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From Ralph Goers <ralph.go...@dslextreme.com>
Subject Re: GPL exception in SWI-Prolog
Date Wed, 18 Feb 2009 14:50:00 GMT
Off hand, I'd say that this resembles Java. You have to run in the  
Java VM and use Java libraries. Java is under the GPL with a similar,  
if not identical, exception clause.  I would guess that since the end  
user would have to obtain SWI-Prolog separately - just as they do with  
Java - that an application that requires the platform would be fine.  
This is also similar, but not identical, to some of the questions that  
have been answered regarding add-ons for JBoss which were also approved.

Ralph

On Feb 18, 2009, at 2:55 AM, Jukka Zitting wrote:

> Hi,
>
> I have a project in Apache Labs where I'm planning to start using
> Prolog as the language and SWI-Prolog [1] as the base platform.
>
> The language implementation in SWI-Prolog is LGPL code, but that
> shouldn't affect any of my code as long as I write standard Prolog.
> AFAIUI the SWI-Prolog runtime would qualify as a system dependency.
>
> The trickier part are the highly useful libraries included in
> SWI-Prolog. They are mostly GPL (v2 or higher) code amended with the
> special exception shown below:
>
> ================================================================
> The Prolog parts of SWI-Prolog are distributed   under  the GPL with  
> the
> following additional clause. This  is  due  to   the  fact  that   
> it  is
> technical impossible to satisfy the LGPL   demand  that the LGPL'ed  
> code
> must be replaceable in the final product.
>
> From the practical point of view you can use the Prolog libraries in
> propriertary code, provided you distribute the source to _changes_ you
> make to the libraries. This has been debated with Richard Stallman.
>
> SWI-Prolog extra clause:
> ------------------------
>
>    As a special exception, if you link this library with other files,
>    compiled with a Free Software compiler, to produce an executable,  
> this
>    library does not by itself cause the resulting executable to be  
> covered
>    by the GNU General Public License. This exception does not however
>    invalidate any other reasons why the executable file might be  
> covered by
>    the GNU General Public License.
> ================================================================
>
> They seem to have borrowed that extra clause from GCC, where the
> compiler automatically embeds the GPL-covered libgcc runtime library
> in the produced executable. In that case the source code that is
> compiled never directly references the GPL code.
>
> However, in my case I would be directly calling these GPL libraries
> from my code, so I'm not sure how well the above exception protects
> me. The "practical point of view" sentence seems to indicate that
> their intention is that I can distribute and license my code as I
> like, but I'm a bit concerned about this interpretation.
>
> So my question is: Can an Apache project use SWI-Prolog?
>
> [1] http://www.swi-prolog.org/
>
> BR,
>
> Jukka Zitting
>
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