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From Jukka Zitting <jukka.zitt...@gmail.com>
Subject GPL exception in SWI-Prolog
Date Wed, 18 Feb 2009 10:55:34 GMT

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/


Jukka Zitting

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