mac installer package deletes old version of ghc
The Mac installer package on the ghc downloads page (the .pkg, not the .tar.bz2), deletes an existing installation of ghc if it already exists. There is no warning of this behaviour on the download page, or in the installer itself.
At least a warning should be given. Even better, would be to leave the existing installation of ghc intact. Best of all would be for the installer to prompt whether you want to keep the old version or delete it.