Scala 3. Kind polymorphism and AnyKind type - any code example?

Question

Scala3 has support for "kind polymorphism". Docs also mention AnyKind type:

AnyKind plays a special role in Scala's subtype system: It is a supertype of all other types no matter what their kind is.

Question:

• can anyone give a working code example how AnyKind generality is useful?

(surprisingly can't find any useful examples so far)

docs

Answer 1

For example the type member MirroredType of scala.deriving.Mirror.Product/Mirror.Sum is actually poly-kinded (although this is not written in the definition of Mirror/Mirror.Sum/Mirror.Product)

sealed trait Mirror:
  type MirroredMonoType
  type MirroredLabel <: String
  type MirroredElemLabels <: Tuple

https://docs.scala-lang.org/scala3/reference/contextual/derivation.html#mirror

The type member MirroredMonoType has always kind *, including being existential (A[?]). But MirroredType can be *

sealed trait A
case class B() extends A
case class C() extends A

val m = summon[Mirror.Sum { type MirroredType = A }]

//scala.deriving.Mirror.Sum{
//  MirroredMonoType = A; MirroredType = A; 
//    MirroredLabel = ("A" : String)
//  ; MirroredElemTypes = (B, C); 
//    MirroredElemLabels = (("B" : String), ("C" : String))
//}

or * => *

sealed trait A[T]
case class B() extends A[Int]
case class C() extends A[String]

val m = summon[Mirror.Sum { type MirroredType[T] = A[T] }]
//val m = summon[Mirror.Sum { type MirroredType = [T] =>> A[T] }]

//scala.deriving.Mirror.Sum{
//  MirroredMonoType = A[?]; MirroredType[T] = A[T]; 
//    MirroredLabel = ("A" : String)
//  ; MirroredElemTypes[T] = (B, C); 
//    MirroredElemLabels = (("B" : String), ("C" : String))
//}

etc.

Notice that MirroredElemTypes is also poly-kinded (MirroredElemTypes = (B, C), MirroredElemTypes[T] = (B, C), ...)

So if I wanted to do something further with a tuple MirroredElemTypes then the only option would be to have upper bound AnyKind

def foo[T <: AnyKind] = ???

foo[m.MirroredElemTypes]

Another example is scala.quoted.Type (thanks to @Max for pointing this out)

abstract class Type[T <: AnyKind]:
  type Underlying = T

https://contributors.scala-lang.org/t/proposal-to-add-kind-polymorphism-to-the-language/2958/16

Miles Sabin. Adding kind-polymorphism to the Scala programming language https://www.youtube.com/watch?v=v6e7rYOXdcM