[Haskell-cafe] Multiple versions of a cabal package Or what's the right way to update?

[Duncan Coutts]

On Sun, 2010-03-14 at 17:50 +0100, david fries wrote:
> Dear Caféistas
> 
> I desperately need your collective knowledge I have been working on
> porting the haskell-platform to the FreeBSD operating system for a while
> now and some versioning issues have come up. 
> 
> FreeBSD is still running on GHC-6.10.4 (6.12 is in the works). The ghc
> package contains a couple of libraries that ghc is built against and
> requires to run. For instance, network-2.2.1.2 is one of them.

You mean the FreeBSD ghc port includes a bundle of extra libraries
including network? Note that the source ghc package does not include
network. The network package an extra library that should be packaged
separately.

> The problem is that the Haskell Platform requires version 2.2.1.4.

Yes, the platform is free to pick that version precisely because it is
not a core package that is distributed with ghc.

> Under normal circumstances we would port network-2.2.1.4 to FreeBSD and
> then specify it as a dependency of the Platform port. But then we end up
> with two (exposed) versions of network installed on the system. 

Sounds like the solution is to remove the network package that is
bundled in the FreeBSD ghc port. I understand it may be impractical for
you to change it at this stage and that perhaps such unbundling has to
wait until you package ghc 6.12.

> In a perfect world, I'd be able to install the required HP dependencies
> without tripping ghc up. Then we could switch over to the platform as a
> foundation of GHC-6.12.
> 
> I have lots of questions. Is it smart to have multiple installed
> versions of the same library? Up until now, we could avoid such
> situations.

It is generally better for distributions not to bundle multiple versions
of a package. For the Haskell Platform we make sure that this is
possible. We never use a different version of a core library than those
that come with the ghc version that HP release targets.

> If it is perhaps a bad idea to update certain "core" libraries, is there
> a way to declare them as such in the cabal file?

Core packages are those that actually come with ghc. All other Haskell
packages should get their own distro package. Then the HP meta-package
depends on the exact versions as appropriate for that HP release.

Duncan