Chris Taylor, Benedict du Boulay, Mukesh Patel
This document constitutes an outline specification for a new tracer for Prolog, the design of which has been guided by an evaluation of the relative strengths and weaknesses of a number of existing Prolog tracers. The new tracer, known as the 'Textual Tree Tracer' (or 'TTT' for short) will produce a 'sideways tree' representation of the execution of a goal, using only textual output, i.e. it will not require the use of any specialised graphics. Its key features include the following: a compact and yet very informative basic form of output, which distinguishes matching events, and several different goal failure modes; clear display of the structure of computation and the flow of control, via the use of a tree representation; extensive use of default controls to limit the quantity of trace output produced; the facility of retrospective inspection of earlier parts of the trace, in order to obtain more detailed information; and a specialised 'database window' which facilitates correlation of the trace with the source code, and shows dynamically any changes to the database resulting from the assertion or retraction of clauses.
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