TreeFilter.hs

data Tree a = Node a (Tree a) (Tree a) | Empty


tFilter :: (Ord a) => (a -> Bool) -> Tree a -> Tree a
tFilter f tree = impl f tree Empty
  where 
    impl f Empty        acc = acc
    impl f (Node x l r) acc = 
      impl f l $ 
      impl f r $ 
        if f x 
          then incl x acc
          else acc 


incl :: Ord a => a -> Tree a -> Tree a
incl x Empty = Node x Empty Empty
incl x t@(Node y l r)
  | x < y     = Node y (incl x l) r
  | x > y     = Node y l (incl x r)
  | otherwise = t