SWI-Prolog and XPCE concentrate on program development by offering an environment that is especially suitable for (rapid) prototyping and debugging. This is supported by the fast incremental SWI-Prolog compiler, the debugger that is capable of debugging compiled code and the built-in commandline editor that supports Prolog specific completion.

Tracing Prolog programs using a traditional 4-port debugger on a terminal is difficult. Large Prolog terms make the trace hard to read, difficult to understand determinism and backtracking, and hard to examine the running clauses. That is why SWI-Prolog provides a source-level debugger.

The graphical tracer

The XPCE-based graphical tracer is the cornerstone of the IDE environment and probably the most useful tool for experts as well as for beginners. It provides three simultaneous views on the status of the environment:

Source code

The current location in the source code is displayed in a window displaying the actual source code or, if the clause is asserted, in a window displaying the decompiled predicate. Colours are used to indicate the status, green meaning normal forward calling, red failure, yellow redo and purple exception.

Bindings

The bindings window displays the binding of the local variables of the selected frame. Variables are indicated by their true name. A concise display, clearly indicating which variables share the same value and removing unbound variables, is provided. Values can be examined by double-clicking.

Stack

The stack-view not only provides the call-stack, but also the choice-point chain. The latter is notably useful to detect (undesired) non-determinism.

The XPCE/SWI-Prolog GUI tracer in action

In this picture, the top-left window indicates the binding. B is not listed as this variable is unbound. The top-right window displays the call-stack. The icon indicates the called predicate is a normal user-defined predicate. The icon indicates the call is deterministic, while the icon indicates the frame has choice-points left. The user can click on any frame to switch both source and bindings window to the clicked frame. The trace was started using

?- gtrace, test_chat.

Starting the source-level debugger

The source-level debugger is controlled by gtrace/0 or gspy/1. These predicates enable the debugger using guitracer/0 and then call trace/0 or spy/1. Like trace/0, gtrace/0 can be called from anywhere in a program to start debugging at a specific location. In the unlikely event that you want to switch back to the traditional tracer in the same session, call noguitracer/0.

?- gspy(dubious/1).
% The graphical front-end will be used for subsequent tracing
Spy point on dubious/1
true.

[debug] ?- go.

Debugging threads

The source-level debugger is also the tool of choice for debugging threads. This is supported by means of tspy/1, tdebug/0 and tnodebug/0. See also the web-server debugging hints