PROLOG PROGRAM VISUALIZATION
This chapter will describe the Prolog Program Visualization Laboratory (PPVL) which provides the first opportunity to study a number of fully implemented tracers within the same environment. PPVL incorporates four SV designs for Prolog: Spy (Byrd, 1980), PTP (Eisenstadt, 1984) , TPM (Eisenstadt and Brayshaw, 1990; Brayshaw and Eisenstadt, 1991) and TTT (Taylor et al., 1992). A description will be given of how each presents the execution of Prolog, and the design issues that the chosen mode of presentation entails. A summary of the four SVs will then be given incorporating general predictions as to how the four SVs could be expected to fare.
PPVL provides an experimental laboratory on which to base a systematic comparison of Prolog tracers. PPVL is implemented in MacPrologTM version 4.5 running on MacintoshTM system 7.1. PPVL provides common interface and navigation as far as is possible, so differences in performance due to the ease of use of different interface technologies are minimised. PPVL also internally records all user activity at the terminal.
These four tracers were chosen for implementation because they illustrate many of the important design issues in the presentation of information. Two particularly important issues which they address are the debate as to how much information should be presented explicitly and implicitly; and also how should a trade off be made between providing a