containers: intersections for Set, along with Semigroup newtype

Alexandre Rodrigues Baldé alexandreR_B at
Sun Dec 6 18:27:18 UTC 2020

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 <libraries-bounces at> em nome de Reed Mullanix <reedmullanix at>
Enviado: Sunday, December 6, 2020 6:20:02 AM
Para: Haskell Libraries <libraries at>
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!
Reed Mullanix
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