Haskell n.8

Haskell.org

@@ -148,3 +148,50 @@ flatten = reverse . go []
         go acc (Elem x) = x:acc
         go acc (List (x:xs)) = go (go acc x) (List xs)
 #+end_src
+*** #8 Eliminate consecutive duplicates of list elements
+
+If a list contains repeated elements they should be replaced with a single copy of the element. The order of the elements should not be changed.
+
+Example:
+
+#+begin_src haskell
+,* (compress '(a a a a b c c a a d e e e e))
+(A B C A D E)
+#+end_src
+
+Example in Haskell:
+
+#+begin_src haskell
+λ> compress "aaaabccaadeeee"
+"abcade"
+#+end_src
+**** Solution
+
+This is not great...
+
+#+begin_src haskell
+compress xs = reverse $ go [] xs
+  where go acc [] = acc
+        go [] (x:xs) = go [x] xs
+        go [a] [x]
+            | a == x = [a]
+            | otherwise = [x,a]
+        go (a:as) [x]
+            | a == x = a:as
+            | otherwise = x:a:as
+        go [a] (x:xs)
+            | a == x = go [a] xs
+            | otherwise = go [x,a] xs
+        go (a:as) (x:xs)
+            | a == x = go (a:as) xs
+            | otherwise = go (x:a:as) xs
+#+end_src
+
+Especially when one of the solutions is the following;
+
+#+begin_src haskell
+import Data.List
+
+compress :: Eq a => [a] -> [a]
+compress = map head . group
+#+end_src