The PDT lets you use all the built-in tracing facilities of SWI-Prolog. The SWI-Prolog tracer is invoked by calling trace before the goal you want to trace. For instance, if you want to trace the execution of p(X,Y,Z) you call:

?- trace, p(X,Y,Z).

What you get as a result of such a call depends on the enabled tracing mode:

• gui tracer (recommended, enabled by default).
• console tracing.

In console tracing mode, each 'RETURN' shows you the next “step” of the logic program's execution. For a full listing of possible console tracing actions hit 'h' and 'RETURN'. Here is how console tracing looks after a series of “step” commands:

A note to Mac users To use the GUI tracer you need to have a X11 server running. On the Mac you need to start it explicitly, it is not on by default.

The GUI tracer is enabled / disabled by using the GUI tracer toggle button in the Prolog Console Toolbar:

Alternatively, the GUI tracer is enabled / disabled by typing guitracer / noguitracer in the Prolog Console ¹⁾:

If GUI tracing is enabled, the query

 ?- trace, p(X,Y,Z).

opens the SWI-Prolog GUI tracer as a new window ( A note to Windows users On Windows the GUI tracer window is intially iconified. Look for its symbol in the task bar and expand it to see the view shown below.) ) that

1. Shows bindings of variables
2. Shows the call stack of the running Prolog process
3. Shows the contents of the prolog source file and the executed goal.
4. Shows the type of call: initial call (green), redo (yellow), fail (red) and success (blue).

The tool bar of the SWI-Prolog GUI tracer offers some useful shortcuts

The buttons in (1) provide - from left to right - the option to show the unifier for the current goal, step into the current goal's execution, step over it or finish the selected goal. The buttons in (2) provide - from left to right – the option to step to the next spy point, add spy points, set a stop at the caret and to remove spy points. For details on other buttons see the respective tool tip.

If a variable's value is too long to fit in the bindings frame (1) you can view it in full in a separate window by double clicking the variable in (1)

ViaEdit > Breakpoints you can define spy points on predicates occurring in the program (by entering the predicate's name/arity and then pressing the left-most button – see red arrow).