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An exception denotes an unpredictable situation at runtime, like "out of disk storage", "read protected file", "user removed disk while reading", "syntax error in user input". These are situation which occur relatively seldom and thus their immediate handling would clutter the code which should describe the regular processing. Since exceptions must be expected at runtime there are also mechanisms for (selectively) handling them. (Control.Exception.try, Control.Exception.catch) Unfortunately Haskell's standard library names common exceptions of IO actions IOError and the module Control.Monad.Error is about exception handling not error handling. In general you should be very careful, not to mix up exceptions with errors. Actually, an unhandled exception is an error.


The great thing about Haskell is, that it is not necessary to hard-wire the exception handling into the language. Everything is already there to implement definition and handling of exceptions nicely. See the implementation in Control.Monad.Error (and please, excuse the misleading name, for now).

There is an old dispute between C++ programmers on whether exceptions or error return codes are the right way. Also Niklaus Wirth considered exceptions to be the reincarnation of GOTO and thus omitted them in his languages. Now Haskell solves the problem the diplomatic way: Function return error codes, but the handling of error codes does not uglify the calling code.

First we implement exception handling for non-monadic functions. Since no IO functions are involved, we can still not handle exceptional situations induced from outside the world, but we can handle situations, where it is unacceptable for the caller to check a priori whether the call can succeed.

data ExAction e a =
     Success a
   | Exception e
   deriving (Show)

instance Monad (ExAction e) where
   return              =  Success
   Exception l >>= _   =  Exception l
   Success  r  >>= k   =  k r

throw :: e -> ExAction e a
throw = Exception

catch :: ExAction e a -> (e -> ExAction e a) -> ExAction e a
catch (Exception  l) h = h l
catch (Success r)    _ = Success r

Now we extend this to monadic functions. This is not restricted to IO, but may be used immediately also for non-deterministic algorithms implemented with the List monad.

newtype ExActionT e m a =
   ExActionT {runExActionT :: m (ExAction e a)}

instance Monad m => Monad (ExActionT e m) where
   return   =  ExActionT . return . Success
   m >>= k  =  ExActionT $
      runExActionT m >>= \ a ->
         case a of
            Exception e -> return (Exception e)
            Success   r -> runExActionT (k r)

throwT :: Monad m => e -> ExActionT e m a
throwT = ExActionT . return . Exception

catchT :: Monad m =>
   ExActionT e m a -> (e -> ExActionT e m a) -> ExActionT e m a
catchT m h = ExActionT $
   runExActionT m >>= \ a ->
      case a of
         Exception l -> runExActionT (h l)
         Success   r -> return (Success r)

bracketT :: Monad m =>
   ExActionT e m h ->
   (h -> ExActionT e m ()) ->
   (h -> ExActionT e m a) ->
   ExActionT e m a
bracketT open close body =
   open >>= (\ h ->
      ExActionT $
         do a <- runExActionT (body h)
            runExActionT (close h)
            return a)

Here are some examples for typical IO functions with explicit exceptions.

data IOException =
   | FileDoesNotExist
   | ReadProtected
   | WriteProtected
   | NoSpaceOnDevice
   deriving (Show, Eq, Enum)

open :: FilePath -> ExActionT IOException IO Handle

close :: Handle -> ExActionT IOException IO ()

read :: Handle -> ExActionT IOException IO String

write :: Handle -> String -> ExActionT IOException IO ()

readText :: FilePath -> ExActionT IOException IO String
readText fileName =
   bracketT (open fileName) close $ \h ->
      read h

Finally we can escape from the Exception monad if we handle the exceptions completely.

main :: IO ()
main =
   do result <- runExActionT (readText "test")
      case result of
         Exception e -> putStrLn ("When reading file 'test' we encountered exception " ++ show e)
         Success x -> putStrLn ("Content of the file 'test'\n" ++ x)

See also