I'd like to add that "sprinkling side effects & vars around tight loops" doesn't generalize to all languages. For isntance, GHC can optimize pure code MUCH more

I'd like to add that "sprinkling side effects & vars around tight loops" doesn't generalize to all languages. For isntance, GHC can optimize pure code MUCH more than impure code. Meaning that with purity you get increased performance in many cases.