Unlike Clojure, Haskell, F#, and the likes, Kotlin is not a pure functional programming language, where immutability is forced; rather, we may refer to Kotlin as a perfect blend of functional programming and OOP languages. It contains the major benefits of both worlds. So, instead of forcing immutability like pure functional programming languages, Kotlin encourages immutability, giving it automatic preference wherever possible.

In other words, Kotlin has immutable variables (val), but no language mechanisms that would guarantee true deep immutability of the state. If a val variable references a mutable object, its contents can still be modified. We will have a more elaborate discussion and a deeper dive on this topic, but first let us have a look at how we can get referential immutability in Kotlin and the differences between var, val, and const val.