foo.hs

import Data.List

data STree a                  =  Empty | Node (STree a) a (STree a)
                                 deriving (Eq, Ord, Show)


bft                ::  [STree a] -> [[a]]
bft                 =  unfoldr f
                        where f x = case x of
                                [] -> Nothing
                                (Empty):xs -> Just ([], xs)
                                (Node l v r):xs -> Just ([v], xs ++ [l] ++ [r])



t = Node (Node (Node Empty 1 Empty) 1 (Node (Node Empty 2 Empty) 2 Empty))
          3
         (Node (Node Empty 5 Empty) 8 Empty)

t2 = Node Empty 2 Empty
-- bft [t] == [[3],[1,8],[1,2,5],[2],[]]

-- tl = (Node (Node Empty 1 Empty) 1 (Node (Node Empty 2 Empty) 2 Empty))
-- tr = (Node (Node Empty 5 Empty) 8 Empty)

-- bft [tl, tr] == [[1,8],[1,2,5],[2],[]]