SWI-Prolog 7.2.0 is available for download. SWI-Prolog version 7 is a
major release, both for new functionality and because it is not fully
compatible with version 6. First, the highlights for the new
functionality:
SWI-Prolog 7 at a glance
- The new dict type and syntax provides both time and space efficient
name-value maps with a pleasant syntax:
tag{key1:value1, key2:value2, ...}
Fields can be accessed using functional notation, as in
writeln(Dict.key1).
- As a consequence, it was necessary to replace the list constructor
.(H,T) by the (also in use by Mercury) '[|]'(H,T). That may seem
drastic, but in practice affects only a few programs, notably doing
functor(Term, F, A) on lists and then selecting further processing
on F == '.', A == 2. To turn lists more into a special construct,
[] is still the empty list, but no longer the same as '[]', i.e.,
[] is not at atom.
- The syntax "..." is now mapped to strings. Strings are compatible
with ECLiPSe (thanks to Joachim Schimpf for all the discussions).
Traditional code-lists are constructed using
.... The flags
double_quotes and back_quotes control this behaviour.
- Thanks to Torbjorn Lager, we have "Pengines", Prolog engines on the
web. This provides a generic API to talk comfortably to a Prolog
server from JavaScript and other Prolog instances. It enabled
SWISH, SWI-Prolog in your browser (http://swish.swi-prolog.org)
as well as http://lpn.swi-prolog.org (Learn Prolog Now! with
embedded SWISH).
- Markus Triska added
clp(b), the boolean constraint solver, improved
his clp(fd) and was before several enhancements to the toplevel
dealing with constraints.
- Thanks to Matt Lilley and Mike Elston, there is CQL, a DSL
(Domain Specific Language) for dealing with SQL. CQL is
developed in an environment where complex SQL databases are
the norm and therefore supports a large subset of SQL and
can deal with tables with thousands of columns and other
stuff that the ocasional SQL user won't expect.
- Matt Lilley improved networking support significantly, including
much better support for SSL and general support for both HTTP and
SOCKS proxy servers.
- Many people have provided add-ons (packs) that rely on SWI-Prolog
version 7. See http://www.swi-prolog.org/pack/list
There are no big changes to the Prolog engine. Notably SWISH has proved
to be a great honeypot for finding ways to crash the system. Many of
these have been fixed and notably stack overflow handling is now much
more robust. Paulo Moura included an extensive portable test suite in
Logtalk which pointed out various small errors. Some of these are still
present, none deemed urgent.
Porting to SWI-Prolog 7
Quite a lot of programs will run unmodified. Notable programs that
extensively use DCGs (grammar rules, -->) may not run unmodified. See
http://www.swi-prolog.org/pldoc/man?section=ext-dquotes-port
for dealing with this. The changes are typically rather straigthforward
and not hard to debug.
The modified list notation does not often lead to portability issues,
but portability issues are typically harder to spot. A good step is to
run the following after loading your program:
?- explain(.).
This lists clauses in the program in which '.' appears. You can of
course also search the sources, but this is relatively hard because the
'.' is used in many contexts (end-of-term, =.., comments, etc).
As a work-around, you might be able to start SWI-Prolog as below. This
restores the traditional list standard and disables the Dict.key
functional notation. It may work well for you if you merely rely on the
core Prolog engine. A growing number of the libraries depends on the new
features and will thus become unusable in this mode.
swipl --traditional
What is next?
As for 7.2.x, I assume we will see some patches dealing with regression
issues. The 7.3.x series are likely to concentrate on strengthening web
functionality and using Prolog as data querying and transformation tool.
Given contributed and demand driven development, it is hard to predict
what will happen. You can influence the process, both by contributing
and by providing funds, either directly or by seeking partnership in
research projects.
Acknowledgements
Many people have made SWI-Prolog version 7 possible. Some are already
mentioned above. I think Anne Ogborn deserves special attention for her
work helping people, getting the Oregon State University Open Source Lab
to host us.swi-prolog.org which provides a backup for the servers
running at the VU University in Amsterdam and finally for advertising
Prolog and Pengines in many places.
Enjoy --- Jan
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