pre_upgrade() {
  if [[ ${1%-*} != ${2%-*} ]]; then
    [[ -d /usr/share/haskell ]] && find /usr/share/haskell -maxdepth 2 -name 'unregister.sh' -exec {} \;
  fi
}

post_upgrade() {
  if [[ ${1%-*} != ${2%-*} ]]; then
    echo '==> All cabalized packages need to be reinstalled now.'
    echo '==> See /usr/share/haskell/ for a tentative list of affected packages.'
  fi
}