Although the example you give works today, it would stop working if mappend ever stopped being a typeclass method of Monoid. I cannot remember if this is on the roadmap. It's certainly not happening any time soon, but it may happen eventually. (It's similar in spirit to removing return from Monad) On Sat, Sep 14, 2019 at 4:48 PM Zemyla wrote:

The point is that someone can write

instance Monoid Thingy where mempty = <something> mappend a b = <something else> mconcat ls = <another thing>

deriving instance Semigroup Thingy via (WrappedMonoid Thingy)

Most of the benefit is not having to look up "stimesMonoid" over and over again.

On Sat, Sep 14, 2019 at 3:36 PM John Villarreal via Libraries wrote:

...

Hello Georgi,

I'm confused about WrappedMonoid. How can something have a Monoid

instance already without having a Semigroup instance in the first place?

...

But also what is the benefit of WrappedMonoid over the standard way of

defining Semigroups and Monoids for your example

...

data List a = Nil | Cons a (List a)

instance Semigroup (List a) where Nil <> ys = ys Cons x xs <> ys = Cons x (xs <> ys)

instance Monoid (List a) where mempty = Nil

?

Cheers, John

On Saturday, 14 September 2019, 19:18:01 UTC, Georgi Lyubenov <

godzbanebane@gmail.com> wrote:

...

Hello!

Sorry if this is not the right place to ask!

As of right now (14.09.2019) Data.Semigroup.WrappedMonoid is slated to

be deprecated.

...

I believe that this newtype is actually useful in tandem with

DerivingVia, as it allows you to derive the Semigroup instance for something that has a Monoid instance already, as discussed in this twitter thread.

...

Would it be possible to *not* deprecate it in the future, and instead

keep it around for exactly this purpose?

...

Cheers, Georgi _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

-- -Andrew Thaddeus Martin