Proposal: Add missing Monoid for ZipList

[Fumiaki Kinoshita]

There is another possible instance:

instance Semigroup a => Monoid (ZipList a) where
  mempty = ZipList []
  mappend (ZipList xs0) (ZipList ys0) = ZipList (mappend xs0 ys0) where
    go (x:xs) (y:ys) = x <> y : go xs ys
    go xs [] = xs
    go [] ys = ys

But +1 to the original one.

2015-07-26 5:50 GMT+09:00 Christopher Allen <cma at bitemyapp.com>:

>
> 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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