[Nix-dev] Re: Building an old GHC for a new Cabal...

[Yury G. Kudryashov]

Jason Dusek wrote:

>   I managed to build GHC 6.12 in Nix on Ubuntu after some help
>   in an earlier discussion on this list. Now, my old Cabal
>   (built through Nix) complains about an invalid package format;
>   this is to be expected if it shells out to `ghc-pkg' to read
>   the package list.
> 
>   When I try to rebuild Cabal, it tries to build it against GHC
>   6.10 instead of 6.12:
>   I do have GHC 6.10 still installed; but why does it not build
>   against the new GHC? Should I delete the `.drv' files? Perhaps
>   these cache something about which GHC to build against?
Deleting .drv files cannot help as new packages do not depend on the old 
state of your ~/.nix-profile etc.

The key warning is "warning: there are multiple derivations named
`cabal-install-0.8.0'; using the first one".

You should use nix-env -i -A ... to select the needed cabal.