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,
127 #ifdef __GLASGOW_HASKELL__
128 setUncaughtExceptionHandler, -- :: (Exception -> IO ()) -> IO ()
129 getUncaughtExceptionHandler -- :: IO (Exception -> IO ())
133 #ifdef __GLASGOW_HASKELL__
136 import {-# SOURCE #-} GHC.Handle
140 import GHC.IOBase as ExceptionBase
141 import GHC.Exception hiding ( Exception )
142 import {-# SOURCE #-} GHC.Conc ( ThreadId(ThreadId) )
143 import Foreign.C.String ( CString, withCString )
147 import Hugs.Exception as ExceptionBase
155 import qualified System.IO.Error as H'98 (catch)
156 import System.IO.Error (ioError)
158 import DIOError -- defn of IOError type
159 import System (ExitCode())
161 -- minimum needed for nhc98 to pretend it has Exceptions
162 data Exception = IOException IOException
163 | ArithException ArithException
164 | ArrayException ArrayException
165 | AsyncException AsyncException
166 | ExitException ExitCode
168 type IOException = IOError
172 instance Show ArithException
173 instance Show ArrayException
174 instance Show AsyncException
176 catch :: IO a -> (Exception -> IO a) -> IO a
177 a `catch` b = a `H'98.catch` (b . IOException)
179 throwIO :: Exception -> IO a
180 throwIO (IOException e) = ioError e
181 throwIO _ = ioError (UserError "Control.Exception.throwIO"
183 throw :: Exception -> a
184 throw = unsafePerformIO . throwIO
186 evaluate :: a -> IO a
187 evaluate x = x `seq` return x
189 assert :: Bool -> a -> a
191 assert False _ = throw (IOException (UserError "" "Assertion failed"))
194 #ifndef __GLASGOW_HASKELL__
195 -- Dummy definitions for implementations lacking asynchonous exceptions
197 block :: IO a -> IO a
199 unblock :: IO a -> IO a
202 blocked = return False
205 -----------------------------------------------------------------------------
206 -- Catching exceptions
208 -- |This is the simplest of the exception-catching functions. It
209 -- takes a single argument, runs it, and if an exception is raised
210 -- the \"handler\" is executed, with the value of the exception passed as an
211 -- argument. Otherwise, the result is returned as normal. For example:
213 -- > catch (openFile f ReadMode)
214 -- > (\e -> hPutStr stderr ("Couldn't open "++f++": " ++ show e))
216 -- For catching exceptions in pure (non-'IO') expressions, see the
217 -- function 'evaluate'.
219 -- Note that due to Haskell\'s unspecified evaluation order, an
220 -- expression may return one of several possible exceptions: consider
221 -- the expression @error \"urk\" + 1 \`div\` 0@. Does
222 -- 'catch' execute the handler passing
223 -- @ErrorCall \"urk\"@, or @ArithError DivideByZero@?
225 -- The answer is \"either\": 'catch' makes a
226 -- non-deterministic choice about which exception to catch. If you
227 -- call it again, you might get a different exception back. This is
228 -- ok, because 'catch' is an 'IO' computation.
230 -- Note that 'catch' catches all types of exceptions, and is generally
231 -- used for \"cleaning up\" before passing on the exception using
232 -- 'throwIO'. It is not good practice to discard the exception and
233 -- continue, without first checking the type of the exception (it
234 -- might be a 'ThreadKilled', for example). In this case it is usually better
235 -- to use 'catchJust' and select the kinds of exceptions to catch.
237 -- Also note that the "Prelude" also exports a function called
238 -- 'Prelude.catch' with a similar type to 'Control.Exception.catch',
239 -- except that the "Prelude" version only catches the IO and user
240 -- families of exceptions (as required by Haskell 98).
242 -- We recommend either hiding the "Prelude" version of 'Prelude.catch'
243 -- when importing "Control.Exception":
245 -- > import Prelude hiding (catch)
247 -- or importing "Control.Exception" qualified, to avoid name-clashes:
249 -- > import qualified Control.Exception as C
251 -- and then using @C.catch@
255 => IO a -- ^ The computation to run
256 -> (e -> IO a) -- ^ Handler to invoke if an exception is raised
258 catch = ExceptionBase.catchException
260 catches :: IO a -> [Handler a] -> IO a
261 catches io handlers = io `catch` catchesHandler handlers
263 catchesHandler :: [Handler a] -> SomeException -> IO a
264 catchesHandler handlers e = foldr tryHandler (throw e) handlers
265 where tryHandler (Handler handler) res
266 = case fromException e of
267 Just e' -> handler e'
270 data Handler a = forall e . Exception e => Handler (e -> IO a)
272 -- | The function 'catchJust' is like 'catch', but it takes an extra
273 -- argument which is an /exception predicate/, a function which
274 -- selects which type of exceptions we\'re interested in.
276 -- > result <- catchJust errorCalls thing_to_try handler
278 -- Any other exceptions which are not matched by the predicate
279 -- are re-raised, and may be caught by an enclosing
280 -- 'catch' or 'catchJust'.
283 => (e -> Maybe b) -- ^ Predicate to select exceptions
284 -> IO a -- ^ Computation to run
285 -> (b -> IO a) -- ^ Handler
287 catchJust p a handler = catch a handler'
288 where handler' e = case p e of
292 -- | A version of 'catch' with the arguments swapped around; useful in
293 -- situations where the code for the handler is shorter. For example:
295 -- > do handle (\e -> exitWith (ExitFailure 1)) $
297 handle :: Exception e => (e -> IO a) -> IO a -> IO a
300 handleAny :: (forall e . Exception e => e -> IO a) -> IO a -> IO a
301 handleAny = flip catchAny
303 -- | A version of 'catchJust' with the arguments swapped around (see
305 handleJust :: Exception e => (e -> Maybe b) -> (b -> IO a) -> IO a -> IO a
306 handleJust p = flip (catchJust p)
308 -----------------------------------------------------------------------------
311 -- | This function maps one exception into another as proposed in the
312 -- paper \"A semantics for imprecise exceptions\".
314 -- Notice that the usage of 'unsafePerformIO' is safe here.
316 mapException :: Exception e => (e -> e) -> a -> a
317 mapException f v = unsafePerformIO (catch (evaluate v)
320 -----------------------------------------------------------------------------
321 -- 'try' and variations.
323 -- | Similar to 'catch', but returns an 'Either' result which is
324 -- @('Right' a)@ if no exception was raised, or @('Left' e)@ if an
325 -- exception was raised and its value is @e@.
327 -- > try a = catch (Right `liftM` a) (return . Left)
329 -- Note: as with 'catch', it is only polite to use this variant if you intend
330 -- to re-throw the exception after performing whatever cleanup is needed.
331 -- Otherwise, 'tryJust' is generally considered to be better.
333 -- Also note that "System.IO.Error" also exports a function called
334 -- 'System.IO.Error.try' with a similar type to 'Control.Exception.try',
335 -- except that it catches only the IO and user families of exceptions
336 -- (as required by the Haskell 98 @IO@ module).
338 try :: Exception e => IO a -> IO (Either e a)
339 try a = catch (a >>= \ v -> return (Right v)) (\e -> return (Left e))
341 -- | A variant of 'try' that takes an exception predicate to select
342 -- which exceptions are caught (c.f. 'catchJust'). If the exception
343 -- does not match the predicate, it is re-thrown.
344 tryJust :: Exception e => (e -> Maybe b) -> IO a -> IO (Either b a)
348 Right v -> return (Right v)
349 Left e -> case p e of
351 Just b -> return (Left b)
353 ignoreExceptions :: IO () -> IO ()
354 ignoreExceptions io = io `catchAny` \_ -> return ()
356 onException :: IO a -> IO () -> IO a
357 onException io what = io `catch` \e -> do what
358 throw (e :: SomeException)
360 -----------------------------------------------------------------------------
361 -- Some Useful Functions
363 -- | When you want to acquire a resource, do some work with it, and
364 -- then release the resource, it is a good idea to use 'bracket',
365 -- because 'bracket' will install the necessary exception handler to
366 -- release the resource in the event that an exception is raised
367 -- during the computation. If an exception is raised, then 'bracket' will
368 -- re-raise the exception (after performing the release).
370 -- A common example is opening a file:
373 -- > (openFile "filename" ReadMode)
375 -- > (\handle -> do { ... })
377 -- The arguments to 'bracket' are in this order so that we can partially apply
380 -- > withFile name mode = bracket (openFile name mode) hClose
384 :: IO a -- ^ computation to run first (\"acquire resource\")
385 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
386 -> (a -> IO c) -- ^ computation to run in-between
387 -> IO c -- returns the value from the in-between computation
388 bracket before after thing =
393 (\e -> do { after a; throw e })
399 -- | A specialised variant of 'bracket' with just a computation to run
402 finally :: IO a -- ^ computation to run first
403 -> IO b -- ^ computation to run afterward (even if an exception
405 -> IO a -- returns the value from the first computation
410 (\e -> do { sequel; throw e })
415 -- | A variant of 'bracket' where the return value from the first computation
417 bracket_ :: IO a -> IO b -> IO c -> IO c
418 bracket_ before after thing = bracket before (const after) (const thing)
420 -- | Like bracket, but only performs the final action if there was an
421 -- exception raised by the in-between computation.
423 :: IO a -- ^ computation to run first (\"acquire resource\")
424 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
425 -> (a -> IO c) -- ^ computation to run in-between
426 -> IO c -- returns the value from the in-between computation
427 bracketOnError before after thing =
432 (\e -> do { after a; throw e })
435 -- -----------------------------------------------------------------------------
436 -- Asynchronous exceptions
440 #AsynchronousExceptions# Asynchronous exceptions are so-called because they arise due to
441 external influences, and can be raised at any point during execution.
442 'StackOverflow' and 'HeapOverflow' are two examples of
443 system-generated asynchronous exceptions.
445 The primary source of asynchronous exceptions, however, is
448 > throwTo :: ThreadId -> Exception -> IO ()
450 'throwTo' (also 'throwDynTo' and 'Control.Concurrent.killThread') allows one
451 running thread to raise an arbitrary exception in another thread. The
452 exception is therefore asynchronous with respect to the target thread,
453 which could be doing anything at the time it receives the exception.
454 Great care should be taken with asynchronous exceptions; it is all too
455 easy to introduce race conditions by the over zealous use of
460 There\'s an implied 'block' around every exception handler in a call
461 to one of the 'catch' family of functions. This is because that is
462 what you want most of the time - it eliminates a common race condition
463 in starting an exception handler, because there may be no exception
464 handler on the stack to handle another exception if one arrives
465 immediately. If asynchronous exceptions are blocked on entering the
466 handler, though, we have time to install a new exception handler
467 before being interrupted. If this weren\'t the default, one would have
468 to write something like
471 > catch (unblock (...))
475 If you need to unblock asynchronous exceptions again in the exception
476 handler, just use 'unblock' as normal.
478 Note that 'try' and friends /do not/ have a similar default, because
479 there is no exception handler in this case. If you want to use 'try'
480 in an asynchronous-exception-safe way, you will need to use
486 Some operations are /interruptible/, which means that they can receive
487 asynchronous exceptions even in the scope of a 'block'. Any function
488 which may itself block is defined as interruptible; this includes
489 'Control.Concurrent.MVar.takeMVar'
490 (but not 'Control.Concurrent.MVar.tryTakeMVar'),
491 and most operations which perform
492 some I\/O with the outside world. The reason for having
493 interruptible operations is so that we can write things like
497 > catch (unblock (...))
501 if the 'Control.Concurrent.MVar.takeMVar' was not interruptible,
503 combination could lead to deadlock, because the thread itself would be
504 blocked in a state where it can\'t receive any asynchronous exceptions.
505 With 'Control.Concurrent.MVar.takeMVar' interruptible, however, we can be
506 safe in the knowledge that the thread can receive exceptions right up
507 until the point when the 'Control.Concurrent.MVar.takeMVar' succeeds.
508 Similar arguments apply for other interruptible operations like
509 'System.IO.openFile'.
512 #if !(__GLASGOW_HASKELL__ || __NHC__)
513 assert :: Bool -> a -> a
515 assert False _ = throw (AssertionFailed "")
519 #ifdef __GLASGOW_HASKELL__
520 {-# NOINLINE uncaughtExceptionHandler #-}
521 uncaughtExceptionHandler :: IORef (SomeException -> IO ())
522 uncaughtExceptionHandler = unsafePerformIO (newIORef defaultHandler)
524 defaultHandler :: SomeException -> IO ()
525 defaultHandler se@(SomeException ex) = do
526 (hFlush stdout) `catchAny` (\ _ -> return ())
527 let msg = case cast ex of
528 Just Deadlock -> "no threads to run: infinite loop or deadlock?"
530 Just (ErrorCall s) -> s
531 _ -> showsPrec 0 se ""
532 withCString "%s" $ \cfmt ->
533 withCString msg $ \cmsg ->
536 -- don't use errorBelch() directly, because we cannot call varargs functions
538 foreign import ccall unsafe "HsBase.h errorBelch2"
539 errorBelch :: CString -> CString -> IO ()
541 setUncaughtExceptionHandler :: (SomeException -> IO ()) -> IO ()
542 setUncaughtExceptionHandler = writeIORef uncaughtExceptionHandler
544 getUncaughtExceptionHandler :: IO (SomeException -> IO ())
545 getUncaughtExceptionHandler = readIORef uncaughtExceptionHandler
548 recSelError, recConError, irrefutPatError, runtimeError,
549 nonExhaustiveGuardsError, patError, noMethodBindingError
550 :: Addr# -> a -- All take a UTF8-encoded C string
552 recSelError s = throw (RecSelError (unpackCStringUtf8# s)) -- No location info unfortunately
553 runtimeError s = error (unpackCStringUtf8# s) -- No location info unfortunately
555 nonExhaustiveGuardsError s = throw (PatternMatchFail (untangle s "Non-exhaustive guards in"))
556 irrefutPatError s = throw (PatternMatchFail (untangle s "Irrefutable pattern failed for pattern"))
557 recConError s = throw (RecConError (untangle s "Missing field in record construction"))
558 noMethodBindingError s = throw (NoMethodError (untangle s "No instance nor default method for class operation"))
559 patError s = throw (PatternMatchFail (untangle s "Non-exhaustive patterns in"))
563 data PatternMatchFail = PatternMatchFail String
564 INSTANCE_TYPEABLE0(PatternMatchFail,patternMatchFailTc,"PatternMatchFail")
566 instance Exception PatternMatchFail
568 instance Show PatternMatchFail where
569 showsPrec _ (PatternMatchFail err) = showString err
573 data RecSelError = RecSelError String
574 INSTANCE_TYPEABLE0(RecSelError,recSelErrorTc,"RecSelError")
576 instance Exception RecSelError
578 instance Show RecSelError where
579 showsPrec _ (RecSelError err) = showString err
583 data RecConError = RecConError String
584 INSTANCE_TYPEABLE0(RecConError,recConErrorTc,"RecConError")
586 instance Exception RecConError
588 instance Show RecConError where
589 showsPrec _ (RecConError err) = showString err
593 data RecUpdError = RecUpdError String
594 INSTANCE_TYPEABLE0(RecUpdError,recUpdErrorTc,"RecUpdError")
596 instance Exception RecUpdError
598 instance Show RecUpdError where
599 showsPrec _ (RecUpdError err) = showString err
603 data NoMethodError = NoMethodError String
604 INSTANCE_TYPEABLE0(NoMethodError,noMethodErrorTc,"NoMethodError")
606 instance Exception NoMethodError
608 instance Show NoMethodError where
609 showsPrec _ (NoMethodError err) = showString err
613 data AssertionFailed = AssertionFailed String
614 INSTANCE_TYPEABLE0(AssertionFailed,assertionFailedTc,"AssertionFailed")
616 instance Exception AssertionFailed
618 instance Show AssertionFailed where
619 showsPrec _ (AssertionFailed err) = showString err
623 data NonTermination = NonTermination
624 INSTANCE_TYPEABLE0(NonTermination,nonTerminationTc,"NonTermination")
626 instance Exception NonTermination
628 instance Show NonTermination where
629 showsPrec _ NonTermination = showString "<<loop>>"
631 -- GHC's RTS calls this
632 nonTermination :: SomeException
633 nonTermination = toException NonTermination
637 data Deadlock = Deadlock
638 INSTANCE_TYPEABLE0(Deadlock,deadlockTc,"Deadlock")
640 instance Exception Deadlock
642 instance Show Deadlock where
643 showsPrec _ Deadlock = showString "<<deadlock>>"
647 data NestedAtomically = NestedAtomically
648 INSTANCE_TYPEABLE0(NestedAtomically,nestedAtomicallyTc,"NestedAtomically")
650 instance Exception NestedAtomically
652 instance Show NestedAtomically where
653 showsPrec _ NestedAtomically = showString "Control.Concurrent.STM.atomically was nested"
655 -- GHC's RTS calls this
656 nestedAtomically :: SomeException
657 nestedAtomically = toException NestedAtomically
661 instance Exception Dynamic
665 assertError :: Addr# -> Bool -> a -> a
666 assertError str pred v
668 | otherwise = throw (AssertionFailed (untangle str "Assertion failed"))
671 (untangle coded message) expects "coded" to be of the form
674 location message details
676 untangle :: Addr# -> String -> String
677 untangle coded message
684 coded_str = unpackCStringUtf8# coded
687 = case (span not_bar coded_str) of { (loc, rest) ->
689 ('|':det) -> (loc, ' ' : det)
695 throwTo :: Exception e => ThreadId -> e -> IO ()
696 throwTo (ThreadId id) ex = IO $ \ s ->
697 case (killThread# id (toException ex) s) of s1 -> (# s1, () #)