cabal install cabal-install with the latest HP on Mac OS X

[Claus Reinke]

>> To my surprise, cabal install began installing
>> an *older* version of itself, together with older
>> versions of dependent libraries to support that.
> 
> This is because the "default" version of cabal-install on hackage
> remains at 0.8.x. As soon as I bump this to 0.10.x then all existing
> users will get prompted to upgrade. I'd like a bit of confidence that
> this will not break everyone's existing setup, so I've not done it
> yet.
..
> This is part of the problem that cabal does not understand that there
> can be two instances of the same version of a package registered at
> once. We will shortly be working on a new solver that does understand
> this.

Others, myself included, have been running into similar 
problems with "cabal install cabal-install" wrecking their 
installed core libs. I was surprised that "install" would do
that.

Handling of multiple installed package instances will be a 
more general solution, but it would be good to know the 
timeline for that feature. If it isn't imminent, then I would 
recommend a short-term safety line:

1. cabal figures out which packages are core ghc dependencies
2. cabal never touches those packages (unless someone reenables
    upgrade, and uses the --force and --yes-really-shoot-myself-
    in-the-knee options;-)

Claus

http://hackage.haskell.org/trac/ghc/ticket/4510#comment:4