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__
130 import {-# SOURCE #-} GHC.Handle
134 import GHC.IOBase as ExceptionBase
135 import GHC.Exception hiding ( Exception )
136 import {-# SOURCE #-} GHC.Conc ( ThreadId(ThreadId) )
137 import Foreign.C.String ( CString, withCString )
141 import Hugs.Exception as ExceptionBase
149 import qualified System.IO.Error as H'98 (catch)
150 import System.IO.Error (ioError)
152 import DIOError -- defn of IOError type
153 import System (ExitCode())
155 -- minimum needed for nhc98 to pretend it has Exceptions
156 data Exception = IOException IOException
157 | ArithException ArithException
158 | ArrayException ArrayException
159 | AsyncException AsyncException
160 | ExitException ExitCode
162 type IOException = IOError
166 instance Show ArithException
167 instance Show ArrayException
168 instance Show AsyncException
170 catch :: IO a -> (Exception -> IO a) -> IO a
171 a `catch` b = a `H'98.catch` (b . IOException)
173 throwIO :: Exception -> IO a
174 throwIO (IOException e) = ioError e
175 throwIO _ = ioError (UserError "Control.Exception.throwIO"
177 throw :: Exception -> a
178 throw = unsafePerformIO . throwIO
180 evaluate :: a -> IO a
181 evaluate x = x `seq` return x
183 assert :: Bool -> a -> a
185 assert False _ = throw (IOException (UserError "" "Assertion failed"))
188 #ifndef __GLASGOW_HASKELL__
189 -- Dummy definitions for implementations lacking asynchonous exceptions
191 block :: IO a -> IO a
193 unblock :: IO a -> IO a
196 blocked = return False
199 -----------------------------------------------------------------------------
200 -- Catching exceptions
202 -- |This is the simplest of the exception-catching functions. It
203 -- takes a single argument, runs it, and if an exception is raised
204 -- the \"handler\" is executed, with the value of the exception passed as an
205 -- argument. Otherwise, the result is returned as normal. For example:
207 -- > catch (openFile f ReadMode)
208 -- > (\e -> hPutStr stderr ("Couldn't open "++f++": " ++ show e))
210 -- For catching exceptions in pure (non-'IO') expressions, see the
211 -- function 'evaluate'.
213 -- Note that due to Haskell\'s unspecified evaluation order, an
214 -- expression may return one of several possible exceptions: consider
215 -- the expression @error \"urk\" + 1 \`div\` 0@. Does
216 -- 'catch' execute the handler passing
217 -- @ErrorCall \"urk\"@, or @ArithError DivideByZero@?
219 -- The answer is \"either\": 'catch' makes a
220 -- non-deterministic choice about which exception to catch. If you
221 -- call it again, you might get a different exception back. This is
222 -- ok, because 'catch' is an 'IO' computation.
224 -- Note that 'catch' catches all types of exceptions, and is generally
225 -- used for \"cleaning up\" before passing on the exception using
226 -- 'throwIO'. It is not good practice to discard the exception and
227 -- continue, without first checking the type of the exception (it
228 -- might be a 'ThreadKilled', for example). In this case it is usually better
229 -- to use 'catchJust' and select the kinds of exceptions to catch.
231 -- Also note that the "Prelude" also exports a function called
232 -- 'Prelude.catch' with a similar type to 'Control.Exception.catch',
233 -- except that the "Prelude" version only catches the IO and user
234 -- families of exceptions (as required by Haskell 98).
236 -- We recommend either hiding the "Prelude" version of 'Prelude.catch'
237 -- when importing "Control.Exception":
239 -- > import Prelude hiding (catch)
241 -- or importing "Control.Exception" qualified, to avoid name-clashes:
243 -- > import qualified Control.Exception as C
245 -- and then using @C.catch@
249 => IO a -- ^ The computation to run
250 -> (e -> IO a) -- ^ Handler to invoke if an exception is raised
252 catch = ExceptionBase.catchException
254 catches :: IO a -> [Handler a] -> IO a
255 catches io handlers = io `catch` catchesHandler handlers
257 catchesHandler :: [Handler a] -> SomeException -> IO a
258 catchesHandler handlers e = foldr tryHandler (throw e) handlers
259 where tryHandler (Handler handler) res
260 = case fromException e of
261 Just e' -> handler e'
264 data Handler a = forall e . Exception e => Handler (e -> IO a)
266 -- | The function 'catchJust' is like 'catch', but it takes an extra
267 -- argument which is an /exception predicate/, a function which
268 -- selects which type of exceptions we\'re interested in.
270 -- > result <- catchJust errorCalls thing_to_try handler
272 -- Any other exceptions which are not matched by the predicate
273 -- are re-raised, and may be caught by an enclosing
274 -- 'catch' or 'catchJust'.
277 => (e -> Maybe b) -- ^ Predicate to select exceptions
278 -> IO a -- ^ Computation to run
279 -> (b -> IO a) -- ^ Handler
281 catchJust p a handler = catch a handler'
282 where handler' e = case p e of
286 -- | A version of 'catch' with the arguments swapped around; useful in
287 -- situations where the code for the handler is shorter. For example:
289 -- > do handle (\e -> exitWith (ExitFailure 1)) $
291 handle :: Exception e => (e -> IO a) -> IO a -> IO a
294 handleAny :: (forall e . Exception e => e -> IO a) -> IO a -> IO a
295 handleAny = flip catchAny
297 -- | A version of 'catchJust' with the arguments swapped around (see
299 handleJust :: Exception e => (e -> Maybe b) -> (b -> IO a) -> IO a -> IO a
300 handleJust p = flip (catchJust p)
302 -----------------------------------------------------------------------------
305 -- | This function maps one exception into another as proposed in the
306 -- paper \"A semantics for imprecise exceptions\".
308 -- Notice that the usage of 'unsafePerformIO' is safe here.
310 mapException :: Exception e => (e -> e) -> a -> a
311 mapException f v = unsafePerformIO (catch (evaluate v)
314 -----------------------------------------------------------------------------
315 -- 'try' and variations.
317 -- | Similar to 'catch', but returns an 'Either' result which is
318 -- @('Right' a)@ if no exception was raised, or @('Left' e)@ if an
319 -- exception was raised and its value is @e@.
321 -- > try a = catch (Right `liftM` a) (return . Left)
323 -- Note: as with 'catch', it is only polite to use this variant if you intend
324 -- to re-throw the exception after performing whatever cleanup is needed.
325 -- Otherwise, 'tryJust' is generally considered to be better.
327 -- Also note that "System.IO.Error" also exports a function called
328 -- 'System.IO.Error.try' with a similar type to 'Control.Exception.try',
329 -- except that it catches only the IO and user families of exceptions
330 -- (as required by the Haskell 98 @IO@ module).
332 try :: Exception e => IO a -> IO (Either e a)
333 try a = catch (a >>= \ v -> return (Right v)) (\e -> return (Left e))
335 -- | A variant of 'try' that takes an exception predicate to select
336 -- which exceptions are caught (c.f. 'catchJust'). If the exception
337 -- does not match the predicate, it is re-thrown.
338 tryJust :: Exception e => (e -> Maybe b) -> IO a -> IO (Either b a)
342 Right v -> return (Right v)
343 Left e -> case p e of
345 Just b -> return (Left b)
347 ignoreExceptions :: IO () -> IO ()
348 ignoreExceptions io = io `catchAny` \_ -> return ()
350 -----------------------------------------------------------------------------
351 -- Some Useful Functions
353 -- | When you want to acquire a resource, do some work with it, and
354 -- then release the resource, it is a good idea to use 'bracket',
355 -- because 'bracket' will install the necessary exception handler to
356 -- release the resource in the event that an exception is raised
357 -- during the computation. If an exception is raised, then 'bracket' will
358 -- re-raise the exception (after performing the release).
360 -- A common example is opening a file:
363 -- > (openFile "filename" ReadMode)
365 -- > (\handle -> do { ... })
367 -- The arguments to 'bracket' are in this order so that we can partially apply
370 -- > withFile name mode = bracket (openFile name mode) hClose
374 :: IO a -- ^ computation to run first (\"acquire resource\")
375 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
376 -> (a -> IO c) -- ^ computation to run in-between
377 -> IO c -- returns the value from the in-between computation
378 bracket before after thing =
383 (\e -> do { after a; throw e })
389 -- | A specialised variant of 'bracket' with just a computation to run
392 finally :: IO a -- ^ computation to run first
393 -> IO b -- ^ computation to run afterward (even if an exception
395 -> IO a -- returns the value from the first computation
400 (\e -> do { sequel; throw e })
405 -- | A variant of 'bracket' where the return value from the first computation
407 bracket_ :: IO a -> IO b -> IO c -> IO c
408 bracket_ before after thing = bracket before (const after) (const thing)
410 -- | Like bracket, but only performs the final action if there was an
411 -- exception raised by the in-between computation.
413 :: IO a -- ^ computation to run first (\"acquire resource\")
414 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
415 -> (a -> IO c) -- ^ computation to run in-between
416 -> IO c -- returns the value from the in-between computation
417 bracketOnError before after thing =
422 (\e -> do { after a; throw e })
425 -- -----------------------------------------------------------------------------
426 -- Asynchronous exceptions
430 #AsynchronousExceptions# Asynchronous exceptions are so-called because they arise due to
431 external influences, and can be raised at any point during execution.
432 'StackOverflow' and 'HeapOverflow' are two examples of
433 system-generated asynchronous exceptions.
435 The primary source of asynchronous exceptions, however, is
438 > throwTo :: ThreadId -> Exception -> IO ()
440 'throwTo' (also 'throwDynTo' and 'Control.Concurrent.killThread') allows one
441 running thread to raise an arbitrary exception in another thread. The
442 exception is therefore asynchronous with respect to the target thread,
443 which could be doing anything at the time it receives the exception.
444 Great care should be taken with asynchronous exceptions; it is all too
445 easy to introduce race conditions by the over zealous use of
450 There\'s an implied 'block' around every exception handler in a call
451 to one of the 'catch' family of functions. This is because that is
452 what you want most of the time - it eliminates a common race condition
453 in starting an exception handler, because there may be no exception
454 handler on the stack to handle another exception if one arrives
455 immediately. If asynchronous exceptions are blocked on entering the
456 handler, though, we have time to install a new exception handler
457 before being interrupted. If this weren\'t the default, one would have
458 to write something like
461 > catch (unblock (...))
465 If you need to unblock asynchronous exceptions again in the exception
466 handler, just use 'unblock' as normal.
468 Note that 'try' and friends /do not/ have a similar default, because
469 there is no exception handler in this case. If you want to use 'try'
470 in an asynchronous-exception-safe way, you will need to use
476 Some operations are /interruptible/, which means that they can receive
477 asynchronous exceptions even in the scope of a 'block'. Any function
478 which may itself block is defined as interruptible; this includes
479 'Control.Concurrent.MVar.takeMVar'
480 (but not 'Control.Concurrent.MVar.tryTakeMVar'),
481 and most operations which perform
482 some I\/O with the outside world. The reason for having
483 interruptible operations is so that we can write things like
487 > catch (unblock (...))
491 if the 'Control.Concurrent.MVar.takeMVar' was not interruptible,
493 combination could lead to deadlock, because the thread itself would be
494 blocked in a state where it can\'t receive any asynchronous exceptions.
495 With 'Control.Concurrent.MVar.takeMVar' interruptible, however, we can be
496 safe in the knowledge that the thread can receive exceptions right up
497 until the point when the 'Control.Concurrent.MVar.takeMVar' succeeds.
498 Similar arguments apply for other interruptible operations like
499 'System.IO.openFile'.
502 #if !(__GLASGOW_HASKELL__ || __NHC__)
503 assert :: Bool -> a -> a
505 assert False _ = throw (AssertionFailed "")
509 #ifdef __GLASGOW_HASKELL__
510 {-# NOINLINE uncaughtExceptionHandler #-}
511 uncaughtExceptionHandler :: IORef (SomeException -> IO ())
512 uncaughtExceptionHandler = unsafePerformIO (newIORef defaultHandler)
514 defaultHandler :: SomeException -> IO ()
515 defaultHandler se@(SomeException ex) = do
516 (hFlush stdout) `catchAny` (\ _ -> return ())
517 let msg = case cast ex of
518 Just Deadlock -> "no threads to run: infinite loop or deadlock?"
520 Just (ErrorCall s) -> s
521 _ -> showsPrec 0 se ""
522 withCString "%s" $ \cfmt ->
523 withCString msg $ \cmsg ->
526 -- don't use errorBelch() directly, because we cannot call varargs functions
528 foreign import ccall unsafe "HsBase.h errorBelch2"
529 errorBelch :: CString -> CString -> IO ()
531 setUncaughtExceptionHandler :: (SomeException -> IO ()) -> IO ()
532 setUncaughtExceptionHandler = writeIORef uncaughtExceptionHandler
534 getUncaughtExceptionHandler :: IO (SomeException -> IO ())
535 getUncaughtExceptionHandler = readIORef uncaughtExceptionHandler
538 recSelError, recConError, irrefutPatError, runtimeError,
539 nonExhaustiveGuardsError, patError, noMethodBindingError
540 :: Addr# -> a -- All take a UTF8-encoded C string
542 recSelError s = throw (RecSelError (unpackCStringUtf8# s)) -- No location info unfortunately
543 runtimeError s = error (unpackCStringUtf8# s) -- No location info unfortunately
545 nonExhaustiveGuardsError s = throw (PatternMatchFail (untangle s "Non-exhaustive guards in"))
546 irrefutPatError s = throw (PatternMatchFail (untangle s "Irrefutable pattern failed for pattern"))
547 recConError s = throw (RecConError (untangle s "Missing field in record construction"))
548 noMethodBindingError s = throw (NoMethodError (untangle s "No instance nor default method for class operation"))
549 patError s = throw (PatternMatchFail (untangle s "Non-exhaustive patterns in"))
553 data PatternMatchFail = PatternMatchFail String
556 instance Exception PatternMatchFail
558 instance Show PatternMatchFail where
559 showsPrec _ (PatternMatchFail err) = showString err
563 data RecSelError = RecSelError String
566 instance Exception RecSelError
568 instance Show RecSelError where
569 showsPrec _ (RecSelError err) = showString err
573 data RecConError = RecConError String
576 instance Exception RecConError
578 instance Show RecConError where
579 showsPrec _ (RecConError err) = showString err
583 data RecUpdError = RecUpdError String
586 instance Exception RecUpdError
588 instance Show RecUpdError where
589 showsPrec _ (RecUpdError err) = showString err
593 data NoMethodError = NoMethodError String
596 instance Exception NoMethodError
598 instance Show NoMethodError where
599 showsPrec _ (NoMethodError err) = showString err
603 data AssertionFailed = AssertionFailed String
606 instance Exception AssertionFailed
608 instance Show AssertionFailed where
609 showsPrec _ (AssertionFailed err) = showString err
613 data NonTermination = NonTermination
616 instance Exception NonTermination
618 instance Show NonTermination where
619 showsPrec _ NonTermination = showString "<<loop>>"
621 -- GHC's RTS calls this
622 nonTermination :: SomeException
623 nonTermination = toException NonTermination
627 data Deadlock = Deadlock
630 instance Exception Deadlock
632 instance Show Deadlock where
633 showsPrec _ Deadlock = showString "<<deadlock>>"
637 data NestedAtomically = NestedAtomically
640 instance Exception NestedAtomically
642 instance Show NestedAtomically where
643 showsPrec _ NestedAtomically = showString "Control.Concurrent.STM.atomically was nested"
645 -- GHC's RTS calls this
646 nestedAtomically :: SomeException
647 nestedAtomically = toException NestedAtomically
651 instance Exception Dynamic
655 assertError :: Addr# -> Bool -> a -> a
656 assertError str pred v
658 | otherwise = throw (AssertionFailed (untangle str "Assertion failed"))
661 (untangle coded message) expects "coded" to be of the form
664 location message details
666 untangle :: Addr# -> String -> String
667 untangle coded message
674 coded_str = unpackCStringUtf8# coded
677 = case (span not_bar coded_str) of { (loc, rest) ->
679 ('|':det) -> (loc, ' ' : det)
685 throwTo :: Exception e => ThreadId -> e -> IO ()
686 throwTo (ThreadId id) ex = IO $ \ s ->
687 case (killThread# id (toException ex) s) of s1 -> (# s1, () #)