2 things that I think would make your life so much easier..
1) Agree with the interviewer before starting to code on the algorithm and problem. I.e. You could say I need function X that would be used like such, and function Y that would loop through such and such. Even better is drawing a high-level example on the board. That way, the interviewer knows that you understood the problem and can be systematic about it. (If you don't have a whiteboard, just write in comment at the top, whatever)
2) Once you start coding, start small.. especially in the REPL. I.e. instead of starting with the whole function, start with the string, then separate it in digits, then sum it up, then work from there. That way, if something doesn't compile, it's perfectly clear what step it was and once you have your function, you're 100% sure it works. TDD style.
1) Agree with the interviewer before starting to code on the algorithm and problem. I.e. You could say I need function X that would be used like such, and function Y that would loop through such and such. Even better is drawing a high-level example on the board. That way, the interviewer knows that you understood the problem and can be systematic about it. (If you don't have a whiteboard, just write in comment at the top, whatever)
2) Once you start coding, start small.. especially in the REPL. I.e. instead of starting with the whole function, start with the string, then separate it in digits, then sum it up, then work from there. That way, if something doesn't compile, it's perfectly clear what step it was and once you have your function, you're 100% sure it works. TDD style.