• Is

Answer 1

My guiding intuition for Alternative comes from parsers which suggest that if one branch of your alternative fails somehow it should be eradicated thus leading to a Longest-style Alternative that probably isn't terrifically useful. This would be unbiased (unlike parsers) but fail on infinite lists.

Then again, all they must do, as you suggest, is form a Monoid. Yours is left biased in a way that ZipList doesn't usually embody, though—you could clearly form the reflected version of your Alternative instance just as easily. As you point out, this is the convention with Maybe as well, but I'm not sure there's any reason for ZipList to follow that convention.

There's no sensible some or many I don't believe, although few Alternatives actually have those—perhaps they'd have been better isolated into a subclass of Alternative.

Frankly, I don't think your suggestion is a bad instance to have, but I don't have any confidence about it being "the" alternative instance implied by having a ZipList. Perhaps it'd be best to see where else this kind of "extension" instance could apply (trees?) and write it as a library.