Author here, thanks for pointing that out! It was a mistake on my part (: I originally wrote this about printf, but decided I should include the section where I implement it, and I decided I didn't want to say anything about side effects, so I changed everything to a version that just returns the new string. In my head this is called "sprintf" (and the notes I based this on, linked in the post, did the same thing) though I can see now how this was confusing to people. Especially since I didn't rewrite it perfectly!