I think this[1] does what you want, unless I'm not understanding your problem description. AFAIK you are correct about protocol extensions not holding state. So

I think this[1] does what you want, unless I'm not understanding your problem description. AFAIK you are correct about protocol extensions not holding state. So I don't think you can implement the `var serialCode = generateSerialCode()` with just a protocol. But you could do it with a protocol + base class. [1]: https://gist.github.com/jayrhynas/49945331e41314fc3ede https://gist.github.com/jayrhynas/49945331e41314fc3ede Edit: I just realized that with this method, the protocol extension is useless, you could just define `move` on Robot. So protocol extensions can give you a mix of abstract & default methods but if you need to inherit state you need to use a base class.