Solve accumulator task

Exercises/exercise-4/Solution.hs

@@ -54,3 +54,16 @@ length' ls = aux
             x : xs -> aux $ map (\y -> (+1) x ) xs
 -- length' und aux sind nicht endrekursiv
 
+-- sieve :: ( a -> a -> Bool ) -> [ a ] -> [ a ]
+-- sieve pred xs = case xs of
+--     [] -> []
+--     x : xs -> x :( sieve pred $ filter ( pred x ) xs )
+sieve :: (a -> a -> Bool) -> [a] -> [a]
+sieve pred xs = realSieve [] xs
+    where
+        realSieve acc [] = acc
+        realSieve acc (x : xs) = realSieve (acc ++ [x]) (filter (pred x) xs)
+
+-- >>> sieve (\ x -> \ y -> y > x) [1, 2, 3, 1, 2, 9, 7]
+-- [1,2,3,9]
+--