Why is day seven so hard?

seven-input.txt

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+$ cd /
+$ ls
+dir a
+14848514 b.txt
+8504156 c.dat
+dir d
+$ cd a
+$ ls
+dir e
+29116 f
+2557 g
+62596 h.lst
+$ cd e
+$ ls
+584 i
+$ cd ..
+$ cd ..
+$ cd d
+$ ls
+4060174 j
+8033020 d.log
+5626152 d.ext
+7214296 k

seven.hs

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+data FS = FS [Inode]
+
+data Inode = Dir Name [Inode] | File Name Size deriving(Show)
+
+type Name = String
+type Size = Int
+
+
+data Command = Ls | Cd Name | CdUp | CdRoot deriving(Show)
+
+
+data Line = LineC Command | LineI Inode deriving(Show)
+
+parseCommand :: String -> Command
+parseCommand ('c':'d':' ':name) =(Cd name)
+parseCommand "ls"               =Ls
+parseCommand "cd .."            =CdUp
+parseCommand "cd /"             =CdRoot
+
+parseInode :: String -> Inode
+parseInode ('d':'i':'r':' ': name) = Dir name []
+parseInode input = File (drop 1 $ dropWhile (/= ' ') input) (read $ takeWhile (/= ' ') input)
+
+
+parseEither :: String -> Line
+parseEither ('$':' ':command) =LineC $  parseCommand command
+parseEither input=            LineI $            parseInode input
+
+
+
+main = do
+  content <- readFile "seven-input.txt"
+  let statements = map parseEither $ lines content
+  print statements