-- Exceptions
Exception(..), ArithException(..), AsyncException(..), ArrayException(..),
- stackOverflow, heapOverflow, throw, throwIO, ioException,
+ stackOverflow, heapOverflow, ioException,
IOError, IOException(..), IOErrorType(..), ioError, userError,
- ExitCode(..)
+ ExitCode(..),
+ throwIO, block, unblock, catch, catchAny, catchException,
+ evaluate,
+ -- The RTS calls this
+ nonTermination,
) where
import GHC.ST
import GHC.List
import GHC.Read
import Foreign.C.Types (CInt)
+import GHC.Exception hiding (Exception)
+import qualified GHC.Exception as Exc
#ifndef __HADDOCK__
import {-# SOURCE #-} Data.Typeable ( showsTypeRep )
-- 'String' argument gives the location of the
-- record update in the source program.
+nonTermination :: SomeException
+nonTermination = toException NonTermination
+
+-- For now at least, make the monolithic Exception type an instance of
+-- the Exception class
+instance Exc.Exception Exception
+
-- |The type of arithmetic exceptions
data ArithException
= Overflow
-- may be prohibited (e.g. 0 on a POSIX-compliant system).
deriving (Eq, Ord, Read, Show)
--- --------------------------------------------------------------------------
--- Primitive throw
-
--- | Throw an exception. Exceptions may be thrown from purely
--- functional code, but may only be caught within the 'IO' monad.
-throw :: Exception -> a
-throw exception = raise# exception
-
--- | A variant of 'throw' that can be used within the 'IO' monad.
---
--- Although 'throwIO' has a type that is an instance of the type of 'throw', the
--- two functions are subtly different:
---
--- > throw e `seq` x ===> throw e
--- > throwIO e `seq` x ===> x
---
--- The first example will cause the exception @e@ to be raised,
--- whereas the second one won\'t. In fact, 'throwIO' will only cause
--- an exception to be raised when it is used within the 'IO' monad.
--- The 'throwIO' variant should be used in preference to 'throw' to
--- raise an exception within the 'IO' monad because it guarantees
--- ordering with respect to other 'IO' operations, whereas 'throw'
--- does not.
-throwIO :: Exception -> IO a
-throwIO err = IO $ raiseIO# err
-
ioException :: IOException -> IO a
-ioException err = IO $ raiseIO# (IOException err)
+ioException err = throwIO (IOException err)
-- | Raise an 'IOError' in the 'IO' monad.
ioError :: IOError -> IO a
data IOMode = ReadMode | WriteMode | AppendMode | ReadWriteMode
deriving (Eq, Ord, Ix, Enum, Read, Show)
\end{code}
+
+%*********************************************************
+%* *
+\subsection{Primitive catch and throwIO}
+%* *
+%*********************************************************
+
+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 :: Exception e => IO a -> (e -> IO a) -> IO a
+catchException (IO io) handler = IO $ catch# io handler'
+ where handler' e = case fromException e of
+ Just e' -> unIO (handler e')
+ Nothing -> raise# e
+
+catchAny :: IO a -> (forall e . Exception e => e -> IO a) -> IO a
+catchAny (IO io) handler = IO $ catch# io handler'
+ where handler' (SomeException e) = unIO (handler e)
+
+-- | A variant of 'throw' that can be used within the 'IO' monad.
+--
+-- Although 'throwIO' has a type that is an instance of the type of 'throw', the
+-- two functions are subtly different:
+--
+-- > throw e `seq` x ===> throw e
+-- > throwIO e `seq` x ===> x
+--
+-- The first example will cause the exception @e@ to be raised,
+-- whereas the second one won\'t. In fact, 'throwIO' will only cause
+-- an exception to be raised when it is used within the 'IO' monad.
+-- The 'throwIO' variant should be used in preference to 'throw' to
+-- raise an exception within the 'IO' monad because it guarantees
+-- ordering with respect to other 'IO' operations, whereas 'throw'
+-- does not.
+throwIO :: Exception e => e -> IO a
+throwIO e = IO (raiseIO# (toException e))
+\end{code}
+
+
+%*********************************************************
+%* *
+\subsection{Controlling asynchronous exception delivery}
+%* *
+%*********************************************************
+
+\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
+\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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