So what exactly are side effects and why should they be avoided? For this discussion, we can define a side effect as some instructions in a function's code that modify the environment outside the scope of this function. The most common example of a side effect is an exception thrown by a program. Throwing an exception is a side effect because if you don't handle it, it will disrupt the program outside the scope of this function. So the program will break at this point and will stop its execution.
Take, for example, the function of the Soda Machine example from the previous chapter. The function that simulates coin-insertion throws an exception if there are no soda cans in the slot machine. So if you try to call that function on an empty soda machine, your program will never proceed past the function call site, because an exception will be thrown. That is...