XPCE is an object oriented system written in C that provides a rich set if built-in classes to build graphical user interfaces. Built-in classes can be instantiated and controlled from Prolog. It is also possible to subclass the built-in classes from Prolog. XPCE is used to build the SWI-Prolog development tools. Notably

• Epilog: the Prolog console.
• PceEmacs: a GNU-Emacs clone written in XPCE+Prolog.
• A graphical front-end for the Prolog debugger.
• The profiler analyses CPU or wall-time usage details on your program.
• The cross-referencing tool finds relations between source files.
• The navigator show files, their (loaded status) and defined predicates.
• The thread monitor inspects running threads and their status.
• A viewer for debug messages.

• XPCE on SDL+Cairo+Pango