17.10. Co- and Contravariant Positions

In the preceding section, you saw that functions are contravariant in their arguments and covariant in their results. Generally, it makes sense to use contravariance for the values an object consumes, and covariance for the values it produces.

If an object does both, then the type should be left invariant. This is generally the case for mutable data structures. For example, in Scala, arrays are invariant. You can’t convert an Array[Student] to an Array[Person] or the other way around. This would not be safe. Consider the following: