About WrappedMonoids deprecation

[Andrew Martin]

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 <zemyla at gmail.com> 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
> <libraries at haskell.org> 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 at 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
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-- 
-Andrew Thaddeus Martin
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