Description: I suggest adding something like this to the post-upgrade of ghc. It means that the libraries that are still compatible will still be available.

This should work, but test it first :)

post_upgrade() {
for HS_DIR in /usr/share/haskell/* ;
do
${HS_DIR}/register.sh --force
done
}

It wouldn't hurt to put that function as the post_install() either, as some libraries perhaps mistakenly do not require ghc.

Pacman should warn and remove/upgrade haskell libraries that are no longer compatible, so the --force option is used without breaking anything. (alphabetical order of the loop will have broken dependencies that get resolved by the end).