-- | 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'.
gcatch :: Exception e => m a -> (e -> m a) -> m a
+ -- | 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'.
gbracket :: m a -> (a -> m b) -> (a -> m c) -> m c
-- 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
+
+ gbracket before after thing =
+ gblock (do
+ a <- before
+ r <- gunblock (thing a) `gonException` after a
+ _ <- after a
+ return r)
- gfinally thing cleanup = do
- r <- thing `gonException` cleanup
- cleanup
- return r
+ a `gfinally` sequel =
+ gblock (do
+ r <- gunblock a `gonException` sequel
+ _ <- sequel
+ return r)
instance ExceptionMonad IO where
gcatch = catch
- gbracket = bracket
- gfinally = finally
-
+ gblock = block
+ gunblock = unblock
gtry :: (ExceptionMonad m, Exception e) => m a -> m (Either e a)
gtry act = gcatch (act >>= \a -> return (Right a))
-- 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
+ do _ <- cleanup
throw (e :: SomeException)