wow, yeah. Here's what I can surmise is happening. since Set doesn't have an Applicative of its own, we are getting the Monoid#applicative instance instead:
scala> implicitly[Unapply[Applicative, Set[Int]]].TC
res0: scalaz.Applicative[_1.M] forSome { val _1: scalaz.Unapply[scalaz.Applicative,Set[Int]] } = scalaz.Monoid$$anon$1@7f5d0856
Since Monoid is defined for types of kind * and applicative is defined for types of kind * -> *, the definition of Applicative in Monoid sorta wedges in an ignored type parameter using a type lambda:
final def applicative: Applicative[({type λ[α]=F})#λ] = new Applicative[({type λ[α]=F})#λ] with SemigroupApply...
Notice there that the type parameter α
of λ
is thrown away, so when Applicative#point is called, which becomes Monoid#zero, instead of it being a Monoid[Set[Option[Int]]] it is a Monoid[Set[Int]].
larsh points out that this has the interesting side-effect of alllowing sequenceU to be (ab)used as sum:
scala> List(1,2,3).sequenceU
res3: Int = 6