Why can't a polymorphic function accept wildcard (existential) types in Scala?-CodePudding

In the example below, I'm wondering why funPoly can't accept the existentially quantified type value outersFromInnersEx, even though funEx can.

case class InnerCassClass[I, E, O](i: I, e: E, o: O)
case class OuterCaseClass[I, E, O](inner: InnerCassClass[I, E, O])


val inner1 = InnerCassClass(5, "foo", 3.3f)
val inner2 = InnerCassClass(4.4f, 6, "bar")


// Doesn't work as expected due to invariance of type paramemters I, E, O (but confirm)
// val outersFromInnersAny: List[OuterCaseClass[Any, Any, Any]] = List(inner1, inner2).map(OuterCaseClass.apply)

val outersFromInnersEx: List[OuterCaseClass[_, _, _]] = List(inner1, inner2).map(OuterCaseClass.apply)

def funPoly[I, E, O](occ: List[OuterCaseClass[I, E, O]]): Unit = ()
def funEx(occ: List[OuterCaseClass[_, _, _]]): Unit = ()


// This doesn't work, but why?
val u1 = funPoly(outersFromInnersEx)

val u2 = funEx(outersFromInnersEx)

Note, I test this in Scala 3 (try online), but largely the problem is the same in Scala 2, though this particular example has other issues in Scala 2.

CodePudding user response：

It about type variances, you can make funPoly works by change

case class InnerCassClass[I, E, O](i: I, e: E, o: O)
case class OuterCaseClass[I, E, O](inner: InnerCassClass[I, E, O])

case class InnerCassClass[ I,  E,  O](i: I, e: E, o: O)
case class OuterCaseClass[ I,  E,  O](inner: InnerCassClass[I, E, O])