[project @ 2005-02-11 11:36:23 by simonmar]

Add bracketOnError

diff --git a/Control/Exception.hs b/Control/Exception.hs
index cb4ba3e..5362610 100644 (file)
--- a/Control/Exception.hs
+++ b/Control/Exception.hs
@@ -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