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|
{-# OPTIONS_GHC -fno-warn-deprecations #-}
module Exception
(
module Control.Exception,
module Exception
)
where
import Control.Exception
import Control.Monad.IO.Class
catchIO :: IO a -> (IOException -> IO a) -> IO a
catchIO = Control.Exception.catch
handleIO :: (IOException -> IO a) -> IO a -> IO a
handleIO = flip catchIO
tryIO :: IO a -> IO (Either IOException a)
tryIO = try
-- | A monad that can catch exceptions. A minimal definition
-- requires a definition of 'gcatch'.
--
-- Implementations on top of 'IO' should implement 'gmask' to
-- eventually call the primitive 'Control.Exception.mask'.
-- These are used for
-- implementations that support asynchronous exceptions. The default
-- implementations of 'gbracket' and 'gfinally' use 'gmask'
-- thus rarely require overriding.
--
class MonadIO m => ExceptionMonad m where
-- | Generalised version of 'Control.Exception.catch', allowing an arbitrary
-- exception handling monad instead of just 'IO'.
gcatch :: Exception e => m a -> (e -> m a) -> m a
-- | Generalised version of 'Control.Exception.mask_', allowing an arbitrary
-- exception handling monad instead of just 'IO'.
gmask :: ((m a -> m a) -> m b) -> m b
-- | Generalised version of 'Control.Exception.bracket', allowing an arbitrary
-- exception handling monad instead of just 'IO'.
gbracket :: m a -> (a -> m b) -> (a -> m c) -> m c
-- | Generalised version of 'Control.Exception.finally', allowing an arbitrary
-- exception handling monad instead of just 'IO'.
gfinally :: m a -> m b -> m a
gbracket before after thing =
gmask $ \restore -> do
a <- before
r <- restore (thing a) `gonException` after a
_ <- after a
return r
a `gfinally` sequel =
gmask $ \restore -> do
r <- restore a `gonException` sequel
_ <- sequel
return r
instance ExceptionMonad IO where
gcatch = Control.Exception.catch
gmask f = mask (\x -> f x)
gtry :: (ExceptionMonad m, Exception e) => m a -> m (Either e a)
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'.
ghandle :: (ExceptionMonad m, Exception e) => (e -> m a) -> m a -> m a
ghandle = flip gcatch
-- | Always executes the first argument. If this throws an exception the
-- second argument is executed and the exception is raised again.
gonException :: (ExceptionMonad m) => m a -> m b -> m a
gonException ioA cleanup = ioA `gcatch` \e ->
do _ <- cleanup
liftIO $ throwIO (e :: SomeException)
|
