1 {-# OPTIONS_GHC -XNoImplicitPrelude #-}
5 -----------------------------------------------------------------------------
7 -- Module : Control.Exception
8 -- Copyright : (c) The University of Glasgow 2001
9 -- License : BSD-style (see the file libraries/base/LICENSE)
11 -- Maintainer : libraries@haskell.org
12 -- Stability : experimental
13 -- Portability : non-portable (extended exceptions)
15 -- This module provides support for raising and catching both built-in
16 -- and user-defined exceptions.
18 -- In addition to exceptions thrown by 'IO' operations, exceptions may
19 -- be thrown by pure code (imprecise exceptions) or by external events
20 -- (asynchronous exceptions), but may only be caught in the 'IO' monad.
21 -- For more details, see:
23 -- * /A semantics for imprecise exceptions/, by Simon Peyton Jones,
24 -- Alastair Reid, Tony Hoare, Simon Marlow, Fergus Henderson,
27 -- * /Asynchronous exceptions in Haskell/, by Simon Marlow, Simon Peyton
28 -- Jones, Andy Moran and John Reppy, in /PLDI'01/.
30 -----------------------------------------------------------------------------
32 module Control.Exception (
34 -- * The Exception type
36 Exception(..), -- instance Eq, Ord, Show, Typeable
37 IOException, -- instance Eq, Ord, Show, Typeable
38 ArithException(..), -- instance Eq, Ord, Show, Typeable
39 ArrayException(..), -- instance Eq, Ord, Show, Typeable
41 AsyncException(..), -- instance Eq, Ord, Show, Typeable
42 NonTermination(..), nonTermination,
43 BlockedOnDeadMVar(..),
44 BlockedIndefinitely(..),
45 NestedAtomically(..), nestedAtomically,
54 -- * Throwing exceptions
55 throwIO, -- :: Exception -> IO a
56 throw, -- :: Exception -> a
57 ioError, -- :: IOError -> IO a
58 #ifdef __GLASGOW_HASKELL__
59 throwTo, -- :: ThreadId -> Exception -> a
62 -- * Catching Exceptions
64 -- |There are several functions for catching and examining
65 -- exceptions; all of them may only be used from within the
68 -- ** The @catch@ functions
69 catch, -- :: IO a -> (Exception -> IO a) -> IO a
72 catchJust, -- :: (Exception -> Maybe b) -> IO a -> (b -> IO a) -> IO a
74 -- ** The @handle@ functions
75 handle, -- :: (Exception -> IO a) -> IO a -> IO a
77 handleJust,-- :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a
79 -- ** The @try@ functions
80 try, -- :: IO a -> IO (Either Exception a)
81 tryJust, -- :: (Exception -> Maybe b) -> a -> IO (Either b a)
85 -- ** The @evaluate@ function
86 evaluate, -- :: a -> IO a
88 -- ** The @mapException@ function
89 mapException, -- :: (Exception -> Exception) -> a -> a
91 -- * Asynchronous Exceptions
95 -- ** Asynchronous exception control
97 -- |The following two functions allow a thread to control delivery of
98 -- asynchronous exceptions during a critical region.
100 block, -- :: IO a -> IO a
101 unblock, -- :: IO a -> IO a
102 blocked, -- :: IO Bool
104 -- *** Applying @block@ to an exception handler
108 -- *** Interruptible operations
114 assert, -- :: Bool -> a -> a
118 bracket, -- :: IO a -> (a -> IO b) -> (a -> IO c) -> IO ()
119 bracket_, -- :: IO a -> IO b -> IO c -> IO ()
122 finally, -- :: IO a -> IO b -> IO a
124 recSelError, recConError, irrefutPatError, runtimeError,
125 nonExhaustiveGuardsError, patError, noMethodBindingError,
128 #ifdef __GLASGOW_HASKELL__
129 setUncaughtExceptionHandler, -- :: (Exception -> IO ()) -> IO ()
130 getUncaughtExceptionHandler -- :: IO (Exception -> IO ())
134 #ifdef __GLASGOW_HASKELL__
137 import {-# SOURCE #-} GHC.Handle
141 import GHC.IOBase as ExceptionBase
142 import GHC.Exception hiding ( Exception )
143 import {-# SOURCE #-} GHC.Conc ( ThreadId(ThreadId) )
144 import Foreign.C.String ( CString, withCString )
148 import Hugs.Exception as ExceptionBase
156 import qualified System.IO.Error as H'98 (catch)
157 import System.IO.Error (ioError)
159 import DIOError -- defn of IOError type
160 import System (ExitCode())
162 -- minimum needed for nhc98 to pretend it has Exceptions
163 data Exception = IOException IOException
164 | ArithException ArithException
165 | ArrayException ArrayException
166 | AsyncException AsyncException
167 | ExitException ExitCode
169 type IOException = IOError
173 instance Show ArithException
174 instance Show ArrayException
175 instance Show AsyncException
177 catch :: IO a -> (Exception -> IO a) -> IO a
178 a `catch` b = a `H'98.catch` (b . IOException)
180 throwIO :: Exception -> IO a
181 throwIO (IOException e) = ioError e
182 throwIO _ = ioError (UserError "Control.Exception.throwIO"
184 throw :: Exception -> a
185 throw = unsafePerformIO . throwIO
187 evaluate :: a -> IO a
188 evaluate x = x `seq` return x
190 assert :: Bool -> a -> a
192 assert False _ = throw (IOException (UserError "" "Assertion failed"))
195 #ifndef __GLASGOW_HASKELL__
196 -- Dummy definitions for implementations lacking asynchonous exceptions
198 block :: IO a -> IO a
200 unblock :: IO a -> IO a
203 blocked = return False
206 -----------------------------------------------------------------------------
207 -- Catching exceptions
209 -- |This is the simplest of the exception-catching functions. It
210 -- takes a single argument, runs it, and if an exception is raised
211 -- the \"handler\" is executed, with the value of the exception passed as an
212 -- argument. Otherwise, the result is returned as normal. For example:
214 -- > catch (openFile f ReadMode)
215 -- > (\e -> hPutStr stderr ("Couldn't open "++f++": " ++ show e))
217 -- For catching exceptions in pure (non-'IO') expressions, see the
218 -- function 'evaluate'.
220 -- Note that due to Haskell\'s unspecified evaluation order, an
221 -- expression may return one of several possible exceptions: consider
222 -- the expression @error \"urk\" + 1 \`div\` 0@. Does
223 -- 'catch' execute the handler passing
224 -- @ErrorCall \"urk\"@, or @ArithError DivideByZero@?
226 -- The answer is \"either\": 'catch' makes a
227 -- non-deterministic choice about which exception to catch. If you
228 -- call it again, you might get a different exception back. This is
229 -- ok, because 'catch' is an 'IO' computation.
231 -- Note that 'catch' catches all types of exceptions, and is generally
232 -- used for \"cleaning up\" before passing on the exception using
233 -- 'throwIO'. It is not good practice to discard the exception and
234 -- continue, without first checking the type of the exception (it
235 -- might be a 'ThreadKilled', for example). In this case it is usually better
236 -- to use 'catchJust' and select the kinds of exceptions to catch.
238 -- Also note that the "Prelude" also exports a function called
239 -- 'Prelude.catch' with a similar type to 'Control.Exception.catch',
240 -- except that the "Prelude" version only catches the IO and user
241 -- families of exceptions (as required by Haskell 98).
243 -- We recommend either hiding the "Prelude" version of 'Prelude.catch'
244 -- when importing "Control.Exception":
246 -- > import Prelude hiding (catch)
248 -- or importing "Control.Exception" qualified, to avoid name-clashes:
250 -- > import qualified Control.Exception as C
252 -- and then using @C.catch@
256 => IO a -- ^ The computation to run
257 -> (e -> IO a) -- ^ Handler to invoke if an exception is raised
259 catch = ExceptionBase.catchException
261 catches :: IO a -> [Handler a] -> IO a
262 catches io handlers = io `catch` catchesHandler handlers
264 catchesHandler :: [Handler a] -> SomeException -> IO a
265 catchesHandler handlers e = foldr tryHandler (throw e) handlers
266 where tryHandler (Handler handler) res
267 = case fromException e of
268 Just e' -> handler e'
271 data Handler a = forall e . Exception e => Handler (e -> IO a)
273 -- | The function 'catchJust' is like 'catch', but it takes an extra
274 -- argument which is an /exception predicate/, a function which
275 -- selects which type of exceptions we\'re interested in.
277 -- > result <- catchJust errorCalls thing_to_try handler
279 -- Any other exceptions which are not matched by the predicate
280 -- are re-raised, and may be caught by an enclosing
281 -- 'catch' or 'catchJust'.
284 => (e -> Maybe b) -- ^ Predicate to select exceptions
285 -> IO a -- ^ Computation to run
286 -> (b -> IO a) -- ^ Handler
288 catchJust p a handler = catch a handler'
289 where handler' e = case p e of
293 -- | A version of 'catch' with the arguments swapped around; useful in
294 -- situations where the code for the handler is shorter. For example:
296 -- > do handle (\e -> exitWith (ExitFailure 1)) $
298 handle :: Exception e => (e -> IO a) -> IO a -> IO a
301 handleAny :: (forall e . Exception e => e -> IO a) -> IO a -> IO a
302 handleAny = flip catchAny
304 -- | A version of 'catchJust' with the arguments swapped around (see
306 handleJust :: Exception e => (e -> Maybe b) -> (b -> IO a) -> IO a -> IO a
307 handleJust p = flip (catchJust p)
309 -----------------------------------------------------------------------------
312 -- | This function maps one exception into another as proposed in the
313 -- paper \"A semantics for imprecise exceptions\".
315 -- Notice that the usage of 'unsafePerformIO' is safe here.
317 mapException :: Exception e => (e -> e) -> a -> a
318 mapException f v = unsafePerformIO (catch (evaluate v)
321 -----------------------------------------------------------------------------
322 -- 'try' and variations.
324 -- | Similar to 'catch', but returns an 'Either' result which is
325 -- @('Right' a)@ if no exception was raised, or @('Left' e)@ if an
326 -- exception was raised and its value is @e@.
328 -- > try a = catch (Right `liftM` a) (return . Left)
330 -- Note: as with 'catch', it is only polite to use this variant if you intend
331 -- to re-throw the exception after performing whatever cleanup is needed.
332 -- Otherwise, 'tryJust' is generally considered to be better.
334 -- Also note that "System.IO.Error" also exports a function called
335 -- 'System.IO.Error.try' with a similar type to 'Control.Exception.try',
336 -- except that it catches only the IO and user families of exceptions
337 -- (as required by the Haskell 98 @IO@ module).
339 try :: Exception e => IO a -> IO (Either e a)
340 try a = catch (a >>= \ v -> return (Right v)) (\e -> return (Left e))
342 -- | A variant of 'try' that takes an exception predicate to select
343 -- which exceptions are caught (c.f. 'catchJust'). If the exception
344 -- does not match the predicate, it is re-thrown.
345 tryJust :: Exception e => (e -> Maybe b) -> IO a -> IO (Either b a)
349 Right v -> return (Right v)
350 Left e -> case p e of
352 Just b -> return (Left b)
354 ignoreExceptions :: IO () -> IO ()
355 ignoreExceptions io = io `catchAny` \_ -> return ()
357 onException :: IO a -> IO () -> IO a
358 onException io what = io `catch` \e -> do what
359 throw (e :: SomeException)
361 -----------------------------------------------------------------------------
362 -- Some Useful Functions
364 -- | When you want to acquire a resource, do some work with it, and
365 -- then release the resource, it is a good idea to use 'bracket',
366 -- because 'bracket' will install the necessary exception handler to
367 -- release the resource in the event that an exception is raised
368 -- during the computation. If an exception is raised, then 'bracket' will
369 -- re-raise the exception (after performing the release).
371 -- A common example is opening a file:
374 -- > (openFile "filename" ReadMode)
376 -- > (\handle -> do { ... })
378 -- The arguments to 'bracket' are in this order so that we can partially apply
381 -- > withFile name mode = bracket (openFile name mode) hClose
385 :: IO a -- ^ computation to run first (\"acquire resource\")
386 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
387 -> (a -> IO c) -- ^ computation to run in-between
388 -> IO c -- returns the value from the in-between computation
389 bracket before after thing =
394 (\e -> do { after a; throw e })
400 -- | A specialised variant of 'bracket' with just a computation to run
403 finally :: IO a -- ^ computation to run first
404 -> IO b -- ^ computation to run afterward (even if an exception
406 -> IO a -- returns the value from the first computation
411 (\e -> do { sequel; throw e })
416 -- | A variant of 'bracket' where the return value from the first computation
418 bracket_ :: IO a -> IO b -> IO c -> IO c
419 bracket_ before after thing = bracket before (const after) (const thing)
421 -- | Like bracket, but only performs the final action if there was an
422 -- exception raised by the in-between computation.
424 :: IO a -- ^ computation to run first (\"acquire resource\")
425 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
426 -> (a -> IO c) -- ^ computation to run in-between
427 -> IO c -- returns the value from the in-between computation
428 bracketOnError before after thing =
433 (\e -> do { after a; throw e })
436 -- -----------------------------------------------------------------------------
437 -- Asynchronous exceptions
441 #AsynchronousExceptions# Asynchronous exceptions are so-called because they arise due to
442 external influences, and can be raised at any point during execution.
443 'StackOverflow' and 'HeapOverflow' are two examples of
444 system-generated asynchronous exceptions.
446 The primary source of asynchronous exceptions, however, is
449 > throwTo :: ThreadId -> Exception -> IO ()
451 'throwTo' (also 'throwDynTo' and 'Control.Concurrent.killThread') allows one
452 running thread to raise an arbitrary exception in another thread. The
453 exception is therefore asynchronous with respect to the target thread,
454 which could be doing anything at the time it receives the exception.
455 Great care should be taken with asynchronous exceptions; it is all too
456 easy to introduce race conditions by the over zealous use of
461 There\'s an implied 'block' around every exception handler in a call
462 to one of the 'catch' family of functions. This is because that is
463 what you want most of the time - it eliminates a common race condition
464 in starting an exception handler, because there may be no exception
465 handler on the stack to handle another exception if one arrives
466 immediately. If asynchronous exceptions are blocked on entering the
467 handler, though, we have time to install a new exception handler
468 before being interrupted. If this weren\'t the default, one would have
469 to write something like
472 > catch (unblock (...))
476 If you need to unblock asynchronous exceptions again in the exception
477 handler, just use 'unblock' as normal.
479 Note that 'try' and friends /do not/ have a similar default, because
480 there is no exception handler in this case. If you want to use 'try'
481 in an asynchronous-exception-safe way, you will need to use
487 Some operations are /interruptible/, which means that they can receive
488 asynchronous exceptions even in the scope of a 'block'. Any function
489 which may itself block is defined as interruptible; this includes
490 'Control.Concurrent.MVar.takeMVar'
491 (but not 'Control.Concurrent.MVar.tryTakeMVar'),
492 and most operations which perform
493 some I\/O with the outside world. The reason for having
494 interruptible operations is so that we can write things like
498 > catch (unblock (...))
502 if the 'Control.Concurrent.MVar.takeMVar' was not interruptible,
504 combination could lead to deadlock, because the thread itself would be
505 blocked in a state where it can\'t receive any asynchronous exceptions.
506 With 'Control.Concurrent.MVar.takeMVar' interruptible, however, we can be
507 safe in the knowledge that the thread can receive exceptions right up
508 until the point when the 'Control.Concurrent.MVar.takeMVar' succeeds.
509 Similar arguments apply for other interruptible operations like
510 'System.IO.openFile'.
513 #if !(__GLASGOW_HASKELL__ || __NHC__)
514 assert :: Bool -> a -> a
516 assert False _ = throw (AssertionFailed "")
520 #ifdef __GLASGOW_HASKELL__
521 {-# NOINLINE uncaughtExceptionHandler #-}
522 uncaughtExceptionHandler :: IORef (SomeException -> IO ())
523 uncaughtExceptionHandler = unsafePerformIO (newIORef defaultHandler)
525 defaultHandler :: SomeException -> IO ()
526 defaultHandler se@(SomeException ex) = do
527 (hFlush stdout) `catchAny` (\ _ -> return ())
528 let msg = case cast ex of
529 Just Deadlock -> "no threads to run: infinite loop or deadlock?"
531 Just (ErrorCall s) -> s
532 _ -> showsPrec 0 se ""
533 withCString "%s" $ \cfmt ->
534 withCString msg $ \cmsg ->
537 -- don't use errorBelch() directly, because we cannot call varargs functions
539 foreign import ccall unsafe "HsBase.h errorBelch2"
540 errorBelch :: CString -> CString -> IO ()
542 setUncaughtExceptionHandler :: (SomeException -> IO ()) -> IO ()
543 setUncaughtExceptionHandler = writeIORef uncaughtExceptionHandler
545 getUncaughtExceptionHandler :: IO (SomeException -> IO ())
546 getUncaughtExceptionHandler = readIORef uncaughtExceptionHandler
549 recSelError, recConError, irrefutPatError, runtimeError,
550 nonExhaustiveGuardsError, patError, noMethodBindingError
551 :: Addr# -> a -- All take a UTF8-encoded C string
553 recSelError s = throw (RecSelError (unpackCStringUtf8# s)) -- No location info unfortunately
554 runtimeError s = error (unpackCStringUtf8# s) -- No location info unfortunately
556 nonExhaustiveGuardsError s = throw (PatternMatchFail (untangle s "Non-exhaustive guards in"))
557 irrefutPatError s = throw (PatternMatchFail (untangle s "Irrefutable pattern failed for pattern"))
558 recConError s = throw (RecConError (untangle s "Missing field in record construction"))
559 noMethodBindingError s = throw (NoMethodError (untangle s "No instance nor default method for class operation"))
560 patError s = throw (PatternMatchFail (untangle s "Non-exhaustive patterns in"))
564 data PatternMatchFail = PatternMatchFail String
565 INSTANCE_TYPEABLE0(PatternMatchFail,patternMatchFailTc,"PatternMatchFail")
567 instance Exception PatternMatchFail
569 instance Show PatternMatchFail where
570 showsPrec _ (PatternMatchFail err) = showString err
574 data RecSelError = RecSelError String
575 INSTANCE_TYPEABLE0(RecSelError,recSelErrorTc,"RecSelError")
577 instance Exception RecSelError
579 instance Show RecSelError where
580 showsPrec _ (RecSelError err) = showString err
584 data RecConError = RecConError String
585 INSTANCE_TYPEABLE0(RecConError,recConErrorTc,"RecConError")
587 instance Exception RecConError
589 instance Show RecConError where
590 showsPrec _ (RecConError err) = showString err
594 data RecUpdError = RecUpdError String
595 INSTANCE_TYPEABLE0(RecUpdError,recUpdErrorTc,"RecUpdError")
597 instance Exception RecUpdError
599 instance Show RecUpdError where
600 showsPrec _ (RecUpdError err) = showString err
604 data NoMethodError = NoMethodError String
605 INSTANCE_TYPEABLE0(NoMethodError,noMethodErrorTc,"NoMethodError")
607 instance Exception NoMethodError
609 instance Show NoMethodError where
610 showsPrec _ (NoMethodError err) = showString err
614 data AssertionFailed = AssertionFailed String
615 INSTANCE_TYPEABLE0(AssertionFailed,assertionFailedTc,"AssertionFailed")
617 instance Exception AssertionFailed
619 instance Show AssertionFailed where
620 showsPrec _ (AssertionFailed err) = showString err
624 data NonTermination = NonTermination
625 INSTANCE_TYPEABLE0(NonTermination,nonTerminationTc,"NonTermination")
627 instance Exception NonTermination
629 instance Show NonTermination where
630 showsPrec _ NonTermination = showString "<<loop>>"
632 -- GHC's RTS calls this
633 nonTermination :: SomeException
634 nonTermination = toException NonTermination
638 data Deadlock = Deadlock
639 INSTANCE_TYPEABLE0(Deadlock,deadlockTc,"Deadlock")
641 instance Exception Deadlock
643 instance Show Deadlock where
644 showsPrec _ Deadlock = showString "<<deadlock>>"
648 data NestedAtomically = NestedAtomically
649 INSTANCE_TYPEABLE0(NestedAtomically,nestedAtomicallyTc,"NestedAtomically")
651 instance Exception NestedAtomically
653 instance Show NestedAtomically where
654 showsPrec _ NestedAtomically = showString "Control.Concurrent.STM.atomically was nested"
656 -- GHC's RTS calls this
657 nestedAtomically :: SomeException
658 nestedAtomically = toException NestedAtomically
662 instance Exception Dynamic
666 assertError :: Addr# -> Bool -> a -> a
667 assertError str pred v
669 | otherwise = throw (AssertionFailed (untangle str "Assertion failed"))
672 (untangle coded message) expects "coded" to be of the form
675 location message details
677 untangle :: Addr# -> String -> String
678 untangle coded message
685 coded_str = unpackCStringUtf8# coded
688 = case (span not_bar coded_str) of { (loc, rest) ->
690 ('|':det) -> (loc, ' ' : det)
696 throwTo :: Exception e => ThreadId -> e -> IO ()
697 throwTo (ThreadId id) ex = IO $ \ s ->
698 case (killThread# id (toException ex) s) of s1 -> (# s1, () #)