where
import Prelude hiding (catch)
-import Control.Exception
#if __GLASGOW_HASKELL__ < 609
-import Data.Typeable ( Typeable )
-
-type SomeException = Exception
-
-onException :: IO a -> IO () -> IO a
-onException io what = io `catch` \e -> do what
- throw e
+import Control.Exception.Extensible as Control.Exception
+#else
+import Control.Exception
#endif
catchIO :: IO a -> (IOException -> IO a) -> IO a
-#if __GLASGOW_HASKELL__ >= 609
catchIO = catch
-#else
-catchIO io handler = io `catch` handler'
- where handler' (IOException ioe) = handler ioe
- handler' e = throw e
-#endif
handleIO :: (IOException -> IO a) -> IO a -> IO a
handleIO = flip catchIO
tryIO :: IO a -> IO (Either IOException a)
-#if __GLASGOW_HASKELL__ >= 609
tryIO = try
-#else
-tryIO io = do ei <- try io
- case ei of
- Right v -> return (Right v)
- Left (IOException ioe) -> return (Left ioe)
- Left e -> throwIO e
-#endif
-- | A monad that can catch exceptions. A minimal definition
-- requires a definition of 'gcatch'.
--
--- Although, 'gbracket' and 'gfinally' could be modelled on top of 'gcatch',
--- they are included in the type class since GHC needs special implementations
--- of these in order to properly handle asynchronous exceptions.
+-- Implementations on top of 'IO' should implement 'gblock' and 'gunblock' to
+-- eventually call the primitives 'Control.Exception.block' and
+-- 'Control.Exception.unblock' respectively. These are used for
+-- implementations that support asynchronous exceptions. The default
+-- implementations of 'gbracket' and 'gfinally' use 'gblock' and 'gunblock'
+-- thus rarely require overriding.
+--
class Monad m => ExceptionMonad m where
+
-- | Generalised version of 'Control.Exception.catch', allowing an arbitrary
-- exception handling monad instead of just 'IO'.
-#if __GLASGOW_HASKELL__ >= 609
gcatch :: Exception e => m a -> (e -> m a) -> m a
-#else
- gcatch :: m a -> (Exception -> m a) -> m a
- gcatchDyn :: Typeable e => m a -> (e -> m a) -> m a
-#endif
+
+ -- | Generalised version of 'Control.Exception.block', allowing an arbitrary
+ -- exception handling monad instead of just 'IO'.
+ gblock :: m a -> m a
+
+ -- | Generalised version of 'Control.Exception.unblock', allowing an
+ -- arbitrary exception handling monad instead of just 'IO'.
+ gunblock :: m a -> m a
-- | Generalised version of 'Control.Exception.bracket', allowing an arbitrary
-- exception handling monad instead of just 'IO'.
-- exception handling monad instead of just 'IO'.
gfinally :: m a -> m b -> m a
- gbracket acquire release in_between = do
- a <- acquire
- r <- in_between a `gonException` release a
- release a
- return r
+ gblock = id
+ gunblock = id
- gfinally thing cleanup = do
- r <- thing `gonException` cleanup
- cleanup
- return r
+ gbracket before after thing =
+ gblock (do
+ a <- before
+ r <- gunblock (thing a) `gonException` after a
+ after a
+ return r)
+
+ a `gfinally` sequel =
+ gblock (do
+ r <- gunblock a `gonException` sequel
+ sequel
+ return r)
instance ExceptionMonad IO where
gcatch = catch
-#if __GLASGOW_HASKELL__ < 609
- gcatchDyn = catchDyn
-#endif
- gbracket = bracket
- gfinally = finally
-
+ gblock = block
+ gunblock = unblock
-#if __GLASGOW_HASKELL__ >= 609
gtry :: (ExceptionMonad m, Exception e) => m a -> m (Either e a)
-#else
-gtry :: (ExceptionMonad m) => m a -> m (Either Exception a)
-#endif
gtry act = gcatch (act >>= \a -> return (Right a))
(\e -> return (Left e))
-- | Generalised version of 'Control.Exception.handle', allowing an arbitrary
-- exception handling monad instead of just 'IO'.
-#if __GLASGOW_HASKELL__ >= 609
ghandle :: (ExceptionMonad m, Exception e) => (e -> m a) -> m a -> m a
-#else
-ghandle :: (ExceptionMonad m) => (Exception -> m a) -> m a -> m a
-#endif
ghandle = flip gcatch
-- | Always executes the first argument. If this throws an exception the
gonException :: (ExceptionMonad m) => m a -> m b -> m a
gonException ioA cleanup = ioA `gcatch` \e ->
do cleanup
-#if __GLASGOW_HASKELL__ >= 609
throw (e :: SomeException)
-#else
- throw e
-#endif
+