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

Am 06.12.20 um 19:58 schrieb Sven Panne:

To me it's just the other way around: It violates aesthetics if it doesn't follow the mathematical definition in all cases, which is why I don't like NonEmpty here.

I think you've got that wrong. x `elem` intersections [] = {definition} forall xs in []. x `elem` xs = {vacuous forall} true Any proposition P(x) is true for all x in []. So the mathematical definition of intersections::[Set a]-> Set a would not be the empty set but the set of all x:a, which in general we have no way to construct. Cheers Ben