Control.Monad.CatchIO
- class MonadIO m => MonadCatchIO m where
- class (Typeable e, Show e) => Exception e where
- toException :: e -> SomeException
- fromException :: SomeException -> Maybe e
- throw :: (MonadCatchIO m, Exception e) => e -> m a
- try :: (MonadCatchIO m, Exception e) => m a -> m (Either e a)
- tryJust :: (MonadCatchIO m, Exception e) => (e -> Maybe b) -> m a -> m (Either b a)
- data Handler m a = forall e . Exception e => Handler (e -> m a)
- catches :: MonadCatchIO m => m a -> [Handler m a] -> m a
Documentation
class MonadIO m => MonadCatchIO m whereSource
Methods
catch :: Exception e => m a -> (e -> m a) -> m aSource
Generalized version of catch
Generalized version of block
Generalized version of unblock
Instances
| MonadCatchIO IO | |
| (Monoid w, MonadCatchIO m) => MonadCatchIO (WriterT w m) | |
| MonadCatchIO m => MonadCatchIO (StateT s m) | |
| (MonadCatchIO m, Error e) => MonadCatchIO (ErrorT e m) | |
| MonadCatchIO m => MonadCatchIO (ReaderT r m) | |
| (Monoid w, MonadCatchIO m) => MonadCatchIO (RWST r w s m) |
class (Typeable e, Show e) => Exception e where
Any type that you wish to throw or catch as an exception must be an
instance of the Exception class. The simplest case is a new exception
type directly below the root:
data MyException = ThisException | ThatException
deriving (Show, Typeable)
instance Exception MyException
The default method definitions in the Exception class do what we need
in this case. You can now throw and catch ThisException and
ThatException as exceptions:
*Main> throw ThisException catch e -> putStrLn ("Caught " ++ show (e :: MyException))
Caught ThisException
In more complicated examples, you may wish to define a whole hierarchy of exceptions:
---------------------------------------------------------------------
-- Make the root exception type for all the exceptions in a compiler
data SomeCompilerException = forall e . Exception e => SomeCompilerException e
deriving Typeable
instance Show SomeCompilerException where
show (SomeCompilerException e) = show e
instance Exception SomeCompilerException
compilerExceptionToException :: Exception e => e -> SomeException
compilerExceptionToException = toException . SomeCompilerException
compilerExceptionFromException :: Exception e => SomeException -> Maybe e
compilerExceptionFromException x = do
SomeCompilerException a <- fromException x
cast a
---------------------------------------------------------------------
-- Make a subhierarchy for exceptions in the frontend of the compiler
data SomeFrontendException = forall e . Exception e => SomeFrontendException e
deriving Typeable
instance Show SomeFrontendException where
show (SomeFrontendException e) = show e
instance Exception SomeFrontendException where
toException = compilerExceptionToException
fromException = compilerExceptionFromException
frontendExceptionToException :: Exception e => e -> SomeException
frontendExceptionToException = toException . SomeFrontendException
frontendExceptionFromException :: Exception e => SomeException -> Maybe e
frontendExceptionFromException x = do
SomeFrontendException a <- fromException x
cast a
---------------------------------------------------------------------
-- Make an exception type for a particular frontend compiler exception
data MismatchedParentheses = MismatchedParentheses
deriving (Typeable, Show)
instance Exception MismatchedParentheses where
toException = frontendExceptionToException
fromException = frontendExceptionFromException
We can now catch a MismatchedParentheses exception as
MismatchedParentheses, SomeFrontendException or
SomeCompilerException, but not other types, e.g. IOException:
*Main> throw MismatchedParentheses catch e -> putStrLn ("Caught " ++ show (e :: MismatchedParentheses))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses catch e -> putStrLn ("Caught " ++ show (e :: SomeFrontendException))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses catch e -> putStrLn ("Caught " ++ show (e :: SomeCompilerException))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses catch e -> putStrLn ("Caught " ++ show (e :: IOException))
*** Exception: MismatchedParentheses
throw :: (MonadCatchIO m, Exception e) => e -> m aSource
Generalized version of throwIO
tryJust :: (MonadCatchIO m, Exception e) => (e -> Maybe b) -> m a -> m (Either b a)Source
Generalized version of tryJust
Generalized version of Handler
catches :: MonadCatchIO m => m a -> [Handler m a] -> m aSource
Generalized version of catches