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 block :: IO a -> IO a
185 unblock :: IO a -> IO a
190 assert :: Bool -> a -> a
192 assert False _ = throw (UserError "" "Assertion failed")
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@
244 catch :: IO a -- ^ The computation to run
245 -> (Exception -> IO a) -- ^ Handler to invoke if an exception is raised
247 catch = ExceptionBase.catchException
249 -- | The function 'catchJust' is like 'catch', but it takes an extra
250 -- argument which is an /exception predicate/, a function which
251 -- selects which type of exceptions we\'re interested in. There are
252 -- some predefined exception predicates for useful subsets of
253 -- exceptions: 'ioErrors', 'arithExceptions', and so on. For example,
254 -- to catch just calls to the 'error' function, we could use
256 -- > result <- catchJust errorCalls thing_to_try handler
258 -- Any other exceptions which are not matched by the predicate
259 -- are re-raised, and may be caught by an enclosing
260 -- 'catch' or 'catchJust'.
262 :: (Exception -> Maybe b) -- ^ Predicate to select exceptions
263 -> IO a -- ^ Computation to run
264 -> (b -> IO a) -- ^ Handler
266 catchJust p a handler = catch a handler'
267 where handler' e = case p e of
271 -- | A version of 'catch' with the arguments swapped around; useful in
272 -- situations where the code for the handler is shorter. For example:
274 -- > do handle (\e -> exitWith (ExitFailure 1)) $
276 handle :: (Exception -> IO a) -> IO a -> IO a
279 -- | A version of 'catchJust' with the arguments swapped around (see
281 handleJust :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a
282 handleJust p = flip (catchJust p)
284 -----------------------------------------------------------------------------
287 -- | This function maps one exception into another as proposed in the
288 -- paper \"A semantics for imprecise exceptions\".
290 -- Notice that the usage of 'unsafePerformIO' is safe here.
292 mapException :: (Exception -> Exception) -> a -> a
293 mapException f v = unsafePerformIO (catch (evaluate v)
296 -----------------------------------------------------------------------------
297 -- 'try' and variations.
299 -- | Similar to 'catch', but returns an 'Either' result which is
300 -- @('Right' a)@ if no exception was raised, or @('Left' e)@ if an
301 -- exception was raised and its value is @e@.
303 -- > try a = catch (Right `liftM` a) (return . Left)
305 -- Note: as with 'catch', it is only polite to use this variant if you intend
306 -- to re-throw the exception after performing whatever cleanup is needed.
307 -- Otherwise, 'tryJust' is generally considered to be better.
309 -- Also note that "System.IO.Error" also exports a function called
310 -- 'System.IO.Error.try' with a similar type to 'Control.Exception.try',
311 -- except that it catches only the IO and user families of exceptions
312 -- (as required by the Haskell 98 @IO@ module).
314 try :: IO a -> IO (Either Exception a)
315 try a = catch (a >>= \ v -> return (Right v)) (\e -> return (Left e))
317 -- | A variant of 'try' that takes an exception predicate to select
318 -- which exceptions are caught (c.f. 'catchJust'). If the exception
319 -- does not match the predicate, it is re-thrown.
320 tryJust :: (Exception -> Maybe b) -> IO a -> IO (Either b a)
324 Right v -> return (Right v)
325 Left e -> case p e of
327 Just b -> return (Left b)
329 -----------------------------------------------------------------------------
330 -- Dynamic exceptions
333 -- #DynamicExceptions# Because the 'Exception' datatype is not extensible, there is an
334 -- interface for throwing and catching exceptions of type 'Dynamic'
335 -- (see "Data.Dynamic") which allows exception values of any type in
336 -- the 'Typeable' class to be thrown and caught.
338 -- | Raise any value as an exception, provided it is in the
340 throwDyn :: Typeable exception => exception -> b
342 throwDyn exception = throw (UserError "" "dynamic exception")
344 throwDyn exception = throw (DynException (toDyn exception))
347 #ifdef __GLASGOW_HASKELL__
348 -- | A variant of 'throwDyn' that throws the dynamic exception to an
349 -- arbitrary thread (GHC only: c.f. 'throwTo').
350 throwDynTo :: Typeable exception => ThreadId -> exception -> IO ()
351 throwDynTo t exception = throwTo t (DynException (toDyn exception))
352 #endif /* __GLASGOW_HASKELL__ */
354 -- | Catch dynamic exceptions of the required type. All other
355 -- exceptions are re-thrown, including dynamic exceptions of the wrong
358 -- When using dynamic exceptions it is advisable to define a new
359 -- datatype to use for your exception type, to avoid possible clashes
360 -- with dynamic exceptions used in other libraries.
362 catchDyn :: Typeable exception => IO a -> (exception -> IO a) -> IO a
364 catchDyn m k = m -- can't catch dyn exceptions in nhc98
366 catchDyn m k = catchException m handle
367 where handle ex = case ex of
368 (DynException dyn) ->
369 case fromDynamic dyn of
370 Just exception -> k exception
375 -----------------------------------------------------------------------------
376 -- Exception Predicates
379 -- These pre-defined predicates may be used as the first argument to
380 -- 'catchJust', 'tryJust', or 'handleJust' to select certain common
381 -- classes of exceptions.
383 ioErrors :: Exception -> Maybe IOError
384 arithExceptions :: Exception -> Maybe ArithException
385 errorCalls :: Exception -> Maybe String
386 assertions :: Exception -> Maybe String
387 dynExceptions :: Exception -> Maybe Dynamic
388 asyncExceptions :: Exception -> Maybe AsyncException
389 userErrors :: Exception -> Maybe String
391 ioErrors (IOException e) = Just e
394 arithExceptions (ArithException e) = Just e
395 arithExceptions _ = Nothing
397 errorCalls (ErrorCall e) = Just e
398 errorCalls _ = Nothing
400 assertions (AssertionFailed e) = Just e
401 assertions _ = Nothing
403 dynExceptions (DynException e) = Just e
404 dynExceptions _ = Nothing
406 asyncExceptions (AsyncException e) = Just e
407 asyncExceptions _ = Nothing
409 userErrors (IOException e) | isUserError e = Just (ioeGetErrorString e)
410 userErrors _ = Nothing
412 -----------------------------------------------------------------------------
413 -- Some Useful Functions
415 -- | When you want to acquire a resource, do some work with it, and
416 -- then release the resource, it is a good idea to use 'bracket',
417 -- because 'bracket' will install the necessary exception handler to
418 -- release the resource in the event that an exception is raised
419 -- during the computation. If an exception is raised, then 'bracket' will
420 -- re-raise the exception (after performing the release).
422 -- A common example is opening a file:
425 -- > (openFile "filename" ReadMode)
427 -- > (\handle -> do { ... })
429 -- The arguments to 'bracket' are in this order so that we can partially apply
432 -- > withFile name mode = bracket (openFile name mode) hClose
436 :: IO a -- ^ computation to run first (\"acquire resource\")
437 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
438 -> (a -> IO c) -- ^ computation to run in-between
439 -> IO c -- returns the value from the in-between computation
440 bracket before after thing =
445 (\e -> do { after a; throw e })
451 -- | A specialised variant of 'bracket' with just a computation to run
454 finally :: IO a -- ^ computation to run first
455 -> IO b -- ^ computation to run afterward (even if an exception
457 -> IO a -- returns the value from the first computation
462 (\e -> do { sequel; throw e })
467 -- | A variant of 'bracket' where the return value from the first computation
469 bracket_ :: IO a -> IO b -> IO c -> IO c
470 bracket_ before after thing = bracket before (const after) (const thing)
472 -- | Like bracket, but only performs the final action if there was an
473 -- exception raised by the in-between computation.
475 :: IO a -- ^ computation to run first (\"acquire resource\")
476 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
477 -> (a -> IO c) -- ^ computation to run in-between
478 -> IO c -- returns the value from the in-between computation
479 bracketOnError before after thing =
484 (\e -> do { after a; throw e })
487 -- -----------------------------------------------------------------------------
488 -- Asynchronous exceptions
492 #AsynchronousExceptions# Asynchronous exceptions are so-called because they arise due to
493 external influences, and can be raised at any point during execution.
494 'StackOverflow' and 'HeapOverflow' are two examples of
495 system-generated asynchronous exceptions.
497 The primary source of asynchronous exceptions, however, is
500 > throwTo :: ThreadId -> Exception -> IO ()
502 'throwTo' (also 'throwDynTo' and 'Control.Concurrent.killThread') allows one
503 running thread to raise an arbitrary exception in another thread. The
504 exception is therefore asynchronous with respect to the target thread,
505 which could be doing anything at the time it receives the exception.
506 Great care should be taken with asynchronous exceptions; it is all too
507 easy to introduce race conditions by the over zealous use of
512 There\'s an implied 'block' around every exception handler in a call
513 to one of the 'catch' family of functions. This is because that is
514 what you want most of the time - it eliminates a common race condition
515 in starting an exception handler, because there may be no exception
516 handler on the stack to handle another exception if one arrives
517 immediately. If asynchronous exceptions are blocked on entering the
518 handler, though, we have time to install a new exception handler
519 before being interrupted. If this weren\'t the default, one would have
520 to write something like
523 > catch (unblock (...))
527 If you need to unblock asynchronous exceptions again in the exception
528 handler, just use 'unblock' as normal.
530 Note that 'try' and friends /do not/ have a similar default, because
531 there is no exception handler in this case. If you want to use 'try'
532 in an asynchronous-exception-safe way, you will need to use
538 Some operations are /interruptible/, which means that they can receive
539 asynchronous exceptions even in the scope of a 'block'. Any function
540 which may itself block is defined as interruptible; this includes
541 'Control.Concurrent.MVar.takeMVar'
542 (but not 'Control.Concurrent.MVar.tryTakeMVar'),
543 and most operations which perform
544 some I\/O with the outside world. The reason for having
545 interruptible operations is so that we can write things like
549 > catch (unblock (...))
553 if the 'Control.Concurrent.MVar.takeMVar' was not interruptible,
555 combination could lead to deadlock, because the thread itself would be
556 blocked in a state where it can\'t receive any asynchronous exceptions.
557 With 'Control.Concurrent.MVar.takeMVar' interruptible, however, we can be
558 safe in the knowledge that the thread can receive exceptions right up
559 until the point when the 'Control.Concurrent.MVar.takeMVar' succeeds.
560 Similar arguments apply for other interruptible operations like
561 'System.IO.openFile'.
564 #if !(__GLASGOW_HASKELL__ || __NHC__)
565 assert :: Bool -> a -> a
567 assert False _ = throw (AssertionFailed "")
571 #ifdef __GLASGOW_HASKELL__
572 {-# NOINLINE uncaughtExceptionHandler #-}
573 uncaughtExceptionHandler :: IORef (Exception -> IO ())
574 uncaughtExceptionHandler = unsafePerformIO (newIORef defaultHandler)
576 defaultHandler :: Exception -> IO ()
577 defaultHandler ex = do
578 (hFlush stdout) `catchException` (\ _ -> return ())
580 Deadlock -> "no threads to run: infinite loop or deadlock?"
582 other -> showsPrec 0 other ""
583 withCString "%s" $ \cfmt ->
584 withCString msg $ \cmsg ->
587 -- don't use errorBelch() directly, because we cannot call varargs functions
589 foreign import ccall unsafe "HsBase.h errorBelch2"
590 errorBelch :: CString -> CString -> IO ()
592 setUncaughtExceptionHandler :: (Exception -> IO ()) -> IO ()
593 setUncaughtExceptionHandler = writeIORef uncaughtExceptionHandler
595 getUncaughtExceptionHandler :: IO (Exception -> IO ())
596 getUncaughtExceptionHandler = readIORef uncaughtExceptionHandler