X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=Control%2FException.hs;h=ee063de8848df7eee72307c227638eda6cff8722;hb=19dfd6fc5b4c1f09b4aee82874bcb179ed6cd0cc;hp=7551e2558231e34ec4577f75392f255c77373ab0;hpb=7316246c1797d46f5757ae156486f09b3db083d9;p=haskell-directory.git

diff --git a/Control/Exception.hs b/Control/Exception.hs
index 7551e25..ee063de 100644
--- a/Control/Exception.hs
+++ b/Control/Exception.hs
@@ -6,7 +6,7 @@
 -- 
 -- Maintainer  :  libraries@haskell.org
 -- Stability   :  experimental
--- Portability :  non-portable
+-- Portability :  non-portable (extended exceptions)
 --
 -- This module provides support for raising and catching both built-in
 -- and user-defined exceptions.
@@ -103,6 +103,7 @@ module Control.Exception (
 
 	bracket,  	-- :: IO a -> (a -> IO b) -> (a -> IO c) -> IO ()
 	bracket_, 	-- :: IO a -> IO b -> IO c -> IO ()
+	bracketOnError,
 
 	finally, 	-- :: IO a -> IO b -> IO a
 	
@@ -390,6 +391,21 @@ a `finally` sequel =
 bracket_ :: IO a -> IO b -> IO c -> IO c
 bracket_ before after thing = bracket before (const after) (const thing)
 
+-- | Like bracket, but only performs the final action if there was an 
+-- exception raised by the in-between computation.
+bracketOnError
+	:: IO a		-- ^ computation to run first (\"acquire resource\")
+	-> (a -> IO b)  -- ^ computation to run last (\"release resource\")
+	-> (a -> IO c)	-- ^ computation to run in-between
+	-> IO c		-- returns the value from the in-between computation
+bracketOnError before after thing =
+  block (do
+    a <- before 
+    catch 
+	(unblock (thing a))
+	(\e -> do { after a; throw e })
+ )
+
 -- -----------------------------------------------------------------------------
 -- Asynchronous exceptions
 
@@ -467,23 +483,6 @@ Similar arguments apply for other interruptible operations like
 'System.IO.openFile'.
 -}
 
--- -----------------------------------------------------------------------------
--- Assert
-
-#ifdef __HADDOCK__
--- | If the first argument evaluates to 'True', then the result is the
--- second argument.  Otherwise an 'AssertionFailed' exception is raised,
--- containing a 'String' with the source file and line number of the
--- call to assert.
---
--- Assertions can normally be turned on or off with a compiler flag
--- (for GHC, assertions are normally on unless the @-fignore-asserts@
--- option is give).  When assertions are turned off, the first
--- argument to 'assert' is ignored, and the second argument is
--- returned as the result.
-assert :: Bool -> a -> a
-#endif
-
 #ifndef __GLASGOW_HASKELL__
 assert :: Bool -> a -> a
 assert True x = x