如何在Haskell中创建树层次结构？

data Tree a = Leaf a | Branch a [Tree a]

printTree :: Show a => Tree a -> IO ()
printTree = putStrLn . unlines . drawTree

drawTree :: Show a => Tree a -> [String]
drawTree (Leaf x) = [show x]
drawTree (Branch x subtrees) = show x : drawSubtrees subtrees

drawSubtrees :: Show a => [Tree a] -> [String]
drawSubtrees [] = []
drawSubtrees [t] = "|" : shift "`- " "   " (drawTree t)
drawSubtrees (t:ts) = "|" : shift "+- " "|  " (drawTree t) ++ drawSubtrees ts

shift :: String -> String -> [String] -> [String]
shift first other = zipWith (++) (first : repeat other)

tree = Branch "A" [Leaf "B", Leaf "C", Branch "D" [Leaf "E", Leaf "F"]]
printTree tree

A
|-- B
|-- C
`-- D
    |-- E
    `-- F