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.

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

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

That should be sufficient.

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.

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.

Here's a shell script I use to uninstall a package. It supports multiple installed versions of GHC and also wipes relevant files (but is provided without warranty, don't blame me if you hose your installation!)

#!/bin/bash -eu
# Usage: ./uninstall.sh [--force | --no-unregister] pkgname-version

# if you set VER in the environment to e.g. "-7.0.1" you can use
# the ghc-pkg associated with a different GHC version
: ${VER:=}

if [ "$#" -lt 1 ]
then
        echo "Usage: $0 [--force | --no-unregister] pkgname-version"
        exit 1
fi

if [ "$1" == "--force" ]
then force=--force; shift; # passed to ghc-pkg unregister
else force=
fi

if [ "$1" == "--no-unregister" ]
then shift # skip unregistering and just delete files
else
        if [ "$(ghc-pkg$VER latest $1)" != "$1" ]
        then
                # full version not specified: list options and exit
                ghc-pkg$VER list $1; exit 1
        fi
        ghc-pkg$VER unregister $force $1
fi

# wipe library files
rm -rfv -- ~/.cabal/lib/$1/ghc-$(ghc$VER --numeric-version)/

# if the directory is left empty, i.e. not on any other GHC version
if rmdir -- ~/.cabal/lib/$1 
then rm -rfv -- ~/.cabal/share/{,doc/}$1 # then wipe the shared files as well
fi