func []  0  res = [res]
func [] _ res = []
func (x:xs) n res = (func xs n res) ++ (func xs (n - x) (x:res))

[[ 6 ], [ 0 ,  6 ], [ 4 ,  2 ], [ 0 ,  4 ,  2 ]]

f xs n = func xs n []
    where func []  0  res = [res]
          func [] _ res = []
          func (x:xs) n res = (func xs n res) ++ (func xs (n - x) (x:res))

f xs n = filter (\x -> n == (sum x)) (subsets xs [] [])
    where subsets []  r1 r2 = r1 ++ r2
          subsets (s:ss) r1 r2 = subsets ss r1' r2'
              where r1' = r1 ++ r2
                    r2' = (map ((:) s) ([]:r2))