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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).
data MonadException e a =
     Success a
   | Exception e
 
instance Monad (MonadException e) where
   return              =  Success
   Exception l >>= _   =  Exception l
   Successt r  >>= k   =  k r
 
throw :: e -> MonadException e a
throw = Exception
 
catch :: MonadException e a -> (e -> MonadException e a) -> MonadException e a
catch (Exception  l) h = h l
catch (Success r)    _ = Success r


2 See also