_[_] kind abstractions

K1.scala

/** we can start with pure structure, not even thinking about specific types here's the structure of
  * a person
  */
trait PersonBase:
  val name, birthdate: Any
  val children: Any // What about this? we know more than just Any!

/** How about abstracting the idea of "something type-indexed"? */
object AbstractKindedTypes:
  opaque type K1[A]                                   = Any
  opaque type K1Impl[F[_], A] >: F[A] <: K1[A] & F[A] = F[A]

export AbstractKindedTypes.*

trait Person extends PersonBase:
  val children: K1[Person]

trait PersonImpl extends Person:
  val name: String
  val birthdate: java.time.LocalDate
  val children: K1Impl[Seq, Person]
end PersonImpl

val samplePerson: PersonImpl =
  new PersonImpl:
    val name      = "John Doe"
    val birthdate = java.time.LocalDate.now()
    val children  = Seq(new PersonImpl:
      val name      = "Jane Doe's son"
      val birthdate = java.time.LocalDate.now()
      val children  = Seq.empty)