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
102 -- *** Applying @block@ to an exception handler
106 -- *** Interruptible operations
112 assert, -- :: Bool -> a -> a
116 bracket, -- :: IO a -> (a -> IO b) -> (a -> IO c) -> IO ()
117 bracket_, -- :: IO a -> IO b -> IO c -> IO ()
120 finally, -- :: IO a -> IO b -> IO a
122 #ifdef __GLASGOW_HASKELL__
123 setUncaughtExceptionHandler, -- :: (Exception -> IO ()) -> IO ()
124 getUncaughtExceptionHandler -- :: IO (Exception -> IO ())
128 #ifdef __GLASGOW_HASKELL__
129 import GHC.Base ( assert )
130 import GHC.Exception as ExceptionBase hiding (catch)
131 import GHC.Conc ( throwTo, ThreadId )
132 import Data.IORef ( IORef, newIORef, readIORef, writeIORef )
133 import Foreign.C.String ( CString, withCString )
134 import System.IO ( stdout, hFlush )
138 import Hugs.Exception as ExceptionBase
141 import Prelude hiding ( catch )
142 import System.IO.Error hiding ( catch, try )
143 import System.IO.Unsafe (unsafePerformIO)
146 -----------------------------------------------------------------------------
147 -- Catching exceptions
149 -- |This is the simplest of the exception-catching functions. It
150 -- takes a single argument, runs it, and if an exception is raised
151 -- the \"handler\" is executed, with the value of the exception passed as an
152 -- argument. Otherwise, the result is returned as normal. For example:
154 -- > catch (openFile f ReadMode)
155 -- > (\e -> hPutStr stderr ("Couldn't open "++f++": " ++ show e))
157 -- For catching exceptions in pure (non-'IO') expressions, see the
158 -- function 'evaluate'.
160 -- Note that due to Haskell\'s unspecified evaluation order, an
161 -- expression may return one of several possible exceptions: consider
162 -- the expression @error \"urk\" + 1 \`div\` 0@. Does
163 -- 'catch' execute the handler passing
164 -- @ErrorCall \"urk\"@, or @ArithError DivideByZero@?
166 -- The answer is \"either\": 'catch' makes a
167 -- non-deterministic choice about which exception to catch. If you
168 -- call it again, you might get a different exception back. This is
169 -- ok, because 'catch' is an 'IO' computation.
171 -- Note that 'catch' catches all types of exceptions, and is generally
172 -- used for \"cleaning up\" before passing on the exception using
173 -- 'throwIO'. It is not good practice to discard the exception and
174 -- continue, without first checking the type of the exception (it
175 -- might be a 'ThreadKilled', for example). In this case it is usually better
176 -- to use 'catchJust' and select the kinds of exceptions to catch.
178 -- Also note that the "Prelude" also exports a function called
179 -- 'Prelude.catch' with a similar type to 'Control.Exception.catch',
180 -- except that the "Prelude" version only catches the IO and user
181 -- families of exceptions (as required by Haskell 98).
183 -- We recommend either hiding the "Prelude" version of 'Prelude.catch'
184 -- when importing "Control.Exception":
186 -- > import Prelude hiding (catch)
188 -- or importing "Control.Exception" qualified, to avoid name-clashes:
190 -- > import qualified Control.Exception as C
192 -- and then using @C.catch@
195 catch :: IO a -- ^ The computation to run
196 -> (Exception -> IO a) -- ^ Handler to invoke if an exception is raised
198 catch = ExceptionBase.catchException
200 -- | The function 'catchJust' is like 'catch', but it takes an extra
201 -- argument which is an /exception predicate/, a function which
202 -- selects which type of exceptions we\'re interested in. There are
203 -- some predefined exception predicates for useful subsets of
204 -- exceptions: 'ioErrors', 'arithExceptions', and so on. For example,
205 -- to catch just calls to the 'error' function, we could use
207 -- > result <- catchJust errorCalls thing_to_try handler
209 -- Any other exceptions which are not matched by the predicate
210 -- are re-raised, and may be caught by an enclosing
211 -- 'catch' or 'catchJust'.
213 :: (Exception -> Maybe b) -- ^ Predicate to select exceptions
214 -> IO a -- ^ Computation to run
215 -> (b -> IO a) -- ^ Handler
217 catchJust p a handler = catch a handler'
218 where handler' e = case p e of
222 -- | A version of 'catch' with the arguments swapped around; useful in
223 -- situations where the code for the handler is shorter. For example:
225 -- > do handle (\e -> exitWith (ExitFailure 1)) $
227 handle :: (Exception -> IO a) -> IO a -> IO a
230 -- | A version of 'catchJust' with the arguments swapped around (see
232 handleJust :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a
233 handleJust p = flip (catchJust p)
235 -----------------------------------------------------------------------------
238 -- | This function maps one exception into another as proposed in the
239 -- paper \"A semantics for imprecise exceptions\".
241 -- Notice that the usage of 'unsafePerformIO' is safe here.
243 mapException :: (Exception -> Exception) -> a -> a
244 mapException f v = unsafePerformIO (catch (evaluate v)
247 -----------------------------------------------------------------------------
248 -- 'try' and variations.
250 -- | Similar to 'catch', but returns an 'Either' result which is
251 -- @('Right' a)@ if no exception was raised, or @('Left' e)@ if an
252 -- exception was raised and its value is @e@.
254 -- > try a = catch (Right `liftM` a) (return . Left)
256 -- Note: as with 'catch', it is only polite to use this variant if you intend
257 -- to re-throw the exception after performing whatever cleanup is needed.
258 -- Otherwise, 'tryJust' is generally considered to be better.
260 -- Also note that "System.IO.Error" also exports a function called
261 -- 'System.IO.Error.try' with a similar type to 'Control.Exception.try',
262 -- except that it catches only the IO and user families of exceptions
263 -- (as required by the Haskell 98 @IO@ module).
265 try :: IO a -> IO (Either Exception a)
266 try a = catch (a >>= \ v -> return (Right v)) (\e -> return (Left e))
268 -- | A variant of 'try' that takes an exception predicate to select
269 -- which exceptions are caught (c.f. 'catchJust'). If the exception
270 -- does not match the predicate, it is re-thrown.
271 tryJust :: (Exception -> Maybe b) -> IO a -> IO (Either b a)
275 Right v -> return (Right v)
276 Left e -> case p e of
278 Just b -> return (Left b)
280 -----------------------------------------------------------------------------
281 -- Dynamic exceptions
284 -- #DynamicExceptions# Because the 'Exception' datatype is not extensible, there is an
285 -- interface for throwing and catching exceptions of type 'Dynamic'
286 -- (see "Data.Dynamic") which allows exception values of any type in
287 -- the 'Typeable' class to be thrown and caught.
289 -- | Raise any value as an exception, provided it is in the
291 throwDyn :: Typeable exception => exception -> b
292 throwDyn exception = throw (DynException (toDyn exception))
294 #ifdef __GLASGOW_HASKELL__
295 -- | A variant of 'throwDyn' that throws the dynamic exception to an
296 -- arbitrary thread (GHC only: c.f. 'throwTo').
297 throwDynTo :: Typeable exception => ThreadId -> exception -> IO ()
298 throwDynTo t exception = throwTo t (DynException (toDyn exception))
299 #endif /* __GLASGOW_HASKELL__ */
301 -- | Catch dynamic exceptions of the required type. All other
302 -- exceptions are re-thrown, including dynamic exceptions of the wrong
305 -- When using dynamic exceptions it is advisable to define a new
306 -- datatype to use for your exception type, to avoid possible clashes
307 -- with dynamic exceptions used in other libraries.
309 catchDyn :: Typeable exception => IO a -> (exception -> IO a) -> IO a
310 catchDyn m k = catchException m handle
311 where handle ex = case ex of
312 (DynException dyn) ->
313 case fromDynamic dyn of
314 Just exception -> k exception
318 -----------------------------------------------------------------------------
319 -- Exception Predicates
322 -- These pre-defined predicates may be used as the first argument to
323 -- 'catchJust', 'tryJust', or 'handleJust' to select certain common
324 -- classes of exceptions.
326 ioErrors :: Exception -> Maybe IOError
327 arithExceptions :: Exception -> Maybe ArithException
328 errorCalls :: Exception -> Maybe String
329 assertions :: Exception -> Maybe String
330 dynExceptions :: Exception -> Maybe Dynamic
331 asyncExceptions :: Exception -> Maybe AsyncException
332 userErrors :: Exception -> Maybe String
334 ioErrors (IOException e) = Just e
337 arithExceptions (ArithException e) = Just e
338 arithExceptions _ = Nothing
340 errorCalls (ErrorCall e) = Just e
341 errorCalls _ = Nothing
343 assertions (AssertionFailed e) = Just e
344 assertions _ = Nothing
346 dynExceptions (DynException e) = Just e
347 dynExceptions _ = Nothing
349 asyncExceptions (AsyncException e) = Just e
350 asyncExceptions _ = Nothing
352 userErrors (IOException e) | isUserError e = Just (ioeGetErrorString e)
353 userErrors _ = Nothing
355 -----------------------------------------------------------------------------
356 -- Some Useful Functions
358 -- | When you want to acquire a resource, do some work with it, and
359 -- then release the resource, it is a good idea to use 'bracket',
360 -- because 'bracket' will install the necessary exception handler to
361 -- release the resource in the event that an exception is raised
362 -- during the computation. If an exception is raised, then 'bracket' will
363 -- re-raise the exception (after performing the release).
365 -- A common example is opening a file:
368 -- > (openFile "filename" ReadMode)
370 -- > (\handle -> do { ... })
372 -- The arguments to 'bracket' are in this order so that we can partially apply
375 -- > withFile name mode = bracket (openFile name mode) hClose
378 :: IO a -- ^ computation to run first (\"acquire resource\")
379 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
380 -> (a -> IO c) -- ^ computation to run in-between
381 -> IO c -- returns the value from the in-between computation
382 bracket before after thing =
387 (\e -> do { after a; throw e })
393 -- | A specialised variant of 'bracket' with just a computation to run
396 finally :: IO a -- ^ computation to run first
397 -> IO b -- ^ computation to run afterward (even if an exception
399 -> IO a -- returns the value from the first computation
404 (\e -> do { sequel; throw e })
409 -- | A variant of 'bracket' where the return value from the first computation
411 bracket_ :: IO a -> IO b -> IO c -> IO c
412 bracket_ before after thing = bracket before (const after) (const thing)
414 -- | Like bracket, but only performs the final action if there was an
415 -- exception raised by the in-between computation.
417 :: IO a -- ^ computation to run first (\"acquire resource\")
418 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
419 -> (a -> IO c) -- ^ computation to run in-between
420 -> IO c -- returns the value from the in-between computation
421 bracketOnError before after thing =
426 (\e -> do { after a; throw e })
429 -- -----------------------------------------------------------------------------
430 -- Asynchronous exceptions
434 #AsynchronousExceptions# Asynchronous exceptions are so-called because they arise due to
435 external influences, and can be raised at any point during execution.
436 'StackOverflow' and 'HeapOverflow' are two examples of
437 system-generated asynchronous exceptions.
439 The primary source of asynchronous exceptions, however, is
442 > throwTo :: ThreadId -> Exception -> IO ()
444 'throwTo' (also 'throwDynTo' and 'Control.Concurrent.killThread') allows one
445 running thread to raise an arbitrary exception in another thread. The
446 exception is therefore asynchronous with respect to the target thread,
447 which could be doing anything at the time it receives the exception.
448 Great care should be taken with asynchronous exceptions; it is all too
449 easy to introduce race conditions by the over zealous use of
454 There\'s an implied 'block' around every exception handler in a call
455 to one of the 'catch' family of functions. This is because that is
456 what you want most of the time - it eliminates a common race condition
457 in starting an exception handler, because there may be no exception
458 handler on the stack to handle another exception if one arrives
459 immediately. If asynchronous exceptions are blocked on entering the
460 handler, though, we have time to install a new exception handler
461 before being interrupted. If this weren\'t the default, one would have
462 to write something like
465 > catch (unblock (...))
469 If you need to unblock asynchronous exceptions again in the exception
470 handler, just use 'unblock' as normal.
472 Note that 'try' and friends /do not/ have a similar default, because
473 there is no exception handler in this case. If you want to use 'try'
474 in an asynchronous-exception-safe way, you will need to use
480 Some operations are /interruptible/, which means that they can receive
481 asynchronous exceptions even in the scope of a 'block'. Any function
482 which may itself block is defined as interruptible; this includes
483 'Control.Concurrent.MVar.takeMVar'
484 (but not 'Control.Concurrent.MVar.tryTakeMVar'),
485 and most operations which perform
486 some I\/O with the outside world. The reason for having
487 interruptible operations is so that we can write things like
491 > catch (unblock (...))
495 if the 'Control.Concurrent.MVar.takeMVar' was not interruptible,
497 combination could lead to deadlock, because the thread itself would be
498 blocked in a state where it can\'t receive any asynchronous exceptions.
499 With 'Control.Concurrent.MVar.takeMVar' interruptible, however, we can be
500 safe in the knowledge that the thread can receive exceptions right up
501 until the point when the 'Control.Concurrent.MVar.takeMVar' succeeds.
502 Similar arguments apply for other interruptible operations like
503 'System.IO.openFile'.
506 #ifndef __GLASGOW_HASKELL__
507 assert :: Bool -> a -> a
509 assert False _ = throw (AssertionFailed "")
513 #ifdef __GLASGOW_HASKELL__
514 {-# NOINLINE uncaughtExceptionHandler #-}
515 uncaughtExceptionHandler :: IORef (Exception -> IO ())
516 uncaughtExceptionHandler = unsafePerformIO (newIORef defaultHandler)
518 defaultHandler :: Exception -> IO ()
519 defaultHandler ex = do
520 (hFlush stdout) `catchException` (\ _ -> return ())
522 Deadlock -> "no threads to run: infinite loop or deadlock?"
524 other -> showsPrec 0 other "\n"
525 withCString "%s" $ \cfmt ->
526 withCString msg $ \cmsg ->
529 foreign import ccall unsafe "RtsMessages.h errorBelch"
530 errorBelch :: CString -> CString -> IO ()
532 setUncaughtExceptionHandler :: (Exception -> IO ()) -> IO ()
533 setUncaughtExceptionHandler = writeIORef uncaughtExceptionHandler
535 getUncaughtExceptionHandler :: IO (Exception -> IO ())
536 getUncaughtExceptionHandler = readIORef uncaughtExceptionHandler