variances.md

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Variance is the correlation of subtyping relationships of complex types and the subtyping relationships of their component types. Scala supports variance annotations of type parameters of generic classes, to allow them to be covariant, contravariant, or invariant if no annotations are used. The use of variance in the type system allows us to make intuitive connections between complex types, whereas the lack of variance can restrict the reuse of a class abstraction.

class Foo[+A] // A covariant class
class Bar[-A] // A contravariant class
class Baz[A]  // An invariant class

Covariance

A type parameter A of a generic class can be made covariant by using the annotation +A. For some class List[+A], making A covariant implies that for two types A and B where A is a subtype of B, then List[A] is a subtype of List[B]. This allows us to make very useful and intuitive subtyping relationships using generics.

Consider this simple class structure:

abstract class Animal {
  def name: String
}
case class Cat(name: String) extends Animal
case class Dog(name: String) extends Animal

Both Cat and Dog are subtypes of Animal. The Scala standard library has a generic immutable sealed abstract class List[+A] class, where the type parameter A is covariant. This means that a List[Cat] is a List[Animal] and a List[Dog] is also a List[Animal]. Intuitively, it makes sense that a list of cats and a list of dogs are each lists of animals, and you should be able to substitute either of them for a List[Animal].

In the following example, the method printAnimalNames will accept a list of animals as an argument and print their names each on a new line. If List[A] were not covariant, the last two method calls would not compile, which would severely limit the usefulness of the printAnimalNames method.

object CovarianceTest extends App {
  def printAnimalNames(animals: List[Animal]): Unit = {
    animals.foreach { animal =>
      println(animal.name)
    }
  }

  val cats: List[Cat] = List(Cat("Whiskers"), Cat("Tom"))
  val dogs: List[Dog] = List(Dog("Fido"), Dog("Rex"))

  printAnimalNames(cats)
  // Whiskers
  // Tom

  printAnimalNames(dogs)
  // Fido
  // Rex
}

Contravariance

A type parameter A of a generic class can be made contravariant by using the annotation -A. This creates a subtyping relationship between the class and its type parameter that is similar, but opposite to what we get with covariance. That is, for some class Writer[-A], making A contravariant implies that for two types A and B where A is a subtype of B, Writer[B] is a subtype of Writer[A].

Consider the Cat, Dog, and Animal classes defined above for the following example:

abstract class Printer[-A] {
  def print(value: A): Unit
}

A Printer[A] is a simple class that knows how to print out some type A. Let's define some subclasses for specific types:

class AnimalPrinter extends Printer[Animal] {
  def print(animal: Animal): Unit =
    println("The animal's name is: " + animal.name)
}

class CatPrinter extends Printer[Cat] {
  def print(cat: Cat): Unit =
    println("The cat's name is: " + cat.name)
}

If a Printer[Cat] knows how to print any Cat to the console, and a Printer[Animal] knows how to print any Animal to the console, it makes sense that a Printer[Animal] would also know how to print any Cat. The inverse relationship does not apply, because a Printer[Cat] does not know how to print any Animal to the console. Therefore, we should be able to substitute a Printer[Animal] for a Printer[Cat], if we wish, and making Printer[A] contravariant allows us to do exactly that.

object ContravarianceTest extends App {
  val myCat: Cat = Cat("Boots")

  def printMyCat(printer: Printer[Cat]): Unit = {
    printer.print(myCat)
  }

  val catPrinter: Printer[Cat] = new CatPrinter
  val animalPrinter: Printer[Animal] = new AnimalPrinter

  printMyCat(catPrinter)
  printMyCat(animalPrinter)
}

The output of this program will be:

The cat's name is: Boots
The animal's name is: Boots

Invariance

Generic classes in Scala are invariant by default. This means that they are neither covariant nor contravariant. In the context of the following example, Container class is invariant. A Container[Cat] is not a Container[Animal], nor is the reverse true.

class Container[A](value: A) {
  private var _value: A = value
  def getValue: A = _value
  def setValue(value: A): Unit = {
    _value = value
  }
}

It may seem like a Container[Cat] should naturally also be a Container[Animal], but allowing a mutable generic class to be covariant would not be safe. In this example, it is very important that Container is invariant. Supposing Container was actually covariant, something like this could happen:

val catContainer: Container[Cat] = new Container(Cat("Felix"))
val animalContainer: Container[Animal] = catContainer
animalContainer.setValue(Dog("Spot"))
val cat: Cat = catContainer.getValue // Oops, we'd end up with a Dog assigned to a Cat

Fortunately, the compiler stops us long before we could get this far.

Other Examples

Another example that can help one understand variance is trait Function1[-T, +R] from the Scala standard library. Function1 represents a function with one parameter, where the first type parameter T represents the parameter type, and the second type parameter R represents the return type. A Function1 is contravariant over its argument type, and covariant over its return type. For this example we'll use the literal notation A => B to represent a Function1[A, B].

Assume the similar Cat, Dog, Animal inheritance tree used earlier, plus the following:

abstract class SmallAnimal extends Animal
case class Mouse(name: String) extends SmallAnimal

Suppose we're working with functions that accept types of animals, and return the types of food they eat. If we would like a Cat => SmallAnimal (because cats eat small animals), but are given a Animal => Mouse instead, our program will still work. Intuitively an Animal => Mouse will still accept a Cat as an argument, because a Cat is an Animal, and it returns a Mouse, which is also a SmallAnimal. Since we can safely and invisibly substitute the former by the latter, we can say Animal => Mouse is a subtype of Cat => SmallAnimal.

Comparison With Other Languages

Variance is supported in different ways by some languages that are similar to Scala. For example, variance annotations in Scala closely resemble those in C#, where the annotations are added when a class abstraction is defined (declaration-site variance). In Java, however, variance annotations are given by clients when a class abstraction is used (use-site variance).