Non-working Haskell :(

nest.hs

travis@localhost ~/Documents/dev/school/language_study/chapter5 $ cat Prettify.hs 
data Doc = Empty
         | Char Char
         | Text String
         | Line
         | Concat Doc Doc
         | Union Doc Doc
           deriving (Show,Eq)
empty :: Doc
empty = Empty

char :: Char -> Doc
char = Char

text :: String -> Doc
text "" = Empty
text s  = Text s

double :: Double -> Doc
double d = text (show d)

line :: Doc
line = Line

(<>) :: Doc -> Doc -> Doc
Empty <> y = y
x <> Empty = x
x <> y = x `Concat` y

compact :: Doc -> String
compact x = transform [x]
    where transform [] = ""
          transform (d:ds) =
              case d of
                Empty        -> transform ds
                Char c       -> c : transform ds
                Text s       -> s ++ transform ds
                Line         -> '\n' : transform ds
                a `Concat` b -> transform (a:b:ds)
                _ `Union` b  -> transform (b:ds)


nest :: Int -> Doc -> Doc
nest i (Char a `Concat` b)
        | isOpenner a = Char a `Concat` nest (i+1) b
        | isCloser a = Char a `Concat` nest (i-1) b
        | otherwise = Char a `Concat` nest i b
nest i (Line `Concat` b) = Line `Concat` Text (take i (cycle ['q'])) `Concat` b
nest i (a `Concat` b) = nest i a `Concat` nest i b
nest _ doc = doc

isOpenner :: Char -> Bool
isOpenner '{' = True 
isOpenner '(' = True 
isOpenner _ = False

isCloser :: Char -> Bool
isCloser '}' = True 
isCloser ')' = True 
isCloser _ = False

main = putStrLn $ compact $ nest 10 $ Text "Hello" <> Char '{' <> Line <> Text "World!" <> Char '}' <> Line <> Text "Bye"