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 System.IO.Error (catch, ioError)
150 import DIOError -- defn of IOError type
152 -- minimum needed for nhc98 to pretend it has Exceptions
153 type Exception = IOError
154 type IOException = IOError
159 throwIO :: Exception -> IO a
161 throw :: Exception -> a
162 throw = unsafePerformIO . throwIO
164 evaluate :: a -> IO a
165 evaluate x = x `seq` return x
167 ioErrors :: Exception -> Maybe IOError
169 arithExceptions :: Exception -> Maybe ArithException
170 arithExceptions = const Nothing
171 errorCalls :: Exception -> Maybe String
172 errorCalls = const Nothing
173 dynExceptions :: Exception -> Maybe Dynamic
174 dynExceptions = const Nothing
175 assertions :: Exception -> Maybe String
176 assertions = const Nothing
177 asyncExceptions :: Exception -> Maybe AsyncException
178 asyncExceptions = const Nothing
179 userErrors :: Exception -> Maybe String
180 userErrors (UserError _ s) = Just s
181 userErrors _ = Nothing
183 assert :: Bool -> a -> a
185 assert False _ = throw (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@
248 catch :: IO a -- ^ The computation to run
249 -> (Exception -> IO a) -- ^ Handler to invoke if an exception is raised
251 catch = ExceptionBase.catchException
253 -- | The function 'catchJust' is like 'catch', but it takes an extra
254 -- argument which is an /exception predicate/, a function which
255 -- selects which type of exceptions we\'re interested in. There are
256 -- some predefined exception predicates for useful subsets of
257 -- exceptions: 'ioErrors', 'arithExceptions', and so on. For example,
258 -- to catch just calls to the 'error' function, we could use
260 -- > result <- catchJust errorCalls thing_to_try handler
262 -- Any other exceptions which are not matched by the predicate
263 -- are re-raised, and may be caught by an enclosing
264 -- 'catch' or 'catchJust'.
266 :: (Exception -> Maybe b) -- ^ Predicate to select exceptions
267 -> IO a -- ^ Computation to run
268 -> (b -> IO a) -- ^ Handler
270 catchJust p a handler = catch a handler'
271 where handler' e = case p e of
275 -- | A version of 'catch' with the arguments swapped around; useful in
276 -- situations where the code for the handler is shorter. For example:
278 -- > do handle (\e -> exitWith (ExitFailure 1)) $
280 handle :: (Exception -> IO a) -> IO a -> IO a
283 -- | A version of 'catchJust' with the arguments swapped around (see
285 handleJust :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a
286 handleJust p = flip (catchJust p)
288 -----------------------------------------------------------------------------
291 -- | This function maps one exception into another as proposed in the
292 -- paper \"A semantics for imprecise exceptions\".
294 -- Notice that the usage of 'unsafePerformIO' is safe here.
296 mapException :: (Exception -> Exception) -> a -> a
297 mapException f v = unsafePerformIO (catch (evaluate v)
300 -----------------------------------------------------------------------------
301 -- 'try' and variations.
303 -- | Similar to 'catch', but returns an 'Either' result which is
304 -- @('Right' a)@ if no exception was raised, or @('Left' e)@ if an
305 -- exception was raised and its value is @e@.
307 -- > try a = catch (Right `liftM` a) (return . Left)
309 -- Note: as with 'catch', it is only polite to use this variant if you intend
310 -- to re-throw the exception after performing whatever cleanup is needed.
311 -- Otherwise, 'tryJust' is generally considered to be better.
313 -- Also note that "System.IO.Error" also exports a function called
314 -- 'System.IO.Error.try' with a similar type to 'Control.Exception.try',
315 -- except that it catches only the IO and user families of exceptions
316 -- (as required by the Haskell 98 @IO@ module).
318 try :: IO a -> IO (Either Exception a)
319 try a = catch (a >>= \ v -> return (Right v)) (\e -> return (Left e))
321 -- | A variant of 'try' that takes an exception predicate to select
322 -- which exceptions are caught (c.f. 'catchJust'). If the exception
323 -- does not match the predicate, it is re-thrown.
324 tryJust :: (Exception -> Maybe b) -> IO a -> IO (Either b a)
328 Right v -> return (Right v)
329 Left e -> case p e of
331 Just b -> return (Left b)
333 -----------------------------------------------------------------------------
334 -- Dynamic exceptions
337 -- #DynamicExceptions# Because the 'Exception' datatype is not extensible, there is an
338 -- interface for throwing and catching exceptions of type 'Dynamic'
339 -- (see "Data.Dynamic") which allows exception values of any type in
340 -- the 'Typeable' class to be thrown and caught.
342 -- | Raise any value as an exception, provided it is in the
344 throwDyn :: Typeable exception => exception -> b
346 throwDyn exception = throw (UserError "" "dynamic exception")
348 throwDyn exception = throw (DynException (toDyn exception))
351 #ifdef __GLASGOW_HASKELL__
352 -- | A variant of 'throwDyn' that throws the dynamic exception to an
353 -- arbitrary thread (GHC only: c.f. 'throwTo').
354 throwDynTo :: Typeable exception => ThreadId -> exception -> IO ()
355 throwDynTo t exception = throwTo t (DynException (toDyn exception))
356 #endif /* __GLASGOW_HASKELL__ */
358 -- | Catch dynamic exceptions of the required type. All other
359 -- exceptions are re-thrown, including dynamic exceptions of the wrong
362 -- When using dynamic exceptions it is advisable to define a new
363 -- datatype to use for your exception type, to avoid possible clashes
364 -- with dynamic exceptions used in other libraries.
366 catchDyn :: Typeable exception => IO a -> (exception -> IO a) -> IO a
368 catchDyn m k = m -- can't catch dyn exceptions in nhc98
370 catchDyn m k = catchException m handle
371 where handle ex = case ex of
372 (DynException dyn) ->
373 case fromDynamic dyn of
374 Just exception -> k exception
379 -----------------------------------------------------------------------------
380 -- Exception Predicates
383 -- These pre-defined predicates may be used as the first argument to
384 -- 'catchJust', 'tryJust', or 'handleJust' to select certain common
385 -- classes of exceptions.
387 ioErrors :: Exception -> Maybe IOError
388 arithExceptions :: Exception -> Maybe ArithException
389 errorCalls :: Exception -> Maybe String
390 assertions :: Exception -> Maybe String
391 dynExceptions :: Exception -> Maybe Dynamic
392 asyncExceptions :: Exception -> Maybe AsyncException
393 userErrors :: Exception -> Maybe String
395 ioErrors (IOException e) = Just e
398 arithExceptions (ArithException e) = Just e
399 arithExceptions _ = Nothing
401 errorCalls (ErrorCall e) = Just e
402 errorCalls _ = Nothing
404 assertions (AssertionFailed e) = Just e
405 assertions _ = Nothing
407 dynExceptions (DynException e) = Just e
408 dynExceptions _ = Nothing
410 asyncExceptions (AsyncException e) = Just e
411 asyncExceptions _ = Nothing
413 userErrors (IOException e) | isUserError e = Just (ioeGetErrorString e)
414 userErrors _ = Nothing
416 -----------------------------------------------------------------------------
417 -- Some Useful Functions
419 -- | When you want to acquire a resource, do some work with it, and
420 -- then release the resource, it is a good idea to use 'bracket',
421 -- because 'bracket' will install the necessary exception handler to
422 -- release the resource in the event that an exception is raised
423 -- during the computation. If an exception is raised, then 'bracket' will
424 -- re-raise the exception (after performing the release).
426 -- A common example is opening a file:
429 -- > (openFile "filename" ReadMode)
431 -- > (\handle -> do { ... })
433 -- The arguments to 'bracket' are in this order so that we can partially apply
436 -- > withFile name mode = bracket (openFile name mode) hClose
440 :: IO a -- ^ computation to run first (\"acquire resource\")
441 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
442 -> (a -> IO c) -- ^ computation to run in-between
443 -> IO c -- returns the value from the in-between computation
444 bracket before after thing =
449 (\e -> do { after a; throw e })
455 -- | A specialised variant of 'bracket' with just a computation to run
458 finally :: IO a -- ^ computation to run first
459 -> IO b -- ^ computation to run afterward (even if an exception
461 -> IO a -- returns the value from the first computation
466 (\e -> do { sequel; throw e })
471 -- | A variant of 'bracket' where the return value from the first computation
473 bracket_ :: IO a -> IO b -> IO c -> IO c
474 bracket_ before after thing = bracket before (const after) (const thing)
476 -- | Like bracket, but only performs the final action if there was an
477 -- exception raised by the in-between computation.
479 :: IO a -- ^ computation to run first (\"acquire resource\")
480 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
481 -> (a -> IO c) -- ^ computation to run in-between
482 -> IO c -- returns the value from the in-between computation
483 bracketOnError before after thing =
488 (\e -> do { after a; throw e })
491 -- -----------------------------------------------------------------------------
492 -- Asynchronous exceptions
496 #AsynchronousExceptions# Asynchronous exceptions are so-called because they arise due to
497 external influences, and can be raised at any point during execution.
498 'StackOverflow' and 'HeapOverflow' are two examples of
499 system-generated asynchronous exceptions.
501 The primary source of asynchronous exceptions, however, is
504 > throwTo :: ThreadId -> Exception -> IO ()
506 'throwTo' (also 'throwDynTo' and 'Control.Concurrent.killThread') allows one
507 running thread to raise an arbitrary exception in another thread. The
508 exception is therefore asynchronous with respect to the target thread,
509 which could be doing anything at the time it receives the exception.
510 Great care should be taken with asynchronous exceptions; it is all too
511 easy to introduce race conditions by the over zealous use of
516 There\'s an implied 'block' around every exception handler in a call
517 to one of the 'catch' family of functions. This is because that is
518 what you want most of the time - it eliminates a common race condition
519 in starting an exception handler, because there may be no exception
520 handler on the stack to handle another exception if one arrives
521 immediately. If asynchronous exceptions are blocked on entering the
522 handler, though, we have time to install a new exception handler
523 before being interrupted. If this weren\'t the default, one would have
524 to write something like
527 > catch (unblock (...))
531 If you need to unblock asynchronous exceptions again in the exception
532 handler, just use 'unblock' as normal.
534 Note that 'try' and friends /do not/ have a similar default, because
535 there is no exception handler in this case. If you want to use 'try'
536 in an asynchronous-exception-safe way, you will need to use
542 Some operations are /interruptible/, which means that they can receive
543 asynchronous exceptions even in the scope of a 'block'. Any function
544 which may itself block is defined as interruptible; this includes
545 'Control.Concurrent.MVar.takeMVar'
546 (but not 'Control.Concurrent.MVar.tryTakeMVar'),
547 and most operations which perform
548 some I\/O with the outside world. The reason for having
549 interruptible operations is so that we can write things like
553 > catch (unblock (...))
557 if the 'Control.Concurrent.MVar.takeMVar' was not interruptible,
559 combination could lead to deadlock, because the thread itself would be
560 blocked in a state where it can\'t receive any asynchronous exceptions.
561 With 'Control.Concurrent.MVar.takeMVar' interruptible, however, we can be
562 safe in the knowledge that the thread can receive exceptions right up
563 until the point when the 'Control.Concurrent.MVar.takeMVar' succeeds.
564 Similar arguments apply for other interruptible operations like
565 'System.IO.openFile'.
568 #if !(__GLASGOW_HASKELL__ || __NHC__)
569 assert :: Bool -> a -> a
571 assert False _ = throw (AssertionFailed "")
575 #ifdef __GLASGOW_HASKELL__
576 {-# NOINLINE uncaughtExceptionHandler #-}
577 uncaughtExceptionHandler :: IORef (Exception -> IO ())
578 uncaughtExceptionHandler = unsafePerformIO (newIORef defaultHandler)
580 defaultHandler :: Exception -> IO ()
581 defaultHandler ex = do
582 (hFlush stdout) `catchException` (\ _ -> return ())
584 Deadlock -> "no threads to run: infinite loop or deadlock?"
586 other -> showsPrec 0 other ""
587 withCString "%s" $ \cfmt ->
588 withCString msg $ \cmsg ->
591 -- don't use errorBelch() directly, because we cannot call varargs functions
593 foreign import ccall unsafe "HsBase.h errorBelch2"
594 errorBelch :: CString -> CString -> IO ()
596 setUncaughtExceptionHandler :: (Exception -> IO ()) -> IO ()
597 setUncaughtExceptionHandler = writeIORef uncaughtExceptionHandler
599 getUncaughtExceptionHandler :: IO (Exception -> IO ())
600 getUncaughtExceptionHandler = readIORef uncaughtExceptionHandler