Side Effects

A side effect is an effect or consequence arising from the evaluation of a function or expression that is different from the values popped from or pushed on to the stack.

Some examples of side-effects are:

• Writing something to a file
• Creating a directory
• Printing a message to the console
• Displaying a graphical element
• Sending a mild electric shock to someone

A function or expression with no side-effects is called pure and a function or expression with side-effects is called impure.