X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Futils%2FException.hs;h=3c7600515a13abc68950bf74c3730b17db71edd1;hb=9aad47f1162666431deee99884c523b1ff69cf98;hp=32422920ac0c2da8355ad95b130c655a2b8ecd18;hpb=1f3a7730cd7f831344d2a3b74a0ce700c382e858;p=ghc-hetmet.git

diff --git a/compiler/utils/Exception.hs b/compiler/utils/Exception.hs
index 3242292..3c76005 100644
--- a/compiler/utils/Exception.hs
+++ b/compiler/utils/Exception.hs
@@ -26,14 +26,27 @@ tryIO = try
 -- | 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
@@ -42,22 +55,26 @@ class Monad m => ExceptionMonad m where
   -- 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))
@@ -72,6 +89,6 @@ ghandle = flip gcatch
 -- 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)