Immutability

The concept of immutability is the CORE of FP, and it means that once you assign a value to something, that value won't change. This is very important, because it eliminates side effects (anything outside of the local function scope), for instance, changing other variables outside the function. Immutability makes it easier to read code, because you know the function that you are using is a pure function. Since your function has a disciplined state and does not change other variables outside of the function, you don't need to look at the code outside the function definition. This sounds like you're not working with state at all, so how would it be possible to write professional applications this way? Well, you will change state but in a very disciplined way. You will create another instance or another pointer to that instance, but you won't change that variable's value. Having immutability is the key to having better, faster, and more correct programs, because you don't need to use locks and your code is parallel by nature.