[Haskell-beginners] Regarding defining cases exhaustively

Hi all, I have this code myMaximumBy :: (a -> a -> Ordering) -> [a] -> a myMaximumBy _ [] = undefined myMaximumBy _ (x:[]) = x myMaximumBy f (x:xs) = case f x maxRestOfList of GT -> x _ -> maxRestOfList where maxRestOfList = myMaximumBy f xs whereby the case myMaximumBy _ [] will never happen because case myMaximumBy _ (x:[]) = x prevents that from happening. However i will still like to define it to keep -Wall warning from complaining. Is using undefined acceptable or is there a better way? -- Best Regards, Boon Hui