[Haskell-cafe] short licensing question

[Duncan Coutts]

On Mon, 2010-01-11 at 09:30 -0800, Andrey Sisoyev wrote:
> 
> Svein Ove Aas wrote:
> > 
> > In this case, LGPL is a problem. It requires you to offer a way to
> > re-link such binaries against new versions/implementations of the
> > library, which in practice requires it to be either open source or
> > dynamically linked.
> > 
> 
> Don't understand that. Can't we just put a constraint: "our binary works
> only with versions x.x.x-y.y.y of this LGPL licensed library"? The Cabal
> provides a way to put such constraint in build-depends...

It's not referring to totally new versions of a library with a different
API/ABI. The LGPL requires that the user of the program is not prevented
from making some small tweak to the LGPL library and relinking it
against your program. Of course if the user makes it ABI incompatible
then that's their fault.

So the LGPL requires it be relinkable, which you can do by providing a
copy of your application as a .o file which can then be linked against
the LGPL lib to make a final program. Or if you're using shared/dynamic
libs then its easier.

If you want to do this in practise with Haskell, GHC and an LGPL Haskell
package then here's what I recommend:

ld -r -x -o main.o ${objects}

Then make your actual executable as:

ghc main.o -o main -package ${pkg1} -package ${pkg2} ... etc

Then you distribute the executable "main" and also the relinkable form
"main.o" to your users, along with a copy of the LGPL license.

Any user can then perform the last step themselves and if they're really
lucky they might get that to work with a slightly modified version of
the LGPL'ed package. In practise of course this isn't that easy for the
user because GHC does not make it easy to make ABI compatible packages.
However it is my understanding that this procedure will comply with the
LGPL.

Duncan