Full Simple Pretty, Haskell

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import Data.List (intercalate)

data Doc = 
      Text String 
    | Nest Int String [Doc] String Int
    deriving (Eq, Show)

getDocFoldedLength :: Doc -> Int
getDocFoldedLength (Text s) = length s
getDocFoldedLength (Nest _ _ _ _ l) = l

getDocFoldedString :: Doc -> String
getDocFoldedString (Nest _ open cs close _) = open ++ intercalate " " (map getDocFoldedString cs) ++ close
getDocFoldedString (Text s) = s

nest :: Int -> String -> [Doc] -> String -> Doc
nest indent open chs close = 
    Nest indent open chs close (length open + length chs - 1 + sum (map getDocFoldedLength chs) + length close)

pretty :: Int -> Doc -> String
pretty w doc = pretty' 0 w doc where
    pretty' i _ (Text s) = replicate i ' ' ++ s
    pretty' i w (Nest j open cs close l) | i + j + l <= w = 
        replicate i ' ' ++ open ++ intercalate " " (map getDocFoldedString cs) ++ close
    pretty' i w (Nest j open cs close l) = 
        replicate i ' ' ++ open ++ "\n" ++ intercalate "\n" (map (pretty' (i + j) w) cs) ++ "\n" ++ replicate i ' ' ++ close

intercalateDocs :: String -> [Doc] -> [Doc]
intercalateDocs _ l | length l < 2 = l
intercalateDocs delim (hd:tl) = case hd of 
    (Text s) -> (Text (s ++ delim)):intercalateDocs delim tl
    otherwise -> hd:intercalateDocs delim tl

data Tree = Tree String [Tree]
    deriving (Eq, Show)

showTree :: Tree -> Doc
showTree (Tree s ts) = if null chs then Text s else nest (1 + length s) (s ++ "[") chs "]" where
	chs = intercalateDocs "," $ map showTree ts

tree = 
    Tree "a" [
        Tree "b" [
            Tree "c" []],
        Tree "d" [
            Tree "e" [],
            Tree "f" [],
            Tree "g" [],
            Tree "h" []
        ],
        Tree "i" []
    ]

main = do
    putStrLn $ show $ showTree tree
    putStrLn $ pretty 3 (showTree tree)
    putStrLn $ pretty 7 (showTree tree)
    putStrLn $ pretty 8 (showTree tree)
    putStrLn $ pretty 20 (showTree tree)
    putStrLn $ pretty 40 (showTree tree)

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Compilation time: 0.72 sec, absolute running time: 0.14 sec, cpu time: 0 sec, memory peak: 3 Mb, absolute service time: 0,87 sec