1 -----------------------------------------------------------------------------
3 -- Module : Control.Exception
4 -- Copyright : (c) The University of Glasgow 2001
5 -- License : BSD-style (see the file libraries/base/LICENSE)
7 -- Maintainer : libraries@haskell.org
8 -- Stability : experimental
9 -- Portability : non-portable (extended exceptions)
11 -- This module provides support for raising and catching both built-in
12 -- and user-defined exceptions.
14 -- In addition to exceptions thrown by 'IO' operations, exceptions may
15 -- be thrown by pure code (imprecise exceptions) or by external events
16 -- (asynchronous exceptions), but may only be caught in the 'IO' monad.
17 -- For more details, see:
19 -- * /A semantics for imprecise exceptions/, by Simon Peyton Jones,
20 -- Alastair Reid, Tony Hoare, Simon Marlow, Fergus Henderson,
23 -- * /Asynchronous exceptions in Haskell/, by Simon Marlow, Simon Peyton
24 -- Jones, Andy Moran and John Reppy, in /PLDI'01/.
26 -----------------------------------------------------------------------------
28 module Control.Exception (
30 -- * The Exception type
31 Exception(..), -- instance Eq, Ord, Show, Typeable
32 IOException, -- instance Eq, Ord, Show, Typeable
33 ArithException(..), -- instance Eq, Ord, Show, Typeable
34 ArrayException(..), -- instance Eq, Ord, Show, Typeable
35 AsyncException(..), -- instance Eq, Ord, Show, Typeable
37 -- * Throwing exceptions
38 throwIO, -- :: Exception -> IO a
39 throw, -- :: Exception -> a
40 ioError, -- :: IOError -> IO a
41 #ifdef __GLASGOW_HASKELL__
42 throwTo, -- :: ThreadId -> Exception -> a
45 -- * Catching Exceptions
47 -- |There are several functions for catching and examining
48 -- exceptions; all of them may only be used from within the
51 -- ** The @catch@ functions
52 catch, -- :: IO a -> (Exception -> IO a) -> IO a
53 catchJust, -- :: (Exception -> Maybe b) -> IO a -> (b -> IO a) -> IO a
55 -- ** The @handle@ functions
56 handle, -- :: (Exception -> IO a) -> IO a -> IO a
57 handleJust,-- :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a
59 -- ** The @try@ functions
60 try, -- :: IO a -> IO (Either Exception a)
61 tryJust, -- :: (Exception -> Maybe b) -> a -> IO (Either b a)
63 -- ** The @evaluate@ function
64 evaluate, -- :: a -> IO a
66 -- ** The @mapException@ function
67 mapException, -- :: (Exception -> Exception) -> a -> a
69 -- ** Exception predicates
73 ioErrors, -- :: Exception -> Maybe IOError
74 arithExceptions, -- :: Exception -> Maybe ArithException
75 errorCalls, -- :: Exception -> Maybe String
76 dynExceptions, -- :: Exception -> Maybe Dynamic
77 assertions, -- :: Exception -> Maybe String
78 asyncExceptions, -- :: Exception -> Maybe AsyncException
79 userErrors, -- :: Exception -> Maybe String
81 -- * Dynamic exceptions
84 throwDyn, -- :: Typeable ex => ex -> b
85 #ifdef __GLASGOW_HASKELL__
86 throwDynTo, -- :: Typeable ex => ThreadId -> ex -> b
88 catchDyn, -- :: Typeable ex => IO a -> (ex -> IO a) -> IO a
90 -- * Asynchronous Exceptions
94 -- ** Asynchronous exception control
96 -- |The following two functions allow a thread to control delivery of
97 -- asynchronous exceptions during a critical region.
99 block, -- :: IO a -> IO a
100 unblock, -- :: IO a -> IO a
101 blocked, -- :: IO Bool
103 -- *** Applying @block@ to an exception handler
107 -- *** Interruptible operations
113 assert, -- :: Bool -> a -> a
117 bracket, -- :: IO a -> (a -> IO b) -> (a -> IO c) -> IO ()
118 bracket_, -- :: IO a -> IO b -> IO c -> IO ()
121 finally, -- :: IO a -> IO b -> IO a
123 #ifdef __GLASGOW_HASKELL__
124 setUncaughtExceptionHandler, -- :: (Exception -> IO ()) -> IO ()
125 getUncaughtExceptionHandler -- :: IO (Exception -> IO ())
129 #ifdef __GLASGOW_HASKELL__
130 import GHC.Base ( assert )
131 import GHC.Exception as ExceptionBase hiding (catch)
132 import GHC.Conc ( throwTo, ThreadId )
133 import Data.IORef ( IORef, newIORef, readIORef, writeIORef )
134 import Foreign.C.String ( CString, withCString )
135 import System.IO ( stdout, hFlush )
139 import Hugs.Exception as ExceptionBase
142 import Prelude hiding ( catch )
143 import System.IO.Error hiding ( catch, try )
144 import System.IO.Unsafe (unsafePerformIO)
148 import qualified System.IO.Error as H'98 (catch)
149 import System.IO.Error (ioError)
151 import DIOError -- defn of IOError type
152 import System (ExitCode())
154 -- minimum needed for nhc98 to pretend it has Exceptions
155 data Exception = IOException IOException
156 | ArithException ArithException
157 | ArrayException ArrayException
158 | AsyncException AsyncException
159 | ExitException ExitCode
161 type IOException = IOError
165 instance Show ArithException
166 instance Show ArrayException
167 instance Show AsyncException
169 catch :: IO a -> (Exception -> IO a) -> IO a
170 a `catch` b = a `H'98.catch` (b . IOException)
172 throwIO :: Exception -> IO a
173 throwIO (IOException e) = ioError e
174 throwIO _ = ioError (UserError "Control.Exception.throwIO"
176 throw :: Exception -> a
177 throw = unsafePerformIO . throwIO
179 evaluate :: a -> IO a
180 evaluate x = x `seq` return x
182 ioErrors :: Exception -> Maybe IOError
183 ioErrors (IOException e) = Just e
185 arithExceptions :: Exception -> Maybe ArithException
186 arithExceptions (ArithException e) = Just e
187 arithExceptions _ = Nothing
188 errorCalls :: Exception -> Maybe String
189 errorCalls = const Nothing
190 dynExceptions :: Exception -> Maybe Dynamic
191 dynExceptions = const Nothing
192 assertions :: Exception -> Maybe String
193 assertions = const Nothing
194 asyncExceptions :: Exception -> Maybe AsyncException
195 asyncExceptions = const Nothing
196 userErrors :: Exception -> Maybe String
197 userErrors (IOException (UserError _ s)) = Just s
198 userErrors _ = Nothing
200 assert :: Bool -> a -> a
202 assert False _ = throw (IOException (UserError "" "Assertion failed"))
205 #ifndef __GLASGOW_HASKELL__
206 -- Dummy definitions for implementations lacking asynchonous exceptions
208 block :: IO a -> IO a
210 unblock :: IO a -> IO a
213 blocked = return False
216 -----------------------------------------------------------------------------
217 -- Catching exceptions
219 -- |This is the simplest of the exception-catching functions. It
220 -- takes a single argument, runs it, and if an exception is raised
221 -- the \"handler\" is executed, with the value of the exception passed as an
222 -- argument. Otherwise, the result is returned as normal. For example:
224 -- > catch (openFile f ReadMode)
225 -- > (\e -> hPutStr stderr ("Couldn't open "++f++": " ++ show e))
227 -- For catching exceptions in pure (non-'IO') expressions, see the
228 -- function 'evaluate'.
230 -- Note that due to Haskell\'s unspecified evaluation order, an
231 -- expression may return one of several possible exceptions: consider
232 -- the expression @error \"urk\" + 1 \`div\` 0@. Does
233 -- 'catch' execute the handler passing
234 -- @ErrorCall \"urk\"@, or @ArithError DivideByZero@?
236 -- The answer is \"either\": 'catch' makes a
237 -- non-deterministic choice about which exception to catch. If you
238 -- call it again, you might get a different exception back. This is
239 -- ok, because 'catch' is an 'IO' computation.
241 -- Note that 'catch' catches all types of exceptions, and is generally
242 -- used for \"cleaning up\" before passing on the exception using
243 -- 'throwIO'. It is not good practice to discard the exception and
244 -- continue, without first checking the type of the exception (it
245 -- might be a 'ThreadKilled', for example). In this case it is usually better
246 -- to use 'catchJust' and select the kinds of exceptions to catch.
248 -- Also note that the "Prelude" also exports a function called
249 -- 'Prelude.catch' with a similar type to 'Control.Exception.catch',
250 -- except that the "Prelude" version only catches the IO and user
251 -- families of exceptions (as required by Haskell 98).
253 -- We recommend either hiding the "Prelude" version of 'Prelude.catch'
254 -- when importing "Control.Exception":
256 -- > import Prelude hiding (catch)
258 -- or importing "Control.Exception" qualified, to avoid name-clashes:
260 -- > import qualified Control.Exception as C
262 -- and then using @C.catch@
265 catch :: IO a -- ^ The computation to run
266 -> (Exception -> IO a) -- ^ Handler to invoke if an exception is raised
268 catch = ExceptionBase.catchException
270 -- | The function 'catchJust' is like 'catch', but it takes an extra
271 -- argument which is an /exception predicate/, a function which
272 -- selects which type of exceptions we\'re interested in. There are
273 -- some predefined exception predicates for useful subsets of
274 -- exceptions: 'ioErrors', 'arithExceptions', and so on. For example,
275 -- to catch just calls to the 'error' function, we could use
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'.
283 :: (Exception -> 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 -> IO a) -> IO a -> IO a
300 -- | A version of 'catchJust' with the arguments swapped around (see
302 handleJust :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a
303 handleJust p = flip (catchJust p)
305 -----------------------------------------------------------------------------
308 -- | This function maps one exception into another as proposed in the
309 -- paper \"A semantics for imprecise exceptions\".
311 -- Notice that the usage of 'unsafePerformIO' is safe here.
313 mapException :: (Exception -> Exception) -> a -> a
314 mapException f v = unsafePerformIO (catch (evaluate v)
317 -----------------------------------------------------------------------------
318 -- 'try' and variations.
320 -- | Similar to 'catch', but returns an 'Either' result which is
321 -- @('Right' a)@ if no exception was raised, or @('Left' e)@ if an
322 -- exception was raised and its value is @e@.
324 -- > try a = catch (Right `liftM` a) (return . Left)
326 -- Note: as with 'catch', it is only polite to use this variant if you intend
327 -- to re-throw the exception after performing whatever cleanup is needed.
328 -- Otherwise, 'tryJust' is generally considered to be better.
330 -- Also note that "System.IO.Error" also exports a function called
331 -- 'System.IO.Error.try' with a similar type to 'Control.Exception.try',
332 -- except that it catches only the IO and user families of exceptions
333 -- (as required by the Haskell 98 @IO@ module).
335 try :: IO a -> IO (Either Exception a)
336 try a = catch (a >>= \ v -> return (Right v)) (\e -> return (Left e))
338 -- | A variant of 'try' that takes an exception predicate to select
339 -- which exceptions are caught (c.f. 'catchJust'). If the exception
340 -- does not match the predicate, it is re-thrown.
341 tryJust :: (Exception -> Maybe b) -> IO a -> IO (Either b a)
345 Right v -> return (Right v)
346 Left e -> case p e of
348 Just b -> return (Left b)
350 -----------------------------------------------------------------------------
351 -- Dynamic exceptions
354 -- #DynamicExceptions# Because the 'Exception' datatype is not extensible, there is an
355 -- interface for throwing and catching exceptions of type 'Dynamic'
356 -- (see "Data.Dynamic") which allows exception values of any type in
357 -- the 'Typeable' class to be thrown and caught.
359 -- | Raise any value as an exception, provided it is in the
361 throwDyn :: Typeable exception => exception -> b
363 throwDyn exception = throw (IOException (UserError "" "dynamic exception"))
365 throwDyn exception = throw (DynException (toDyn exception))
368 #ifdef __GLASGOW_HASKELL__
369 -- | A variant of 'throwDyn' that throws the dynamic exception to an
370 -- arbitrary thread (GHC only: c.f. 'throwTo').
371 throwDynTo :: Typeable exception => ThreadId -> exception -> IO ()
372 throwDynTo t exception = throwTo t (DynException (toDyn exception))
373 #endif /* __GLASGOW_HASKELL__ */
375 -- | Catch dynamic exceptions of the required type. All other
376 -- exceptions are re-thrown, including dynamic exceptions of the wrong
379 -- When using dynamic exceptions it is advisable to define a new
380 -- datatype to use for your exception type, to avoid possible clashes
381 -- with dynamic exceptions used in other libraries.
383 catchDyn :: Typeable exception => IO a -> (exception -> IO a) -> IO a
385 catchDyn m k = m -- can't catch dyn exceptions in nhc98
387 catchDyn m k = catchException m handle
388 where handle ex = case ex of
389 (DynException dyn) ->
390 case fromDynamic dyn of
391 Just exception -> k exception
396 -----------------------------------------------------------------------------
397 -- Exception Predicates
400 -- These pre-defined predicates may be used as the first argument to
401 -- 'catchJust', 'tryJust', or 'handleJust' to select certain common
402 -- classes of exceptions.
404 ioErrors :: Exception -> Maybe IOError
405 arithExceptions :: Exception -> Maybe ArithException
406 errorCalls :: Exception -> Maybe String
407 assertions :: Exception -> Maybe String
408 dynExceptions :: Exception -> Maybe Dynamic
409 asyncExceptions :: Exception -> Maybe AsyncException
410 userErrors :: Exception -> Maybe String
412 ioErrors (IOException e) = Just e
415 arithExceptions (ArithException e) = Just e
416 arithExceptions _ = Nothing
418 errorCalls (ErrorCall e) = Just e
419 errorCalls _ = Nothing
421 assertions (AssertionFailed e) = Just e
422 assertions _ = Nothing
424 dynExceptions (DynException e) = Just e
425 dynExceptions _ = Nothing
427 asyncExceptions (AsyncException e) = Just e
428 asyncExceptions _ = Nothing
430 userErrors (IOException e) | isUserError e = Just (ioeGetErrorString e)
431 userErrors _ = Nothing
433 -----------------------------------------------------------------------------
434 -- Some Useful Functions
436 -- | When you want to acquire a resource, do some work with it, and
437 -- then release the resource, it is a good idea to use 'bracket',
438 -- because 'bracket' will install the necessary exception handler to
439 -- release the resource in the event that an exception is raised
440 -- during the computation. If an exception is raised, then 'bracket' will
441 -- re-raise the exception (after performing the release).
443 -- A common example is opening a file:
446 -- > (openFile "filename" ReadMode)
448 -- > (\handle -> do { ... })
450 -- The arguments to 'bracket' are in this order so that we can partially apply
453 -- > withFile name mode = bracket (openFile name mode) hClose
457 :: IO a -- ^ computation to run first (\"acquire resource\")
458 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
459 -> (a -> IO c) -- ^ computation to run in-between
460 -> IO c -- returns the value from the in-between computation
461 bracket before after thing =
466 (\e -> do { after a; throw e })
472 -- | A specialised variant of 'bracket' with just a computation to run
475 finally :: IO a -- ^ computation to run first
476 -> IO b -- ^ computation to run afterward (even if an exception
478 -> IO a -- returns the value from the first computation
483 (\e -> do { sequel; throw e })
488 -- | A variant of 'bracket' where the return value from the first computation
490 bracket_ :: IO a -> IO b -> IO c -> IO c
491 bracket_ before after thing = bracket before (const after) (const thing)
493 -- | Like bracket, but only performs the final action if there was an
494 -- exception raised by the in-between computation.
496 :: IO a -- ^ computation to run first (\"acquire resource\")
497 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
498 -> (a -> IO c) -- ^ computation to run in-between
499 -> IO c -- returns the value from the in-between computation
500 bracketOnError before after thing =
505 (\e -> do { after a; throw e })
508 -- -----------------------------------------------------------------------------
509 -- Asynchronous exceptions
513 #AsynchronousExceptions# Asynchronous exceptions are so-called because they arise due to
514 external influences, and can be raised at any point during execution.
515 'StackOverflow' and 'HeapOverflow' are two examples of
516 system-generated asynchronous exceptions.
518 The primary source of asynchronous exceptions, however, is
521 > throwTo :: ThreadId -> Exception -> IO ()
523 'throwTo' (also 'throwDynTo' and 'Control.Concurrent.killThread') allows one
524 running thread to raise an arbitrary exception in another thread. The
525 exception is therefore asynchronous with respect to the target thread,
526 which could be doing anything at the time it receives the exception.
527 Great care should be taken with asynchronous exceptions; it is all too
528 easy to introduce race conditions by the over zealous use of
533 There\'s an implied 'block' around every exception handler in a call
534 to one of the 'catch' family of functions. This is because that is
535 what you want most of the time - it eliminates a common race condition
536 in starting an exception handler, because there may be no exception
537 handler on the stack to handle another exception if one arrives
538 immediately. If asynchronous exceptions are blocked on entering the
539 handler, though, we have time to install a new exception handler
540 before being interrupted. If this weren\'t the default, one would have
541 to write something like
544 > catch (unblock (...))
548 If you need to unblock asynchronous exceptions again in the exception
549 handler, just use 'unblock' as normal.
551 Note that 'try' and friends /do not/ have a similar default, because
552 there is no exception handler in this case. If you want to use 'try'
553 in an asynchronous-exception-safe way, you will need to use
559 Some operations are /interruptible/, which means that they can receive
560 asynchronous exceptions even in the scope of a 'block'. Any function
561 which may itself block is defined as interruptible; this includes
562 'Control.Concurrent.MVar.takeMVar'
563 (but not 'Control.Concurrent.MVar.tryTakeMVar'),
564 and most operations which perform
565 some I\/O with the outside world. The reason for having
566 interruptible operations is so that we can write things like
570 > catch (unblock (...))
574 if the 'Control.Concurrent.MVar.takeMVar' was not interruptible,
576 combination could lead to deadlock, because the thread itself would be
577 blocked in a state where it can\'t receive any asynchronous exceptions.
578 With 'Control.Concurrent.MVar.takeMVar' interruptible, however, we can be
579 safe in the knowledge that the thread can receive exceptions right up
580 until the point when the 'Control.Concurrent.MVar.takeMVar' succeeds.
581 Similar arguments apply for other interruptible operations like
582 'System.IO.openFile'.
585 #if !(__GLASGOW_HASKELL__ || __NHC__)
586 assert :: Bool -> a -> a
588 assert False _ = throw (AssertionFailed "")
592 #ifdef __GLASGOW_HASKELL__
593 {-# NOINLINE uncaughtExceptionHandler #-}
594 uncaughtExceptionHandler :: IORef (Exception -> IO ())
595 uncaughtExceptionHandler = unsafePerformIO (newIORef defaultHandler)
597 defaultHandler :: Exception -> IO ()
598 defaultHandler ex = do
599 (hFlush stdout) `catchException` (\ _ -> return ())
601 Deadlock -> "no threads to run: infinite loop or deadlock?"
603 other -> showsPrec 0 other ""
604 withCString "%s" $ \cfmt ->
605 withCString msg $ \cmsg ->
608 -- don't use errorBelch() directly, because we cannot call varargs functions
610 foreign import ccall unsafe "HsBase.h errorBelch2"
611 errorBelch :: CString -> CString -> IO ()
613 setUncaughtExceptionHandler :: (Exception -> IO ()) -> IO ()
614 setUncaughtExceptionHandler = writeIORef uncaughtExceptionHandler
616 getUncaughtExceptionHandler :: IO (Exception -> IO ())
617 getUncaughtExceptionHandler = readIORef uncaughtExceptionHandler