XPCE/SWI-Prolog --- Graphical tracer front-end

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 very fast incremental SWI-Prolog compiler, the debugger that is capable of debugging compiled code, GNU-readline based command-line editor and XPCE's support for dynamic modification of the running environment.

Though sufficient to support expert (Prolog) users, the current support is not satisfactory for novice users raised with a mouse.

In addition, even being a Prolog expert, tracing Prolog programs using a traditional 4-port debugger in a terminal is difficult. Large Prolog terms make the trace hard to read, it is difficult to understand determinism and backtracking and hard to examine the running clauses.

The GUI tracer

The XPCE/SWI-Prolog GUI-Tracer is the cornerstone of the future 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 forwards calling, red failure, yellow redo and purple exception.
Bindings
The bindings window displayes the binding of the local variables of the selected frame. Variables are indicated by their true name. A consise display, clearly indicating which variable 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.

Starting the GUI-Tracer

The tracer is loaded using the (autoload) predicate guitracer/0. Executing this installs hooks into the SWI-Prolog tracer that will cause any normal trace actions to be redirected to the GUI tracer. The interface appears whenever the tracer is started: using trace/0, spy/1 or by putting a call to trace/0 somewhere in your source-code: 

?- guitracer.
% The graphical front-end will be used for subsequent tracing

Yes
?- spy(dubious/1).
Spy point on dubious/1

Yes
[debug] ?- go.

Home This page is maintained using the chpp macro language