Proposal: Add missing Monoid for ZipList

Christopher Allen cma at
Sat Jul 25 20:50:40 UTC 2015

There's a Monoid that matches what the Applicative for ZipList does that
seems to be missing.

instance Monoid a => Monoid (ZipList a) where
  mempty  = pure mempty
  mappend = liftA2 mappend

It's been brought up before:

Not only is it useful when it's the Monoid you want, but it serves an
educational purpose for highlighting the relationship between Monoid and
Applicative as well.

Are there any good reasons not to have it?

I'd like to limit discussion to two weeks.
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