Functional programming is based on the simple premise that your functions should not have side effects; they are considered evil in this paradigm. If a function has side effects we call it a procedure, so functions do not have side effects. We consider that a function has a side effect if it modifies a mutable data structure or variable, uses IO, throws an exception or halts an error; all of these things are considered side effects. The reason why side effects are bad is because, if you had them, a function can be unpredictable depending on the state of the system; when a function has no side effects we can execute it anytime, it will always return the same result, given the same input.
Referential Transparency
A function that returns always the same result for the same input is called a pure function. A pure function, therefore, is a function with no observable side effects, if there are any side effects on a function the evaluation could return different results even if we invoke it with the same arguments.
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