AsyncException(..), -- instance Eq, Ord, Show, Typeable
-- * Throwing exceptions
+ throwIO, -- :: Exception -> IO a
throw, -- :: Exception -> a
- ioError, -- :: Exception -> IO a
+ ioError, -- :: IOError -> IO a
+#ifdef __GLASGOW_HASKELL__
throwTo, -- :: ThreadId -> Exception -> a
+#endif
-- * Catching Exceptions
-- ** The @evaluate@ function
evaluate, -- :: a -> IO a
+ -- ** The @mapException@ function
+ mapException, -- :: (Exception -> Exception) -> a -> a
+
-- ** Exception predicates
-- $preds
-- $dynamic
throwDyn, -- :: Typeable ex => ex -> b
+#ifdef __GLASGOW_HASKELL__
throwDynTo, -- :: Typeable ex => ThreadId -> ex -> b
+#endif
catchDyn, -- :: Typeable ex => IO a -> (ex -> IO a) -> IO a
-- * Asynchronous Exceptions
bracket, -- :: IO a -> (a -> IO b) -> (a -> IO c) -> IO ()
bracket_, -- :: IO a -> IO b -> IO c -> IO ()
+ bracketOnError,
- finally, -- :: IO a -> IO b -> IO b
-
+ finally, -- :: IO a -> IO b -> IO a
+
+#ifdef __GLASGOW_HASKELL__
+ setUncaughtExceptionHandler, -- :: (Exception -> IO ()) -> IO ()
+ getUncaughtExceptionHandler -- :: IO (Exception -> IO ())
+#endif
) where
#ifdef __GLASGOW_HASKELL__
-import Prelude hiding (catch)
-import System.IO.Error
import GHC.Base ( assert )
-import GHC.Exception hiding (try, catch, bracket, bracket_)
+import GHC.Exception as ExceptionBase hiding (catch)
import GHC.Conc ( throwTo, ThreadId )
-import GHC.IOBase ( IO(..) )
+import Data.IORef ( IORef, newIORef, readIORef, writeIORef )
+import Foreign.C.String ( CString, withCString )
+import System.IO ( stdout, hFlush )
#endif
#ifdef __HUGS__
-import Prelude hiding ( catch )
-import PrelPrim ( catchException
- , Exception(..)
- , throw
- , ArithException(..)
- , AsyncException(..)
- , assert
- )
+import Hugs.Exception as ExceptionBase
#endif
+import Prelude hiding ( catch )
+import System.IO.Error hiding ( catch, try )
+import System.IO.Unsafe (unsafePerformIO)
import Data.Dynamic
-#include "Dynamic.h"
-INSTANCE_TYPEABLE0(Exception,exceptionTc,"Exception")
-INSTANCE_TYPEABLE0(IOException,ioExceptionTc,"IOException")
-INSTANCE_TYPEABLE0(ArithException,arithExceptionTc,"ArithException")
-INSTANCE_TYPEABLE0(ArrayException,arrayExceptionTc,"ArrayException")
-INSTANCE_TYPEABLE0(AsyncException,asyncExceptionTc,"AsyncException")
-
-----------------------------------------------------------------------------
-- Catching exceptions
--
-- Note that 'catch' catches all types of exceptions, and is generally
-- used for \"cleaning up\" before passing on the exception using
--- 'ioError'. It is not good practice to discard the exception and
+-- 'throwIO'. It is not good practice to discard the exception and
-- continue, without first checking the type of the exception (it
-- might be a 'ThreadKilled', for example). In this case it is usually better
-- to use 'catchJust' and select the kinds of exceptions to catch.
--
--- Also note that The "Prelude" also exports a
--- function called 'catch' which has the same type as
--- 'Exception.catch', the difference being that the
--- "Prelude" version only catches the IO and user
+-- Also note that the "Prelude" also exports a function called
+-- 'Prelude.catch' with a similar type to 'Control.Exception.catch',
+-- except that the "Prelude" version only catches the IO and user
-- families of exceptions (as required by Haskell 98). We recommend
-- either hiding the "Prelude" version of
--- 'catch' when importing
+-- 'Prelude.catch' when importing
-- "Control.Exception", or importing
-- "Control.Exception" qualified, to avoid name-clashes.
catch :: IO a -- ^ The computation to run
-> (Exception -> IO a) -- ^ Handler to invoke if an exception is raised
-> IO a
-catch = GHC.Exception.catchException
+catch = ExceptionBase.catchException
-- | The function 'catchJust' is like 'catch', but it takes an extra
-- argument which is an /exception predicate/, a function which
-- exceptions: 'ioErrors', 'arithExceptions', and so on. For example,
-- to catch just calls to the 'error' function, we could use
--
--- > result \<- catchJust errorCalls thing_to_try handler
+-- > result <- catchJust errorCalls thing_to_try handler
--
-- Any other exceptions which are not matched by the predicate
-- are re-raised, and may be caught by an enclosing
handleJust p = flip (catchJust p)
-----------------------------------------------------------------------------
--- evaluate
+-- 'mapException'
--- | Forces its argument to be evaluated, and returns the result in
--- the 'IO' monad. It can be used to order evaluation with respect to
--- other 'IO' operations; its semantics are given by
---
--- > evaluate undefined `seq` return () ==> return ()
--- > catch (evaluate undefined) (\e -> return ()) ==> return ()
---
--- NOTE: @(evaluate a)@ is /not/ the same as @(a \`seq\` return a)@.
-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
+-- | This function maps one exception into another as proposed in the
+-- paper \"A semantics for imprecise exceptions\".
+
+-- Notice that the usage of 'unsafePerformIO' is safe here.
+
+mapException :: (Exception -> Exception) -> a -> a
+mapException f v = unsafePerformIO (catch (evaluate v)
+ (\x -> throw (f x)))
-----------------------------------------------------------------------------
-- 'try' and variations.
-- | Similar to 'catch', but returns an 'Either' result which is
--- @(Right a)@ if no exception was raised, or @(Left e)@ if an
+-- @('Right' a)@ if no exception was raised, or @('Left' e)@ if an
-- exception was raised and its value is @e@.
--
-- > try a = catch (Right \`liftM\` a) (return . Left)
-- to re-throw the exception after performing whatever cleanup is needed.
-- Otherwise, 'tryJust' is generally considered to be better.
--
+-- Also note that "System.IO.Error" also exports a function called
+-- 'System.IO.Error.try' with a similar type to 'Control.Exception.try',
+-- except that it catches only the IO and user families of exceptions
+-- (as required by the Haskell 98 @IO@ module).
+
try :: IO a -> IO (Either Exception a)
try a = catch (a >>= \ v -> return (Right v)) (\e -> return (Left e))
throwDyn :: Typeable exception => exception -> b
throwDyn exception = throw (DynException (toDyn exception))
+#ifdef __GLASGOW_HASKELL__
-- | A variant of 'throwDyn' that throws the dynamic exception to an
--- arbitrary thread (c.f. 'throwTo').
+-- arbitrary thread (GHC only: c.f. 'throwTo').
throwDynTo :: Typeable exception => ThreadId -> exception -> IO ()
throwDynTo t exception = throwTo t (DynException (toDyn exception))
+#endif /* __GLASGOW_HASKELL__ */
-- | Catch dynamic exceptions of the required type. All other
-- exceptions are re-thrown, including dynamic exceptions of the wrong
ioErrors :: Exception -> Maybe IOError
arithExceptions :: Exception -> Maybe ArithException
errorCalls :: Exception -> Maybe String
-dynExceptions :: Exception -> Maybe Dynamic
assertions :: Exception -> Maybe String
+dynExceptions :: Exception -> Maybe Dynamic
asyncExceptions :: Exception -> Maybe AsyncException
userErrors :: Exception -> Maybe String
-ioErrors e@(IOException _) = Just e
+ioErrors (IOException e) = Just e
ioErrors _ = Nothing
arithExceptions (ArithException e) = Just e
asyncExceptions (AsyncException e) = Just e
asyncExceptions _ = Nothing
-userErrors e@IOException{} | isUserError e = Just (ioeGetErrorString e)
+userErrors (IOException e) | isUserError e = Just (ioeGetErrorString e)
userErrors _ = Nothing
-----------------------------------------------------------------------------
bracket_ :: IO a -> IO b -> IO c -> IO c
bracket_ before after thing = bracket before (const after) (const thing)
+-- | Like bracket, but only performs the final action if there was an
+-- exception raised by the in-between computation.
+bracketOnError
+ :: IO a -- ^ computation to run first (\"acquire resource\")
+ -> (a -> IO b) -- ^ computation to run last (\"release resource\")
+ -> (a -> IO c) -- ^ computation to run in-between
+ -> IO c -- returns the value from the in-between computation
+bracketOnError before after thing =
+ block (do
+ a <- before
+ catch
+ (unblock (thing a))
+ (\e -> do { after a; throw e })
+ )
+
-- -----------------------------------------------------------------------------
-- Asynchronous exceptions
> throwTo :: ThreadId -> Exception -> IO ()
-'throwTo' (also 'throwDynTo' and 'Concurrent.killThread') allows one
+'throwTo' (also 'throwDynTo' and 'Control.Concurrent.killThread') allows one
running thread to raise an arbitrary exception in another thread. The
exception is therefore asynchronous with respect to the target thread,
which could be doing anything at the time it receives the exception.
Some operations are /interruptible/, which means that they can receive
asynchronous exceptions even in the scope of a 'block'. Any function
which may itself block is defined as interruptible; this includes
-'takeMVar' (but not 'tryTakeMVar'), and most operations which perform
-some I\/O with the outside world.. The reason for having
+'Control.Concurrent.MVar.takeMVar'
+(but not 'Control.Concurrent.MVar.tryTakeMVar'),
+and most operations which perform
+some I\/O with the outside world. The reason for having
interruptible operations is so that we can write things like
> block (
> (\e -> ...)
> )
-if the 'takeMVar' was not interruptible, then this particular
+if the 'Control.Concurrent.MVar.takeMVar' was not interruptible,
+then this particular
combination could lead to deadlock, because the thread itself would be
blocked in a state where it can\'t receive any asynchronous exceptions.
-With 'takeMVar' interruptible, however, we can be
+With 'Control.Concurrent.MVar.takeMVar' interruptible, however, we can be
safe in the knowledge that the thread can receive exceptions right up
-until the point when the 'takeMVar' succeeds.
+until the point when the 'Control.Concurrent.MVar.takeMVar' succeeds.
Similar arguments apply for other interruptible operations like
-'IO.openFile'.
+'System.IO.openFile'.
-}
--- -----------------------------------------------------------------------------
--- Assert
-
-#ifdef __HADDOCK__
--- | If the first argument evaluates to 'True', then the result is the
--- second argument. Otherwise an 'Assertion' exception is raised,
--- containing a 'String' with the source file and line number of the
--- call to assert.
---
--- Assertions can normally be turned on or off with a compiler flag
--- (for GHC, assertions are normally on unless the @-fignore-asserts@
--- option is give). When assertions are turned off, the first
--- argument to 'assert' is ignored, and the second argument is
--- returned as the result.
+#ifndef __GLASGOW_HASKELL__
assert :: Bool -> a -> a
+assert True x = x
+assert False _ = throw (AssertionFailed "")
+#endif
+
+
+#ifdef __GLASGOW_HASKELL__
+{-# NOINLINE uncaughtExceptionHandler #-}
+uncaughtExceptionHandler :: IORef (Exception -> IO ())
+uncaughtExceptionHandler = unsafePerformIO (newIORef defaultHandler)
+ where
+ defaultHandler :: Exception -> IO ()
+ defaultHandler ex = do
+ (hFlush stdout) `catchException` (\ _ -> return ())
+ let msg = case ex of
+ Deadlock -> "no threads to run: infinite loop or deadlock?"
+ ErrorCall s -> s
+ other -> showsPrec 0 other "\n"
+ withCString "%s" $ \cfmt ->
+ withCString msg $ \cmsg ->
+ errorBelch cfmt cmsg
+
+foreign import ccall unsafe errorBelch :: CString -> CString -> IO ()
+
+setUncaughtExceptionHandler :: (Exception -> IO ()) -> IO ()
+setUncaughtExceptionHandler = writeIORef uncaughtExceptionHandler
+
+getUncaughtExceptionHandler :: IO (Exception -> IO ())
+getUncaughtExceptionHandler = readIORef uncaughtExceptionHandler
#endif
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