Nicely printing/showing a binary tree in Haskell

Question

I have a tree data type:

data Tree a b = Branch b (Tree a b) (Tree a b) | Leaf a

...and I need to make it an instance of Show, wit

Answer 1

You might study the drawTree function in the base Data.Tree module. Just shamelessly importing it would give you something like this:

import Data.Tree hiding (Tree )
data Tree a b = Branch b (Tree a b) (Tree a b) 
              | Leaf a deriving (Eq,Ord,Show)

toDataTree (Leaf a) = Node a []
toDataTree (Branch b cs ds) = Node b [toDataTree cs, toDataTree ds]

d = Branch "1" (Branch "11" (Leaf "111") (Leaf "112")) 
               (Branch "12" (Leaf "121") (Leaf "122"))

e = toDataTree d
f = putStrLn $ drawTree e

{-
*Main> f
1
|
+- 11
|  |
|  +- 111
|  |
|  `- 112
|
`- 12
   |
   +- 121
   |
   `- 122
-}