[Chris2048]

Allowing the extra constraint "doesn't have a side effect" is more dangerous than clarity in this case because it increases what a dev needs to know to modify the code, and allows for a bug to easily be introduced if the validate code is modified to have side-effects.

And though a test might be added to check for this, "Was this function run" is easier to determine than "Does this function have side-effects".

[guelo]

That's why all your functions should not have a side effect if at all possible. And if they do it should be stated in the function's name. Maybe something like validateAndLogIt().