Простая задачка из раздела С++: напечатать все буквы(по одному разу) из массива строк в алфавитном порядке

import Prelude hiding (lookup)  
import Data.Map hiding (foldl,filter, map)
 
calculate :: (Ord element, Num numberOfElements) => 
    [element] -> [(element, numberOfElements)]
calculate = toList.foldl (\acc x ->
    case x`lookup`acc of
        Nothing -> insert x 1 acc
        Just i  -> insert x (i + 1) acc
    ) empty
 
-- И само решение
task :: [String] -> String
task = map fst.calculate. concat

inc :: Int -> [Int] -> [Int]
inc n acc | (n==0)    = (head acc)+1 : (drop 1 acc)
          | (n==255)  = (init acc) ++ [(last acc)+1]
          | otherwise = h ++ [p] ++ t
            where h=take n acc
                  p=(acc !! n)+1
                  t=drop (n+1) acc 
 
accept :: String -> [Int] -> [Int]
accept s arr = foldl (\ acc x -> inc (ord x) acc)  arr s
 
task :: String -> String
task s = foldl (\ r n -> if ((q !! n) /= 0) then r ++ [chr n] else r) "" [0..255]
         where q=accept s (take 256 $ repeat 0)
 
Main> task "abrakadabra"
"abdkr"

import Data.List
 
uniq :: String -> String
uniq = sort . foldl (\ acc x -> if (x `elem` acc) then acc else x:acc) ""

import qualified Data.Set as Set
 
task :: [String] -> String
task = Set.elems . Set.fromList . concat

main :: IO ()
main = print $ task ["Apple","Apricot","Lime","Mango"] -- "ALMacegilmnoprt"

λ> Set.fromList $ concat ["Apple","Apricot","Lime","Mango"]
fromList "ALMacegilmnoprt"
it :: Set.Set Char

res = sort.nub.concat