import Data.Ord
import Data.List

-- | Sorting a List by frequency of their Elements
sortByFreq :: Eq a => [a] -> [a]
sortByFreq lis = sortF $ getFreq lis $ map (\x -> (x, 0)) $ nub lis
  where getFreq [] freq = freq
        getFreq (x:xs) freq = getFreq xs $ map (\(elem, freq) ->
                                                 if x == elem
                                                 then (elem, freq + 1)
                                                 else (elem, freq)) freq
        sortF = sortBy (comparing snd)