Re: [PATCH 02/14] gnu: Add qtimageformats.

[Andreas Enge]

On Fri, Jun 10, 2016 at 12:30:45PM -0400, Leo Famulari wrote:
> On Fri, Jun 10, 2016 at 03:10:13PM +0300, Efraim Flashner wrote:
> > +(define-public qtimageformats
> > +  (package (inherit qtsvg)
> These patches make me realize my understanding of package inheritance is
> unclear.
> 
> If qtsvg is updated, but no other changes are made, will qtimageformats
> (and all the other packages that inherit from qtsvg) need to be rebuilt
> as well?
> 
> Looking at `guix refresh -l` and `guix gc --references` suggests the
> answer is "no".

My understanding is that not the package definition, but the derivation
counts. So for instance, if the version is increased in qtsvg, this new
version also appears in qtimageformats, but if it is set back there, not
even the package definition has changed.

So this should be safe; in a sense, the inheritance from qtsvg creates a
"qt build system"; since this is so far only used internally to build qt
itself, I think it is okay to not expose it to the exterior world.

However, another question: Are all the inputs for qtbase needed for all the
other modules, too, or could the list be trimmed?

Andreas