The main difference between functional and imperative programming is that imperative programming produces side effects while functional programming doesn't. In imperative programming, the expressions are evaluated and its resulting value is assigned to variables. So, when we group series of expressions into a function, the resulting value depends upon the state of variables at that point in time. This is called side effects. Because of the continuous changes in state, the order of evaluation matters. In functional programming world, destructive assignment is forbidden and each time an assignment happens a new variable is induced.