Prelude> myMaximumBy (_ _ -> GT) [1..10]
1
Prelude> myMaximumBy (_ _ -> LT) [1..10]
10

myMaximumBy :: (a -> a -> Ordering) -> [a] -> a
myMaximumBy f (x:xs) = foldl doF x xs
  where
    doF a b
     | f a b == GT = a
     | otherwise   = b