\begin{code}
-{-# OPTIONS_GHC -fno-implicit-prelude #-}
+{-# OPTIONS_GHC -XNoImplicitPrelude #-}
{-# OPTIONS_HADDOCK hide #-}
-----------------------------------------------------------------------------
-- |
-----------------------------------------------------------------------------
-- #hide
-module GHC.Exception
- ( module GHC.Exception,
- Exception(..), AsyncException(..),
- IOException(..), ArithException(..), ArrayException(..),
- throw, throwIO, ioError )
- where
+module GHC.Exception where
+import Data.Maybe
+import {-# SOURCE #-} Data.Typeable
import GHC.Base
-import GHC.IOBase
+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
-catchException (IO m) k = IO $ \s -> catch# m (\ex -> unIO (k ex)) s
+ toException = SomeException
+ fromException (SomeException e) = cast e
--- | 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 (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 with 'Control.Exception.throwTo' 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'.
---
--- Threads created by 'Control.Concurrent.forkIO' inherit the blocked
--- state from the parent; that is, to start a thread in blocked mode,
--- use @block $ forkIO ...@. This is particularly useful if you need to
--- establish an exception handler in the forked thread before any
--- asynchronous exceptions are received.
-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
-
-block (IO io) = IO $ blockAsyncExceptions# io
-unblock (IO io) = IO $ unblockAsyncExceptions# io
+-- | 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}
-\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}
projects / ghc-base.git / blobdiff