cat in haskell

cat.hs

import System.IO
import System.Environment
import Data.Maybe
import System.Directory

main :: IO ()
main = do
	hSetBuffering stdout NoBuffering
	args <- getArgs
	let files = case args of
		[] -> [Nothing]
		_  -> map (\f -> Just f) args
	mapM_ cat files

-- if nothing, just cat stdin
cat :: Maybe FilePath -> IO ()
cat file = do
	exists <- case file of
		Just f -> checkExists f
		_      -> return True
	readable <- if exists then
		case file of
			Just f -> checkReadable f
			_      -> return True
	else
		return False
	if exists && readable then do
		handle <- maybe (return stdin) (\f -> openBinaryFile f ReadMode) file
		c <- hGetContents handle
		hSetBuffering handle NoBuffering
		putStr c
	else
		return ()

-- check if file exists
checkExists :: FilePath -> IO Bool
checkExists file = do
	exists <- doesFileExist file
	if not exists then do
		hPutStrLn stderr  $ "File does not exist: " ++ file
		return False
	else
		return True

-- check if file is readable
checkReadable file = do
	p <- getPermissions file -- this may fail when accessing permissions is not allowed :(
	if not (readable p) then do
		hPutStrLn stderr $ "File is not readable: " ++ file
		return False
	else
		return True