It would have to be very close to as powerful as deploying code in an APK, in other words few limitations on what kind of code can be run via the REPL. There are two big limitations with Android here, as far as I understand it: there's a fair amount that you're not allowed to do dynamically (like create new Activities), and Dalvik has limitations with dynamic class generation. To be honest I haven't experimented with it much, but I was disappointed that the Clojure and Ruby REPLs didn't quite deliver what I was hoping they would (no fault of the project authors)