Variance (Covariant/Contravariant)
Variance describes whether a box of one type can stand in for a box of a related type, like a basket of cats counting as a basket of animals. Scala uses plus and minus signs to say how.
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Syntax
class Box[+T](value: T) // covariant
trait Handler[-T] // contravariantVariance (Covariant/Contravariant)
Covariance, declared with class Box[+T], lets Box[Dog] be used where Box[Animal] is expected, preserving the subtype relationship of the type parameter.
Contravariance, declared with class Printer[-T], works the opposite direction -- a Printer[Animal] can be used where a Printer[Dog] is expected, since it can handle anything an Animal-or-more-general printer could. By default (no +/-), a type parameter is invariant, meaning no such substitution is allowed at all.
Note:
Covariant (+T) type parameters are generally restricted to output/return positions only -- using one as a method parameter type usually causes a compile error.
Example: Variance (Covariant/Contravariant)
class Animal(val name: String)
class Dog(name: String) extends Animal(name)
class Box[+T](val item: T)
object Main extends App {
val dogBox: Box[Dog] = new Box(new Dog("Rex"))
val animalBox: Box[Animal] = dogBox // allowed because Box is covariant
println(animalBox.item.name)
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Related Topics
{# common_mistakes/chapter_summary/browser_support: on Hindi pages the
view already swaps in the hi_ translation fields (or blanks these
out if untranslated), so this renders correctly for both languages
without a lang_code check here. #}
Common Mistakes
- Declaring a mutable field of a covariant type, like
class Box[+T](var item: T), which does not compile. - Assuming
Box[Dog]is aBox[Animal]without+T, since Scala classes are invariant by default. - Mixing up
+Tand-T, when covariance is for producers and contravariance for consumers.
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