add System.Posix.Types to default nhc98 build

[haskell-directory.git] / Control / Exception.hs

diff --git a/Control/Exception.hs b/Control/Exception.hs
index cb4ba3e..1a40780 100644 (file)
--- a/Control/Exception.hs
+++ b/Control/Exception.hs
@@ -6,11 +6,23 @@
 -- 
 -- 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.
 --
+-- In addition to exceptions thrown by 'IO' operations, exceptions may
+-- be thrown by pure code (imprecise exceptions) or by external events
+-- (asynchronous exceptions), but may only be caught in the 'IO' monad.
+-- For more details, see:
+--
+--  * /A semantics for imprecise exceptions/, by Simon Peyton Jones,
+--    Alastair Reid, Tony Hoare, Simon Marlow, Fergus Henderson,
+--    in /PLDI'99/.
+--
+--  * /Asynchronous exceptions in Haskell/, by Simon Marlow, Simon Peyton
+--    Jones, Andy Moran and John Reppy, in /PLDI'01/.
+--
 -----------------------------------------------------------------------------
 
 module Control.Exception (
@@ -103,6 +115,7 @@ 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
        
@@ -139,7 +152,7 @@ import Data.Dynamic
 -- argument.  Otherwise, the result is returned as normal.  For example:
 --
 -- >   catch (openFile f ReadMode) 
--- >       (\e -> hPutStr stderr (\"Couldn\'t open \"++f++\": \" ++ show e))
+-- >       (\e -> hPutStr stderr ("Couldn't open "++f++": " ++ show e))
 --
 -- For catching exceptions in pure (non-'IO') expressions, see the
 -- function 'evaluate'.
@@ -165,11 +178,19 @@ import Data.Dynamic
 -- Also note that the "Prelude" also exports a function called
 -- 'Prelude.catch' with a similar type to 'Control.Exception.catch',
 -- except that the "Prelude" version only catches the IO and user
--- families of exceptions (as required by Haskell 98).  We recommend
--- either hiding the "Prelude" version of
--- 'Prelude.catch' when importing
--- "Control.Exception", or importing
--- "Control.Exception" qualified, to avoid name-clashes.
+-- families of exceptions (as required by Haskell 98).  
+--
+-- We recommend either hiding the "Prelude" version of 'Prelude.catch'
+-- when importing "Control.Exception": 
+--
+-- > import Prelude hiding (catch)
+--
+-- or importing "Control.Exception" qualified, to avoid name-clashes:
+--
+-- > import qualified Control.Exception as C
+--
+-- and then using @C.catch@
+--
 
 catch          :: IO a                 -- ^ The computation to run
        -> (Exception -> IO a)  -- ^ Handler to invoke if an exception is raised
@@ -230,7 +251,7 @@ mapException f v = unsafePerformIO (catch (evaluate v)
 -- @('Right' a)@ if no exception was raised, or @('Left' e)@ if an
 -- exception was raised and its value is @e@.
 --
--- >  try a = catch (Right \`liftM\` a) (return . Left)
+-- >  try a = catch (Right `liftM` a) (return . Left)
 --
 -- Note: as with 'catch', it is only polite to use this variant if you intend
 -- to re-throw the exception after performing whatever cleanup is needed.
@@ -351,7 +372,7 @@ userErrors _ = Nothing
 -- The arguments to 'bracket' are in this order so that we can partially apply 
 -- it, e.g.:
 --
--- > withFile name = bracket (openFile name) hClose
+-- > withFile name mode = bracket (openFile name mode) hClose
 --
 bracket 
        :: IO a         -- ^ computation to run first (\"acquire resource\")
@@ -390,6 +411,21 @@ 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
 
@@ -490,7 +526,8 @@ uncaughtExceptionHandler = unsafePerformIO (newIORef defaultHandler)
           withCString msg $ \cmsg ->
             errorBelch cfmt cmsg
 
-foreign import ccall unsafe errorBelch :: CString -> CString -> IO ()
+foreign import ccall unsafe "RtsMessages.h errorBelch"
+   errorBelch :: CString -> CString -> IO ()
 
 setUncaughtExceptionHandler :: (Exception -> IO ()) -> IO ()
 setUncaughtExceptionHandler = writeIORef uncaughtExceptionHandler