Outline Proposal for a Prolog `Textual Tree Tracer' (TTT)

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 clause 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.