The command-query separation (CQS) principle states that every function should either be a command that performs an action or a query that returns a value. But never both at the same time.

I.e. your functions should either have no side effects (be referentially transparent) and return a value. Or they should have side effects, but then their return type should be void.

Your command functions are allowed to call your query functions, but your query functions are not allowed to call your command functions.

This is one of my favorite programming principles. If you apply it, your functions will make more sense.

When you follow this principle, you will also find that your code becomes easier to read and debug. While looking for a bug regarding a computational error, you will probably look at your query functions and can ignore your command functions. While looking for a bug regarding side effects (persistence for example), you will look at your command functions.