[Haskell-beginners] cabal and dependencies

[Andres Loeh]

Hi Manfred.

> I did 'ghc-pkg check' which showed nothing. Fine!
>
> Then I did 'cabal install gitit' and after successfully installing gitit
> I ran 'ghc-pkg check' again and got:

[lots of broken packages deleted]

Yes. That can happen with current cabal-install.

> Why is this happening?

The dependency resolution algorithm will try to make a consistent
install plan that allows gitit to be run. However, in order to achieve
that, it may decide that it's necessary to reinstall some existing
packages in a new configuration (with other dependency or flag
settings). Such reinstalls will then overwrite the previous package in
the store, and if that previous package was depended on by something
else, these other things will break.

While ghc-pkg currently allows both foo-1.0 and foo-2.0 to be in the
store at the same time, it doesn't allow two different configurations
of foo-2.0 in the store at the same time.

> Could I prevent it somehow?

Currently, you can only discover the danger of this happening in
advance: You can run "cabal install --dry-run -v gitit", and then
you'll get an annotated install plan printed out. If you notice lines
like

HTTP-4000.1.1 (reinstall) changes: network-2.3.0.2 -> 2.2.1.10

then you are in danger. If you only see lines that say "(new package)"
or "(new version)", you're safe.

In the new dependency resolver I'm currently implementing for Cabal,
which you can preview at

  http://darcs.haskell.org/cabal-branches/cabal-modular-solver/

there will be an option "--avoid-reinstalls" that tries to avoid
reinstalls (but perhaps at the cost of selecting older versions of
certain packages to avoid them). Also, the next release of Cabal will
probably add a warning if an install plan contains reinstalls.

The proper solution would be to lift the limitation in the GHC package
store and allow arbitrarily many variants of packages to coexist
peacefully, so that even a "reinstall" won't have to remove an old
version. I hope we're getting around to implement this sooner or
later.

Cheers,
  Andres