Focused Live Programming with Loop Seeds

Live programming is a paradigm in which the programmer can visualize the runtime values of the program each time the program changes. The promise of live programming depends on using test cases to run the program and thereby provide these runtime values. In this paper we show that in some situations test cases are insufficient in a fundamental way, in that there are no test inputs that can drive certain incomplete loops to produce useful data, a problem we call the loop-datavoid problem. The problem stems from the fact that useful data inside the loop might only be produced after the loop has been fully written. To solve this problem, we propose a paradigm called Focused Live Programming with Loop Seeds, in which the programmer provides hypothetical values to start a loop iteration, and then the programming environment focuses the live visualization on this hypothetical loop iteration. We introduce the loop-datavoid problem, present our proposed solution, explain it in detail, and then present the results of a user study.