gistfile1.scala

trait StorageComponent {
  type User <: UserLike

  def storeUser(user: User)
  def retrieveUser(id: Int): Option[User]

  trait UserLike {
    def id: Int
    def hash: Vector[Byte]
  }
}

trait MySQLStorageComponent extends StorageComponent {
  override def storeUser(user: User) { ... }
  override def retrieveUser(id: Int): Option[User] = ...

  case class User(id: Int, hash: Vector[Byte]) extends UserLike
}