Re: containers: intersections for Set, along with Semigroup newtype

In a perfect world we would use something like Foldable1 from semigroupoids, but sadly that is not in base. However, this is exactly why I proposed the Intersection newtype, as it would make it nice and easy to use with a Foldable1 like type class. On Sun, Dec 6, 2020 at 10:27 AM Alexandre Rodrigues Baldé < alexandreR_B@outlook.com> wrote:

Sounds like a good idea, could it be possible to use a typeclass instead of `NonEmpty (Set a)`?

I recall needing this a few times, but not over a NonEmpty.

*De:* Libraries em nome de Reed Mullanix < reedmullanix@gmail.com> *Enviado:* Sunday, December 6, 2020 6:20:02 AM *Para:* Haskell Libraries *Assunto:* containers: intersections for Set, along with Semigroup newtype

Hey all,

I've found myself reaching for the following function a couple of times now, so I figured it might make a good addition.

intersections :: Ord a => NonEmpty (Set a) -> Set a intersections (s :| ss) = Foldable.foldl' intersection s ss

In a similar vein, we may as well add the following newtype + instance combo:

newtype Intersection a = Intersection { getIntersection :: Set a }

instance (Ord a) => Semigroup (Intersection a) where (Intersection a) <> (Intersection b) = Intersection $ intersection a b stimes = stimesIdempotent

Would love to hear everyone's thoughts on this!

Thanks

Reed Mullanix