[Nix-dev] Python 2 versus 3

[Eelco Dolstra]

Hi,

Andres Loeh wrote:

> Probably, renaming to python3 is the most pragmatic option available.
> 
> But in general, I think this is due to the actually rather
> short-sighted assumption that upgrades are desired from a package
> pkg-m to a package pkg-n if an only if m<n.
> 
> This is, in general, not true. The python situation is a case where
> installing python-3 over python-2 automatically is almost never
> desired. However, there are other situations where the assumption is
> incorrect as well, the most frequent being renames of packages, but
> also random changes in the version numbering scheme. In such cases, we
> might suddenly want to upgrade from pkg-17.3 to gkp-4.5.

This is possible by marking the higher version as low priority (lowPrio), which
should cover most use cases.  The "priority" meta-attribute here plays a role
similar to the "epoch" in RPM packages.

> It would be absolutely amazing if the Nix expression language would
> just let us specify how the directed set of version upgrades should
> look like. 

Add a "meta.versionComparator" attribute specifying a comparison function? :-)

> I.e., we'd need a way to specify that a package constitutes
> an upgrade for another even if the names are unrelated.

I've been wanting to add a meta-attribute to support package renames for a long
time.  E.g. to rename the current "MPlayer" package to "mplayer", we'd change
the name attribute to "mplayer-<version>", and add a meta attribute like this:

  meta.upgrades = [ "MPlayer" ];

-- 
Eelco Dolstra | http://www.st.ewi.tudelft.nl/~dolstra/