The lens design pattern

We already mentioned before that, in Scala, objects are immutable. You can, of course, make sure that a specific class has its fields declared as vars but this is discouraged and considered bad practice. After all, immutability is good and we should try to aim for it.

The lens design pattern was created specifically for that purpose and allows us to overcome the immutability limitation and at the same time preserve the code readability. In the following subsections, we will start with some code that doesn't use the lens design pattern and we will go step by step to show how to use it and how it improves our applications.

Lens example

In order to show the lens design pattern in practice, we will create a class hierarchy that is usually seen in enterprise applications. Let's imagine that we are building a system for a library that can be used by the employees of different companies. We might end up with the following classes:

case class Country(name: String, code: String...