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
11 -- This module provides support for raising and catching both built-in
12 -- and user-defined exceptions.
14 -----------------------------------------------------------------------------
16 module Control.Exception (
18 -- * The Exception type
19 Exception(..), -- instance Eq, Ord, Show, Typeable
20 IOException, -- instance Eq, Ord, Show, Typeable
21 ArithException(..), -- instance Eq, Ord, Show, Typeable
22 ArrayException(..), -- instance Eq, Ord, Show, Typeable
23 AsyncException(..), -- instance Eq, Ord, Show, Typeable
25 -- * Throwing exceptions
26 throwIO, -- :: Exception -> IO a
27 throw, -- :: Exception -> a
28 ioError, -- :: IOError -> IO a
29 #ifdef __GLASGOW_HASKELL__
30 throwTo, -- :: ThreadId -> Exception -> a
33 -- * Catching Exceptions
35 -- |There are several functions for catching and examining
36 -- exceptions; all of them may only be used from within the
39 -- ** The @catch@ functions
40 catch, -- :: IO a -> (Exception -> IO a) -> IO a
41 catchJust, -- :: (Exception -> Maybe b) -> IO a -> (b -> IO a) -> IO a
43 -- ** The @handle@ functions
44 handle, -- :: (Exception -> IO a) -> IO a -> IO a
45 handleJust,-- :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a
47 -- ** The @try@ functions
48 try, -- :: IO a -> IO (Either Exception a)
49 tryJust, -- :: (Exception -> Maybe b) -> a -> IO (Either b a)
51 -- ** The @evaluate@ function
52 evaluate, -- :: a -> IO a
54 -- ** The @mapException@ function
55 mapException, -- :: (Exception -> Exception) -> a -> a
57 -- ** Exception predicates
61 ioErrors, -- :: Exception -> Maybe IOError
62 arithExceptions, -- :: Exception -> Maybe ArithException
63 errorCalls, -- :: Exception -> Maybe String
64 dynExceptions, -- :: Exception -> Maybe Dynamic
65 assertions, -- :: Exception -> Maybe String
66 asyncExceptions, -- :: Exception -> Maybe AsyncException
67 userErrors, -- :: Exception -> Maybe String
69 -- * Dynamic exceptions
72 throwDyn, -- :: Typeable ex => ex -> b
73 #ifdef __GLASGOW_HASKELL__
74 throwDynTo, -- :: Typeable ex => ThreadId -> ex -> b
76 catchDyn, -- :: Typeable ex => IO a -> (ex -> IO a) -> IO a
78 -- * Asynchronous Exceptions
82 -- ** Asynchronous exception control
84 -- |The following two functions allow a thread to control delivery of
85 -- asynchronous exceptions during a critical region.
87 block, -- :: IO a -> IO a
88 unblock, -- :: IO a -> IO a
90 -- *** Applying @block@ to an exception handler
94 -- *** Interruptible operations
100 assert, -- :: Bool -> a -> a
104 bracket, -- :: IO a -> (a -> IO b) -> (a -> IO c) -> IO ()
105 bracket_, -- :: IO a -> IO b -> IO c -> IO ()
107 finally, -- :: IO a -> IO b -> IO a
109 #ifdef __GLASGOW_HASKELL__
110 setUncaughtExceptionHandler, -- :: (Exception -> IO ()) -> IO ()
111 getUncaughtExceptionHandler -- :: IO (Exception -> IO ())
115 #ifdef __GLASGOW_HASKELL__
116 import GHC.Base ( assert )
117 import GHC.Exception as ExceptionBase hiding (catch)
118 import GHC.Conc ( throwTo, ThreadId )
119 import Data.IORef ( IORef, newIORef, readIORef, writeIORef )
120 import Foreign.C.String ( CString, withCString )
121 import System.IO ( stdout, hFlush )
125 import Hugs.Exception as ExceptionBase
128 import Prelude hiding ( catch )
129 import System.IO.Error hiding ( catch, try )
130 import System.IO.Unsafe (unsafePerformIO)
133 -----------------------------------------------------------------------------
134 -- Catching exceptions
136 -- |This is the simplest of the exception-catching functions. It
137 -- takes a single argument, runs it, and if an exception is raised
138 -- the \"handler\" is executed, with the value of the exception passed as an
139 -- argument. Otherwise, the result is returned as normal. For example:
141 -- > catch (openFile f ReadMode)
142 -- > (\e -> hPutStr stderr (\"Couldn\'t open \"++f++\": \" ++ show e))
144 -- For catching exceptions in pure (non-'IO') expressions, see the
145 -- function 'evaluate'.
147 -- Note that due to Haskell\'s unspecified evaluation order, an
148 -- expression may return one of several possible exceptions: consider
149 -- the expression @error \"urk\" + 1 \`div\` 0@. Does
150 -- 'catch' execute the handler passing
151 -- @ErrorCall \"urk\"@, or @ArithError DivideByZero@?
153 -- The answer is \"either\": 'catch' makes a
154 -- non-deterministic choice about which exception to catch. If you
155 -- call it again, you might get a different exception back. This is
156 -- ok, because 'catch' is an 'IO' computation.
158 -- Note that 'catch' catches all types of exceptions, and is generally
159 -- used for \"cleaning up\" before passing on the exception using
160 -- 'throwIO'. It is not good practice to discard the exception and
161 -- continue, without first checking the type of the exception (it
162 -- might be a 'ThreadKilled', for example). In this case it is usually better
163 -- to use 'catchJust' and select the kinds of exceptions to catch.
165 -- Also note that the "Prelude" also exports a function called
166 -- 'Prelude.catch' with a similar type to 'Control.Exception.catch',
167 -- except that the "Prelude" version only catches the IO and user
168 -- families of exceptions (as required by Haskell 98). We recommend
169 -- either hiding the "Prelude" version of
170 -- 'Prelude.catch' when importing
171 -- "Control.Exception", or importing
172 -- "Control.Exception" qualified, to avoid name-clashes.
174 catch :: IO a -- ^ The computation to run
175 -> (Exception -> IO a) -- ^ Handler to invoke if an exception is raised
177 catch = ExceptionBase.catchException
179 -- | The function 'catchJust' is like 'catch', but it takes an extra
180 -- argument which is an /exception predicate/, a function which
181 -- selects which type of exceptions we\'re interested in. There are
182 -- some predefined exception predicates for useful subsets of
183 -- exceptions: 'ioErrors', 'arithExceptions', and so on. For example,
184 -- to catch just calls to the 'error' function, we could use
186 -- > result <- catchJust errorCalls thing_to_try handler
188 -- Any other exceptions which are not matched by the predicate
189 -- are re-raised, and may be caught by an enclosing
190 -- 'catch' or 'catchJust'.
192 :: (Exception -> Maybe b) -- ^ Predicate to select exceptions
193 -> IO a -- ^ Computation to run
194 -> (b -> IO a) -- ^ Handler
196 catchJust p a handler = catch a handler'
197 where handler' e = case p e of
201 -- | A version of 'catch' with the arguments swapped around; useful in
202 -- situations where the code for the handler is shorter. For example:
204 -- > do handle (\e -> exitWith (ExitFailure 1)) $
206 handle :: (Exception -> IO a) -> IO a -> IO a
209 -- | A version of 'catchJust' with the arguments swapped around (see
211 handleJust :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a
212 handleJust p = flip (catchJust p)
214 -----------------------------------------------------------------------------
217 -- | This function maps one exception into another as proposed in the
218 -- paper \"A semantics for imprecise exceptions\".
220 -- Notice that the usage of 'unsafePerformIO' is safe here.
222 mapException :: (Exception -> Exception) -> a -> a
223 mapException f v = unsafePerformIO (catch (evaluate v)
226 -----------------------------------------------------------------------------
227 -- 'try' and variations.
229 -- | Similar to 'catch', but returns an 'Either' result which is
230 -- @('Right' a)@ if no exception was raised, or @('Left' e)@ if an
231 -- exception was raised and its value is @e@.
233 -- > try a = catch (Right \`liftM\` a) (return . Left)
235 -- Note: as with 'catch', it is only polite to use this variant if you intend
236 -- to re-throw the exception after performing whatever cleanup is needed.
237 -- Otherwise, 'tryJust' is generally considered to be better.
239 -- Also note that "System.IO.Error" also exports a function called
240 -- 'System.IO.Error.try' with a similar type to 'Control.Exception.try',
241 -- except that it catches only the IO and user families of exceptions
242 -- (as required by the Haskell 98 @IO@ module).
244 try :: IO a -> IO (Either Exception a)
245 try a = catch (a >>= \ v -> return (Right v)) (\e -> return (Left e))
247 -- | A variant of 'try' that takes an exception predicate to select
248 -- which exceptions are caught (c.f. 'catchJust'). If the exception
249 -- does not match the predicate, it is re-thrown.
250 tryJust :: (Exception -> Maybe b) -> IO a -> IO (Either b a)
254 Right v -> return (Right v)
255 Left e -> case p e of
257 Just b -> return (Left b)
259 -----------------------------------------------------------------------------
260 -- Dynamic exceptions
263 -- #DynamicExceptions# Because the 'Exception' datatype is not extensible, there is an
264 -- interface for throwing and catching exceptions of type 'Dynamic'
265 -- (see "Data.Dynamic") which allows exception values of any type in
266 -- the 'Typeable' class to be thrown and caught.
268 -- | Raise any value as an exception, provided it is in the
270 throwDyn :: Typeable exception => exception -> b
271 throwDyn exception = throw (DynException (toDyn exception))
273 #ifdef __GLASGOW_HASKELL__
274 -- | A variant of 'throwDyn' that throws the dynamic exception to an
275 -- arbitrary thread (GHC only: c.f. 'throwTo').
276 throwDynTo :: Typeable exception => ThreadId -> exception -> IO ()
277 throwDynTo t exception = throwTo t (DynException (toDyn exception))
278 #endif /* __GLASGOW_HASKELL__ */
280 -- | Catch dynamic exceptions of the required type. All other
281 -- exceptions are re-thrown, including dynamic exceptions of the wrong
284 -- When using dynamic exceptions it is advisable to define a new
285 -- datatype to use for your exception type, to avoid possible clashes
286 -- with dynamic exceptions used in other libraries.
288 catchDyn :: Typeable exception => IO a -> (exception -> IO a) -> IO a
289 catchDyn m k = catchException m handle
290 where handle ex = case ex of
291 (DynException dyn) ->
292 case fromDynamic dyn of
293 Just exception -> k exception
297 -----------------------------------------------------------------------------
298 -- Exception Predicates
301 -- These pre-defined predicates may be used as the first argument to
302 -- 'catchJust', 'tryJust', or 'handleJust' to select certain common
303 -- classes of exceptions.
305 ioErrors :: Exception -> Maybe IOError
306 arithExceptions :: Exception -> Maybe ArithException
307 errorCalls :: Exception -> Maybe String
308 assertions :: Exception -> Maybe String
309 dynExceptions :: Exception -> Maybe Dynamic
310 asyncExceptions :: Exception -> Maybe AsyncException
311 userErrors :: Exception -> Maybe String
313 ioErrors (IOException e) = Just e
316 arithExceptions (ArithException e) = Just e
317 arithExceptions _ = Nothing
319 errorCalls (ErrorCall e) = Just e
320 errorCalls _ = Nothing
322 assertions (AssertionFailed e) = Just e
323 assertions _ = Nothing
325 dynExceptions (DynException e) = Just e
326 dynExceptions _ = Nothing
328 asyncExceptions (AsyncException e) = Just e
329 asyncExceptions _ = Nothing
331 userErrors (IOException e) | isUserError e = Just (ioeGetErrorString e)
332 userErrors _ = Nothing
334 -----------------------------------------------------------------------------
335 -- Some Useful Functions
337 -- | When you want to acquire a resource, do some work with it, and
338 -- then release the resource, it is a good idea to use 'bracket',
339 -- because 'bracket' will install the necessary exception handler to
340 -- release the resource in the event that an exception is raised
341 -- during the computation. If an exception is raised, then 'bracket' will
342 -- re-raise the exception (after performing the release).
344 -- A common example is opening a file:
347 -- > (openFile "filename" ReadMode)
349 -- > (\handle -> do { ... })
351 -- The arguments to 'bracket' are in this order so that we can partially apply
354 -- > withFile name = bracket (openFile name) hClose
357 :: IO a -- ^ computation to run first (\"acquire resource\")
358 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
359 -> (a -> IO c) -- ^ computation to run in-between
360 -> IO c -- returns the value from the in-between computation
361 bracket before after thing =
366 (\e -> do { after a; throw e })
372 -- | A specialised variant of 'bracket' with just a computation to run
375 finally :: IO a -- ^ computation to run first
376 -> IO b -- ^ computation to run afterward (even if an exception
378 -> IO a -- returns the value from the first computation
383 (\e -> do { sequel; throw e })
388 -- | A variant of 'bracket' where the return value from the first computation
390 bracket_ :: IO a -> IO b -> IO c -> IO c
391 bracket_ before after thing = bracket before (const after) (const thing)
393 -- -----------------------------------------------------------------------------
394 -- Asynchronous exceptions
398 #AsynchronousExceptions# Asynchronous exceptions are so-called because they arise due to
399 external influences, and can be raised at any point during execution.
400 'StackOverflow' and 'HeapOverflow' are two examples of
401 system-generated asynchronous exceptions.
403 The primary source of asynchronous exceptions, however, is
406 > throwTo :: ThreadId -> Exception -> IO ()
408 'throwTo' (also 'throwDynTo' and 'Control.Concurrent.killThread') allows one
409 running thread to raise an arbitrary exception in another thread. The
410 exception is therefore asynchronous with respect to the target thread,
411 which could be doing anything at the time it receives the exception.
412 Great care should be taken with asynchronous exceptions; it is all too
413 easy to introduce race conditions by the over zealous use of
418 There\'s an implied 'block' around every exception handler in a call
419 to one of the 'catch' family of functions. This is because that is
420 what you want most of the time - it eliminates a common race condition
421 in starting an exception handler, because there may be no exception
422 handler on the stack to handle another exception if one arrives
423 immediately. If asynchronous exceptions are blocked on entering the
424 handler, though, we have time to install a new exception handler
425 before being interrupted. If this weren\'t the default, one would have
426 to write something like
429 > catch (unblock (...))
433 If you need to unblock asynchronous exceptions again in the exception
434 handler, just use 'unblock' as normal.
436 Note that 'try' and friends /do not/ have a similar default, because
437 there is no exception handler in this case. If you want to use 'try'
438 in an asynchronous-exception-safe way, you will need to use
444 Some operations are /interruptible/, which means that they can receive
445 asynchronous exceptions even in the scope of a 'block'. Any function
446 which may itself block is defined as interruptible; this includes
447 'Control.Concurrent.MVar.takeMVar'
448 (but not 'Control.Concurrent.MVar.tryTakeMVar'),
449 and most operations which perform
450 some I\/O with the outside world. The reason for having
451 interruptible operations is so that we can write things like
455 > catch (unblock (...))
459 if the 'Control.Concurrent.MVar.takeMVar' was not interruptible,
461 combination could lead to deadlock, because the thread itself would be
462 blocked in a state where it can\'t receive any asynchronous exceptions.
463 With 'Control.Concurrent.MVar.takeMVar' interruptible, however, we can be
464 safe in the knowledge that the thread can receive exceptions right up
465 until the point when the 'Control.Concurrent.MVar.takeMVar' succeeds.
466 Similar arguments apply for other interruptible operations like
467 'System.IO.openFile'.
470 -- -----------------------------------------------------------------------------
474 -- | If the first argument evaluates to 'True', then the result is the
475 -- second argument. Otherwise an 'AssertionFailed' exception is raised,
476 -- containing a 'String' with the source file and line number of the
479 -- Assertions can normally be turned on or off with a compiler flag
480 -- (for GHC, assertions are normally on unless the @-fignore-asserts@
481 -- option is give). When assertions are turned off, the first
482 -- argument to 'assert' is ignored, and the second argument is
483 -- returned as the result.
484 assert :: Bool -> a -> a
487 #ifndef __GLASGOW_HASKELL__
488 assert :: Bool -> a -> a
490 assert False _ = throw (AssertionFailed "")
494 #ifdef __GLASGOW_HASKELL__
495 {-# NOINLINE uncaughtExceptionHandler #-}
496 uncaughtExceptionHandler :: IORef (Exception -> IO ())
497 uncaughtExceptionHandler = unsafePerformIO (newIORef defaultHandler)
499 defaultHandler :: Exception -> IO ()
500 defaultHandler ex = do
501 (hFlush stdout) `catchException` (\ _ -> return ())
503 Deadlock -> "no threads to run: infinite loop or deadlock?"
505 other -> showsPrec 0 other "\n"
506 withCString "%s" $ \cfmt ->
507 withCString msg $ \cmsg ->
510 foreign import ccall unsafe errorBelch :: CString -> CString -> IO ()
512 setUncaughtExceptionHandler :: (Exception -> IO ()) -> IO ()
513 setUncaughtExceptionHandler = writeIORef uncaughtExceptionHandler
515 getUncaughtExceptionHandler :: IO (Exception -> IO ())
516 getUncaughtExceptionHandler = readIORef uncaughtExceptionHandler