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?