Unable to make args mandatory about cmdargs HOT 4 CLOSED

data Args =
   ...
   | Dump { output     :: String
          , cabalFiles :: [FilePath]
          }
   deriving (Data, Typeable, Show, Eq)


dumpArgs :: Args
dumpArgs = Dump
   { output     = def &= explicit &= typ "FILE" &= name "output" &= name "o"
                      &= help "Save libraries with lower bounds to file, if empty, then it's written to stdout."
   , cabalFiles = def &= args &= typ "CABAL_FILE"
   }


 $> cabal-bounds dump --help
 cabal-bounds dump [OPTIONS]

 Flags:
   -o --output=ITEM            Save libraries with lower bounds to file, if
                               empty, then it's written to stdout.
   -c --cabalfiles=CABAL_FILE
 Common flags:
   -h --help                   Display help message
   -v --version                Print version information
      --numeric-version        Print just the version number