Exploring pure functions

When a function always returns the same value for a given parameter or parameter set and never modifies anything outside of the function scope (side effects), that function is called a pure function; in fact, you can replace no-argument pure functions with constants. The concept of a pure function is completely based on mathematical functions, for instance, in the mathematical function y = f(x), for a given value of x, y (both constants).

Let's take a look at the following example:

class Calculator {
    var anyVariable: Int = 0
 
    fun add(a: Int, b: Int): Int = a + b         // pure function
    fun multiply(a: Int, b: Int): Int = a * b    // pure function
    fun subtract(a: Int, b: Int): Int = a - b    // pure function
    fun divide(a: Int, b: Int): Int = a / b      // pure function
 
    fun anyFunction(x: Int): Int {               // not a pure function...