Introduce an 'ExceptionMonad' class.

[ghc-hetmet.git] / compiler / utils / Exception.hs

diff --git a/compiler/utils/Exception.hs b/compiler/utils/Exception.hs
index a316c56..8d5d438 100644 (file)
--- a/compiler/utils/Exception.hs
+++ b/compiler/utils/Exception.hs
@@ -10,6 +10,8 @@ import Prelude hiding (catch)
 import Control.Exception
 
 #if __GLASGOW_HASKELL__ < 609
+import Data.Typeable ( Typeable )
+
 type SomeException = Exception
 
 onException :: IO a -> IO () -> IO a
@@ -40,3 +42,74 @@ tryIO io = do ei <- try io
                   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.
+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.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 acquire release in_between = do
+      a <- acquire
+      r <- in_between a `gonException` release a
+      release a
+      return r
+
+  gfinally thing cleanup = do
+      r <- thing `gonException` cleanup
+      cleanup
+      return r
+
+instance ExceptionMonad IO where
+  gcatch    = catch
+#if __GLASGOW_HASKELL__ < 609
+  gcatchDyn = catchDyn
+#endif
+  gbracket  = bracket
+  gfinally  = finally
+
+
+#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
+-- 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
+#if __GLASGOW_HASKELL__ >= 609
+                                throw (e :: SomeException)
+#else
+                                throw e
+#endif