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Well, if somebody breaks the kernel so that printing doesn't work, it will most likely return something strange, in the case of Clojure, an exception.

A pure function always returns the same result because it only depends on its input parameters. An impure IO function does not.

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I see that if there's a kernel panic, or power outage that f(x) might return something insane or nothing at all, but those aren't things I'm usually worried about. What's the practical upside to this?

Is breaking the kernel something I should actually consider likely?

Compiler optimization are allowed to change how often a pure function is called, and you can freely change it in your code. If it had a printf inside it, you'd need a way to say you don't care about the side effect of printing a specific line one time, and not zero or two times.

Of course there is lots of code that has side effects in implementation but not in their interface. Like malloc, or a read only data structure that has an internal cache it updates.

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