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Exception

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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.

1 Implementation

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).

First for non-monadic functions.

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 monadic functions.

This is not restricted to IO, but may also immediately used for non-deterministic algorithms implemented with
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 example signatures for typical IO functions with explicit exceptions.

data IOException =
     DiskFull
   | 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 exception 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)


2 See also