What convinced me that avoiding mutations as much as possible was having to deal with code like that (in java): public class MyBigClass() { private

What convinced me that avoiding mutations as much as possible was having to deal with code like that (in java): public class MyBigClass() { private int a; private int b; //hundreds of lines of code private int calculateSomething() { //do something a = a+1; return b*b; } //hundreds lines of code private int calculateSomethingElse() { int c = calculateSomething(); return a+c; } //hundreds of lines of code public returnSomething() { return calculateSomethingElse(); } } In this example. It's hard because you have to remember constantly which functions are changing your classes before you use them, and if you haven't written the functions, it's very easy to introduce a bug by involuntarily modify the states using the function calculateSomething, just wanting its output and not the modification of a.