{
  fail,
  haskellPackages,
  nixpkgs1803,
  runCommand,
  unpack',
}:

with builtins;
{ dir, field }:
import (
  runCommand "cabal-field-${field}.nix"
    {
      inherit dir field;
      buildInputs = [ fail ];
    }
    ''
      set   -e
      set   -o pipefail
      shopt -s nullglob

      cd "$dir" || fail "Couldn't cd to '$dir'"

      VAL=""
      for F in *.cabal
      do
        LINES=$(grep -i "^$field\s*:" < "$F") ||
          fail "No '$field' lines in '$F'"

        VAL=$(echo "$LINES" | head -n1       |
                              cut -d ':' -f2 |
                              sed -e 's/\s//g')
      done
      [[ -n "$VAL" ]] || {
        echo "Couldn't find Cabal field '$field' in '$dir'" 1>&2
        exit 1
      }
      echo "\"$VAL\"" > "$out"
    ''
)