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, withCStringLen )
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 #include "Typeable.h"
134 INSTANCE_TYPEABLE0(Exception,exceptionTc,"Exception")
135 INSTANCE_TYPEABLE0(IOException,ioExceptionTc,"IOException")
136 INSTANCE_TYPEABLE0(ArithException,arithExceptionTc,"ArithException")
137 INSTANCE_TYPEABLE0(ArrayException,arrayExceptionTc,"ArrayException")
138 INSTANCE_TYPEABLE0(AsyncException,asyncExceptionTc,"AsyncException")
140 -----------------------------------------------------------------------------
141 -- Catching exceptions
143 -- |This is the simplest of the exception-catching functions. It
144 -- takes a single argument, runs it, and if an exception is raised
145 -- the \"handler\" is executed, with the value of the exception passed as an
146 -- argument. Otherwise, the result is returned as normal. For example:
148 -- > catch (openFile f ReadMode)
149 -- > (\e -> hPutStr stderr (\"Couldn\'t open \"++f++\": \" ++ show e))
151 -- For catching exceptions in pure (non-'IO') expressions, see the
152 -- function 'evaluate'.
154 -- Note that due to Haskell\'s unspecified evaluation order, an
155 -- expression may return one of several possible exceptions: consider
156 -- the expression @error \"urk\" + 1 \`div\` 0@. Does
157 -- 'catch' execute the handler passing
158 -- @ErrorCall \"urk\"@, or @ArithError DivideByZero@?
160 -- The answer is \"either\": 'catch' makes a
161 -- non-deterministic choice about which exception to catch. If you
162 -- call it again, you might get a different exception back. This is
163 -- ok, because 'catch' is an 'IO' computation.
165 -- Note that 'catch' catches all types of exceptions, and is generally
166 -- used for \"cleaning up\" before passing on the exception using
167 -- 'throwIO'. It is not good practice to discard the exception and
168 -- continue, without first checking the type of the exception (it
169 -- might be a 'ThreadKilled', for example). In this case it is usually better
170 -- to use 'catchJust' and select the kinds of exceptions to catch.
172 -- Also note that the "Prelude" also exports a
173 -- function called 'Prelude.catch' which has the same type as
174 -- 'Control.Exception.catch', the difference being that the
175 -- "Prelude" version only catches the IO and user
176 -- families of exceptions (as required by Haskell 98). We recommend
177 -- either hiding the "Prelude" version of
178 -- 'Prelude.catch' when importing
179 -- "Control.Exception", or importing
180 -- "Control.Exception" qualified, to avoid name-clashes.
182 catch :: IO a -- ^ The computation to run
183 -> (Exception -> IO a) -- ^ Handler to invoke if an exception is raised
185 catch = ExceptionBase.catchException
187 -- | The function 'catchJust' is like 'catch', but it takes an extra
188 -- argument which is an /exception predicate/, a function which
189 -- selects which type of exceptions we\'re interested in. There are
190 -- some predefined exception predicates for useful subsets of
191 -- exceptions: 'ioErrors', 'arithExceptions', and so on. For example,
192 -- to catch just calls to the 'error' function, we could use
194 -- > result <- catchJust errorCalls thing_to_try handler
196 -- Any other exceptions which are not matched by the predicate
197 -- are re-raised, and may be caught by an enclosing
198 -- 'catch' or 'catchJust'.
200 :: (Exception -> Maybe b) -- ^ Predicate to select exceptions
201 -> IO a -- ^ Computation to run
202 -> (b -> IO a) -- ^ Handler
204 catchJust p a handler = catch a handler'
205 where handler' e = case p e of
209 -- | A version of 'catch' with the arguments swapped around; useful in
210 -- situations where the code for the handler is shorter. For example:
212 -- > do handle (\e -> exitWith (ExitFailure 1)) $
214 handle :: (Exception -> IO a) -> IO a -> IO a
217 -- | A version of 'catchJust' with the arguments swapped around (see
219 handleJust :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a
220 handleJust p = flip (catchJust p)
222 -----------------------------------------------------------------------------
225 -- | This function maps one exception into another as proposed in the
226 -- paper \"A semantics for imprecise exceptions\".
228 -- Notice that the usage of 'unsafePerformIO' is safe here.
230 mapException :: (Exception -> Exception) -> a -> a
231 mapException f v = unsafePerformIO (catch (evaluate v)
234 -----------------------------------------------------------------------------
235 -- 'try' and variations.
237 -- | Similar to 'catch', but returns an 'Either' result which is
238 -- @('Right' a)@ if no exception was raised, or @('Left' e)@ if an
239 -- exception was raised and its value is @e@.
241 -- > try a = catch (Right \`liftM\` a) (return . Left)
243 -- Note: as with 'catch', it is only polite to use this variant if you intend
244 -- to re-throw the exception after performing whatever cleanup is needed.
245 -- Otherwise, 'tryJust' is generally considered to be better.
247 try :: IO a -> IO (Either Exception a)
248 try a = catch (a >>= \ v -> return (Right v)) (\e -> return (Left e))
250 -- | A variant of 'try' that takes an exception predicate to select
251 -- which exceptions are caught (c.f. 'catchJust'). If the exception
252 -- does not match the predicate, it is re-thrown.
253 tryJust :: (Exception -> Maybe b) -> IO a -> IO (Either b a)
257 Right v -> return (Right v)
258 Left e -> case p e of
260 Just b -> return (Left b)
262 -----------------------------------------------------------------------------
263 -- Dynamic exceptions
266 -- #DynamicExceptions# Because the 'Exception' datatype is not extensible, there is an
267 -- interface for throwing and catching exceptions of type 'Dynamic'
268 -- (see "Data.Dynamic") which allows exception values of any type in
269 -- the 'Typeable' class to be thrown and caught.
271 -- | Raise any value as an exception, provided it is in the
273 throwDyn :: Typeable exception => exception -> b
274 throwDyn exception = throw (DynException (toDyn exception))
276 #ifdef __GLASGOW_HASKELL__
277 -- | A variant of 'throwDyn' that throws the dynamic exception to an
278 -- arbitrary thread (GHC only: c.f. 'throwTo').
279 throwDynTo :: Typeable exception => ThreadId -> exception -> IO ()
280 throwDynTo t exception = throwTo t (DynException (toDyn exception))
281 #endif /* __GLASGOW_HASKELL__ */
283 -- | Catch dynamic exceptions of the required type. All other
284 -- exceptions are re-thrown, including dynamic exceptions of the wrong
287 -- When using dynamic exceptions it is advisable to define a new
288 -- datatype to use for your exception type, to avoid possible clashes
289 -- with dynamic exceptions used in other libraries.
291 catchDyn :: Typeable exception => IO a -> (exception -> IO a) -> IO a
292 catchDyn m k = catchException m handle
293 where handle ex = case ex of
294 (DynException dyn) ->
295 case fromDynamic dyn of
296 Just exception -> k exception
300 -----------------------------------------------------------------------------
301 -- Exception Predicates
304 -- These pre-defined predicates may be used as the first argument to
305 -- 'catchJust', 'tryJust', or 'handleJust' to select certain common
306 -- classes of exceptions.
308 ioErrors :: Exception -> Maybe IOError
309 arithExceptions :: Exception -> Maybe ArithException
310 errorCalls :: Exception -> Maybe String
311 assertions :: Exception -> Maybe String
312 dynExceptions :: Exception -> Maybe Dynamic
313 asyncExceptions :: Exception -> Maybe AsyncException
314 userErrors :: Exception -> Maybe String
316 ioErrors (IOException e) = Just e
319 arithExceptions (ArithException e) = Just e
320 arithExceptions _ = Nothing
322 errorCalls (ErrorCall e) = Just e
323 errorCalls _ = Nothing
325 assertions (AssertionFailed e) = Just e
326 assertions _ = Nothing
328 dynExceptions (DynException e) = Just e
329 dynExceptions _ = Nothing
331 asyncExceptions (AsyncException e) = Just e
332 asyncExceptions _ = Nothing
334 userErrors (IOException e) | isUserError e = Just (ioeGetErrorString e)
335 userErrors _ = Nothing
337 -----------------------------------------------------------------------------
338 -- Some Useful Functions
340 -- | When you want to acquire a resource, do some work with it, and
341 -- then release the resource, it is a good idea to use 'bracket',
342 -- because 'bracket' will install the necessary exception handler to
343 -- release the resource in the event that an exception is raised
344 -- during the computation. If an exception is raised, then 'bracket' will
345 -- re-raise the exception (after performing the release).
347 -- A common example is opening a file:
350 -- > (openFile "filename" ReadMode)
352 -- > (\handle -> do { ... })
354 -- The arguments to 'bracket' are in this order so that we can partially apply
357 -- > withFile name = bracket (openFile name) hClose
360 :: IO a -- ^ computation to run first (\"acquire resource\")
361 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
362 -> (a -> IO c) -- ^ computation to run in-between
363 -> IO c -- returns the value from the in-between computation
364 bracket before after thing =
369 (\e -> do { after a; throw e })
375 -- | A specialised variant of 'bracket' with just a computation to run
378 finally :: IO a -- ^ computation to run first
379 -> IO b -- ^ computation to run afterward (even if an exception
381 -> IO a -- returns the value from the first computation
386 (\e -> do { sequel; throw e })
391 -- | A variant of 'bracket' where the return value from the first computation
393 bracket_ :: IO a -> IO b -> IO c -> IO c
394 bracket_ before after thing = bracket before (const after) (const thing)
396 -- -----------------------------------------------------------------------------
397 -- Asynchronous exceptions
401 #AsynchronousExceptions# Asynchronous exceptions are so-called because they arise due to
402 external influences, and can be raised at any point during execution.
403 'StackOverflow' and 'HeapOverflow' are two examples of
404 system-generated asynchronous exceptions.
406 The primary source of asynchronous exceptions, however, is
409 > throwTo :: ThreadId -> Exception -> IO ()
411 'throwTo' (also 'throwDynTo' and 'Control.Concurrent.killThread') allows one
412 running thread to raise an arbitrary exception in another thread. The
413 exception is therefore asynchronous with respect to the target thread,
414 which could be doing anything at the time it receives the exception.
415 Great care should be taken with asynchronous exceptions; it is all too
416 easy to introduce race conditions by the over zealous use of
421 There\'s an implied 'block' around every exception handler in a call
422 to one of the 'catch' family of functions. This is because that is
423 what you want most of the time - it eliminates a common race condition
424 in starting an exception handler, because there may be no exception
425 handler on the stack to handle another exception if one arrives
426 immediately. If asynchronous exceptions are blocked on entering the
427 handler, though, we have time to install a new exception handler
428 before being interrupted. If this weren\'t the default, one would have
429 to write something like
432 > catch (unblock (...))
436 If you need to unblock asynchronous exceptions again in the exception
437 handler, just use 'unblock' as normal.
439 Note that 'try' and friends /do not/ have a similar default, because
440 there is no exception handler in this case. If you want to use 'try'
441 in an asynchronous-exception-safe way, you will need to use
447 Some operations are /interruptible/, which means that they can receive
448 asynchronous exceptions even in the scope of a 'block'. Any function
449 which may itself block is defined as interruptible; this includes
450 'Control.Concurrent.MVar.takeMVar'
451 (but not 'Control.Concurrent.MVar.tryTakeMVar'),
452 and most operations which perform
453 some I\/O with the outside world. The reason for having
454 interruptible operations is so that we can write things like
458 > catch (unblock (...))
462 if the 'Control.Concurrent.MVar.takeMVar' was not interruptible,
464 combination could lead to deadlock, because the thread itself would be
465 blocked in a state where it can\'t receive any asynchronous exceptions.
466 With 'Control.Concurrent.MVar.takeMVar' interruptible, however, we can be
467 safe in the knowledge that the thread can receive exceptions right up
468 until the point when the 'Control.Concurrent.MVar.takeMVar' succeeds.
469 Similar arguments apply for other interruptible operations like
470 'System.IO.openFile'.
473 -- -----------------------------------------------------------------------------
477 -- | If the first argument evaluates to 'True', then the result is the
478 -- second argument. Otherwise an 'AssertionFailed' exception is raised,
479 -- containing a 'String' with the source file and line number of the
482 -- Assertions can normally be turned on or off with a compiler flag
483 -- (for GHC, assertions are normally on unless the @-fignore-asserts@
484 -- option is give). When assertions are turned off, the first
485 -- argument to 'assert' is ignored, and the second argument is
486 -- returned as the result.
487 assert :: Bool -> a -> a
490 #ifndef __GLASGOW_HASKELL__
491 assert :: Bool -> a -> a
493 assert False _ = throw (AssertionFailed "")
497 #ifdef __GLASGOW_HASKELL__
498 {-# NOINLINE uncaughtExceptionHandler #-}
499 uncaughtExceptionHandler :: IORef (Exception -> IO ())
500 uncaughtExceptionHandler = unsafePerformIO (newIORef defaultHandler)
502 defaultHandler :: Exception -> IO ()
503 defaultHandler ex = do
504 (hFlush stdout) `catchException` (\ _ -> return ())
506 Deadlock -> "no threads to run: infinite loop or deadlock?"
508 other -> showsPrec 0 other "\n"
509 withCStringLen ("Fail: "++msg) $ \(cstr,len) -> writeErrString cstr len
511 foreign import ccall unsafe "writeErrString__"
512 writeErrString :: CString -> Int -> IO ()
514 setUncaughtExceptionHandler :: (Exception -> IO ()) -> IO ()
515 setUncaughtExceptionHandler = writeIORef uncaughtExceptionHandler
517 getUncaughtExceptionHandler :: IO (Exception -> IO ())
518 getUncaughtExceptionHandler = readIORef uncaughtExceptionHandler