In Scala, arrays are invariant by default and immutable collections (or container types) are covariant or [+A]. Since they are immutable all of your potential type errors will be discovered during compile time as opposed to runtime. Another possible option is to define a container as contravariant ([-A]). Contravariance means that a container wi…

describe_type_variance.scala

case class InvariantContainer[A]()
case class CovariantContainer[+A]()
case class ContravariantContainer[-A]()

class Person
class User extends Person
class Admin extends User

val inv1: InvariantContainer[User] = InvariantContainer[User] // works

val inv2: InvariantContainer[User] = InvariantContainer[Admin] // doesn't work

val cov1: CovariantContainer[User] = CovariantContainer[User] // works

val cov2: CovariantContainer[User] = CovariantContainer[Admin] // works

val con1: ContravariantContainer[User] = ContravariantContainer[User] // works

val con2: ContravariantContainer[User] = ContravariantContainer[Admin] // doesn't work

val con3: ContravariantContainer[User] = ContravariantContainer[Person] // works