--
-- 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
- IOException, -- instance Eq, Ord, Show, Typeable
- ArithException(..), -- instance Eq, Ord, Show, Typeable
- ArrayException(..), -- instance Eq, Ord, Show, Typeable
- AsyncException(..), -- instance Eq, Ord, Show, Typeable
+ -- * The Exception type
+ Exception(..), -- instance Eq, Ord, Show, Typeable
+ IOException, -- instance Eq, Ord, Show, Typeable
+ ArithException(..), -- instance Eq, Ord, Show, Typeable
+ ArrayException(..), -- instance Eq, Ord, Show, Typeable
+ AsyncException(..), -- instance Eq, Ord, Show, Typeable
+
+ -- * Throwing exceptions
+ throwIO, -- :: Exception -> IO a
+ throw, -- :: Exception -> a
+ ioError, -- :: IOError -> IO a
+#ifdef __GLASGOW_HASKELL__
+ throwTo, -- :: ThreadId -> Exception -> a
+#endif
- -- * Throwing exceptions
- throw, -- :: Exception -> a
- ioError, -- :: Exception -> IO a
- throwTo, -- :: ThreadId -> Exception -> a
+ -- * Catching Exceptions
- -- * Catching Exceptions
+ -- |There are several functions for catching and examining
+ -- exceptions; all of them may only be used from within the
+ -- 'IO' monad.
- -- |There are several functions for catching and examining
- -- exceptions; all of them may only be used from within the
- -- 'IO' monad.
+ -- ** The @catch@ functions
+ catch, -- :: IO a -> (Exception -> IO a) -> IO a
+ catchAny,
+ catchJust, -- :: (Exception -> Maybe b) -> IO a -> (b -> IO a) -> IO a
- -- ** The @catch@ functions
- catch, -- :: IO a -> (Exception -> IO a) -> IO a
- catchJust, -- :: (Exception -> Maybe b) -> IO a -> (b -> IO a) -> IO a
+ -- ** The @handle@ functions
+ handle, -- :: (Exception -> IO a) -> IO a -> IO a
+ handleJust,-- :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a
- -- ** The @handle@ functions
- handle, -- :: (Exception -> IO a) -> IO a -> IO a
- handleJust,-- :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a
+ -- ** The @try@ functions
+ try, -- :: IO a -> IO (Either Exception a)
+ tryJust, -- :: (Exception -> Maybe b) -> a -> IO (Either b a)
- -- ** The @try@ functions
- try, -- :: IO a -> IO (Either Exception a)
- tryJust, -- :: (Exception -> Maybe b) -> a -> IO (Either b a)
+ -- ** The @evaluate@ function
+ evaluate, -- :: a -> IO a
- -- ** The @evaluate@ function
- evaluate, -- :: a -> IO a
+ -- ** The @mapException@ function
+ mapException, -- :: (Exception -> Exception) -> a -> a
- -- ** Exception predicates
-
- -- $preds
+ -- ** Exception predicates
+
+ -- $preds
- ioErrors, -- :: Exception -> Maybe IOError
- arithExceptions, -- :: Exception -> Maybe ArithException
- errorCalls, -- :: Exception -> Maybe String
- dynExceptions, -- :: Exception -> Maybe Dynamic
- assertions, -- :: Exception -> Maybe String
- asyncExceptions, -- :: Exception -> Maybe AsyncException
- userErrors, -- :: Exception -> Maybe String
+ ioErrors, -- :: Exception -> Maybe IOError
+ arithExceptions, -- :: Exception -> Maybe ArithException
+ errorCalls, -- :: Exception -> Maybe String
+ dynExceptions, -- :: Exception -> Maybe Dynamic
+ assertions, -- :: Exception -> Maybe String
+ asyncExceptions, -- :: Exception -> Maybe AsyncException
+ userErrors, -- :: Exception -> Maybe String
- -- * Dynamic exceptions
+ -- * Dynamic exceptions
- -- $dynamic
- throwDyn, -- :: Typeable ex => ex -> b
- throwDynTo, -- :: Typeable ex => ThreadId -> ex -> b
- catchDyn, -- :: Typeable ex => IO a -> (ex -> IO a) -> IO a
-
- -- * Asynchronous Exceptions
+ -- $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
- -- $async
+ -- $async
- -- ** Asynchronous exception control
+ -- ** Asynchronous exception control
- -- |The following two functions allow a thread to control delivery of
- -- asynchronous exceptions during a critical region.
+ -- |The following two functions allow a thread to control delivery of
+ -- asynchronous exceptions during a critical region.
block, -- :: IO a -> IO a
unblock, -- :: IO a -> IO a
+ blocked, -- :: IO Bool
- -- *** Applying @block@ to an exception handler
+ -- *** Applying @block@ to an exception handler
- -- $block_handler
+ -- $block_handler
- -- *** Interruptible operations
+ -- *** Interruptible operations
- -- $interruptible
+ -- $interruptible
- -- * Assertions
+ -- * Assertions
- assert, -- :: Bool -> a -> a
+ assert, -- :: Bool -> a -> a
- -- * Utilities
+ -- * Utilities
- bracket, -- :: IO a -> (a -> IO b) -> (a -> IO c) -> IO ()
- bracket_, -- :: IO a -> IO b -> IO c -> IO ()
-
- finally, -- :: IO a -> IO b -> IO b
+ 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 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.Conc ( throwTo, ThreadId )
-import GHC.IOBase ( IO(..) )
+import GHC.Base ( assert )
+import GHC.IOBase
+import GHC.Exception as ExceptionBase hiding (Exception, catch)
+import GHC.Conc ( throwTo, ThreadId )
+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")
+#ifdef __NHC__
+import qualified System.IO.Error as H'98 (catch)
+import System.IO.Error (ioError)
+import IO (bracket)
+import DIOError -- defn of IOError type
+import System (ExitCode())
+
+-- minimum needed for nhc98 to pretend it has Exceptions
+data Exception = IOException IOException
+ | ArithException ArithException
+ | ArrayException ArrayException
+ | AsyncException AsyncException
+ | ExitException ExitCode
+ deriving Show
+type IOException = IOError
+data ArithException
+data ArrayException
+data AsyncException
+instance Show ArithException
+instance Show ArrayException
+instance Show AsyncException
+
+catch :: IO a -> (Exception -> IO a) -> IO a
+a `catch` b = a `H'98.catch` (b . IOException)
+
+throwIO :: Exception -> IO a
+throwIO (IOException e) = ioError e
+throwIO _ = ioError (UserError "Control.Exception.throwIO"
+ "unknown exception")
+throw :: Exception -> a
+throw = unsafePerformIO . throwIO
+
+evaluate :: a -> IO a
+evaluate x = x `seq` return x
+
+ioErrors :: Exception -> Maybe IOError
+ioErrors (IOException e) = Just e
+ioErrors _ = Nothing
+arithExceptions :: Exception -> Maybe ArithException
+arithExceptions (ArithException e) = Just e
+arithExceptions _ = 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 (IOException (UserError _ s)) = Just s
+userErrors _ = Nothing
+
+assert :: Bool -> a -> a
+assert True x = x
+assert False _ = throw (IOException (UserError "" "Assertion failed"))
+#endif
+
+#ifndef __GLASGOW_HASKELL__
+-- Dummy definitions for implementations lacking asynchonous exceptions
+
+block :: IO a -> IO a
+block = id
+unblock :: IO a -> IO a
+unblock = id
+blocked :: IO Bool
+blocked = return False
+#endif
-----------------------------------------------------------------------------
-- Catching exceptions
-- 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'.
--
-- 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
--- 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.
-
-catch :: IO a -- ^ The computation to run
- -> (Exception -> IO a) -- ^ Handler to invoke if an exception is raised
- -> IO a
-catch = GHC.Exception.catchException
-
+-- 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
-- 'catch' or 'catchJust'.
catchJust
- :: (Exception -> Maybe b) -- ^ Predicate to select exceptions
- -> IO a -- ^ Computation to run
- -> (b -> IO a) -- ^ Handler
- -> IO a
+ :: (Exception -> Maybe b) -- ^ Predicate to select exceptions
+ -> IO a -- ^ Computation to run
+ -> (b -> IO a) -- ^ Handler
+ -> IO a
catchJust p a handler = catch a handler'
where handler' e = case p e of
- Nothing -> throw e
- Just b -> handler b
+ Nothing -> throw e
+ Just b -> handler b
-- | A version of 'catch' with the arguments swapped around; useful in
-- situations where the code for the handler is shorter. For example:
--
-- > do handle (\e -> exitWith (ExitFailure 1)) $
--- > ...
-handle :: (Exception -> IO a) -> IO a -> IO a
+-- > ...
+handle :: (Exception -> IO a) -> IO a -> IO a
handle = flip catch
-- | A version of 'catchJust' with the arguments swapped around (see
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)
+-- > 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))
tryJust p a = do
r <- try a
case r of
- Right v -> return (Right v)
- Left e -> case p e of
- Nothing -> throw e
- Just b -> return (Left b)
+ Right v -> return (Right v)
+ Left e -> case p e of
+ Nothing -> throw e
+ Just b -> return (Left b)
-----------------------------------------------------------------------------
-- Dynamic exceptions
-- $dynamic
--- Because the 'Exception' datatype is not extensible, there is an
+-- #DynamicExceptions# Because the 'Exception' datatype is not extensible, there is an
-- interface for throwing and catching exceptions of type 'Dynamic'
-- (see "Data.Dynamic") which allows exception values of any type in
-- the 'Typeable' class to be thrown and caught.
-- | Raise any value as an exception, provided it is in the
-- 'Typeable' class.
throwDyn :: Typeable exception => exception -> b
+#ifdef __NHC__
+throwDyn exception = throw (IOException (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) ->
- case fromDynamic dyn of
- Just exception -> k exception
- Nothing -> throw ex
- _ -> throw ex
+ (DynException dyn) ->
+ case fromDynamic dyn of
+ Just exception -> k exception
+ Nothing -> throw ex
+ _ -> throw ex
+#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.
-
-ioErrors :: Exception -> Maybe IOError
-arithExceptions :: Exception -> Maybe ArithException
-errorCalls :: Exception -> Maybe String
-dynExceptions :: Exception -> Maybe Dynamic
-assertions :: Exception -> Maybe String
-asyncExceptions :: Exception -> Maybe AsyncException
-userErrors :: Exception -> Maybe String
-
-ioErrors e@(IOException _) = Just e
+#ifndef __NHC__
+ioErrors :: Exception -> Maybe IOError
+arithExceptions :: Exception -> Maybe ArithException
+errorCalls :: Exception -> Maybe String
+assertions :: Exception -> Maybe String
+dynExceptions :: Exception -> Maybe Dynamic
+asyncExceptions :: Exception -> Maybe AsyncException
+userErrors :: Exception -> Maybe String
+
+ioErrors (IOException e) = Just e
ioErrors _ = Nothing
arithExceptions (ArithException e) = Just e
asyncExceptions (AsyncException e) = Just e
asyncExceptions _ = Nothing
-userErrors e | isUserError e = Just (ioeGetErrorString e)
+userErrors (IOException e) | isUserError e = Just (ioeGetErrorString e)
userErrors _ = Nothing
-
+#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\")
- -> (a -> IO c) -- ^ computation to run in-between
- -> IO c -- returns the value from the in-between computation
+ :: 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
bracket before after thing =
block (do
a <- before
r <- catch
- (unblock (thing a))
- (\e -> do { after a; throw e })
+ (unblock (thing a))
+ (\e -> do { after a; throw e })
after a
return r
)
-
+#endif
-- | A specialised variant of 'bracket' with just a computation to run
-- afterward.
--
-finally :: IO a -- ^ computation to run first
- -> IO b -- ^ computation to run afterward (even if an exception
- -- was raised)
- -> IO a -- returns the value from the first computation
+finally :: IO a -- ^ computation to run first
+ -> IO b -- ^ computation to run afterward (even if an exception
+ -- was raised)
+ -> IO a -- returns the value from the first computation
a `finally` sequel =
block (do
r <- catch
- (unblock a)
- (\e -> do { sequel; throw e })
+ (unblock a)
+ (\e -> do { sequel; throw e })
sequel
return r
)
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
{- $async
-Asynchronous exceptions are so-called because they arise due to
+ #AsynchronousExceptions# Asynchronous exceptions are so-called because they arise due to
external influences, and can be raised at any point during execution.
'StackOverflow' and 'HeapOverflow' are two examples of
system-generated 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
+
+
+#ifdef __GLASGOW_HASKELL__
+{-# NOINLINE uncaughtExceptionHandler #-}
+uncaughtExceptionHandler :: IORef (Exception -> IO ())
+uncaughtExceptionHandler = unsafePerformIO (newIORef defaultHandler)
+ where
+ defaultHandler :: Exception -> IO ()
+ defaultHandler ex = do
+ (hFlush stdout) `catchAny` (\ _ -> 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
+
+-- don't use errorBelch() directly, because we cannot call varargs functions
+-- using the FFI.
+foreign import ccall unsafe "HsBase.h errorBelch2"
+ errorBelch :: CString -> CString -> IO ()
+
+setUncaughtExceptionHandler :: (Exception -> IO ()) -> IO ()
+setUncaughtExceptionHandler = writeIORef uncaughtExceptionHandler
+
+getUncaughtExceptionHandler :: IO (Exception -> IO ())
+getUncaughtExceptionHandler = readIORef uncaughtExceptionHandler
#endif
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