A visual debugger for pure Prolog

Abstract

This work involves the design and coding of an interpreter for pure Prolog and building a visual debugger for it. Most of the available Prolog interpreters contain some tracing facilities. They do not incorporate, however, a comprehensive visual debugger. The interpreter performs the operations of parsing, unification, resolution, and search in a state-space representation of the Prolog program. The visual debugger incorporates the graphical visualization and the manipulation of the SLD resolution tree. The user visualizes the execution of a pure Prolog program and interacts with the program inside a windowing environment. The program execution may be viewed without interruption or the execution can be stopped at any moment in time. At this point the “snapshot” can be scrutinized with the help of break-points and data displays. This software aims itself to those who wish to observe the actual process of predicate unification, substitution, resolution and goal matching in a Prolog program and to visually interact with the interpreter using a highly friendly and pleasing user interface. An advanced feature, referred to as debugging on the tree, provides the user with the ability to insert break-points directly on the SLD tree, to choose the path of execution, and change the search mode. The resultant search algorithm can be a mixture of depth-first and breadth-first search, avoiding infinite search paths.