A visual Lisp debugging environment

Graphics provides a useful tool for visualizing the behavior of computer programs. Diagrams can be be used to illustrate both the control structure of the program and the data structures operated upon by the program.; The integration of graphics into two traditional debugging tools, a program stepper and a data structure inspector, is described. Diagrams for both systems are produced using the same hierarchical layout package and make use of similar mechanisms for pruning and elision of the display. As part of the program stepper, an integrated framework has been developed to support both prospective (active) debugging methodologies such as stepping and tracing, and inspective (passive) methodologies such as backtracing and program history. This framework makes use of both hooks in the interpreter and annotations to the code. It allows multiple, arbitrary trace functions to be invoked at each evaluation step, and can be used in both compiled and interpreted code.