There are a number of reasons why it might be better to choose a
commercial, or another free, Prolog system:
SWI-Prolog comes with no warranties
Although the developers or the community often provide a work-around or
a fix for a bug, there is no place you can go to for guaranteed support.
However, the full source archive is available and can be used to compile
and debug SWI-Prolog using free tools on all major platforms. Users
requiring more support should ensure access to knowledgeable developers.
Performance is your first concern
Various free and commercial systems have better performance. But,
`standard' Prolog benchmarks disregard many factors that are often
critical to the performance of large applications. SWI-Prolog is not
good at fast calling of simple predicates and if-then-else selection
based on simple built-in tests, but it is fast with dynamic code,
meta-calling and predicates that contain large numbers of clauses. Many
of SWI-Prolog's built-in predicates are written in C and have excellent
You need features not offered by SWI-Prolog
Although SWI-Prolog has many features, it also lacks some important
features. The most well known is probably tablingFreire et
al., 1997. If you require additional features and you
have resources, be it financial or expertise, please contact the
On the other hand, SWI-Prolog offers some facilities that are widely
appreciated by users:
SWI-Prolog provides a good command line environment, including `Do What
I Mean', autocompletion, history and a tracer that operates on single
key strokes. The system automatically recompiles modified parts of the
source code using the make/0
command. The system can be instructed to open an arbitrary editor on the
right file and line based on its source database. It ships with various
graphical tools and can be combined with the SWI-Prolog editor, PDT
(Eclipse plugin for Prolog) or GNU-Emacs.
Even very large applications can be loaded in seconds on most machines.
If this is not enough, there is the Quick Load Format. See qcompile/1
Transparent compiled code
SWI-Prolog compiled code can be treated just as interpreted code: you
can list it, trace it, etc. This implies you do not have to decide
beforehand whether a module should be loaded for debugging or not, and
the performance of debugged code is close to that of normal operation.
Source level debugger
The source level debugger provides a good overview of your current
location in the search tree, variable bindings, your source code and
open choice points. Choice point inspection provides meaningful insight
to both novices and experienced users. Avoiding unintended choice points
often provides a huge increase in performance and a huge saving in
SWI-Prolog offers an execution profiler with either textual output or
graphical output. Finding and improving hotspots in a Prolog program may
result in huge speedups.
SWI-Prolog can easily be integrated with C, supporting non-determinism
in Prolog calling C as well as C calling Prolog (see section
11). It can also be embedded in external programs (see
section 11.5). System
predicates can be redefined locally to provide compatibility with other
Robust support for multiple threads may improve performance and is a key
enabling factor for deploying Prolog in server applications.
SWI-Prolog ships with many extension packages that provide robust
interfaces to processes, encryption, TCP/IP, TIPC, ODBC, SGML/XML/HTML,
RDF, HTTP, graphics and much more.
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