A New Way of Debugging Lisp Programs

A major cause of software unreliability is the misapplication of primitive operations, such as taking the car of nil, dividing by zero, or using an invalid array index. These errors are traditionally discovered by extensive testing and debugging, but this approach is unsatisfactory because it is time-consuming and may not identify all potential errors. We suggest to address this problem with sophisticated static debugging systems. Recent advances in proof technology have brought such advanced static debugging systems within reach. Methods like control-flow analysis or set-based analysis establish invariants that can identify potentially faulty program operations. Past research, however, only focused on the synthesis of the invariants and completely neglected their presentation to the programmer. We believe that the programmer must be able to inspect the inferred invariants and to browse their underlying proof. Then, if some set invariant contains surprising elements, the programmer can determine whether the result is a weakness in the proof system or whether it uncovers a flaw in the program. This paper presents MrSpidey, a user-friendly, interactive static debugger that is completely integrated into DrScheme, our program development environment. MrSpidey exposes those operations that may signal errors during an execution; it describes the sets of values the program expressions may assume; and it also provides a graphical explanation of how values flow through the program. Using MrSpidey, the programmer can easily identify and eliminate the causes of potential run-time errors. Experimental results support our belief that MrSpidey expedites the process of program debugging.