Pure functions

Pure functions don't have side effects, nor memory, nor I/O. Pure functions have many properties, including referential transparency, caching (memoization), and others (we'll cover these features in the next chapters).

It is possible to write pure functions in Kotlin, but the compiler doesn't enforce it as in other languages. It is up to you to create pure functions to enjoy its benefits. Because Kotlin doesn't enforce pure functions, many programmers said that Kotlin isn't a real functional programming tool, and maybe they are right. Yes, Kotlin doesn't enforce pure functional programming, and that gives you great flexibility including the ability to write in a purely functional style, if you wish.