On 8/23/2010 4:53 PM, Bert Freudenberg wrote:
> On 24.08.2010, at 01:44, Levente Uzonyi wrote:
>> But that doesn't work. For example 311Deprecated depends on Traits, but Traits is at the end of the list while 311Deprecated is the first non-dummy package.
>
> Then, obviously, the update map is wrong :)

No, it's not. The update map isn't ordered to allow reloading all the 
packages after they've been unloaded. The update is ordered to allow 
loading all the updates *after the previous update map*. In fact, it can 
and does happen that it switches the ordering of two packages between 
updates. For example, consider moving class Process from Kernel to 
System. To do this you would need the System package to be loaded before 
the Kernel. But if you would load these from first principles, almost 
certainly you would load Kernel before System.

So the order in the update map is fairly irrelevant for the order which 
you would use to load all these packages from first principles. The 
update map simply cannot be used for this purpose.

The real question here is if we should allow updating a subset of 
packages simply by skipping over the packages you haven't present in 
your image? This could be done but would come at the price that we could 
no longer add new "required" packages and let people fetch them 
automatically (because they wouldn't be present in the image they would 
not be loaded).

In addition, allowing to update subsets would require us to be *much* 
more careful with changing package dependencies - as you can see from 
the failing package dependency tests in the current trunk the 
dependencies *do* change but if we want to allow updating subsets then 
we have to be "dependency clean" in every single update.

The latter is really the reason why I'm not in favor of allowing this. 
Right now we have a solution that basically says when you update 
incrementally, you update an image with all the packages present which 
keeps it nice and simple. At the release points, we spend the additional 
time to make sure all the dependencies are in the right place and we can 
unload things but we don't have to worry about accidentally introducing 
a dependency in the middle and getting angry messages from people whose 
system we broke since they only update a subset of the packages.

Makes sense? If not, then let's discuss ways in which we can improve things.

Cheers,
   - Andreas