\begin{code}
-{-# OPTIONS -fno-implicit-prelude #-}
+{-# OPTIONS_GHC -XNoImplicitPrelude #-}
+{-# OPTIONS_HADDOCK hide #-}
-----------------------------------------------------------------------------
-- |
-- Module : GHC.Exception
--
-----------------------------------------------------------------------------
-#ifndef __HUGS__
-module GHC.Exception
- ( module GHC.Exception,
- Exception(..), AsyncException(..),
- IOException(..), ArithException(..), ArrayException(..),
- throw, ioError )
- where
-
-import Data.Either
+-- #hide
+module GHC.Exception where
+import Data.Maybe
+import {-# SOURCE #-} Data.Typeable
import GHC.Base
-import GHC.IOBase
-
-#endif
+import GHC.Show
\end{code}
%*********************************************************
-%* *
-\subsection{Primitive catch}
-%* *
+%* *
+\subsection{Exceptions}
+%* *
%*********************************************************
-catchException used to handle the passing around of the state to the
-action and the handler. This turned out to be a bad idea - it meant
-that we had to wrap both arguments in thunks so they could be entered
-as normal (remember IO returns an unboxed pair...).
-
-Now catch# has type
-
- catch# :: IO a -> (b -> IO a) -> IO a
-
-(well almost; the compiler doesn't know about the IO newtype so we
-have to work around that in the definition of catchException below).
-
\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
-catch m k = catchException m handler
- where handler err@(IOException _) = k err
- handler other = throw other
-\end{code}
-
+data SomeException = forall e . Exception e => SomeException e
+ deriving Typeable
-%*********************************************************
-%* *
-\subsection{Try and bracket}
-%* *
-%*********************************************************
-
-The construct @try comp@ exposes errors which occur within a
-computation, and which are not fully handled. It always succeeds.
+instance Show SomeException where
+ showsPrec p (SomeException e) = showsPrec p e
-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)
+class (Typeable e, Show e) => Exception e where
+ toException :: e -> SomeException
+ fromException :: SomeException -> Maybe e
-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
+ toException = SomeException
+ fromException (SomeException e) = cast 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
+instance Exception SomeException where
+ toException se = se
+ fromException = Just
\end{code}
-
%*********************************************************
-%* *
-\subsection{Controlling asynchronous exception delivery}
-%* *
+%* *
+\subsection{Primitive throw}
+%* *
%*********************************************************
\begin{code}
-#ifndef __HUGS__
-block :: IO a -> IO a
-block (IO io) = IO $ blockAsyncExceptions# io
-
-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
+-- | Throw an exception. Exceptions may be thrown from purely
+-- functional code, but may only be caught within the 'IO' monad.
+throw :: Exception e => e -> a
+throw e = raise# (toException e)
\end{code}
-
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