Haskell Walk Directory Recursively

walk.hs

import System.Directory
import System.FilePath
import Control.Monad
import Control.Exception
import GHC.IO.Encoding
-- Gets a root directory and a callback,
-- calls callback for root directory and every subdirectory recursively
-- callback signature root -> files -> dirs -> IO () 
walkDirectory :: String -> (String -> [String] -> [String] -> IO ()) -> IO ()
walkDirectory rootDir callback = do
    putStrLn $ "Processing " ++ rootDir
    contents <- catch (listDirectory rootDir) (\e -> do
            putStrLn $ show (e :: IOException)
            return []
        )
    let fullPaths = map (rootDir </>) contents
    files <- filterM doesFileExist fullPaths
    dirs <- filterM doesDirectoryExist fullPaths
    callback rootDir files dirs
    mapM_ (flip walkDirectory callback) dirs


main = do
    setLocaleEncoding utf8
    walkDirectory "." $ \root files dirs -> do
        let txtFiles = filter (\fileName -> takeExtension fileName == ".txt") files
        let numtxtFiles = length txtFiles
        if numtxtFiles > 3 then
            putStrLn $ root ++ ":" ++ show numtxtFiles
        else
            return ()