打印出一棵玫瑰树……？

Question

I am currently learning Haskell in one of my online classes at the university. I now have to write a program that can work with a rose tree. Right now it looks like this:

data Rose a = Rose a [Rose a]

testTree1 = Rose 4 [Rose 5 [Rose 1 [], Rose 2 [Rose 7 [], Rose 8 []], Rose 3 []], Rose 6 []]

class Pretty a where
    pretty :: a -> String


instance Pretty (Rose a) where
    pretty (Rose v []) = "Test"
    pretty (Rose v t ) = "Test" ++ (concat(map ("\n+--"++) (map (pretty) t)))

On calling putStrLn (pretty (testTree1)) the result should look like this:

4
+-- 5
|   +-- 1
|   +-- 2
|   |   +-- 7
|   |   +-- 8
|   +-- 3
+-- 6

My current function has the following result:

Test
+--Test
+--Test
+--Test
+--Test
+--Test
+--Test
+--Test

So I have these two questions.

1. How can I put the real value of v in the String instead of the placeholder Test , when I don't know which type the v will have?
2. I don't understand why the Test s in my result all have the same alignment. Of course I am not using the | yet, but shouldn't the +-- appear more than one time in some lines?

Thx for your help:)

Answer 1

If you want to use the value of v in the output, then Rose a only has a Pretty instance if a does as well, in which case you can use pretty to convert v .

instance  Pretty (Rose a) where
    pretty (Rose v []) = 
    pretty (Rose v t ) =  ++ concat(map ("\n+--"++) (map pretty t)))

For your example, you also need to provide a concrete type for testTree1 and to define an appropriate instance for Pretty . For example,

testTree1 :: Rose Int
testTree1 = ...

instance Pretty Int where
    pretty = show