Proposal: Add missing Monoid for ZipList

[David Feuer]

We could add a newtype for this in Data.Monoid.

newtype Appl f a = Appl (f a)
instance (Applicative f, Monoid a) => Monoid (Appl f a) where
  mempty = Appl $ pure mempty
  mplus (Appl x) (Appl y) = Appl $ liftA2 mplus x y
On Jul 25, 2015 4:50 PM, "Christopher Allen" <cma at bitemyapp.com> wrote:

>
> http://hackage.haskell.org/package/base-4.8.1.0/docs/Control-Applicative.html#t:ZipList
>
> 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:
>
> https://mail.haskell.org/pipermail/haskell-cafe/2009-October/067861.html
>
> 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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