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@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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