--
-- Maintainer : libraries@haskell.org
-- Stability : experimental
--- Portability : non-portable
+-- Portability : non-portable (extended exceptions)
--
-- This module provides support for raising and catching both built-in
-- and user-defined exceptions.
--
+-- In addition to exceptions thrown by 'IO' operations, exceptions may
+-- be thrown by pure code (imprecise exceptions) or by external events
+-- (asynchronous exceptions), but may only be caught in the 'IO' monad.
+-- For more details, see:
+--
+-- * /A semantics for imprecise exceptions/, by Simon Peyton Jones,
+-- Alastair Reid, Tony Hoare, Simon Marlow, Fergus Henderson,
+-- in /PLDI'99/.
+--
+-- * /Asynchronous exceptions in Haskell/, by Simon Marlow, Simon Peyton
+-- Jones, Andy Moran and John Reppy, in /PLDI'01/.
+--
-----------------------------------------------------------------------------
module Control.Exception (
-- * The Exception type
Exception(..), -- instance Eq, Ord, Show, Typeable
-#ifdef __GLASGOW_HASKELL__
IOException, -- instance Eq, Ord, Show, Typeable
ArithException(..), -- instance Eq, Ord, Show, Typeable
ArrayException(..), -- instance Eq, Ord, Show, Typeable
AsyncException(..), -- instance Eq, Ord, Show, Typeable
-#endif
-- * Throwing exceptions
throwIO, -- :: Exception -> IO a
-#ifndef __HUGS__
throw, -- :: Exception -> a
-#endif
ioError, -- :: IOError -> IO a
-#ifndef __HUGS__
+#ifdef __GLASGOW_HASKELL__
throwTo, -- :: ThreadId -> Exception -> a
#endif
-- $preds
ioErrors, -- :: Exception -> Maybe IOError
-#ifndef __HUGS__
arithExceptions, -- :: Exception -> Maybe ArithException
errorCalls, -- :: Exception -> Maybe String
dynExceptions, -- :: Exception -> Maybe Dynamic
assertions, -- :: Exception -> Maybe String
asyncExceptions, -- :: Exception -> Maybe AsyncException
-#endif /* __HUGS__ */
userErrors, -- :: Exception -> Maybe String
-#ifdef __GLASGOW_HASKELL__
-- * Dynamic exceptions
-- $dynamic
throwDyn, -- :: Typeable ex => ex -> b
+#ifdef __GLASGOW_HASKELL__
throwDynTo, -- :: Typeable ex => ThreadId -> ex -> b
- catchDyn, -- :: Typeable ex => IO a -> (ex -> IO a) -> IO a
#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 GHC.Base ( assert )
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 System.IO.Unsafe (unsafePerformIO)
import Data.Dynamic
-#include "Dynamic.h"
-INSTANCE_TYPEABLE0(Exception,exceptionTc,"Exception")
-#ifdef __GLASGOW_HASKELL__
-INSTANCE_TYPEABLE0(IOException,ioExceptionTc,"IOException")
-INSTANCE_TYPEABLE0(ArithException,arithExceptionTc,"ArithException")
-INSTANCE_TYPEABLE0(ArrayException,arrayExceptionTc,"ArrayException")
-INSTANCE_TYPEABLE0(AsyncException,asyncExceptionTc,"AsyncException")
-#endif
+#ifdef __NHC__
+import System.IO.Error (catch, ioError)
+import IO (bracket)
+import DIOError -- defn of IOError type
-#ifdef __HUGS__
--- This is as close as Hugs gets to providing throw
-throw :: Exception -> IO a
-throw = throwIO
+-- minimum needed for nhc98 to pretend it has Exceptions
+type Exception = IOError
+type IOException = IOError
+data ArithException
+data ArrayException
+data AsyncException
+
+throwIO :: Exception -> IO a
+throwIO = ioError
+throw :: Exception -> a
+throw = unsafePerformIO . throwIO
+
+evaluate :: a -> IO a
+evaluate x = x `seq` return x
+
+ioErrors :: Exception -> Maybe IOError
+ioErrors e = Just e
+arithExceptions :: Exception -> Maybe ArithException
+arithExceptions = const Nothing
+errorCalls :: Exception -> Maybe String
+errorCalls = const Nothing
+dynExceptions :: Exception -> Maybe Dynamic
+dynExceptions = const Nothing
+assertions :: Exception -> Maybe String
+assertions = const Nothing
+asyncExceptions :: Exception -> Maybe AsyncException
+asyncExceptions = const Nothing
+userErrors :: Exception -> Maybe String
+userErrors (UserError _ s) = Just s
+userErrors _ = Nothing
+
+block :: IO a -> IO a
+block = id
+unblock :: IO a -> IO a
+unblock = id
+
+assert :: Bool -> a -> a
+assert True x = x
+assert False _ = throw (UserError "" "Assertion failed")
#endif
-----------------------------------------------------------------------------
-- argument. Otherwise, the result is returned as normal. For example:
--
-- > catch (openFile f ReadMode)
--- > (\e -> hPutStr stderr (\"Couldn\'t open \"++f++\": \" ++ show e))
+-- > (\e -> hPutStr stderr ("Couldn't open "++f++": " ++ show e))
--
-- For catching exceptions in pure (non-'IO') expressions, see the
-- function 'evaluate'.
-- 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
--- families of exceptions (as required by Haskell 98). We recommend
--- either hiding the "Prelude" version of
--- 'catch' when importing
--- "Control.Exception", or importing
--- "Control.Exception" qualified, to avoid name-clashes.
-
+-- 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 'Prelude.catch'
+-- when importing "Control.Exception":
+--
+-- > import Prelude hiding (catch)
+--
+-- or importing "Control.Exception" qualified, to avoid name-clashes:
+--
+-- > import qualified Control.Exception as C
+--
+-- and then using @C.catch@
+--
+#ifndef __NHC__
catch :: IO a -- ^ The computation to run
-> (Exception -> IO a) -- ^ Handler to invoke if an exception is raised
-> IO a
catch = ExceptionBase.catchException
-
+#endif
-- | The function 'catchJust' is like 'catch', but it takes an extra
-- argument which is an /exception predicate/, a function which
-- selects which type of exceptions we\'re interested in. There are
-- 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
-
--- | 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)@.
-#ifdef __GLASGOW_HASKELL__
-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
-#endif
-
------------------------------------------------------------------------------
-- 'mapException'
-- | This function maps one exception into another as proposed in the
--- paper "A semantics for imprecise exceptions".
+-- paper \"A semantics for imprecise exceptions\".
-- Notice that the usage of 'unsafePerformIO' is safe here.
-- '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)
+-- > try a = catch (Right `liftM` a) (return . Left)
--
-- Note: as with 'catch', it is only polite to use this variant if you intend
-- 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))
Nothing -> throw e
Just b -> return (Left b)
-#ifdef __GLASGOW_HASKELL__
-----------------------------------------------------------------------------
-- Dynamic exceptions
-- | Raise any value as an exception, provided it is in the
-- 'Typeable' class.
throwDyn :: Typeable exception => exception -> b
+#ifdef __NHC__
+throwDyn exception = throw (UserError "" "dynamic exception")
+#else
throwDyn exception = throw (DynException (toDyn exception))
+#endif
+#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
-- with dynamic exceptions used in other libraries.
--
catchDyn :: Typeable exception => IO a -> (exception -> IO a) -> IO a
+#ifdef __NHC__
+catchDyn m k = m -- can't catch dyn exceptions in nhc98
+#else
catchDyn m k = catchException m handle
where handle ex = case ex of
(DynException dyn) ->
Just exception -> k exception
Nothing -> throw ex
_ -> throw ex
-#endif /* __GLASGOW_HASKELL__ */
+#endif
-----------------------------------------------------------------------------
-- Exception Predicates
-- These pre-defined predicates may be used as the first argument to
-- 'catchJust', 'tryJust', or 'handleJust' to select certain common
-- classes of exceptions.
-
-#ifdef __GLASGOW_HASKELL__
+#ifndef __NHC__
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
-#endif /* __GLASGOW_HASKELL__ */
-#ifdef __GLASGOW_HASKELL__
ioErrors (IOException e) = Just e
ioErrors _ = Nothing
userErrors (IOException e) | isUserError e = Just (ioeGetErrorString e)
userErrors _ = Nothing
-#endif /* __GLASGOW_HASKELL__ */
-
+#endif
-----------------------------------------------------------------------------
-- Some Useful Functions
-- The arguments to 'bracket' are in this order so that we can partially apply
-- it, e.g.:
--
--- > withFile name = bracket (openFile name) hClose
+-- > withFile name mode = bracket (openFile name mode) hClose
--
+#ifndef __NHC__
bracket
:: IO a -- ^ computation to run first (\"acquire resource\")
-> (a -> IO b) -- ^ computation to run last (\"release resource\")
after a
return r
)
-
+#endif
-- | A specialised variant of 'bracket' with just a computation to run
-- afterward.
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.
+#if !(__GLASGOW_HASKELL__ || __NHC__)
assert :: Bool -> a -> a
+assert True x = x
+assert False _ = throw (AssertionFailed "")
#endif
-#ifndef __GLASGOW_HASKELL__
-assert :: Bool -> a -> a
-assert True x = x
-assert False _ = error "Assertion failure"
+
+#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 ""
+ withCString "%s" $ \cfmt ->
+ withCString msg $ \cmsg ->
+ errorBelch cfmt cmsg
+
+foreign import ccall unsafe "RtsMessages.h errorBelch"
+ errorBelch :: CString -> CString -> IO ()
+
+setUncaughtExceptionHandler :: (Exception -> IO ()) -> IO ()
+setUncaughtExceptionHandler = writeIORef uncaughtExceptionHandler
+
+getUncaughtExceptionHandler :: IO (Exception -> IO ())
+getUncaughtExceptionHandler = readIORef uncaughtExceptionHandler
#endif
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