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New stable version: SWI-Prolog 7
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Picture of user Jan Wielemaker.

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

  • 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


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.


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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