Past exam of the year 1 of the computer science course of the University of Oxford / 2017 / Imperative programming (resit) / 6 / c / Solution

Immutable Scala List[+A] is covariant: if Juice is a subtype of Drink, a list of juices can be used as a list of drinks. Scala Array[A] is invariant because it permits both reading and writing elements.

If an array of juices could be treated as an array of drinks, writing a beer through that widened reference would violate the original juice-array contract. This illustrates covariant type parameter versus generic type invariance; immutability permits the first relationship for lists.