Rejig the extensible exceptions so there is less circular importing

[ghc-base.git] / GHC / Exception.lhs

diff --git a/GHC/Exception.lhs b/GHC/Exception.lhs
index 3e26cdc..a285542 100644 (file)
--- a/GHC/Exception.lhs
+++ b/GHC/Exception.lhs
@@ -1,5 +1,6 @@
 \begin{code}
-{-# OPTIONS -fno-implicit-prelude #-}
+{-# OPTIONS_GHC -XNoImplicitPrelude #-}
+{-# OPTIONS_HADDOCK hide #-}
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  GHC.Exception
@@ -14,84 +15,50 @@
 -- 
 -----------------------------------------------------------------------------
 
-#ifndef __HUGS__
-module GHC.Exception 
-       ( module GHC.Exception, 
-         Exception(..), AsyncException(..), 
-         IOException(..), ArithException(..), ArrayException(..),
-         throw, throwIO, ioError ) 
-  where
+-- #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
+\begin{code}
+data SomeException = forall e . Exception e => SomeException e
+    deriving Typeable
 
-    catch# :: IO a -> (b -> IO a) -> IO a
+instance Show SomeException where
+    showsPrec p (SomeException e) = showsPrec p e
 
-(well almost; the compiler doesn't know about the IO newtype so we
-have to work around that in the definition of catchException below).
+class (Typeable e, Show e) => Exception e where
+    toException   :: e -> SomeException
+    fromException :: SomeException -> Maybe e
 
-\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
+    toException = SomeException
+    fromException (SomeException e) = cast e
 
-catch           :: IO a -> (IOError -> IO a) -> IO a 
-catch m k      =  catchException m handler
-  where handler (IOException err)   = k err
-       handler other               = throw other
+instance Exception SomeException where
+    toException se = se
+    fromException = Just
 \end{code}
 
-
 %*********************************************************
-%*                                                     *
-\subsection{Controlling asynchronous exception delivery}
-%*                                                     *
+%*                                                      *
+\subsection{Primitive throw}
+%*                                                      *
 %*********************************************************
 
 \begin{code}
--- | 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
-
--- | 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
-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}
 
-