How can I uninstall a version of a Cabal package?

Happstack Lite is breaking on me because it's getting blaze-html version 0.5 and it wants version 0.4. Cabal says that both versions 0.4.3.4 and 0.5.0.0 are installed. I want to remove the 0.5.0.0 and use just the older version. But cabal does not have an "uninstall" command, and when I try ghc-pkg unregister --force blaze-html, ghc-pkg says my command has been ignored.

What do I do?

UPDATE: Don't believe it. Although ghc-pkg claims to ignore the command, the command isn't ignored. And with Don Stewart's accepted answer you can remove exactly the version you wish to eliminate.

Solution 1:

You can ghc-pkg unregister a specific version, like so:

$ ghc-pkg unregister --force regex-compat-0.95.1

That should be sufficient.

Solution 2:

If you are outside a sandbox:

ghc-pkg unregister --force regex-compat-0.95.1

If you are inside a cabal sandbox:

cabal sandbox hc-pkg -- unregister attoparsec --force

The first -- is the argument separator for hc-pkg. This runs ghc-pkg in a sandbox aware manner.

Solution 3:

There is also the cabal-uninstall package which provides a cabal-uninstall command. It unregisters the package and deletes the folder. It is worth mentioning though that it passes --force to ghc-pkg unregister so it can break other packages.