A guitar that teaches you how to play

I was just thinking -- do these things exist yet? That would be totally cool. A guitar that basically guided you through lessons and evaluated your progress. It would need some way of "hearing" the notes you were playing. The lessons could be stored in flash memory. You could get special cards that taught you the particular things you wanted to learn.

In fact, it would be neat to have all the instruments work this way. Imagine, "band in a box!" All the instruments you need and instruction in how to play them. The instruction is built into the instruments.

Update: OK, so apparently there are some early-stage "teaching" guitars. My first impression is that they look a little clunkier than what I was imaginging. For one thing, I was imagining that the guitar would talk to you, working through each lesson kind of like an audio practice tape. However, it would move at the learner's pace, stopping where necessary and allowing repetition when needed. If the learner was quick, the program would speed up, too. It should probably connect to your computer via Bluetooth. However, I'm picturing an interface more like that of a game than a piece of office software. At least for beginners, it should be guiding the learner through the process.

This is obviously not a core part of the idea but I'm imagining one of those guitars with a built in speaker. The idea would be to create an instrument for the absolute beginner that would be useful with a minimum of tweaking.