Professional Scala

Variance

Variance provides a way to constrain parameterized types. It defines a subtyping relationship between parameterized types based on the subtyping relationship of their component types.

Imagine that you have the following class hierarchy:

class Tool
class HandTool extends Tool
class PowerTool extends Tool
class Hammer extends HandTool
class Screwdriver extends HandTool
class Driller extends PowerTool
If we define a generic box:
trait Box[T] {
  def get: T
}

How can Box of Tools relate to one another? Scala provides three ways:

• Covariant: Box[Hammer] <: Box[Tool] if Hammer <: Tool
• Contravariant: Box[Tool] <: Box[Hammer] if Tool <: Hammer
• Invariant: There's no subtyping relationship between Box[Tool] and Box[Hammer] independently of the subtyping ...