Library for tree output of data for analysis - haskell

Question

For convenient analysis of data I'd like to use a library which for the following code:
data SomeType = A [String] Int | B | C Int deriving (Eq, Ord, Show)
main = do
let theData = A ["a", "b", "c"] 9 : C 3 : B : []
putStr $ treeString theData -- `treeString` is the implied library function
would produce an output similar to the following:
- A:
| - - a
| | - b
| | - c
| - 9
- C:
| - 3
- B
Is there such a library? Or maybe a better approach to such a problem?

Answer 1

Data.Tree has drawTree and drawForest functions with similar formatting, so you can write a function to convert your data structure to a Tree String and then use drawTree.
import Data.Tree
data SomeType = A [String] Int | B | C Int deriving (Eq, Ord, Show)
toTree :: SomeType -> Tree String
toTree (A xs n) = Node "A" [Node "*" (map (flip Node []) xs), Node (show n) []]
toTree B = Node "B" []
toTree (C n) = Node "C" [Node (show n) []]
main = do
let theData = A ["a", "b", "c"] 9 : C 3 : B : []
putStr $ drawTree (Node "*" (map toTree theData))
Output:
*
|
+- A
| |
| +- *
| | |
| | +- a
| | |
| | +- b
| | |
| | `- c
| |
| `- 9
|
+- C
| |
| `- 3
|
`- B

Answer 2

Adding to hammar's answer. Here's how one can do a generic conversion to Data.Tree:
import Data.Tree
import Data.Generics
import Control.Applicative
dataTree = fix . genericTree
where
genericTree :: Data a => a -> Tree String
genericTree = dflt `extQ` string
where
string x = Node x []
dflt a = Node (showConstr (toConstr a)) (gmapQ genericTree a)
fix (Node name forest)
| name == "(:)"
, a : b : [] <- forest
= Node "*" $ (fix a) : (subForest $ fix b)
| otherwise = Node name $ fix <$> forest
But for this to work on one's data types, they must have an instance of Data, which can easily be achieved by adding a {-# LANGUAGE DeriveDataTypeable #-} pragma and making the type derive from Typeable and Data like so:
data SomeType = A [String] Int | B | C Int | D [[String]]
deriving (Typeable, Data)