不怕错误的猫

-- compile with: ghc --make cat.hs
import System.IO
import System.IO.Error
import System.Environment

cat :: String -> IO ()
cat fn = do
  contents <- readFile fn
  putStr contents

handler :: IOError -> IO ()
handler e
    | isDoesNotExistError e =
        case ioeGetFileName e of
          Just path -> putStrLn $ path ++ ": file not found"
          Nothing -> putStrLn "Oops! File unknown."
    | otherwise = ioError e

main = do args <- getArgs
          if null args
              then interact id
              else mapM_ (\fn -> cat fn `catch` handler) args