〈Haskell Tutorial（23）Exception 的 catch 與 handle〉的解答

Finally.hs

import Data.Typeable
import Prelude hiding (catch, readFile)
import Control.Exception hiding (finally)
import System.Environment
import System.IO hiding (readFile)

readFile :: FilePath -> IO String
readFile fileName =    
   bracket (openFile fileName ReadMode) hClose (\handle -> do    
       contents <- hGetContents handle
       evaluate contents
       return contents)

main =
    (do (fileName:_) <- getArgs
        contents <- readFile fileName
        putStrLn contents)
    `catches`
        [Handler (\(_::IOException) -> putStrLn "發生 IOException ..."),
         Handler (\(e::SomeException) -> print $ typeOf e)]
    `finally` (putStrLn "finally 執行最後資源善後")

finally :: IO a -> IO b -> IO a
finally a1 a2 = bracket (return ()) (\_ -> a2) (\_ -> a1)