I want to get the instance of a singleton type in Scala, is this possible?
Example: (I know it can be done easier in this case)
sealed trait Animal
case object Dog extends Animal
case object Cat extends Animal
trait Person {
def name: String
// Is there a "FavoriteAnimal.instance" syntax?
def mostImportantThings = (FavoriteAnimal.instance, name)
protected type FavoriteAnimal <: Animal with scala.Singleton
}
case class DogPerson(override val name: String) extends Person {
override type FavoriteAnimal = Dog.type
}
case class CatPerson(override val name: String) extends Person {
override type FavoriteAnimal = Cat.type
}
Using shapeless.Witness
correct syntax is
sealed trait Animal
case object Dog extends Animal
case object Cat extends Animal
trait Person {
def name: String
def mostImportantThings(implicit
witness: Witness.Aux[FavoriteAnimal]
): (FavoriteAnimal, String) = (witness.value, name)
protected type FavoriteAnimal <: Animal with scala.Singleton
}
case class DogPerson(override val name: String) extends Person {
override type FavoriteAnimal = Dog.type
}
case class CatPerson(override val name: String) extends Person {
override type FavoriteAnimal = Cat.type
}
DogPerson("A Dog Person").mostImportantThings // (Dog, A Dog Person)
Unfortunately in the current version of Shapeless (2.3.3) there is a bug and this code doesn't compile. But after fix it does.