Proposal: Add missing Monoid for ZipList

Roman Cheplyaka roma at
Sat Jul 25 21:45:09 UTC 2015

This is a neat idea, and potentially useful. +1.

If this gets through, the instance should include documentation
describing how it works (probably mentioning zipWith) and contrasting it
with the Monoid [a] instance.

On 25/07/15 23:50, Christopher Allen wrote:
> 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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