XPCE is a toolkit for the development of Graphical User Interfaces
(GUIs). XPCE aims at the rapid development of fast and well-structured
GUIs that are portable accross UNIX/X11 and Windows
(Windows-NT/2000/XP/Vista). It achieves these goals by providing a high
level of abstraction for specifying the GUI, a smooth integration with
Prolog and a set of development tools for browsing the class- and
object-world, with constant access to the documentation and your
XPCE provides for two complementary ways of programming. The built-in
and library classes may simply be used from your Prolog code, or new
XPCE classes can be defined in Prolog using natural Prolog syntax. The
first is often used to `do something small', while the latter provides
excellent structuring for large graphical applications.
Prolog and graphics
Prolog is normally associated with AI, natural language processing,
databases and similar tasks. Interactive programs may have subtasks for
which Prolog is `the tool-to-use'. Such applications are often
implemented in other languages and Prolog is used as an embedded engine
to deal with these tasks.
Prolog is very suitable for building graphical applications. Its
powerful meta-programming capabilities allow for clear and concise
declarations of properties and dependencies in the application. Its
interactive development environment allows for fast prototyping while
the application is running and its automatic data and memory management
makes development fast and reliable.
The two most important drawbacks of Prolog, speed and relatively large
footprint and startup time of very small applications do not matter for
Alternatives for Graphics in Prolog
A couple of approaches have been taken to arrive at graphical
applications that include Prolog. We will review them below.
Since quite some time, Prolog implementations provide powerful
interfaces to C and mechanisms to embed the Prolog system in an
application written in another language. The advantage of this
approach is freedom of choice for your GUI platform. Disadvantage is
that debugging and developing the hybrid environment is generally
cumbersome, and you lose many the advantages of Prolog as a GUI
language outlined above.
- Using a Prolog with built-in graphics
Various Prolog implementations that originate from the PC or Mac
world provide graphical primitives. They are generally non-portable.
- Window-system API
In this approach, the C-API from the window-system is provided from
Prolog with minimal changes. ProXt, distributed with Quintus Prolog,
is an example of this approach. Advantages are that C-programmers
with experience with the API have no trouble using it. The resulting
GUI can be fully compliant to the platform standards. Disadvantages
are the generally low-level of abstraction, while Prolog is not the
most suitable language to add the very CPU-intensive graphics
manipulations required by some applications. APIs designed for C
generally do not provide sufficient data integrity to allow for
complicated modifications at runtime or handling `retry' or `abort'
from Prolog. Finally, the result is not portable.
- External GUI languages
Another common approach is to use an external GUI language such as
Tcl/Tk, and connect the interface to Prolog using pipes. Advantages
and disadvantages are similar to `Embedding'. Intensive
communication between the GUI and Prolog application can cause poor
The dynamic object-system of XPCE provides the ideal platform. It
guarantees an acceptable level of integrity at the data level to
allow for interactive development many Prolog users like so much. It
provides object-oriented programming using a natural syntax from
Prolog, which is very suitable for implementing large GUIs. The
tight integration ensures good performance and finally, abstraction
over the native Window-system API provides applications that are
fully portable between UNIX and PCs.