Get instance of singleton type in scala

Question

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
}

@Thilo It's not in Scaladoc (still) but it is the type extended by singleton types of objects and literals (see docs.scala-lang.org/sips/42.type.html). — Alexey Romanov
– Alexey Romanov, Commented Feb 20, 2019 at 17:13
@AlexeyRomanov That's not just not in Scaladoc, it is not implemented in mainline Scala 2.12, either, is it? — Thilo
– Thilo, Commented Feb 20, 2019 at 23:30
@Thilo it is implemented, try def onlySingleton(arg: Singleton): String = arg.toString: scastie.scala-lang.org/kU481gG3Rtih0rt6dEF2ag — dtech
– dtech, Commented Feb 21, 2019 at 7:32
There are certainly other types which don't have a .class file. Don't remember which at the moment. But I don't know any which are as undocumented. — Alexey Romanov
– Alexey Romanov, Commented Feb 21, 2019 at 20:31

Dmytro Mitin · Accepted Answer · 2019-02-22 22:02:29Z

3

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.

answered Feb 22, 2019 at 22:02

Dmytro Mitin

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Thilo · Accepted Answer · 2019-02-20 13:02:47Z

2

Maybe you want something like

sealed trait Animal
case object Dog extends Animal
case object Cat extends Animal

trait Person[A <: Animal] {
  def name: String
  def animal: A
  def mostImportantThings = (animal, name)
}

case class DogPerson(override val name: String) extends Person[Dog.type] {
  override val animal = Dog
}

case class CatPerson(override val name: String) extends Person[Cat.type] {
  override val animal = Cat
}

answered Feb 20, 2019 at 13:02

Thilo

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3 Comments

dtech Over a year ago

I know this is possible, I was more curious whether my exact use-case is possible. Your approach is not ideal in the real situation I'm trying solve

Thilo Over a year ago

Can you describe that use-case in more detail? It seems that if you at compile time already know the type of the singleton object, then you might just as well write your code to specify the singleton instead of the type (because it is easier to go from singleton to type than the other way around). Without more knowledge of your use case, the end result appears equivalent.

dtech Over a year ago

it's partly interest, party that this would allow me to prevent changing a decent amount of code since the type is already available, but the instance is not.

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