-% ------------------------------------------------------------------------------
-% $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 #-}
-
-#ifndef __HUGS__
+{-# OPTIONS_GHC -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.
+--
+-----------------------------------------------------------------------------
+
+-- #hide
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
-
-#endif
\end{code}
%*********************************************************
\begin{code}
catchException :: IO a -> (Exception -> IO a) -> IO a
-#ifdef __HUGS__
-catchException m k = ST (\s -> unST m s `primCatch'` \ err -> unST (k err) s)
-#else
catchException (IO m) k = IO $ \s -> catch# m (\ex -> unIO (k ex)) s
-#endif
-catch :: IO a -> (Exception -> IO a) -> IO a
+-- | The 'catch' function establishes a handler that receives any 'IOError'
+-- raised in the action protected by 'catch'. An 'IOError' is caught by
+-- the most recent handler established by 'catch'. These handlers are
+-- not selective: all 'IOError's are caught. Exception propagation
+-- must be explicitly provided in a handler by re-raising any unwanted
+-- exceptions. For example, in
+--
+-- > f = catch g (\e -> if IO.isEOFError e then return [] else ioError e)
+--
+-- the function @f@ returns @[]@ when an end-of-file exception
+-- (cf. 'System.IO.Error.isEOFError') occurs in @g@; otherwise, the
+-- exception is propagated to the next outer handler.
+--
+-- When an exception propagates outside the main program, the Haskell
+-- system prints the associated 'IOError' value and exits the program.
+--
+-- Non-I\/O exceptions are not caught by this variant; to catch all
+-- exceptions, use 'Control.Exception.catch' from "Control.Exception".
+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
-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
+block (IO io) = IO $ blockAsyncExceptions# io
+unblock (IO io) = IO $ unblockAsyncExceptions# io
\end{code}
-
+\begin{code}
+-- | Forces its argument to be evaluated when the resultant 'IO' action
+-- is executed. It can be used to order evaluation with respect to
+-- other 'IO' operations; its semantics are given by
+--
+-- > evaluate x `seq` y ==> y
+-- > evaluate x `catch` f ==> (return $! x) `catch` f
+-- > evaluate x >>= f ==> (return $! x) >>= f
+--
+-- /Note:/ the first equation implies that @(evaluate x)@ is /not/ the
+-- same as @(return $! x)@. A correct definition is
+--
+-- > evaluate x = (return $! x) >>= return
+--
+evaluate :: a -> IO a
+evaluate a = IO $ \s -> case a `seq` () of () -> (# s, a #)
+ -- NB. can't write
+ -- a `seq` (# s, a #)
+ -- because we can't have an unboxed tuple as a function argument
+\end{code}
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