Documents.scala

// Represents a named collection of documents of type A
trait Documents[A] {
  def name: String
  def take(target: Int): Documents[A]

  // [...]
}

// Represents a collection of named collections of documents.
// A is the type of a document, D that of a named collection of documents
case class Data[A, D[_] <: Documents[_]](docs: List[D[A]]) {
  // Makes sure no D contains more than target documents
  def cap(target: Int): Data[A, Documents] = {
    // Fails to compile: the map returns a D[_]
    //copy(cats = cats.map(_.take(target)))

    // Compiles
    Data(docs.map(_.asInstanceOf[Documents[A]].take(target)))
  }

  // [...]
}