The iPhone 4S hits stores today and its killer app is called Siri.

Billed as a personal digital assistant you can talk to, Siri understands voice commands, questions and even casual chit-chat. It uses artificial intelligence to help you get what you want in the word.

If you need to book a flight, find a restaurant in the neighborhood or make an appointment, Siri can help. It’s also not perfect — sometimes there are misunderstandings. But here’s the thing Siri learns from its mistakes.

It listens.

And as anyone who has ever been stuck in a bad relationship can tell you, these are rare qualities indeed.

But computers that learn are not as rare as they once were. Artificial intelligence is going mainstream. It is being used by IBM to make Watson — the computer that is already a “Jeopardy” champion — even smarter. It’s being used by Google to make Google translate apps and search more accurate, and it’s even being used by Microsoft in Kinect to teach the toy to learn to recognize individual people in the room.

None of this stuff is perfect, but it is getting better.

“The thing about Siri that’s interesting is that it’s not like this sci-fi future that we’ve been promised,” said Josh Topolosky, editor of This is My Next. “But what’s cool about it is that you can see that future using this thing. I mean, you know that in five years this is going to be so good, it’ll be like talking to a human.”

And maybe that’s why the researcher that built Siri used to call it HAL.