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
71 catchJust, -- :: (Exception -> Maybe b) -> IO a -> (b -> IO a) -> IO a
73 -- ** The @handle@ functions
74 handle, -- :: (Exception -> IO a) -> IO a -> IO a
75 handleJust,-- :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a
77 -- ** The @try@ functions
78 try, -- :: IO a -> IO (Either Exception a)
79 tryJust, -- :: (Exception -> Maybe b) -> a -> IO (Either b a)
82 -- ** The @evaluate@ function
83 evaluate, -- :: a -> IO a
85 -- ** The @mapException@ function
86 mapException, -- :: (Exception -> Exception) -> a -> a
88 -- * Asynchronous Exceptions
92 -- ** Asynchronous exception control
94 -- |The following two functions allow a thread to control delivery of
95 -- asynchronous exceptions during a critical region.
97 block, -- :: IO a -> IO a
98 unblock, -- :: IO a -> IO a
99 blocked, -- :: IO Bool
101 -- *** Applying @block@ to an exception handler
105 -- *** Interruptible operations
111 assert, -- :: Bool -> a -> a
115 bracket, -- :: IO a -> (a -> IO b) -> (a -> IO c) -> IO ()
116 bracket_, -- :: IO a -> IO b -> IO c -> IO ()
119 finally, -- :: IO a -> IO b -> IO a
121 recSelError, recConError, irrefutPatError, runtimeError,
122 nonExhaustiveGuardsError, patError, noMethodBindingError,
126 #ifdef __GLASGOW_HASKELL__
131 import GHC.IOBase as ExceptionBase
132 import GHC.Exception hiding ( Exception )
133 import GHC.Conc ( ThreadId(ThreadId) )
137 import Hugs.Exception as ExceptionBase
145 import qualified System.IO.Error as H'98 (catch)
146 import System.IO.Error (ioError)
148 import DIOError -- defn of IOError type
149 import System (ExitCode())
151 -- minimum needed for nhc98 to pretend it has Exceptions
152 data Exception = IOException IOException
153 | ArithException ArithException
154 | ArrayException ArrayException
155 | AsyncException AsyncException
156 | ExitException ExitCode
158 type IOException = IOError
162 instance Show ArithException
163 instance Show ArrayException
164 instance Show AsyncException
166 catch :: IO a -> (Exception -> IO a) -> IO a
167 a `catch` b = a `H'98.catch` (b . IOException)
169 throwIO :: Exception -> IO a
170 throwIO (IOException e) = ioError e
171 throwIO _ = ioError (UserError "Control.Exception.throwIO"
173 throw :: Exception -> a
174 throw = unsafePerformIO . throwIO
176 evaluate :: a -> IO a
177 evaluate x = x `seq` return x
179 assert :: Bool -> a -> a
181 assert False _ = throw (IOException (UserError "" "Assertion failed"))
184 #ifndef __GLASGOW_HASKELL__
185 -- Dummy definitions for implementations lacking asynchonous exceptions
187 block :: IO a -> IO a
189 unblock :: IO a -> IO a
192 blocked = return False
195 -----------------------------------------------------------------------------
196 -- Catching exceptions
198 -- |This is the simplest of the exception-catching functions. It
199 -- takes a single argument, runs it, and if an exception is raised
200 -- the \"handler\" is executed, with the value of the exception passed as an
201 -- argument. Otherwise, the result is returned as normal. For example:
203 -- > catch (openFile f ReadMode)
204 -- > (\e -> hPutStr stderr ("Couldn't open "++f++": " ++ show e))
206 -- For catching exceptions in pure (non-'IO') expressions, see the
207 -- function 'evaluate'.
209 -- Note that due to Haskell\'s unspecified evaluation order, an
210 -- expression may return one of several possible exceptions: consider
211 -- the expression @error \"urk\" + 1 \`div\` 0@. Does
212 -- 'catch' execute the handler passing
213 -- @ErrorCall \"urk\"@, or @ArithError DivideByZero@?
215 -- The answer is \"either\": 'catch' makes a
216 -- non-deterministic choice about which exception to catch. If you
217 -- call it again, you might get a different exception back. This is
218 -- ok, because 'catch' is an 'IO' computation.
220 -- Note that 'catch' catches all types of exceptions, and is generally
221 -- used for \"cleaning up\" before passing on the exception using
222 -- 'throwIO'. It is not good practice to discard the exception and
223 -- continue, without first checking the type of the exception (it
224 -- might be a 'ThreadKilled', for example). In this case it is usually better
225 -- to use 'catchJust' and select the kinds of exceptions to catch.
227 -- Also note that the "Prelude" also exports a function called
228 -- 'Prelude.catch' with a similar type to 'Control.Exception.catch',
229 -- except that the "Prelude" version only catches the IO and user
230 -- families of exceptions (as required by Haskell 98).
232 -- We recommend either hiding the "Prelude" version of 'Prelude.catch'
233 -- when importing "Control.Exception":
235 -- > import Prelude hiding (catch)
237 -- or importing "Control.Exception" qualified, to avoid name-clashes:
239 -- > import qualified Control.Exception as C
241 -- and then using @C.catch@
245 => IO a -- ^ The computation to run
246 -> (e -> IO a) -- ^ Handler to invoke if an exception is raised
248 catch = ExceptionBase.catchException
250 catches :: IO a -> [Handler a] -> IO a
251 catches io handlers = io `catch` catchesHandler handlers
253 catchesHandler :: [Handler a] -> SomeException -> IO a
254 catchesHandler handlers e = foldr tryHandler (throw e) handlers
255 where tryHandler (Handler handler) res
256 = case fromException e of
257 Just e' -> handler e'
260 data Handler a = forall e . Exception e => Handler (e -> IO a)
262 -- | The function 'catchJust' is like 'catch', but it takes an extra
263 -- argument which is an /exception predicate/, a function which
264 -- selects which type of exceptions we\'re interested in.
266 -- > result <- catchJust errorCalls thing_to_try handler
268 -- Any other exceptions which are not matched by the predicate
269 -- are re-raised, and may be caught by an enclosing
270 -- 'catch' or 'catchJust'.
273 => (e -> Maybe b) -- ^ Predicate to select exceptions
274 -> IO a -- ^ Computation to run
275 -> (b -> IO a) -- ^ Handler
277 catchJust p a handler = catch a handler'
278 where handler' e = case p e of
282 -- | A version of 'catch' with the arguments swapped around; useful in
283 -- situations where the code for the handler is shorter. For example:
285 -- > do handle (\e -> exitWith (ExitFailure 1)) $
287 handle :: Exception e => (e -> IO a) -> IO a -> IO a
290 -- | A version of 'catchJust' with the arguments swapped around (see
292 handleJust :: Exception e => (e -> Maybe b) -> (b -> IO a) -> IO a -> IO a
293 handleJust p = flip (catchJust p)
295 -----------------------------------------------------------------------------
298 -- | This function maps one exception into another as proposed in the
299 -- paper \"A semantics for imprecise exceptions\".
301 -- Notice that the usage of 'unsafePerformIO' is safe here.
303 mapException :: Exception e => (e -> e) -> a -> a
304 mapException f v = unsafePerformIO (catch (evaluate v)
307 -----------------------------------------------------------------------------
308 -- 'try' and variations.
310 -- | Similar to 'catch', but returns an 'Either' result which is
311 -- @('Right' a)@ if no exception was raised, or @('Left' e)@ if an
312 -- exception was raised and its value is @e@.
314 -- > try a = catch (Right `liftM` a) (return . Left)
316 -- Note: as with 'catch', it is only polite to use this variant if you intend
317 -- to re-throw the exception after performing whatever cleanup is needed.
318 -- Otherwise, 'tryJust' is generally considered to be better.
320 -- Also note that "System.IO.Error" also exports a function called
321 -- 'System.IO.Error.try' with a similar type to 'Control.Exception.try',
322 -- except that it catches only the IO and user families of exceptions
323 -- (as required by the Haskell 98 @IO@ module).
325 try :: Exception e => IO a -> IO (Either e a)
326 try a = catch (a >>= \ v -> return (Right v)) (\e -> return (Left e))
328 -- | A variant of 'try' that takes an exception predicate to select
329 -- which exceptions are caught (c.f. 'catchJust'). If the exception
330 -- does not match the predicate, it is re-thrown.
331 tryJust :: Exception e => (e -> Maybe b) -> IO a -> IO (Either b a)
335 Right v -> return (Right v)
336 Left e -> case p e of
338 Just b -> return (Left b)
340 onException :: IO a -> IO b -> IO a
341 onException io what = io `catch` \e -> do what
342 throw (e :: SomeException)
344 -----------------------------------------------------------------------------
345 -- Some Useful Functions
347 -- | When you want to acquire a resource, do some work with it, and
348 -- then release the resource, it is a good idea to use 'bracket',
349 -- because 'bracket' will install the necessary exception handler to
350 -- release the resource in the event that an exception is raised
351 -- during the computation. If an exception is raised, then 'bracket' will
352 -- re-raise the exception (after performing the release).
354 -- A common example is opening a file:
357 -- > (openFile "filename" ReadMode)
359 -- > (\handle -> do { ... })
361 -- The arguments to 'bracket' are in this order so that we can partially apply
364 -- > withFile name mode = bracket (openFile name mode) hClose
368 :: IO a -- ^ computation to run first (\"acquire resource\")
369 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
370 -> (a -> IO c) -- ^ computation to run in-between
371 -> IO c -- returns the value from the in-between computation
372 bracket before after thing =
375 r <- unblock (thing a) `onException` after a
381 -- | A specialised variant of 'bracket' with just a computation to run
384 finally :: IO a -- ^ computation to run first
385 -> IO b -- ^ computation to run afterward (even if an exception
387 -> IO a -- returns the value from the first computation
390 r <- unblock a `onException` sequel
395 -- | A variant of 'bracket' where the return value from the first computation
397 bracket_ :: IO a -> IO b -> IO c -> IO c
398 bracket_ before after thing = bracket before (const after) (const thing)
400 -- | Like bracket, but only performs the final action if there was an
401 -- exception raised by the in-between computation.
403 :: IO a -- ^ computation to run first (\"acquire resource\")
404 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
405 -> (a -> IO c) -- ^ computation to run in-between
406 -> IO c -- returns the value from the in-between computation
407 bracketOnError before after thing =
410 unblock (thing a) `onException` after a
413 -- -----------------------------------------------------------------------------
414 -- Asynchronous exceptions
418 #AsynchronousExceptions# Asynchronous exceptions are so-called because they arise due to
419 external influences, and can be raised at any point during execution.
420 'StackOverflow' and 'HeapOverflow' are two examples of
421 system-generated asynchronous exceptions.
423 The primary source of asynchronous exceptions, however, is
426 > throwTo :: ThreadId -> Exception -> IO ()
428 'throwTo' (also 'throwDynTo' and 'Control.Concurrent.killThread') allows one
429 running thread to raise an arbitrary exception in another thread. The
430 exception is therefore asynchronous with respect to the target thread,
431 which could be doing anything at the time it receives the exception.
432 Great care should be taken with asynchronous exceptions; it is all too
433 easy to introduce race conditions by the over zealous use of
438 There\'s an implied 'block' around every exception handler in a call
439 to one of the 'catch' family of functions. This is because that is
440 what you want most of the time - it eliminates a common race condition
441 in starting an exception handler, because there may be no exception
442 handler on the stack to handle another exception if one arrives
443 immediately. If asynchronous exceptions are blocked on entering the
444 handler, though, we have time to install a new exception handler
445 before being interrupted. If this weren\'t the default, one would have
446 to write something like
449 > catch (unblock (...))
453 If you need to unblock asynchronous exceptions again in the exception
454 handler, just use 'unblock' as normal.
456 Note that 'try' and friends /do not/ have a similar default, because
457 there is no exception handler in this case. If you want to use 'try'
458 in an asynchronous-exception-safe way, you will need to use
464 Some operations are /interruptible/, which means that they can receive
465 asynchronous exceptions even in the scope of a 'block'. Any function
466 which may itself block is defined as interruptible; this includes
467 'Control.Concurrent.MVar.takeMVar'
468 (but not 'Control.Concurrent.MVar.tryTakeMVar'),
469 and most operations which perform
470 some I\/O with the outside world. The reason for having
471 interruptible operations is so that we can write things like
475 > catch (unblock (...))
479 if the 'Control.Concurrent.MVar.takeMVar' was not interruptible,
481 combination could lead to deadlock, because the thread itself would be
482 blocked in a state where it can\'t receive any asynchronous exceptions.
483 With 'Control.Concurrent.MVar.takeMVar' interruptible, however, we can be
484 safe in the knowledge that the thread can receive exceptions right up
485 until the point when the 'Control.Concurrent.MVar.takeMVar' succeeds.
486 Similar arguments apply for other interruptible operations like
487 'System.IO.openFile'.
490 #if !(__GLASGOW_HASKELL__ || __NHC__)
491 assert :: Bool -> a -> a
493 assert False _ = throw (AssertionFailed "")
496 recSelError, recConError, irrefutPatError, runtimeError,
497 nonExhaustiveGuardsError, patError, noMethodBindingError
498 :: Addr# -> a -- All take a UTF8-encoded C string
500 recSelError s = throw (RecSelError (unpackCStringUtf8# s)) -- No location info unfortunately
501 runtimeError s = error (unpackCStringUtf8# s) -- No location info unfortunately
503 nonExhaustiveGuardsError s = throw (PatternMatchFail (untangle s "Non-exhaustive guards in"))
504 irrefutPatError s = throw (PatternMatchFail (untangle s "Irrefutable pattern failed for pattern"))
505 recConError s = throw (RecConError (untangle s "Missing field in record construction"))
506 noMethodBindingError s = throw (NoMethodError (untangle s "No instance nor default method for class operation"))
507 patError s = throw (PatternMatchFail (untangle s "Non-exhaustive patterns in"))
511 data PatternMatchFail = PatternMatchFail String
512 INSTANCE_TYPEABLE0(PatternMatchFail,patternMatchFailTc,"PatternMatchFail")
514 instance Exception PatternMatchFail
516 instance Show PatternMatchFail where
517 showsPrec _ (PatternMatchFail err) = showString err
521 data RecSelError = RecSelError String
522 INSTANCE_TYPEABLE0(RecSelError,recSelErrorTc,"RecSelError")
524 instance Exception RecSelError
526 instance Show RecSelError where
527 showsPrec _ (RecSelError err) = showString err
531 data RecConError = RecConError String
532 INSTANCE_TYPEABLE0(RecConError,recConErrorTc,"RecConError")
534 instance Exception RecConError
536 instance Show RecConError where
537 showsPrec _ (RecConError err) = showString err
541 data RecUpdError = RecUpdError String
542 INSTANCE_TYPEABLE0(RecUpdError,recUpdErrorTc,"RecUpdError")
544 instance Exception RecUpdError
546 instance Show RecUpdError where
547 showsPrec _ (RecUpdError err) = showString err
551 data NoMethodError = NoMethodError String
552 INSTANCE_TYPEABLE0(NoMethodError,noMethodErrorTc,"NoMethodError")
554 instance Exception NoMethodError
556 instance Show NoMethodError where
557 showsPrec _ (NoMethodError err) = showString err
561 data AssertionFailed = AssertionFailed String
562 INSTANCE_TYPEABLE0(AssertionFailed,assertionFailedTc,"AssertionFailed")
564 instance Exception AssertionFailed
566 instance Show AssertionFailed where
567 showsPrec _ (AssertionFailed err) = showString err
571 data NonTermination = NonTermination
572 INSTANCE_TYPEABLE0(NonTermination,nonTerminationTc,"NonTermination")
574 instance Exception NonTermination
576 instance Show NonTermination where
577 showsPrec _ NonTermination = showString "<<loop>>"
579 -- GHC's RTS calls this
580 nonTermination :: SomeException
581 nonTermination = toException NonTermination
585 data NestedAtomically = NestedAtomically
586 INSTANCE_TYPEABLE0(NestedAtomically,nestedAtomicallyTc,"NestedAtomically")
588 instance Exception NestedAtomically
590 instance Show NestedAtomically where
591 showsPrec _ NestedAtomically = showString "Control.Concurrent.STM.atomically was nested"
593 -- GHC's RTS calls this
594 nestedAtomically :: SomeException
595 nestedAtomically = toException NestedAtomically
599 instance Exception Dynamic
603 assertError :: Addr# -> Bool -> a -> a
604 assertError str pred v
606 | otherwise = throw (AssertionFailed (untangle str "Assertion failed"))
609 (untangle coded message) expects "coded" to be of the form
612 location message details
614 untangle :: Addr# -> String -> String
615 untangle coded message
622 coded_str = unpackCStringUtf8# coded
625 = case (span not_bar coded_str) of { (loc, rest) ->
627 ('|':det) -> (loc, ' ' : det)
633 throwTo :: Exception e => ThreadId -> e -> IO ()
634 throwTo (ThreadId id) ex = IO $ \ s ->
635 case (killThread# id (toException ex) s) of s1 -> (# s1, () #)