Haskell 99 problems - 10 to 15

gistfile1.hs

import Data.List

-- problem 11
data Element a = Multiple a Int | Single a deriving (Show, Eq)  

encodeModified :: Eq a => [a] -> [Element a]
encodeModified [] = []
encodeModified l = map (\x -> if (length x > 1) then Multiple (head x) (length x) else Single (head x)) $ group l 


-- problem 12
decodeModified :: Eq a => [Element a] -> [a]
decodeModified [] = []
decodeModified l = concatMap innerDecode l 
				   where
				   innerDecode (Single v) = [v]
				   innerDecode (Multiple v t) = replicate t v


-- problem 13
encodeBis :: Eq a => [a] -> [(Int, a)]
encodeBis l = foldl helper [] l
			  where
			  helper [] e = [(1, e)] 
			  helper acc e
			    | e == (snd $ last acc) = init acc ++ [((fst $ last acc) + 1 , snd $ last acc)]
			    | otherwise = acc ++ [(1, e)]


encodeDirect :: Eq a => [a] -> [Element a]
encodeDirect [] = []
encodeDirect l = concatMap innerEncode (encodeBis l)
                 where 
                 innerEncode (1, v) = [Single v]
                 innerEncode (t, v) = [Multiple v t]


-- problem 14 
myDuplicate :: [a] -> [a]
myDuplicate = foldl (\acc x -> acc ++ (replicate 2 x)) [] 

myDuplicateEvenSimpler :: [a] -> [a]
myDuplicateEvenSimpler = foldl (\acc x -> acc ++ [x,x]) []


--problem 15
myRepli :: [a] -> Int -> [a]
myRepli l v = foldl (\acc x -> acc ++ (replicate v x)) [] l

myDuplicateUsingRepli l = myRepli l 2