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(link to Error vs. Exception)
(Improved some grammatical ambiguities)
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Unfortunately Haskell's standard library names common exceptions of IO actions <hask>IOError</hask>
 
Unfortunately Haskell's standard library names common exceptions of IO actions <hask>IOError</hask>
 
and the module <hask>Control.Monad.Error</hask> is about exception handling not error handling.
 
and the module <hask>Control.Monad.Error</hask> is about exception handling not error handling.
In general you should be very careful, not to mix up exceptions with [[error]]s.
+
In general you should be very careful not to mix up exceptions with [[error]]s.
 
Actually, an unhandled exception is an [[error]].
 
Actually, an unhandled exception is an [[error]].
   
 
== Implementation ==
 
== Implementation ==
   
The great thing about Haskell is, that it is not necessary to hard-wire the exception handling into the language.
+
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.
+
Everything is already there to implement the definition and handling of exceptions nicely.
See the implementation in <hask>Control.Monad.Error</hask> (and please, excuse the misleading name, for now).
+
See the implementation in <hask>Control.Monad.Error</hask> (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.
 
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.
 
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:
+
Haskell solves the problem a diplomatic way:
Function return error codes, but the handling of error codes does not uglify the calling code.
+
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.
 
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,
+
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.
+
but we can handle situations where it is unacceptable for the caller to check a priori whether the call can succeed.
 
<haskell>
 
<haskell>
 
data Exceptional e a =
 
data Exceptional e a =

Revision as of 03:58, 13 March 2010

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 the 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. 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
List
monad.
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 =
     DiskFull
   | 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)


2 Implementation

3 See also