[eigen] About the BSD license

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I thought that I'd keep the list informed of what happened over the
past few days.

I've been discussing licenses with VTK developers (BSD license) who
are considering Eigen,

According to them, basically, proprietary software companies won't
accept to depend on LGPL software. Obviously I don't agree with them,
since Eigen _is_ used by several such companies. And moreover there
are plenty of examples of large industrial project that have LGPL
components (for example the core components of WebKit, WebCore and
JavaScriptCore, are LGPL, and that doesn't prevent Apple from
accepting that deep into its software stack).

The interesting thing that came out of this is that it forced me to
look deeper into the LGPL (as a result the FAQ was updated) and I am
now entirely certain that in the special case of a headers-only like
Eigen, the LGPL license is equivalent to BSD + "no proprietary forks".

So the only change that the BSD license would bring would be:
1) allowing proprietary forks.
2) having a simpler license.

1) is simply not wanted. I had a conversation with Gael about this and
we're sure that we don't want to allow proprietary forks.

2) would be nice, but can't be as important as BSD fans have it sound
like. Licenses are complex, that's life. I sure would like to have a
simpler license, so I wrote to a FSFE guy about that, but so far there
doesn't seem to exist any simple weak copyleft license. And simplicity
is not, in my opinion, important enough to give up our weak-copyleft.

I'll add that the typical succesful BSD-licensed project doesn't look
like Eigen. Eigen is, so far, a 100% volunteer project. Even if it got
support from a company in the future, it is a hard requirement that it
remains forever a friendly place for volunteers to contribute. Some of
us have expressed that they may not want to contribute to a
BSD-licensed project, out of fear that their work may be misused by a
third party (proprietary forks). I'm not sure how much I share this
concern, but it is clearly a legitimate concern. That's got to have
something to do with why successful BSD-licensed projects are
typically developed by full-time paid developers.

Conclusion: move along, nothing to see here. Looks like the LGPL is
the right match for us and for our users, and users who are interested
enough in Eigen should be able to overcome a little phobia of matches
of the .*GPL regular expression.


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