Monoid for ZipList

[Andrew Martin]

There is another possible instance. We can instead write:

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

This behaves differently, and it is also law-abiding.

On Thu, May 3, 2018 at 5:22 AM, 박신환 <ndospark320 at naver.com> wrote:

> As *instance Alternative ZipList* is defined since 4.11.0.0:
>
> *instance Alternative <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/GHC.Base.html#Alternative> ZipList <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicative.html#ZipList> where
>     empty <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/GHC.Base.html#empty> = ZipList <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicative.html#ZipList> []
>     ZipList <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicative.html#ZipList> xs <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicative.html#local-6989586621679319881> <|> <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/GHC.Base.html#%3C%7C%3E> ZipList <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicative.html#ZipList> ys <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicative.html#local-6989586621679319882> = ZipList <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicative.html#ZipList> (xs <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicative.html#local-6989586621679319881> ++ <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/GHC.Base.html#%2B%2B> drop <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/GHC.List.html#drop> (length <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Data.Foldable.html#length> xs <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicative.html#local-6989586621679319881>) ys <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicative.html#local-6989586621679319882>*
> *)*
> It seems perfectly fine to make *Monoid* for *ZipList* as followings:
>
>
>
>
>
>
>
> *instance Semigroup a => Semigroup (ZipList a) where    ZipList []     <> ZipList ys     = ZipList ys    ZipList xs     <> ZipList []     = ZipList xs    ZipList (x:xs) <> ZipList (y:ys) = ZipList (x <> y : ZipList xs <> ZipList ys)instance Semigroup a => Monoid (ZipList a) where    mempty = ZipList []*
>
> Note that this semantic is similar to that of *Maybe*​.
>
>
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-- 
-Andrew Thaddeus Martin
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