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 #ifdef __GLASGOW_HASKELL__
21 IOException, -- instance Eq, Ord, Show, Typeable
22 ArithException(..), -- instance Eq, Ord, Show, Typeable
23 ArrayException(..), -- instance Eq, Ord, Show, Typeable
24 AsyncException(..), -- instance Eq, Ord, Show, Typeable
27 -- * Throwing exceptions
29 throwIO, -- :: Exception -> IO a
31 throw, -- :: Exception -> a
32 ioError, -- :: Exception -> IO a
33 throwTo, -- :: ThreadId -> Exception -> a
36 -- * Catching Exceptions
38 -- |There are several functions for catching and examining
39 -- exceptions; all of them may only be used from within the
42 -- ** The @catch@ functions
43 catch, -- :: IO a -> (Exception -> IO a) -> IO a
44 catchJust, -- :: (Exception -> Maybe b) -> IO a -> (b -> IO a) -> IO a
46 -- ** The @handle@ functions
47 handle, -- :: (Exception -> IO a) -> IO a -> IO a
48 handleJust,-- :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a
50 -- ** The @try@ functions
51 try, -- :: IO a -> IO (Either Exception a)
52 tryJust, -- :: (Exception -> Maybe b) -> a -> IO (Either b a)
54 -- ** The @evaluate@ function
55 evaluate, -- :: a -> IO a
57 -- ** Exception predicates
61 ioErrors, -- :: Exception -> Maybe IOError
63 arithExceptions, -- :: Exception -> Maybe ArithException
64 errorCalls, -- :: Exception -> Maybe String
65 dynExceptions, -- :: Exception -> Maybe Dynamic
66 assertions, -- :: Exception -> Maybe String
67 asyncExceptions, -- :: Exception -> Maybe AsyncException
69 userErrors, -- :: Exception -> Maybe String
71 #ifdef __GLASGOW_HASKELL__
72 -- * Dynamic exceptions
75 throwDyn, -- :: Typeable ex => ex -> b
76 throwDynTo, -- :: Typeable ex => ThreadId -> ex -> b
77 catchDyn, -- :: Typeable ex => IO a -> (ex -> IO a) -> IO a
80 -- * Asynchronous Exceptions
84 -- ** Asynchronous exception control
86 -- |The following two functions allow a thread to control delivery of
87 -- asynchronous exceptions during a critical region.
89 block, -- :: IO a -> IO a
90 unblock, -- :: IO a -> IO a
92 -- *** Applying @block@ to an exception handler
96 -- *** Interruptible operations
102 assert, -- :: Bool -> a -> a
106 bracket, -- :: IO a -> (a -> IO b) -> (a -> IO c) -> IO ()
107 bracket_, -- :: IO a -> IO b -> IO c -> IO ()
109 finally, -- :: IO a -> IO b -> IO b
113 #ifdef __GLASGOW_HASKELL__
114 import GHC.Base ( assert )
115 import GHC.Exception as ExceptionBase hiding (try, catch, bracket, bracket_)
116 import GHC.Conc ( throwTo, ThreadId )
117 import GHC.IOBase ( IO(..) )
121 import Hugs.Exception hiding ( evaluate )
122 import qualified Hugs.Exception as ExceptionBase
125 import Prelude hiding ( catch )
126 import System.IO.Error
130 INSTANCE_TYPEABLE0(Exception,exceptionTc,"Exception")
131 #ifdef __GLASGOW_HASKELL__
132 INSTANCE_TYPEABLE0(IOException,ioExceptionTc,"IOException")
133 INSTANCE_TYPEABLE0(ArithException,arithExceptionTc,"ArithException")
134 INSTANCE_TYPEABLE0(ArrayException,arrayExceptionTc,"ArrayException")
135 INSTANCE_TYPEABLE0(AsyncException,asyncExceptionTc,"AsyncException")
139 -- This is as close as Hugs gets to providing throw
140 throw :: Exception -> IO a
144 -----------------------------------------------------------------------------
145 -- Catching exceptions
147 -- |This is the simplest of the exception-catching functions. It
148 -- takes a single argument, runs it, and if an exception is raised
149 -- the \"handler\" is executed, with the value of the exception passed as an
150 -- argument. Otherwise, the result is returned as normal. For example:
152 -- > catch (openFile f ReadMode)
153 -- > (\e -> hPutStr stderr (\"Couldn\'t open \"++f++\": \" ++ show e))
155 -- For catching exceptions in pure (non-'IO') expressions, see the
156 -- function 'evaluate'.
158 -- Note that due to Haskell\'s unspecified evaluation order, an
159 -- expression may return one of several possible exceptions: consider
160 -- the expression @error \"urk\" + 1 \`div\` 0@. Does
161 -- 'catch' execute the handler passing
162 -- @ErrorCall \"urk\"@, or @ArithError DivideByZero@?
164 -- The answer is \"either\": 'catch' makes a
165 -- non-deterministic choice about which exception to catch. If you
166 -- call it again, you might get a different exception back. This is
167 -- ok, because 'catch' is an 'IO' computation.
169 -- Note that 'catch' catches all types of exceptions, and is generally
170 -- used for \"cleaning up\" before passing on the exception using
171 -- 'ioError'. It is not good practice to discard the exception and
172 -- continue, without first checking the type of the exception (it
173 -- might be a 'ThreadKilled', for example). In this case it is usually better
174 -- to use 'catchJust' and select the kinds of exceptions to catch.
176 -- Also note that The "Prelude" also exports a
177 -- function called 'catch' which has the same type as
178 -- 'Exception.catch', the difference being that the
179 -- "Prelude" version only catches the IO and user
180 -- families of exceptions (as required by Haskell 98). We recommend
181 -- either hiding the "Prelude" version of
182 -- 'catch' when importing
183 -- "Control.Exception", or importing
184 -- "Control.Exception" qualified, to avoid name-clashes.
186 catch :: IO a -- ^ The computation to run
187 -> (Exception -> IO a) -- ^ Handler to invoke if an exception is raised
189 catch = ExceptionBase.catchException
191 -- | The function 'catchJust' is like 'catch', but it takes an extra
192 -- argument which is an /exception predicate/, a function which
193 -- selects which type of exceptions we\'re interested in. There are
194 -- some predefined exception predicates for useful subsets of
195 -- exceptions: 'ioErrors', 'arithExceptions', and so on. For example,
196 -- to catch just calls to the 'error' function, we could use
198 -- > result \<- catchJust errorCalls thing_to_try handler
200 -- Any other exceptions which are not matched by the predicate
201 -- are re-raised, and may be caught by an enclosing
202 -- 'catch' or 'catchJust'.
204 :: (Exception -> Maybe b) -- ^ Predicate to select exceptions
205 -> IO a -- ^ Computation to run
206 -> (b -> IO a) -- ^ Handler
208 catchJust p a handler = catch a handler'
209 where handler' e = case p e of
213 -- | A version of 'catch' with the arguments swapped around; useful in
214 -- situations where the code for the handler is shorter. For example:
216 -- > do handle (\e -> exitWith (ExitFailure 1)) $
218 handle :: (Exception -> IO a) -> IO a -> IO a
221 -- | A version of 'catchJust' with the arguments swapped around (see
223 handleJust :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a
224 handleJust p = flip (catchJust p)
226 -----------------------------------------------------------------------------
229 -- | Forces its argument to be evaluated, and returns the result in
230 -- the 'IO' monad. It can be used to order evaluation with respect to
231 -- other 'IO' operations; its semantics are given by
233 -- > evaluate undefined `seq` return () ==> return ()
234 -- > catch (evaluate undefined) (\e -> return ()) ==> return ()
236 -- NOTE: @(evaluate a)@ is /not/ the same as @(a \`seq\` return a)@.
237 evaluate :: a -> IO a
238 #if defined(__GLASGOW_HASKELL__)
239 evaluate a = IO $ \s -> case a `seq` () of () -> (# s, a #)
241 -- a `seq` (# s, a #)
242 -- because we can't have an unboxed tuple as a function argument
243 #elif defined(__HUGS__)
244 evaluate = ExceptionBase.evaluate
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 try :: IO a -> IO (Either Exception a)
261 try a = catch (a >>= \ v -> return (Right v)) (\e -> return (Left e))
263 -- | A variant of 'try' that takes an exception predicate to select
264 -- which exceptions are caught (c.f. 'catchJust'). If the exception
265 -- does not match the predicate, it is re-thrown.
266 tryJust :: (Exception -> Maybe b) -> IO a -> IO (Either b a)
270 Right v -> return (Right v)
271 Left e -> case p e of
273 Just b -> return (Left b)
275 #ifdef __GLASGOW_HASKELL__
276 -----------------------------------------------------------------------------
277 -- Dynamic exceptions
280 -- #DynamicExceptions# Because the 'Exception' datatype is not extensible, there is an
281 -- interface for throwing and catching exceptions of type 'Dynamic'
282 -- (see "Data.Dynamic") which allows exception values of any type in
283 -- the 'Typeable' class to be thrown and caught.
285 -- | Raise any value as an exception, provided it is in the
287 throwDyn :: Typeable exception => exception -> b
288 throwDyn exception = throw (DynException (toDyn exception))
290 -- | A variant of 'throwDyn' that throws the dynamic exception to an
291 -- arbitrary thread (c.f. 'throwTo').
292 throwDynTo :: Typeable exception => ThreadId -> exception -> IO ()
293 throwDynTo t exception = throwTo t (DynException (toDyn exception))
295 -- | Catch dynamic exceptions of the required type. All other
296 -- exceptions are re-thrown, including dynamic exceptions of the wrong
299 -- When using dynamic exceptions it is advisable to define a new
300 -- datatype to use for your exception type, to avoid possible clashes
301 -- with dynamic exceptions used in other libraries.
303 catchDyn :: Typeable exception => IO a -> (exception -> IO a) -> IO a
304 catchDyn m k = catchException m handle
305 where handle ex = case ex of
306 (DynException dyn) ->
307 case fromDynamic dyn of
308 Just exception -> k exception
311 #endif /* __GLASGOW_HASKELL__ */
313 -----------------------------------------------------------------------------
314 -- Exception Predicates
317 -- These pre-defined predicates may be used as the first argument to
318 -- 'catchJust', 'tryJust', or 'handleJust' to select certain common
319 -- classes of exceptions.
321 #ifdef __GLASGOW_HASKELL__
322 ioErrors :: Exception -> Maybe IOError
323 arithExceptions :: Exception -> Maybe ArithException
324 errorCalls :: Exception -> Maybe String
325 dynExceptions :: Exception -> Maybe Dynamic
326 assertions :: Exception -> Maybe String
327 asyncExceptions :: Exception -> Maybe AsyncException
328 userErrors :: Exception -> Maybe String
329 #endif /* __GLASGOW_HASKELL__ */
331 #ifdef __GLASGOW_HASKELL__
332 ioErrors e@(IOException _) = Just e
335 arithExceptions (ArithException e) = Just e
336 arithExceptions _ = Nothing
338 errorCalls (ErrorCall e) = Just e
339 errorCalls _ = Nothing
341 assertions (AssertionFailed e) = Just e
342 assertions _ = Nothing
344 dynExceptions (DynException e) = Just e
345 dynExceptions _ = Nothing
347 asyncExceptions (AsyncException e) = Just e
348 asyncExceptions _ = Nothing
350 userErrors e@IOException{} | isUserError e = Just (ioeGetErrorString e)
351 userErrors _ = Nothing
352 #endif /* __GLASGOW_HASKELL__ */
354 -----------------------------------------------------------------------------
355 -- Some Useful Functions
357 -- | When you want to acquire a resource, do some work with it, and
358 -- then release the resource, it is a good idea to use 'bracket',
359 -- because 'bracket' will install the necessary exception handler to
360 -- release the resource in the event that an exception is raised
361 -- during the computation. If an exception is raised, then 'bracket' will
362 -- re-raise the exception (after performing the release).
364 -- A common example is opening a file:
367 -- > (openFile "filename" ReadMode)
369 -- > (\handle -> do { ... })
371 -- The arguments to 'bracket' are in this order so that we can partially apply
374 -- > withFile name = bracket (openFile name) hClose
377 :: IO a -- ^ computation to run first (\"acquire resource\")
378 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
379 -> (a -> IO c) -- ^ computation to run in-between
380 -> IO c -- returns the value from the in-between computation
381 bracket before after thing =
386 (\e -> do { after a; throw e })
392 -- | A specialised variant of 'bracket' with just a computation to run
395 finally :: IO a -- ^ computation to run first
396 -> IO b -- ^ computation to run afterward (even if an exception
398 -> IO a -- returns the value from the first computation
403 (\e -> do { sequel; throw e })
408 -- | A variant of 'bracket' where the return value from the first computation
410 bracket_ :: IO a -> IO b -> IO c -> IO c
411 bracket_ before after thing = bracket before (const after) (const thing)
413 -- -----------------------------------------------------------------------------
414 -- Asynchronous exceptions
418 #AsynchronousExceptions# Asynchronous exceptions are so-called because they arise due to
419 external influences, and can be raised at any point during execution.
420 'StackOverflow' and 'HeapOverflow' are two examples of
421 system-generated asynchronous exceptions.
423 The primary source of asynchronous exceptions, however, is
426 > throwTo :: ThreadId -> Exception -> IO ()
428 'throwTo' (also 'throwDynTo' and 'Concurrent.killThread') allows one
429 running thread to raise an arbitrary exception in another thread. The
430 exception is therefore asynchronous with respect to the target thread,
431 which could be doing anything at the time it receives the exception.
432 Great care should be taken with asynchronous exceptions; it is all too
433 easy to introduce race conditions by the over zealous use of
438 There\'s an implied 'block' around every exception handler in a call
439 to one of the 'catch' family of functions. This is because that is
440 what you want most of the time - it eliminates a common race condition
441 in starting an exception handler, because there may be no exception
442 handler on the stack to handle another exception if one arrives
443 immediately. If asynchronous exceptions are blocked on entering the
444 handler, though, we have time to install a new exception handler
445 before being interrupted. If this weren\'t the default, one would have
446 to write something like
449 > catch (unblock (...))
453 If you need to unblock asynchronous exceptions again in the exception
454 handler, just use 'unblock' as normal.
456 Note that 'try' and friends /do not/ have a similar default, because
457 there is no exception handler in this case. If you want to use 'try'
458 in an asynchronous-exception-safe way, you will need to use
464 Some operations are /interruptible/, which means that they can receive
465 asynchronous exceptions even in the scope of a 'block'. Any function
466 which may itself block is defined as interruptible; this includes
467 'takeMVar' (but not 'tryTakeMVar'), and most operations which perform
468 some I\/O with the outside world.. The reason for having
469 interruptible operations is so that we can write things like
473 > catch (unblock (...))
477 if the 'takeMVar' was not interruptible, then this particular
478 combination could lead to deadlock, because the thread itself would be
479 blocked in a state where it can\'t receive any asynchronous exceptions.
480 With 'takeMVar' interruptible, however, we can be
481 safe in the knowledge that the thread can receive exceptions right up
482 until the point when the 'takeMVar' succeeds.
483 Similar arguments apply for other interruptible operations like
487 -- -----------------------------------------------------------------------------
491 -- | If the first argument evaluates to 'True', then the result is the
492 -- second argument. Otherwise an 'Assertion' exception is raised,
493 -- containing a 'String' with the source file and line number of the
496 -- Assertions can normally be turned on or off with a compiler flag
497 -- (for GHC, assertions are normally on unless the @-fignore-asserts@
498 -- option is give). When assertions are turned off, the first
499 -- argument to 'assert' is ignored, and the second argument is
500 -- returned as the result.
501 assert :: Bool -> a -> a
504 #ifndef __GLASGOW_HASKELL__
505 assert :: Bool -> a -> a
507 assert False _ = error "Assertion failure"