Creating preorder tree by folding list

tree.hs

data Tree a = Leaf | Branch a (Tree a) (Tree a) deriving Show

insertLeftBalanced :: a -> Tree a -> Tree a
insertLeftBalanced x Leaf = Branch x Leaf Leaf
insertLeftBalanced x (Branch m Leaf r) = Branch m (Branch x Leaf Leaf) r
insertLeftBalanced x (Branch m l Leaf) = Branch m l (Branch x Leaf Leaf)
insertLeftBalanced x (Branch m l r) = Branch m (insertLeftBalanced x l) r

listToTree :: [a] -> Tree a
listToTree xs = foldl (flip insertLeftBalanced) Leaf xs


-- cf. http://stackoverflow.com/a/19083798/1346276
indent :: [String] -> [String]
indent = map ("  "++)

layoutTree :: Show a => Tree a -> [String]
layoutTree Leaf = []  -- wow, that was easy
layoutTree (Branch here left right) 
         = indent (layoutTree right) ++ [show here] ++ indent (layoutTree left)

prettyTree :: Show a => Tree a -> String
prettyTree = unlines . layoutTree

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