import System.Directory

--

find1 :: FilePath -> IO ()
find1 dir =
  do fs <- getDirectoryContents dir
     sequence_ [ found (dir ++ "/" ++ f)
               | f <- fs
               , f `notElem` [".",".."]
               ]
 where
  found file =
    do print file
       b <- doesDirectoryExist file
       if b then
         find1 file
        else
         return ()

--

find2 :: FilePath -> IO [FilePath]
find2 dir =
  do fs <- getDirectoryContents dir
     fss <- sequence [ found (dir ++ "/" ++ f)
                     | f <- fs
                     , f `notElem` [".",".."]
                     ]
     return (concat fss)
 where
  found file =
    do b <- doesDirectoryExist file
       if b then
         do fs <- find2 file
            return (file:fs)
        else
         return [file]

--

data File
  = Dir String [File]
  | File String
 deriving ( Show, Eq )

type FileSystem = [File]

find3 :: FilePath -> IO FileSystem
find3 dir =
  do fs <- getDirectoryContents dir
     sequence [ found dir f
              | f <- fs
              , f `notElem` [".",".."]
              ]
 where
  found dir f =
    do b <- doesDirectoryExist file
       if b then
         do fs <- find3 file
            return (Dir f fs)
        else
         return (File f)
   where
    file = dir ++ "/" ++ f

--

keepFile :: (String -> Bool) -> File -> Maybe File
keepFile p (File f) | p f       = Just (File f)
                    | otherwise = Nothing
keepFile p (Dir d fs)           = Just (Dir d (keep p fs))

keep :: (String -> Bool) -> FileSystem -> FileSystem
keep p files = [ file | file <- files, Just file' <- [keepFile p file] ]