Defining a generic to be a case class - scala

Question

In this example, I want the generic T to be a case class and a DAOEntity with id, so in the abstract implementation, I can use the copy method.
How to define it?
trait DAOEntity {
def id: String
}
// How to define this generic to force the use of a `case class` to have access to `copy`?
abstract class DAO[T <: DAOEntity] {
def storeInUppercase(entity: T): T = entity.copy(id = entity.id)
}
case class MyEntity(id: String) extends DAOEntity
class MyEntityDAO extends DAO[MyEntity] {
// Other stuff
}

Answer 1

There is no way to know if a type is a case class or not.
And even if there was, you won't get the copy method. The language doesn't provide a way to abstract over constructor; thus neither copy and factories (apply on companions) by extension. Which makes sense, what would be the type signature of such function?
What you can do instead is create a factory-like typeclass and ask for that:
trait DAOFactory[T <: DAOEntity] {
def copy(oldEntity: T, newId: String): T
}
object DAOFactory {
def instance[T <: DAOEntity](f: (T, String) => T): DAOFactory[T] =
new DAOFactory[T] {
override final def copy(oldEntity: T, newId: String): T =
f(oldEntity, newId)
}
}
Which can be used like this:
abstract class DAO[T <: DAOEntity](implicit factory: DAOFactory[T]) {
def storeInUppercase(entity: T): T =
factory.copy(
oldEntity = entity,
newId = entity.id.toUpperCase
)
}
And entities would provide the instance like this:
final case class MyEntity(id: String, age: Int) extends DAOEntity
object MyEntity {
implicit final val MyEntityFactory: DAOFactory[MyEntity] =
DAOFactory.instance {
case (oldEntity, newId) =>
oldEntity.copy(id = newId)
}
}
// This compile thanks to the instance in the companion object.
object MyEntityDAO extends DAO[MyEntity]
You can see the code running here.