1 {-# OPTIONS_GHC -XNoImplicitPrelude #-}
3 -----------------------------------------------------------------------------
5 -- Module : Control.Exception
6 -- Copyright : (c) The University of Glasgow 2001
7 -- License : BSD-style (see the file libraries/base/LICENSE)
9 -- Maintainer : libraries@haskell.org
10 -- Stability : experimental
11 -- Portability : non-portable (extended exceptions)
13 -- This module provides support for raising and catching both built-in
14 -- and user-defined exceptions.
16 -- In addition to exceptions thrown by 'IO' operations, exceptions may
17 -- be thrown by pure code (imprecise exceptions) or by external events
18 -- (asynchronous exceptions), but may only be caught in the 'IO' monad.
19 -- For more details, see:
21 -- * /A semantics for imprecise exceptions/, by Simon Peyton Jones,
22 -- Alastair Reid, Tony Hoare, Simon Marlow, Fergus Henderson,
25 -- * /Asynchronous exceptions in Haskell/, by Simon Marlow, Simon Peyton
26 -- Jones, Andy Moran and John Reppy, in /PLDI'01/.
28 -- * /An Extensible Dynamically-Typed Hierarchy of Exceptions/,
29 -- by Simon Marlow, in /Haskell '06/.
31 -----------------------------------------------------------------------------
33 module Control.Exception (
35 -- * The Exception type
41 Exception(..), -- class
42 IOException, -- instance Eq, Ord, Show, Typeable, Exception
43 ArithException(..), -- instance Eq, Ord, Show, Typeable, Exception
44 ArrayException(..), -- instance Eq, Ord, Show, Typeable, Exception
46 AsyncException(..), -- instance Eq, Ord, Show, Typeable, Exception
48 #if __GLASGOW_HASKELL__ || __HUGS__
53 System.ExitCode(), -- instance Exception
56 BlockedIndefinitelyOnMVar(..),
57 BlockedIndefinitelyOnSTM(..),
66 -- * Throwing exceptions
70 #ifdef __GLASGOW_HASKELL__
74 -- * Catching Exceptions
78 -- ** Catching all exceptions
82 -- ** The @catch@ functions
87 -- ** The @handle@ functions
91 -- ** The @try@ functions
95 -- ** The @evaluate@ function
98 -- ** The @mapException@ function
101 -- * Asynchronous Exceptions
105 -- ** Asynchronous exception control
107 -- |The following functions allow a thread to control delivery of
108 -- asynchronous exceptions during a critical region.
113 uninterruptibleMask_,
117 -- ** (deprecated) Asynchronous exception control
123 -- *** Applying @block@ to an exception handler
127 -- *** Interruptible operations
146 import Control.Exception.Base
148 #ifdef __GLASGOW_HASKELL__
152 import Prelude hiding (catch)
156 import System (ExitCode())
159 -- | You need this when using 'catches'.
160 data Handler a = forall e . Exception e => Handler (e -> IO a)
163 Sometimes you want to catch two different sorts of exception. You could
166 > f = expr `catch` \ (ex :: ArithException) -> handleArith ex
167 > `catch` \ (ex :: IOException) -> handleIO ex
169 However, there are a couple of problems with this approach. The first is
170 that having two exception handlers is inefficient. However, the more
171 serious issue is that the second exception handler will catch exceptions
172 in the first, e.g. in the example above, if @handleArith@ throws an
173 @IOException@ then the second exception handler will catch it.
175 Instead, we provide a function 'catches', which would be used thus:
177 > f = expr `catches` [Handler (\ (ex :: ArithException) -> handleArith ex),
178 > Handler (\ (ex :: IOException) -> handleIO ex)]
180 catches :: IO a -> [Handler a] -> IO a
181 catches io handlers = io `catch` catchesHandler handlers
183 catchesHandler :: [Handler a] -> SomeException -> IO a
184 catchesHandler handlers e = foldr tryHandler (throw e) handlers
185 where tryHandler (Handler handler) res
186 = case fromException e of
187 Just e' -> handler e'
190 -- -----------------------------------------------------------------------------
191 -- Catching exceptions
195 There are several functions for catching and examining
196 exceptions; all of them may only be used from within the
199 Here's a rule of thumb for deciding which catch-style function to
202 * If you want to do some cleanup in the event that an exception
203 is raised, use 'finally', 'bracket' or 'onException'.
205 * To recover after an exception and do something else, the best
206 choice is to use one of the 'try' family.
208 * ... unless you are recovering from an asynchronous exception, in which
209 case use 'catch' or 'catchJust'.
211 The difference between using 'try' and 'catch' for recovery is that in
212 'catch' the handler is inside an implicit 'block' (see \"Asynchronous
213 Exceptions\") which is important when catching asynchronous
214 exceptions, but when catching other kinds of exception it is
215 unnecessary. Furthermore it is possible to accidentally stay inside
216 the implicit 'block' by tail-calling rather than returning from the
217 handler, which is why we recommend using 'try' rather than 'catch' for
218 ordinary exception recovery.
220 A typical use of 'tryJust' for recovery looks like this:
222 > do r <- tryJust (guard . isDoesNotExistError) $ getEnv "HOME"
229 -- -----------------------------------------------------------------------------
230 -- Asynchronous exceptions
234 #AsynchronousExceptions# Asynchronous exceptions are so-called because they arise due to
235 external influences, and can be raised at any point during execution.
236 'StackOverflow' and 'HeapOverflow' are two examples of
237 system-generated asynchronous exceptions.
239 The primary source of asynchronous exceptions, however, is
242 > throwTo :: ThreadId -> Exception -> IO ()
244 'throwTo' (also 'throwDynTo' and 'Control.Concurrent.killThread') allows one
245 running thread to raise an arbitrary exception in another thread. The
246 exception is therefore asynchronous with respect to the target thread,
247 which could be doing anything at the time it receives the exception.
248 Great care should be taken with asynchronous exceptions; it is all too
249 easy to introduce race conditions by the over zealous use of
254 There\'s an implied 'mask' around every exception handler in a call
255 to one of the 'catch' family of functions. This is because that is
256 what you want most of the time - it eliminates a common race condition
257 in starting an exception handler, because there may be no exception
258 handler on the stack to handle another exception if one arrives
259 immediately. If asynchronous exceptions are blocked on entering the
260 handler, though, we have time to install a new exception handler
261 before being interrupted. If this weren\'t the default, one would have
262 to write something like
264 > block $ \restore ->
265 > catch (restore (...))
268 If you need to unblock asynchronous exceptions again in the exception
269 handler, just use 'unblock' as normal.
271 Note that 'try' and friends /do not/ have a similar default, because
272 there is no exception handler in this case. Don't use 'try' for
273 recovering from an asynchronous exception.
279 Some operations are /interruptible/, which means that they can receive
280 asynchronous exceptions even in the scope of a 'block'. Any function
281 which may itself block is defined as interruptible; this includes
282 'Control.Concurrent.MVar.takeMVar'
283 (but not 'Control.Concurrent.MVar.tryTakeMVar'),
284 and most operations which perform
285 some I\/O with the outside world. The reason for having
286 interruptible operations is so that we can write things like
288 > mask $ \restore -> do
290 > catch (restore (...))
293 if the 'Control.Concurrent.MVar.takeMVar' was not interruptible,
295 combination could lead to deadlock, because the thread itself would be
296 blocked in a state where it can\'t receive any asynchronous exceptions.
297 With 'Control.Concurrent.MVar.takeMVar' interruptible, however, we can be
298 safe in the knowledge that the thread can receive exceptions right up
299 until the point when the 'Control.Concurrent.MVar.takeMVar' succeeds.
300 Similar arguments apply for other interruptible operations like
301 'System.IO.openFile'.
306 It is possible to catch all exceptions, by using the type 'SomeException':
308 > catch f (\e -> ... (e :: SomeException) ...)
310 HOWEVER, this is normally not what you want to do!
312 For example, suppose you want to read a file, but if it doesn't exist
313 then continue as if it contained \"\". You might be tempted to just
314 catch all exceptions and return \"\" in the handler. However, this has
315 all sorts of undesirable consequences. For example, if the user
316 presses control-C at just the right moment then the 'UserInterrupt'
317 exception will be caught, and the program will continue running under
318 the belief that the file contains \"\". Similarly, if another thread
319 tries to kill the thread reading the file then the 'ThreadKilled'
320 exception will be ignored.
322 Instead, you should only catch exactly the exceptions that you really
323 want. In this case, this would likely be more specific than even
324 \"any IO exception\"; a permissions error would likely also want to be
325 handled differently. Instead, you would probably want something like:
327 > e <- tryJust (guard . isDoesNotExistError) (readFile f)
328 > let str = either (const "") id e
330 There are occassions when you really do need to catch any sort of
331 exception. However, in most cases this is just so you can do some
332 cleaning up; you aren't actually interested in the exception itself.
333 For example, if you open a file then you want to close it again,
334 whether processing the file executes normally or throws an exception.
335 However, in these cases you can use functions like 'bracket', 'finally'
336 and 'onException', which never actually pass you the exception, but
337 just call the cleanup functions at the appropriate points.
339 But sometimes you really do need to catch any exception, and actually
340 see what the exception is. One example is at the very top-level of a
341 program, you may wish to catch any exception, print it to a logfile or
342 the screen, and then exit gracefully. For these cases, you can use
343 'catch' (or one of the other exception-catching functions) with the
344 'SomeException' type.