Review best practice advice on side-effects

[grahamison]

The note says "It’s best practice for your function to either return an object or have a side-effect, but not both". Is this always true? E.g. you might want to return an object and display something (e.g. print a summary, display a plot, print a message).

[pbbgss]

I think it is true - signalling (messages, errors, warnings, progress bars etc) is the exception. If you want a summary or a plot of a returned object that should be via a separate step

[grahamison]

Could we add a sentence on why it is best practice/where the advice comes from? And could printing messages be an exception too?