What are the best practices for using variance in Scala?

What are the best practices for using variance in Scala?

When using variance in Scala, it's important to understand the concepts of covariance, contravariance, and invariance. Covariance allows you to use a subtype in place of a supertype, while contravariance allows the opposite. Always prefer using covariance for immutable collections and contravariance for function parameters.

Can you give an example of covariance in Scala?

Sure! In Scala, you can declare a class as covariant by using the '+' symbol. For example, class Box[+A] allows you to create a Box[Cat] where Cat is a subtype of Animal. This way, you can treat Box[Cat] as a Box[Animal].

What about contravariance? How does it work?

Contravariance is indicated by the '-' symbol. For instance, if you have a class Processor[-A], you can pass a Processor[Animal] where a Processor[Cat] is expected. This is useful for function parameters where you want to accept a broader range of types.

Are there any common pitfalls to avoid?

Yes, a common pitfall is misusing variance annotations. For example, using covariance with mutable collections can lead to runtime exceptions. Always ensure that the type's behavior aligns with the variance you choose.