SWI-Prolog 7.4.2 Reference Manual: Section 3.5

## 3.5 The Graphical Debugger

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.

### 3.5.1 Invoking the window-based debugger

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 particulary 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)`.