#summary Definition of side effect.
#labels Glossary

= Side Effects =

A side effect is an effect or consequence arising from the evaluation of a [Function function] or [Expression 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 [Purity pure] and a function or expression with side-effects is called [Purity impure].