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 setUncatchedExceptionHandler, -- :: (Exception -> IO ()) -> IO ()
110 getUncatchedExceptionHandler -- :: IO (Exception -> IO ())
113 #ifdef __GLASGOW_HASKELL__
114 import GHC.Base ( assert )
115 import GHC.Exception as ExceptionBase hiding (catch)
116 import GHC.Conc ( throwTo, ThreadId )
117 import GHC.IOBase ( IO(..), IORef(..), newIORef, readIORef, writeIORef )
118 import GHC.Handle ( stdout, hFlush )
119 import Foreign.C.String ( CString, withCStringLen )
123 import Hugs.Exception as ExceptionBase
126 import Prelude hiding ( catch )
127 import System.IO.Error hiding ( catch, try )
128 import System.IO.Unsafe (unsafePerformIO)
131 #include "Typeable.h"
132 INSTANCE_TYPEABLE0(Exception,exceptionTc,"Exception")
133 INSTANCE_TYPEABLE0(IOException,ioExceptionTc,"IOException")
134 INSTANCE_TYPEABLE0(ArithException,arithExceptionTc,"ArithException")
135 INSTANCE_TYPEABLE0(ArrayException,arrayExceptionTc,"ArrayException")
136 INSTANCE_TYPEABLE0(AsyncException,asyncExceptionTc,"AsyncException")
138 -----------------------------------------------------------------------------
139 -- Catching exceptions
141 -- |This is the simplest of the exception-catching functions. It
142 -- takes a single argument, runs it, and if an exception is raised
143 -- the \"handler\" is executed, with the value of the exception passed as an
144 -- argument. Otherwise, the result is returned as normal. For example:
146 -- > catch (openFile f ReadMode)
147 -- > (\e -> hPutStr stderr (\"Couldn\'t open \"++f++\": \" ++ show e))
149 -- For catching exceptions in pure (non-'IO') expressions, see the
150 -- function 'evaluate'.
152 -- Note that due to Haskell\'s unspecified evaluation order, an
153 -- expression may return one of several possible exceptions: consider
154 -- the expression @error \"urk\" + 1 \`div\` 0@. Does
155 -- 'catch' execute the handler passing
156 -- @ErrorCall \"urk\"@, or @ArithError DivideByZero@?
158 -- The answer is \"either\": 'catch' makes a
159 -- non-deterministic choice about which exception to catch. If you
160 -- call it again, you might get a different exception back. This is
161 -- ok, because 'catch' is an 'IO' computation.
163 -- Note that 'catch' catches all types of exceptions, and is generally
164 -- used for \"cleaning up\" before passing on the exception using
165 -- 'throwIO'. It is not good practice to discard the exception and
166 -- continue, without first checking the type of the exception (it
167 -- might be a 'ThreadKilled', for example). In this case it is usually better
168 -- to use 'catchJust' and select the kinds of exceptions to catch.
170 -- Also note that The "Prelude" also exports a
171 -- function called 'catch' which has the same type as
172 -- 'Control.Exception.catch', the difference being that the
173 -- "Prelude" version only catches the IO and user
174 -- families of exceptions (as required by Haskell 98). We recommend
175 -- either hiding the "Prelude" version of
176 -- 'catch' when importing
177 -- "Control.Exception", or importing
178 -- "Control.Exception" qualified, to avoid name-clashes.
180 catch :: IO a -- ^ The computation to run
181 -> (Exception -> IO a) -- ^ Handler to invoke if an exception is raised
183 catch = ExceptionBase.catchException
185 -- | The function 'catchJust' is like 'catch', but it takes an extra
186 -- argument which is an /exception predicate/, a function which
187 -- selects which type of exceptions we\'re interested in. There are
188 -- some predefined exception predicates for useful subsets of
189 -- exceptions: 'ioErrors', 'arithExceptions', and so on. For example,
190 -- to catch just calls to the 'error' function, we could use
192 -- > result <- catchJust errorCalls thing_to_try handler
194 -- Any other exceptions which are not matched by the predicate
195 -- are re-raised, and may be caught by an enclosing
196 -- 'catch' or 'catchJust'.
198 :: (Exception -> Maybe b) -- ^ Predicate to select exceptions
199 -> IO a -- ^ Computation to run
200 -> (b -> IO a) -- ^ Handler
202 catchJust p a handler = catch a handler'
203 where handler' e = case p e of
207 -- | A version of 'catch' with the arguments swapped around; useful in
208 -- situations where the code for the handler is shorter. For example:
210 -- > do handle (\e -> exitWith (ExitFailure 1)) $
212 handle :: (Exception -> IO a) -> IO a -> IO a
215 -- | A version of 'catchJust' with the arguments swapped around (see
217 handleJust :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a
218 handleJust p = flip (catchJust p)
220 -----------------------------------------------------------------------------
223 -- | Forces its argument to be evaluated, and returns the result in
224 -- the 'IO' monad. It can be used to order evaluation with respect to
225 -- other 'IO' operations; its semantics are given by
227 -- > evaluate undefined `seq` return () ==> return ()
228 -- > catch (evaluate undefined) (\e -> return ()) ==> return ()
230 -- NOTE: @(evaluate a)@ is /not/ the same as @(a \`seq\` return a)@.
231 #ifdef __GLASGOW_HASKELL__
232 evaluate :: a -> IO a
233 evaluate a = IO $ \s -> case a `seq` () of () -> (# s, a #)
235 -- a `seq` (# s, a #)
236 -- because we can't have an unboxed tuple as a function argument
239 -----------------------------------------------------------------------------
242 -- | This function maps one exception into another as proposed in the
243 -- paper \"A semantics for imprecise exceptions\".
245 -- Notice that the usage of 'unsafePerformIO' is safe here.
247 mapException :: (Exception -> Exception) -> a -> a
248 mapException f v = unsafePerformIO (catch (evaluate v)
251 -----------------------------------------------------------------------------
252 -- 'try' and variations.
254 -- | Similar to 'catch', but returns an 'Either' result which is
255 -- @(Right a)@ if no exception was raised, or @(Left e)@ if an
256 -- exception was raised and its value is @e@.
258 -- > try a = catch (Right \`liftM\` a) (return . Left)
260 -- Note: as with 'catch', it is only polite to use this variant if you intend
261 -- to re-throw the exception after performing whatever cleanup is needed.
262 -- Otherwise, 'tryJust' is generally considered to be better.
264 try :: IO a -> IO (Either Exception a)
265 try a = catch (a >>= \ v -> return (Right v)) (\e -> return (Left e))
267 -- | A variant of 'try' that takes an exception predicate to select
268 -- which exceptions are caught (c.f. 'catchJust'). If the exception
269 -- does not match the predicate, it is re-thrown.
270 tryJust :: (Exception -> Maybe b) -> IO a -> IO (Either b a)
274 Right v -> return (Right v)
275 Left e -> case p e of
277 Just b -> return (Left b)
279 -----------------------------------------------------------------------------
280 -- Dynamic exceptions
283 -- #DynamicExceptions# Because the 'Exception' datatype is not extensible, there is an
284 -- interface for throwing and catching exceptions of type 'Dynamic'
285 -- (see "Data.Dynamic") which allows exception values of any type in
286 -- the 'Typeable' class to be thrown and caught.
288 -- | Raise any value as an exception, provided it is in the
290 throwDyn :: Typeable exception => exception -> b
291 throwDyn exception = throw (DynException (toDyn exception))
293 #ifdef __GLASGOW_HASKELL__
294 -- | A variant of 'throwDyn' that throws the dynamic exception to an
295 -- arbitrary thread (GHC only: c.f. 'throwTo').
296 throwDynTo :: Typeable exception => ThreadId -> exception -> IO ()
297 throwDynTo t exception = throwTo t (DynException (toDyn exception))
298 #endif /* __GLASGOW_HASKELL__ */
300 -- | Catch dynamic exceptions of the required type. All other
301 -- exceptions are re-thrown, including dynamic exceptions of the wrong
304 -- When using dynamic exceptions it is advisable to define a new
305 -- datatype to use for your exception type, to avoid possible clashes
306 -- with dynamic exceptions used in other libraries.
308 catchDyn :: Typeable exception => IO a -> (exception -> IO a) -> IO a
309 catchDyn m k = catchException m handle
310 where handle ex = case ex of
311 (DynException dyn) ->
312 case fromDynamic dyn of
313 Just exception -> k exception
317 -----------------------------------------------------------------------------
318 -- Exception Predicates
321 -- These pre-defined predicates may be used as the first argument to
322 -- 'catchJust', 'tryJust', or 'handleJust' to select certain common
323 -- classes of exceptions.
325 ioErrors :: Exception -> Maybe IOError
326 arithExceptions :: Exception -> Maybe ArithException
327 errorCalls :: Exception -> Maybe String
328 assertions :: Exception -> Maybe String
329 dynExceptions :: Exception -> Maybe Dynamic
330 asyncExceptions :: Exception -> Maybe AsyncException
331 userErrors :: Exception -> Maybe String
333 ioErrors (IOException e) = Just e
336 arithExceptions (ArithException e) = Just e
337 arithExceptions _ = Nothing
339 errorCalls (ErrorCall e) = Just e
340 errorCalls _ = Nothing
342 assertions (AssertionFailed e) = Just e
343 assertions _ = Nothing
345 dynExceptions (DynException e) = Just e
346 dynExceptions _ = Nothing
348 asyncExceptions (AsyncException e) = Just e
349 asyncExceptions _ = Nothing
351 userErrors (IOException e) | isUserError e = Just (ioeGetErrorString e)
352 userErrors _ = Nothing
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 'Control.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 'Control.Concurrent.MVar.takeMVar'
468 (but not 'Control.Concurrent.MVar.tryTakeMVar'),
469 and most operations which perform
470 some I\/O with the outside world. The reason for having
471 interruptible operations is so that we can write things like
475 > catch (unblock (...))
479 if the 'Control.Concurrent.MVar.takeMVar' was not interruptible,
481 combination could lead to deadlock, because the thread itself would be
482 blocked in a state where it can\'t receive any asynchronous exceptions.
483 With 'Control.Concurrent.MVar.takeMVar' interruptible, however, we can be
484 safe in the knowledge that the thread can receive exceptions right up
485 until the point when the 'Control.Concurrent.MVar.takeMVar' succeeds.
486 Similar arguments apply for other interruptible operations like
487 'System.IO.openFile'.
490 -- -----------------------------------------------------------------------------
494 -- | If the first argument evaluates to 'True', then the result is the
495 -- second argument. Otherwise an 'AssertionFailed' exception is raised,
496 -- containing a 'String' with the source file and line number of the
499 -- Assertions can normally be turned on or off with a compiler flag
500 -- (for GHC, assertions are normally on unless the @-fignore-asserts@
501 -- option is give). When assertions are turned off, the first
502 -- argument to 'assert' is ignored, and the second argument is
503 -- returned as the result.
504 assert :: Bool -> a -> a
507 #ifndef __GLASGOW_HASKELL__
508 assert :: Bool -> a -> a
510 assert False _ = throw (AssertionFailed "")
514 {-# NOINLINE uncatchedExceptionHandler #-}
515 uncatchedExceptionHandler :: IORef (Exception -> IO ())
516 uncatchedExceptionHandler = 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 withCStringLen ("Fail: "++msg) $ \(cstr,len) -> writeErrString cstr len
527 foreign import ccall unsafe "writeErrString__"
528 writeErrString :: CString -> Int -> IO ()
530 setUncatchedExceptionHandler :: (Exception -> IO ()) -> IO ()
531 setUncatchedExceptionHandler = writeIORef uncatchedExceptionHandler
533 getUncatchedExceptionHandler :: IO (Exception -> IO ())
534 getUncatchedExceptionHandler = readIORef uncatchedExceptionHandler