data Tree t = Leaf | Node t (Tree t) (Tree t) deriving (Show)

exT :: Tree Int
exT = Node 1 (Node 2 (Node 4 Leaf Leaf) (Node 5 Leaf Leaf)) (Node 3 (Node 6 Leaf Leaf) (Node 7 Leaf Leaf))

bfsH :: Tree t -> [[t]]
bfsH Leaf = []
bfsH (Node a l r) = [a]: (zipWithButKeepRest (++) (bfsH l) (bfsH r))
  
-- This version of zipWith simply returns the remainder of the non-empty list if one list is empty. THIS IS NOT THE USUAL DEFINITION!
zipWithButKeepRest f [] x = x
zipWithButKeepRest f x [] = x
zipWithButKeepRest f (x:xs) (y:ys) = (f x y) : zipWithButKeepRest f xs ys

foldTree :: b
         -> (t -> b -> b -> b)
         -> Tree t
         -> b
foldTree fLeaf fNode = go
  where go Leaf = fLeaf
        go (Node a l r) = fNode a (go l) (go r)
        
bfsByFold :: Tree t -> [[t]]        
bfsByFold = foldTree ([]) (\x l r -> [x]: (zipWithButKeepRest (++) l r))