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