but, in your foo example, presumably complex_function() returned some data that was important, and foo was set to null for a reason, so that when you call do_stuff() you need to know was it supposed to be called on using the result of complex_function, or should it not be run at all because there is a missing null check.

I guess what you are saying is that foo should not have been changed, but a new variable created called "can_do_stuff" and it should have been checked before the call do_stuff()

I think the old Carmack article linked somewhere below makes a very good case for why pure functions are a good thing, and I see the value of making small atomic changes to an apps state, but ultimately, almost every applications job is to mutate data.