Filenames.hs

import Control.Exception ( SomeException(..), catch )
import Control.Monad     ( when )
import Data.Foldable     ( for_ )
import System.Directory  ( doesDirectoryExist
                         , doesPathExist
                         , listDirectory
                         , pathIsSymbolicLink
                         )
import System.FilePath   ( (</>) )

main = filenames "/"

filenames rootPath = do
    yes <- doesPathExist rootPath
    link <- pathIsSymbolicLink rootPath
    when (yes && not link) $ do
      dir <- doesDirectoryExist rootPath
      if dir
      then do
        list <- listDirectory rootPath
        for_ list $ \file -> do
          filenames (rootPath </> file)
      else do
        putStrLn rootPath
  `catch` \(SomeException e) ->
    return ()