Visual Metaphors for Understanding Logic Program Execution

A classic notion in logic programming is the separation of logic and control. Logic is for problem solving; control is for directing inference. However, practical experience in the classroom suggests that problem-solving students nonetheless devote much eeort to understanding control issues such as eliminating looping behaviours and improving program eeciency. In the case of Prolog, this requires a clear understanding of the operation of both uniication and backtracking. Students often try to get this understand by tracing executions, but the common four-port debugger used in Prolog is not as helpful as it could be. In particular, it provides information in low bandwidth textual form. This paper describes a new visualization system for logic programming that uses colour tagging to trace uniication through the Prolog proof tree. A user can dynamically \tag" a term or subterm with a colour that is immediately propagrated through the displayed tree. The colour is also propagated through the proof tree on subsequent execution steps. This \colour uniication" has an interesting relationship with usual Prolog uniication. Initial experiences reveal several interesting visual metaphors that assist understanding of logic program execution. Experience to date also suggests new directions for visualization of logic programs.