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 setUncatchedExceptionHandler, -- :: (Exception -> IO ()) -> IO ()
111 getUncatchedExceptionHandler -- :: 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 GHC.IOBase ( IO(..), IORef(..), newIORef, readIORef, writeIORef )
120 import GHC.Handle ( stdout, hFlush )
124 import Hugs.Exception as ExceptionBase
127 import Foreign.C.String ( CString, withCStringLen )
129 import Prelude hiding ( catch )
130 import System.IO.Error hiding ( catch, try )
131 import System.IO.Unsafe (unsafePerformIO)
134 #include "Typeable.h"
135 INSTANCE_TYPEABLE0(Exception,exceptionTc,"Exception")
136 INSTANCE_TYPEABLE0(IOException,ioExceptionTc,"IOException")
137 INSTANCE_TYPEABLE0(ArithException,arithExceptionTc,"ArithException")
138 INSTANCE_TYPEABLE0(ArrayException,arrayExceptionTc,"ArrayException")
139 INSTANCE_TYPEABLE0(AsyncException,asyncExceptionTc,"AsyncException")
141 -----------------------------------------------------------------------------
142 -- Catching exceptions
144 -- |This is the simplest of the exception-catching functions. It
145 -- takes a single argument, runs it, and if an exception is raised
146 -- the \"handler\" is executed, with the value of the exception passed as an
147 -- argument. Otherwise, the result is returned as normal. For example:
149 -- > catch (openFile f ReadMode)
150 -- > (\e -> hPutStr stderr (\"Couldn\'t open \"++f++\": \" ++ show e))
152 -- For catching exceptions in pure (non-'IO') expressions, see the
153 -- function 'evaluate'.
155 -- Note that due to Haskell\'s unspecified evaluation order, an
156 -- expression may return one of several possible exceptions: consider
157 -- the expression @error \"urk\" + 1 \`div\` 0@. Does
158 -- 'catch' execute the handler passing
159 -- @ErrorCall \"urk\"@, or @ArithError DivideByZero@?
161 -- The answer is \"either\": 'catch' makes a
162 -- non-deterministic choice about which exception to catch. If you
163 -- call it again, you might get a different exception back. This is
164 -- ok, because 'catch' is an 'IO' computation.
166 -- Note that 'catch' catches all types of exceptions, and is generally
167 -- used for \"cleaning up\" before passing on the exception using
168 -- 'throwIO'. It is not good practice to discard the exception and
169 -- continue, without first checking the type of the exception (it
170 -- might be a 'ThreadKilled', for example). In this case it is usually better
171 -- to use 'catchJust' and select the kinds of exceptions to catch.
173 -- Also note that The "Prelude" also exports a
174 -- function called 'catch' which has the same type as
175 -- 'Control.Exception.catch', the difference being that the
176 -- "Prelude" version only catches the IO and user
177 -- families of exceptions (as required by Haskell 98). We recommend
178 -- either hiding the "Prelude" version of
179 -- 'catch' when importing
180 -- "Control.Exception", or importing
181 -- "Control.Exception" qualified, to avoid name-clashes.
183 catch :: IO a -- ^ The computation to run
184 -> (Exception -> IO a) -- ^ Handler to invoke if an exception is raised
186 catch = ExceptionBase.catchException
188 -- | The function 'catchJust' is like 'catch', but it takes an extra
189 -- argument which is an /exception predicate/, a function which
190 -- selects which type of exceptions we\'re interested in. There are
191 -- some predefined exception predicates for useful subsets of
192 -- exceptions: 'ioErrors', 'arithExceptions', and so on. For example,
193 -- to catch just calls to the 'error' function, we could use
195 -- > result <- catchJust errorCalls thing_to_try handler
197 -- Any other exceptions which are not matched by the predicate
198 -- are re-raised, and may be caught by an enclosing
199 -- 'catch' or 'catchJust'.
201 :: (Exception -> Maybe b) -- ^ Predicate to select exceptions
202 -> IO a -- ^ Computation to run
203 -> (b -> IO a) -- ^ Handler
205 catchJust p a handler = catch a handler'
206 where handler' e = case p e of
210 -- | A version of 'catch' with the arguments swapped around; useful in
211 -- situations where the code for the handler is shorter. For example:
213 -- > do handle (\e -> exitWith (ExitFailure 1)) $
215 handle :: (Exception -> IO a) -> IO a -> IO a
218 -- | A version of 'catchJust' with the arguments swapped around (see
220 handleJust :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a
221 handleJust p = flip (catchJust p)
223 -----------------------------------------------------------------------------
226 -- | Forces its argument to be evaluated, and returns the result in
227 -- the 'IO' monad. It can be used to order evaluation with respect to
228 -- other 'IO' operations; its semantics are given by
230 -- > evaluate undefined `seq` return () ==> return ()
231 -- > catch (evaluate undefined) (\e -> return ()) ==> return ()
233 -- NOTE: @(evaluate a)@ is /not/ the same as @(a \`seq\` return a)@.
234 #ifdef __GLASGOW_HASKELL__
235 evaluate :: a -> IO a
236 evaluate a = IO $ \s -> case a `seq` () of () -> (# s, a #)
238 -- a `seq` (# s, a #)
239 -- because we can't have an unboxed tuple as a function argument
242 -----------------------------------------------------------------------------
245 -- | This function maps one exception into another as proposed in the
246 -- paper \"A semantics for imprecise exceptions\".
248 -- Notice that the usage of 'unsafePerformIO' is safe here.
250 mapException :: (Exception -> Exception) -> a -> a
251 mapException f v = unsafePerformIO (catch (evaluate v)
254 -----------------------------------------------------------------------------
255 -- 'try' and variations.
257 -- | Similar to 'catch', but returns an 'Either' result which is
258 -- @(Right a)@ if no exception was raised, or @(Left e)@ if an
259 -- exception was raised and its value is @e@.
261 -- > try a = catch (Right \`liftM\` a) (return . Left)
263 -- Note: as with 'catch', it is only polite to use this variant if you intend
264 -- to re-throw the exception after performing whatever cleanup is needed.
265 -- Otherwise, 'tryJust' is generally considered to be better.
267 try :: IO a -> IO (Either Exception a)
268 try a = catch (a >>= \ v -> return (Right v)) (\e -> return (Left e))
270 -- | A variant of 'try' that takes an exception predicate to select
271 -- which exceptions are caught (c.f. 'catchJust'). If the exception
272 -- does not match the predicate, it is re-thrown.
273 tryJust :: (Exception -> Maybe b) -> IO a -> IO (Either b a)
277 Right v -> return (Right v)
278 Left e -> case p e of
280 Just b -> return (Left b)
282 -----------------------------------------------------------------------------
283 -- Dynamic exceptions
286 -- #DynamicExceptions# Because the 'Exception' datatype is not extensible, there is an
287 -- interface for throwing and catching exceptions of type 'Dynamic'
288 -- (see "Data.Dynamic") which allows exception values of any type in
289 -- the 'Typeable' class to be thrown and caught.
291 -- | Raise any value as an exception, provided it is in the
293 throwDyn :: Typeable exception => exception -> b
294 throwDyn exception = throw (DynException (toDyn exception))
296 #ifdef __GLASGOW_HASKELL__
297 -- | A variant of 'throwDyn' that throws the dynamic exception to an
298 -- arbitrary thread (GHC only: c.f. 'throwTo').
299 throwDynTo :: Typeable exception => ThreadId -> exception -> IO ()
300 throwDynTo t exception = throwTo t (DynException (toDyn exception))
301 #endif /* __GLASGOW_HASKELL__ */
303 -- | Catch dynamic exceptions of the required type. All other
304 -- exceptions are re-thrown, including dynamic exceptions of the wrong
307 -- When using dynamic exceptions it is advisable to define a new
308 -- datatype to use for your exception type, to avoid possible clashes
309 -- with dynamic exceptions used in other libraries.
311 catchDyn :: Typeable exception => IO a -> (exception -> IO a) -> IO a
312 catchDyn m k = catchException m handle
313 where handle ex = case ex of
314 (DynException dyn) ->
315 case fromDynamic dyn of
316 Just exception -> k exception
320 -----------------------------------------------------------------------------
321 -- Exception Predicates
324 -- These pre-defined predicates may be used as the first argument to
325 -- 'catchJust', 'tryJust', or 'handleJust' to select certain common
326 -- classes of exceptions.
328 ioErrors :: Exception -> Maybe IOError
329 arithExceptions :: Exception -> Maybe ArithException
330 errorCalls :: Exception -> Maybe String
331 assertions :: Exception -> Maybe String
332 dynExceptions :: Exception -> Maybe Dynamic
333 asyncExceptions :: Exception -> Maybe AsyncException
334 userErrors :: Exception -> Maybe String
336 ioErrors (IOException e) = Just e
339 arithExceptions (ArithException e) = Just e
340 arithExceptions _ = Nothing
342 errorCalls (ErrorCall e) = Just e
343 errorCalls _ = Nothing
345 assertions (AssertionFailed e) = Just e
346 assertions _ = Nothing
348 dynExceptions (DynException e) = Just e
349 dynExceptions _ = Nothing
351 asyncExceptions (AsyncException e) = Just e
352 asyncExceptions _ = Nothing
354 userErrors (IOException e) | isUserError e = Just (ioeGetErrorString e)
355 userErrors _ = Nothing
357 -----------------------------------------------------------------------------
358 -- Some Useful Functions
360 -- | When you want to acquire a resource, do some work with it, and
361 -- then release the resource, it is a good idea to use 'bracket',
362 -- because 'bracket' will install the necessary exception handler to
363 -- release the resource in the event that an exception is raised
364 -- during the computation. If an exception is raised, then 'bracket' will
365 -- re-raise the exception (after performing the release).
367 -- A common example is opening a file:
370 -- > (openFile "filename" ReadMode)
372 -- > (\handle -> do { ... })
374 -- The arguments to 'bracket' are in this order so that we can partially apply
377 -- > withFile name = bracket (openFile name) hClose
380 :: IO a -- ^ computation to run first (\"acquire resource\")
381 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
382 -> (a -> IO c) -- ^ computation to run in-between
383 -> IO c -- returns the value from the in-between computation
384 bracket before after thing =
389 (\e -> do { after a; throw e })
395 -- | A specialised variant of 'bracket' with just a computation to run
398 finally :: IO a -- ^ computation to run first
399 -> IO b -- ^ computation to run afterward (even if an exception
401 -> IO a -- returns the value from the first computation
406 (\e -> do { sequel; throw e })
411 -- | A variant of 'bracket' where the return value from the first computation
413 bracket_ :: IO a -> IO b -> IO c -> IO c
414 bracket_ before after thing = bracket before (const after) (const thing)
416 -- -----------------------------------------------------------------------------
417 -- Asynchronous exceptions
421 #AsynchronousExceptions# Asynchronous exceptions are so-called because they arise due to
422 external influences, and can be raised at any point during execution.
423 'StackOverflow' and 'HeapOverflow' are two examples of
424 system-generated asynchronous exceptions.
426 The primary source of asynchronous exceptions, however, is
429 > throwTo :: ThreadId -> Exception -> IO ()
431 'throwTo' (also 'throwDynTo' and 'Control.Concurrent.killThread') allows one
432 running thread to raise an arbitrary exception in another thread. The
433 exception is therefore asynchronous with respect to the target thread,
434 which could be doing anything at the time it receives the exception.
435 Great care should be taken with asynchronous exceptions; it is all too
436 easy to introduce race conditions by the over zealous use of
441 There\'s an implied 'block' around every exception handler in a call
442 to one of the 'catch' family of functions. This is because that is
443 what you want most of the time - it eliminates a common race condition
444 in starting an exception handler, because there may be no exception
445 handler on the stack to handle another exception if one arrives
446 immediately. If asynchronous exceptions are blocked on entering the
447 handler, though, we have time to install a new exception handler
448 before being interrupted. If this weren\'t the default, one would have
449 to write something like
452 > catch (unblock (...))
456 If you need to unblock asynchronous exceptions again in the exception
457 handler, just use 'unblock' as normal.
459 Note that 'try' and friends /do not/ have a similar default, because
460 there is no exception handler in this case. If you want to use 'try'
461 in an asynchronous-exception-safe way, you will need to use
467 Some operations are /interruptible/, which means that they can receive
468 asynchronous exceptions even in the scope of a 'block'. Any function
469 which may itself block is defined as interruptible; this includes
470 'Control.Concurrent.MVar.takeMVar'
471 (but not 'Control.Concurrent.MVar.tryTakeMVar'),
472 and most operations which perform
473 some I\/O with the outside world. The reason for having
474 interruptible operations is so that we can write things like
478 > catch (unblock (...))
482 if the 'Control.Concurrent.MVar.takeMVar' was not interruptible,
484 combination could lead to deadlock, because the thread itself would be
485 blocked in a state where it can\'t receive any asynchronous exceptions.
486 With 'Control.Concurrent.MVar.takeMVar' interruptible, however, we can be
487 safe in the knowledge that the thread can receive exceptions right up
488 until the point when the 'Control.Concurrent.MVar.takeMVar' succeeds.
489 Similar arguments apply for other interruptible operations like
490 'System.IO.openFile'.
493 -- -----------------------------------------------------------------------------
497 -- | If the first argument evaluates to 'True', then the result is the
498 -- second argument. Otherwise an 'AssertionFailed' exception is raised,
499 -- containing a 'String' with the source file and line number of the
502 -- Assertions can normally be turned on or off with a compiler flag
503 -- (for GHC, assertions are normally on unless the @-fignore-asserts@
504 -- option is give). When assertions are turned off, the first
505 -- argument to 'assert' is ignored, and the second argument is
506 -- returned as the result.
507 assert :: Bool -> a -> a
510 #ifndef __GLASGOW_HASKELL__
511 assert :: Bool -> a -> a
513 assert False _ = throw (AssertionFailed "")
517 #ifdef __GLASGOW_HASKELL__
518 {-# NOINLINE uncatchedExceptionHandler #-}
519 uncatchedExceptionHandler :: IORef (Exception -> IO ())
520 uncatchedExceptionHandler = unsafePerformIO (newIORef defaultHandler)
522 defaultHandler :: Exception -> IO ()
523 defaultHandler ex = do
524 (hFlush stdout) `catchException` (\ _ -> return ())
526 Deadlock -> "no threads to run: infinite loop or deadlock?"
528 other -> showsPrec 0 other "\n"
529 withCStringLen ("Fail: "++msg) $ \(cstr,len) -> writeErrString cstr len
531 foreign import ccall unsafe "writeErrString__"
532 writeErrString :: CString -> Int -> IO ()
534 setUncatchedExceptionHandler :: (Exception -> IO ()) -> IO ()
535 setUncatchedExceptionHandler = writeIORef uncatchedExceptionHandler
537 getUncatchedExceptionHandler :: IO (Exception -> IO ())
538 getUncatchedExceptionHandler = readIORef uncatchedExceptionHandler