SWI-Prolog offers two debuggers. One is the traditional text
console-based 4-port Prolog tracer and the other is a window-based
source level debugger. The window-based debugger requires XPCE
installed. It operates based on the prolog_trace_interception/4
hook and other low-level functionality described in chapter
B.
Window-based tracing provides a much better overview due to the
eminent relation to your source code, a clear list of named variables
and their bindings as well as a graphical overview of the call and
choice point stack. There are some drawbacks though. Using a textual
trace on the console, one can scroll back and examine the past, while
the graphical debugger just presents a (much better) overview of the
current state.
Whether the text-based or window-based debugger is used is controlled
using the predicates guitracer/0
and noguitracer/0.
Entering debug mode is controlled using the normal predicates for this: trace/0
and
spy/1.
In addition, PceEmacs prolog mode provides the command
Prolog/Break at (Control-c b) to insert a
break-point at a specific location in the source code.
The graphical tracer is particularly useful for debugging threads.
The tracer must be loaded from the main thread before it
can be used from a background thread.
- guitracer
- This predicate installs the above-mentioned hooks that redirect tracing
to the window-based environment. No window appears. The debugger window
appears as actual tracing is started through trace/0,
by hitting a spy point defined by spy/1
or a break point defined using the PceEmacs command Prolog/Break
at (
Control-c b).
- noguitracer
- Disable the hooks installed by guitracer/0,
reverting to normal text console-based tracing.
- gtrace
- Utility defined as
guitracer,trace.
- gdebug
- Utility defined as
guitracer,debug.
- gspy(+Predicate)
- Utility defined as
guitracer,spy(Predicate).
