1 {-# OPTIONS_GHC -XNoImplicitPrelude #-}
2 -----------------------------------------------------------------------------
4 -- Module : Control.Exception
5 -- Copyright : (c) The University of Glasgow 2001
6 -- License : BSD-style (see the file libraries/base/LICENSE)
8 -- Maintainer : libraries@haskell.org
9 -- Stability : experimental
10 -- Portability : non-portable (extended exceptions)
12 -- This module provides support for raising and catching both built-in
13 -- and user-defined exceptions.
15 -- In addition to exceptions thrown by 'IO' operations, exceptions may
16 -- be thrown by pure code (imprecise exceptions) or by external events
17 -- (asynchronous exceptions), but may only be caught in the 'IO' monad.
18 -- For more details, see:
20 -- * /A semantics for imprecise exceptions/, by Simon Peyton Jones,
21 -- Alastair Reid, Tony Hoare, Simon Marlow, Fergus Henderson,
24 -- * /Asynchronous exceptions in Haskell/, by Simon Marlow, Simon Peyton
25 -- Jones, Andy Moran and John Reppy, in /PLDI'01/.
27 -----------------------------------------------------------------------------
29 module Control.Exception (
31 -- * The Exception type
33 Exception(..), -- instance Eq, Ord, Show, Typeable
34 IOException, -- instance Eq, Ord, Show, Typeable
35 ArithException(..), -- instance Eq, Ord, Show, Typeable
36 ArrayException(..), -- instance Eq, Ord, Show, Typeable
38 AsyncException(..), -- instance Eq, Ord, Show, Typeable
39 NonTermination(..), nonTermination,
40 BlockedOnDeadMVar(..),
41 BlockedIndefinitely(..),
42 NestedAtomically(..), nestedAtomically,
50 -- * Throwing exceptions
51 throwIO, -- :: Exception -> IO a
52 throw, -- :: Exception -> a
53 ioError, -- :: IOError -> IO a
54 #ifdef __GLASGOW_HASKELL__
55 throwTo, -- :: ThreadId -> Exception -> a
58 -- * Catching Exceptions
60 -- |There are several functions for catching and examining
61 -- exceptions; all of them may only be used from within the
64 -- ** The @catch@ functions
65 catch, -- :: IO a -> (Exception -> IO a) -> IO a
68 catchJust, -- :: (Exception -> Maybe b) -> IO a -> (b -> IO a) -> IO a
70 -- ** The @handle@ functions
71 handle, -- :: (Exception -> IO a) -> IO a -> IO a
73 handleJust,-- :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a
75 -- ** The @try@ functions
76 try, -- :: IO a -> IO (Either Exception a)
77 tryJust, -- :: (Exception -> Maybe b) -> a -> IO (Either b a)
80 -- ** The @evaluate@ function
81 evaluate, -- :: a -> IO a
83 -- ** The @mapException@ function
84 mapException, -- :: (Exception -> Exception) -> a -> a
86 -- * Asynchronous Exceptions
90 -- ** Asynchronous exception control
92 -- |The following two functions allow a thread to control delivery of
93 -- asynchronous exceptions during a critical region.
95 block, -- :: IO a -> IO a
96 unblock, -- :: IO a -> IO a
97 blocked, -- :: IO Bool
99 -- *** Applying @block@ to an exception handler
103 -- *** Interruptible operations
109 assert, -- :: Bool -> a -> a
113 bracket, -- :: IO a -> (a -> IO b) -> (a -> IO c) -> IO ()
114 bracket_, -- :: IO a -> IO b -> IO c -> IO ()
117 finally, -- :: IO a -> IO b -> IO a
119 recSelError, recConError, irrefutPatError, runtimeError,
120 nonExhaustiveGuardsError, patError, noMethodBindingError,
122 #ifdef __GLASGOW_HASKELL__
123 setUncaughtExceptionHandler, -- :: (Exception -> IO ()) -> IO ()
124 getUncaughtExceptionHandler -- :: IO (Exception -> IO ())
128 #ifdef __GLASGOW_HASKELL__
131 import {-# SOURCE #-} GHC.Handle
135 import GHC.IOBase as ExceptionBase
136 import GHC.Exception hiding ( Exception )
137 import {-# SOURCE #-} GHC.Conc ( ThreadId(ThreadId) )
138 import Foreign.C.String ( CString, withCString )
142 import Hugs.Exception as ExceptionBase
150 import qualified System.IO.Error as H'98 (catch)
151 import System.IO.Error (ioError)
153 import DIOError -- defn of IOError type
154 import System (ExitCode())
156 -- minimum needed for nhc98 to pretend it has Exceptions
157 data Exception = IOException IOException
158 | ArithException ArithException
159 | ArrayException ArrayException
160 | AsyncException AsyncException
161 | ExitException ExitCode
163 type IOException = IOError
167 instance Show ArithException
168 instance Show ArrayException
169 instance Show AsyncException
171 catch :: IO a -> (Exception -> IO a) -> IO a
172 a `catch` b = a `H'98.catch` (b . IOException)
174 throwIO :: Exception -> IO a
175 throwIO (IOException e) = ioError e
176 throwIO _ = ioError (UserError "Control.Exception.throwIO"
178 throw :: Exception -> a
179 throw = unsafePerformIO . throwIO
181 evaluate :: a -> IO a
182 evaluate x = x `seq` return x
184 assert :: Bool -> a -> a
186 assert False _ = throw (IOException (UserError "" "Assertion failed"))
189 #ifndef __GLASGOW_HASKELL__
190 -- Dummy definitions for implementations lacking asynchonous exceptions
192 block :: IO a -> IO a
194 unblock :: IO a -> IO a
197 blocked = return False
200 -----------------------------------------------------------------------------
201 -- Catching exceptions
203 -- |This is the simplest of the exception-catching functions. It
204 -- takes a single argument, runs it, and if an exception is raised
205 -- the \"handler\" is executed, with the value of the exception passed as an
206 -- argument. Otherwise, the result is returned as normal. For example:
208 -- > catch (openFile f ReadMode)
209 -- > (\e -> hPutStr stderr ("Couldn't open "++f++": " ++ show e))
211 -- For catching exceptions in pure (non-'IO') expressions, see the
212 -- function 'evaluate'.
214 -- Note that due to Haskell\'s unspecified evaluation order, an
215 -- expression may return one of several possible exceptions: consider
216 -- the expression @error \"urk\" + 1 \`div\` 0@. Does
217 -- 'catch' execute the handler passing
218 -- @ErrorCall \"urk\"@, or @ArithError DivideByZero@?
220 -- The answer is \"either\": 'catch' makes a
221 -- non-deterministic choice about which exception to catch. If you
222 -- call it again, you might get a different exception back. This is
223 -- ok, because 'catch' is an 'IO' computation.
225 -- Note that 'catch' catches all types of exceptions, and is generally
226 -- used for \"cleaning up\" before passing on the exception using
227 -- 'throwIO'. It is not good practice to discard the exception and
228 -- continue, without first checking the type of the exception (it
229 -- might be a 'ThreadKilled', for example). In this case it is usually better
230 -- to use 'catchJust' and select the kinds of exceptions to catch.
232 -- Also note that the "Prelude" also exports a function called
233 -- 'Prelude.catch' with a similar type to 'Control.Exception.catch',
234 -- except that the "Prelude" version only catches the IO and user
235 -- families of exceptions (as required by Haskell 98).
237 -- We recommend either hiding the "Prelude" version of 'Prelude.catch'
238 -- when importing "Control.Exception":
240 -- > import Prelude hiding (catch)
242 -- or importing "Control.Exception" qualified, to avoid name-clashes:
244 -- > import qualified Control.Exception as C
246 -- and then using @C.catch@
250 => IO a -- ^ The computation to run
251 -> (e -> IO a) -- ^ Handler to invoke if an exception is raised
253 catch = ExceptionBase.catchException
255 catches :: IO a -> [Handler a] -> IO a
256 catches io handlers = io `catch` catchesHandler handlers
258 catchesHandler :: [Handler a] -> SomeException -> IO a
259 catchesHandler handlers e = foldr tryHandler (throw e) handlers
260 where tryHandler (Handler handler) res
261 = case fromException e of
262 Just e' -> handler e'
265 data Handler a = forall e . Exception e => Handler (e -> IO a)
267 -- | The function 'catchJust' is like 'catch', but it takes an extra
268 -- argument which is an /exception predicate/, a function which
269 -- selects which type of exceptions we\'re interested in.
271 -- > result <- catchJust errorCalls thing_to_try handler
273 -- Any other exceptions which are not matched by the predicate
274 -- are re-raised, and may be caught by an enclosing
275 -- 'catch' or 'catchJust'.
278 => (e -> Maybe b) -- ^ Predicate to select exceptions
279 -> IO a -- ^ Computation to run
280 -> (b -> IO a) -- ^ Handler
282 catchJust p a handler = catch a handler'
283 where handler' e = case p e of
287 -- | A version of 'catch' with the arguments swapped around; useful in
288 -- situations where the code for the handler is shorter. For example:
290 -- > do handle (\e -> exitWith (ExitFailure 1)) $
292 handle :: Exception e => (e -> IO a) -> IO a -> IO a
295 handleAny :: (forall e . Exception e => e -> IO a) -> IO a -> IO a
296 handleAny = flip catchAny
298 -- | A version of 'catchJust' with the arguments swapped around (see
300 handleJust :: Exception e => (e -> Maybe b) -> (b -> IO a) -> IO a -> IO a
301 handleJust p = flip (catchJust p)
303 -----------------------------------------------------------------------------
306 -- | This function maps one exception into another as proposed in the
307 -- paper \"A semantics for imprecise exceptions\".
309 -- Notice that the usage of 'unsafePerformIO' is safe here.
311 mapException :: Exception e => (e -> e) -> a -> a
312 mapException f v = unsafePerformIO (catch (evaluate v)
315 -----------------------------------------------------------------------------
316 -- 'try' and variations.
318 -- | Similar to 'catch', but returns an 'Either' result which is
319 -- @('Right' a)@ if no exception was raised, or @('Left' e)@ if an
320 -- exception was raised and its value is @e@.
322 -- > try a = catch (Right `liftM` a) (return . Left)
324 -- Note: as with 'catch', it is only polite to use this variant if you intend
325 -- to re-throw the exception after performing whatever cleanup is needed.
326 -- Otherwise, 'tryJust' is generally considered to be better.
328 -- Also note that "System.IO.Error" also exports a function called
329 -- 'System.IO.Error.try' with a similar type to 'Control.Exception.try',
330 -- except that it catches only the IO and user families of exceptions
331 -- (as required by the Haskell 98 @IO@ module).
333 try :: Exception e => IO a -> IO (Either e a)
334 try a = catch (a >>= \ v -> return (Right v)) (\e -> return (Left e))
336 -- | A variant of 'try' that takes an exception predicate to select
337 -- which exceptions are caught (c.f. 'catchJust'). If the exception
338 -- does not match the predicate, it is re-thrown.
339 tryJust :: Exception e => (e -> Maybe b) -> IO a -> IO (Either b a)
343 Right v -> return (Right v)
344 Left e -> case p e of
346 Just b -> return (Left b)
348 ignoreExceptions :: IO () -> IO ()
349 ignoreExceptions io = io `catchAny` \_ -> return ()
351 -----------------------------------------------------------------------------
352 -- Some Useful Functions
354 -- | When you want to acquire a resource, do some work with it, and
355 -- then release the resource, it is a good idea to use 'bracket',
356 -- because 'bracket' will install the necessary exception handler to
357 -- release the resource in the event that an exception is raised
358 -- during the computation. If an exception is raised, then 'bracket' will
359 -- re-raise the exception (after performing the release).
361 -- A common example is opening a file:
364 -- > (openFile "filename" ReadMode)
366 -- > (\handle -> do { ... })
368 -- The arguments to 'bracket' are in this order so that we can partially apply
371 -- > withFile name mode = bracket (openFile name mode) hClose
375 :: IO a -- ^ computation to run first (\"acquire resource\")
376 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
377 -> (a -> IO c) -- ^ computation to run in-between
378 -> IO c -- returns the value from the in-between computation
379 bracket before after thing =
384 (\e -> do { after a; throw e })
390 -- | A specialised variant of 'bracket' with just a computation to run
393 finally :: IO a -- ^ computation to run first
394 -> IO b -- ^ computation to run afterward (even if an exception
396 -> IO a -- returns the value from the first computation
401 (\e -> do { sequel; throw e })
406 -- | A variant of 'bracket' where the return value from the first computation
408 bracket_ :: IO a -> IO b -> IO c -> IO c
409 bracket_ before after thing = bracket before (const after) (const thing)
411 -- | Like bracket, but only performs the final action if there was an
412 -- exception raised by the in-between computation.
414 :: IO a -- ^ computation to run first (\"acquire resource\")
415 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
416 -> (a -> IO c) -- ^ computation to run in-between
417 -> IO c -- returns the value from the in-between computation
418 bracketOnError before after thing =
423 (\e -> do { after a; throw e })
426 -- -----------------------------------------------------------------------------
427 -- Asynchronous exceptions
431 #AsynchronousExceptions# Asynchronous exceptions are so-called because they arise due to
432 external influences, and can be raised at any point during execution.
433 'StackOverflow' and 'HeapOverflow' are two examples of
434 system-generated asynchronous exceptions.
436 The primary source of asynchronous exceptions, however, is
439 > throwTo :: ThreadId -> Exception -> IO ()
441 'throwTo' (also 'throwDynTo' and 'Control.Concurrent.killThread') allows one
442 running thread to raise an arbitrary exception in another thread. The
443 exception is therefore asynchronous with respect to the target thread,
444 which could be doing anything at the time it receives the exception.
445 Great care should be taken with asynchronous exceptions; it is all too
446 easy to introduce race conditions by the over zealous use of
451 There\'s an implied 'block' around every exception handler in a call
452 to one of the 'catch' family of functions. This is because that is
453 what you want most of the time - it eliminates a common race condition
454 in starting an exception handler, because there may be no exception
455 handler on the stack to handle another exception if one arrives
456 immediately. If asynchronous exceptions are blocked on entering the
457 handler, though, we have time to install a new exception handler
458 before being interrupted. If this weren\'t the default, one would have
459 to write something like
462 > catch (unblock (...))
466 If you need to unblock asynchronous exceptions again in the exception
467 handler, just use 'unblock' as normal.
469 Note that 'try' and friends /do not/ have a similar default, because
470 there is no exception handler in this case. If you want to use 'try'
471 in an asynchronous-exception-safe way, you will need to use
477 Some operations are /interruptible/, which means that they can receive
478 asynchronous exceptions even in the scope of a 'block'. Any function
479 which may itself block is defined as interruptible; this includes
480 'Control.Concurrent.MVar.takeMVar'
481 (but not 'Control.Concurrent.MVar.tryTakeMVar'),
482 and most operations which perform
483 some I\/O with the outside world. The reason for having
484 interruptible operations is so that we can write things like
488 > catch (unblock (...))
492 if the 'Control.Concurrent.MVar.takeMVar' was not interruptible,
494 combination could lead to deadlock, because the thread itself would be
495 blocked in a state where it can\'t receive any asynchronous exceptions.
496 With 'Control.Concurrent.MVar.takeMVar' interruptible, however, we can be
497 safe in the knowledge that the thread can receive exceptions right up
498 until the point when the 'Control.Concurrent.MVar.takeMVar' succeeds.
499 Similar arguments apply for other interruptible operations like
500 'System.IO.openFile'.
503 #if !(__GLASGOW_HASKELL__ || __NHC__)
504 assert :: Bool -> a -> a
506 assert False _ = throw (AssertionFailed "")
510 #ifdef __GLASGOW_HASKELL__
511 {-# NOINLINE uncaughtExceptionHandler #-}
512 uncaughtExceptionHandler :: IORef (SomeException -> IO ())
513 uncaughtExceptionHandler = unsafePerformIO (newIORef defaultHandler)
515 defaultHandler :: SomeException -> IO ()
516 defaultHandler se@(SomeException ex) = do
517 (hFlush stdout) `catchAny` (\ _ -> return ())
518 let msg = case cast ex of
519 Just Deadlock -> "no threads to run: infinite loop or deadlock?"
521 Just (ErrorCall s) -> s
522 _ -> showsPrec 0 se ""
523 withCString "%s" $ \cfmt ->
524 withCString msg $ \cmsg ->
527 -- don't use errorBelch() directly, because we cannot call varargs functions
529 foreign import ccall unsafe "HsBase.h errorBelch2"
530 errorBelch :: CString -> CString -> IO ()
532 setUncaughtExceptionHandler :: (SomeException -> IO ()) -> IO ()
533 setUncaughtExceptionHandler = writeIORef uncaughtExceptionHandler
535 getUncaughtExceptionHandler :: IO (SomeException -> IO ())
536 getUncaughtExceptionHandler = readIORef uncaughtExceptionHandler
539 recSelError, recConError, irrefutPatError, runtimeError,
540 nonExhaustiveGuardsError, patError, noMethodBindingError
541 :: Addr# -> a -- All take a UTF8-encoded C string
543 recSelError s = throw (RecSelError (unpackCStringUtf8# s)) -- No location info unfortunately
544 runtimeError s = error (unpackCStringUtf8# s) -- No location info unfortunately
546 nonExhaustiveGuardsError s = throw (PatternMatchFail (untangle s "Non-exhaustive guards in"))
547 irrefutPatError s = throw (PatternMatchFail (untangle s "Irrefutable pattern failed for pattern"))
548 recConError s = throw (RecConError (untangle s "Missing field in record construction"))
549 noMethodBindingError s = throw (NoMethodError (untangle s "No instance nor default method for class operation"))
550 patError s = throw (PatternMatchFail (untangle s "Non-exhaustive patterns in"))
554 data PatternMatchFail = PatternMatchFail String
557 instance Exception PatternMatchFail
559 instance Show PatternMatchFail where
560 showsPrec _ (PatternMatchFail err) = showString err
564 data RecSelError = RecSelError String
567 instance Exception RecSelError
569 instance Show RecSelError where
570 showsPrec _ (RecSelError err) = showString err
574 data RecConError = RecConError String
577 instance Exception RecConError
579 instance Show RecConError where
580 showsPrec _ (RecConError err) = showString err
584 data RecUpdError = RecUpdError String
587 instance Exception RecUpdError
589 instance Show RecUpdError where
590 showsPrec _ (RecUpdError err) = showString err
594 data NoMethodError = NoMethodError String
597 instance Exception NoMethodError
599 instance Show NoMethodError where
600 showsPrec _ (NoMethodError err) = showString err
604 data AssertionFailed = AssertionFailed String
607 instance Exception AssertionFailed
609 instance Show AssertionFailed where
610 showsPrec _ (AssertionFailed err) = showString err
614 data NonTermination = NonTermination
617 instance Exception NonTermination
619 instance Show NonTermination where
620 showsPrec _ NonTermination = showString "<<loop>>"
622 -- GHC's RTS calls this
623 nonTermination :: SomeException
624 nonTermination = toException NonTermination
628 data Deadlock = Deadlock
631 instance Exception Deadlock
633 instance Show Deadlock where
634 showsPrec _ Deadlock = showString "<<deadlock>>"
638 data NestedAtomically = NestedAtomically
641 instance Exception NestedAtomically
643 instance Show NestedAtomically where
644 showsPrec _ NestedAtomically = showString "Control.Concurrent.STM.atomically was nested"
646 -- GHC's RTS calls this
647 nestedAtomically :: SomeException
648 nestedAtomically = toException NestedAtomically
652 instance Exception Dynamic
656 assertError :: Addr# -> Bool -> a -> a
657 assertError str pred v
659 | otherwise = throw (AssertionFailed (untangle str "Assertion failed"))
662 (untangle coded message) expects "coded" to be of the form
665 location message details
667 untangle :: Addr# -> String -> String
668 untangle coded message
675 coded_str = unpackCStringUtf8# coded
678 = case (span not_bar coded_str) of { (loc, rest) ->
680 ('|':det) -> (loc, ' ' : det)
686 throwTo :: Exception e => ThreadId -> e -> IO ()
687 throwTo (ThreadId id) ex = IO $ \ s ->
688 case (killThread# id (toException ex) s) of s1 -> (# s1, () #)