[project @ 2002-12-12 13:42:46 by ross]

[haskell-directory.git] / GHC / Exception.lhs

diff --git a/GHC/Exception.lhs b/GHC/Exception.lhs
index 50af792..3e26cdc 100644 (file)
--- a/GHC/Exception.lhs
+++ b/GHC/Exception.lhs
@@ -1,24 +1,27 @@
-% ------------------------------------------------------------------------------
-% $Id: Exception.lhs,v 1.3 2002/02/05 17:32:26 simonmar Exp $
-%
-% (c) The University of Glasgow, 1998-2000
-%
-
-Exceptions and exception-handling functions.
-
 \begin{code}
 {-# OPTIONS -fno-implicit-prelude #-}
+-----------------------------------------------------------------------------
+-- |
+-- Module      :  GHC.Exception
+-- Copyright   :  (c) The University of Glasgow, 1998-2002
+-- License     :  see libraries/base/LICENSE
+-- 
+-- Maintainer  :  cvs-ghc@haskell.org
+-- Stability   :  internal
+-- Portability :  non-portable (GHC extensions)
+--
+-- Exceptions and exception-handling functions.
+-- 
+-----------------------------------------------------------------------------
 
 #ifndef __HUGS__
 module GHC.Exception 
        ( module GHC.Exception, 
          Exception(..), AsyncException(..), 
          IOException(..), ArithException(..), ArrayException(..),
-         throw, ioError ) 
+         throw, throwIO, ioError ) 
   where
 
-import Data.Either
-
 import GHC.Base
 import GHC.IOBase
 
@@ -51,70 +54,41 @@ catchException m k =  ST (\s -> unST m s `primCatch'` \ err -> unST (k err) s)
 catchException (IO m) k =  IO $ \s -> catch# m (\ex -> unIO (k ex)) s
 #endif
 
-catch           :: IO a -> (Exception -> IO a) -> IO a 
+catch           :: IO a -> (IOError -> IO a) -> IO a 
 catch m k      =  catchException m handler
-  where handler err@(IOException _) = k err
+  where handler (IOException err)   = k err
        handler other               = throw other
 \end{code}
 
 
 %*********************************************************
 %*                                                     *
-\subsection{Try and bracket}
-%*                                                     *
-%*********************************************************
-
-The construct @try comp@ exposes errors which occur within a
-computation, and which are not fully handled.  It always succeeds.
-
-These are the IO-only try/bracket.  For the full exception try/bracket
-see hslibs/lang/Exception.lhs.
-
-\begin{code}
-try            :: IO a -> IO (Either Exception a)
-try f          =  catch (do r <- f
-                            return (Right r))
-                        (return . Left)
-
-bracket        :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c
-bracket before after m = do
-        x  <- before
-        rs <- try (m x)
-        after x
-        case rs of
-           Right r -> return r
-           Left  e -> ioError e
-
--- variant of the above where middle computation doesn't want x
-bracket_        :: IO a -> (a -> IO b) -> IO c -> IO c
-bracket_ before after m = do
-         x  <- before
-         rs <- try m
-         after x
-         case rs of
-            Right r -> return r
-            Left  e -> ioError e
-\end{code}
-
-
-%*********************************************************
-%*                                                     *
 \subsection{Controlling asynchronous exception delivery}
 %*                                                     *
 %*********************************************************
 
 \begin{code}
-#ifndef __HUGS__
+-- | Applying 'block' to a computation will
+-- execute that computation with asynchronous exceptions
+-- /blocked/.  That is, any thread which
+-- attempts to raise an exception in the current thread will be
+-- blocked until asynchronous exceptions are enabled again.  There\'s
+-- no need to worry about re-enabling asynchronous exceptions; that is
+-- done automatically on exiting the scope of
+-- 'block'.
 block :: IO a -> IO a
-block (IO io) = IO $ blockAsyncExceptions# io
 
+-- | To re-enable asynchronous exceptions inside the scope of
+-- 'block', 'unblock' can be
+-- used.  It scopes in exactly the same way, so on exit from
+-- 'unblock' asynchronous exception delivery will
+-- be disabled again.
 unblock :: IO a -> IO a
+
+#ifndef __HUGS__
+block (IO io) = IO $ blockAsyncExceptions# io
 unblock (IO io) = IO $ unblockAsyncExceptions# io
 #else
--- Not implemented yet in Hugs.
-block :: IO a -> IO a
-block (IO io) = IO io
-
 unblock :: IO a -> IO a
 unblock (IO io) = IO io
 #endif