set functions from sicp for haskell

set_functions.hs

intersection_set _ [] = []
intersection_set [] _ = []
intersection_set (x:xs) ys
     | element_of_set x ys = x : intersection_set xs ys
     | otherwise = intersection_set xs ys

element_of_set _ [] = False
element_of_set x (y:ys)
   | x == y = True
   | otherwise = element_of_set x ys


adjoin_set x ys
  | element_of_set x ys = ys
  | otherwise = x : ys