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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.

) Unfortunately Haskell's standard library names common exceptions of IO actions
and the module
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.

1 Implementation

1.1 Exception monad

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 the definition and handling of exceptions nicely.

See the implementation in
(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. Haskell solves the problem a diplomatic way: Functions 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 still cannot 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 Exceptional e a =
     Success a
   | Exception e
   deriving (Show)
instance Monad (Exceptional e) where
   return              =  Success
   Exception l >>= _   =  Exception l
   Success  r  >>= k   =  k r
throw :: e -> Exceptional e a
throw = Exception
catch :: Exceptional e a -> (e -> Exceptional e a) -> Exceptional 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
newtype ExceptionalT e m a =
   ExceptionalT {runExceptionalT :: m (Exceptional e a)}
instance Monad m => Monad (ExceptionalT e m) where
   return   =  ExceptionalT . return . Success
   m >>= k  =  ExceptionalT $
      runExceptionalT m >>= \ a ->
         case a of
            Exception e -> return (Exception e)
            Success   r -> runExceptionalT (k r)
throwT :: Monad m => e -> ExceptionalT e m a
throwT = ExceptionalT . return . Exception
catchT :: Monad m =>
   ExceptionalT e m a -> (e -> ExceptionalT e m a) -> ExceptionalT e m a
catchT m h = ExceptionalT $
   runExceptionalT m >>= \ a ->
      case a of
         Exception l -> runExceptionalT (h l)
         Success   r -> return (Success r)
bracketT :: Monad m =>
   ExceptionalT e m h ->
   (h -> ExceptionalT e m ()) ->
   (h -> ExceptionalT e m a) ->
   ExceptionalT e m a
bracketT open close body =
   open >>= (\ h ->
      ExceptionalT $
         do a <- runExceptionalT (body h)
            runExceptionalT (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 -> ExceptionalT IOException IO Handle
close :: Handle -> ExceptionalT IOException IO ()
read :: Handle -> ExceptionalT IOException IO String
write :: Handle -> String -> ExceptionalT IOException IO ()
readText :: FilePath -> ExceptionalT 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 <- runExceptionalT (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)

Package explicit-exception
Repository darcs get

2 See also