Any useful program has side-effects. IMHO the point is to isolate the part of the code that has the side-effects as much as possible, and keep the rest purely f

Any useful program has side-effects. IMHO the point is to isolate the part of the code that has the side-effects as much as possible, and keep the rest purely functionsl. That makes it easier to debug, test, and create good abstractions. Long term it is a very good approach.