post_install() {
    cd /usr/share/doc/ghc/libraries
    /usr/share/doc/ghc/libraries/gen_contents_index
}

post_upgrade() {
    cd /usr/share/doc/ghc/libraries
    /usr/share/doc/ghc/libraries/gen_contents_index
}

post_remove() {
    cd /usr/share/doc/ghc/libraries
    /usr/share/doc/ghc/libraries/gen_contents_index
}

op=$1
shift
$op $*