Thanks for the added information. I'll argue that `Process.exit!` does modify the state of the program in place (which isn't the typical thing one thinks of wh

Thanks for the added information. I'll argue that `Process.exit!` does modify the state of the program in place (which isn't the typical thing one thinks of when thinking of values). Without the bang, it's just a function call that throws an error, which can be caught and handled, so the logical state machine is unchanged. With the bang, it replaces the set of states the current program can be in to a single exit node.