{-# LANGUAGE Trustworthy #-} {-# LANGUAGE NoImplicitPrelude, ExistentialQuantification #-} ----------------------------------------------------------------------------- -- | -- Module : Control.Exception -- Copyright : (c) The University of Glasgow 2001 -- License : BSD-style (see the file libraries/base/LICENSE) -- -- Maintainer : libraries@haskell.org -- Stability : experimental -- Portability : non-portable (extended exceptions) -- -- This module provides support for raising and catching both built-in -- and user-defined exceptions. -- -- In addition to exceptions thrown by 'IO' operations, exceptions may -- be thrown by pure code (imprecise exceptions) or by external events -- (asynchronous exceptions), but may only be caught in the 'IO' monad. -- For more details, see: -- -- * /A semantics for imprecise exceptions/, by Simon Peyton Jones, -- Alastair Reid, Tony Hoare, Simon Marlow, Fergus Henderson, -- in /PLDI'99/. -- -- * /Asynchronous exceptions in Haskell/, by Simon Marlow, Simon Peyton -- Jones, Andy Moran and John Reppy, in /PLDI'01/. -- -- * /An Extensible Dynamically-Typed Hierarchy of Exceptions/, -- by Simon Marlow, in /Haskell '06/. -- ----------------------------------------------------------------------------- module Control.Exception ( -- * The Exception type SomeException(..), Exception(..), -- class IOException, -- instance Eq, Ord, Show, Typeable, Exception ArithException(..), -- instance Eq, Ord, Show, Typeable, Exception ArrayException(..), -- instance Eq, Ord, Show, Typeable, Exception AssertionFailed(..), SomeAsyncException(..), AsyncException(..), -- instance Eq, Ord, Show, Typeable, Exception asyncExceptionToException, asyncExceptionFromException, NonTermination(..), NestedAtomically(..), BlockedIndefinitelyOnMVar(..), BlockedIndefinitelyOnSTM(..), Deadlock(..), NoMethodError(..), PatternMatchFail(..), RecConError(..), RecSelError(..), RecUpdError(..), ErrorCall(..), -- * Throwing exceptions throw, throwIO, ioError, throwTo, -- * Catching Exceptions -- $catching -- ** Catching all exceptions -- $catchall -- ** The @catch@ functions catch, catches, Handler(..), catchJust, -- ** The @handle@ functions handle, handleJust, -- ** The @try@ functions try, tryJust, -- ** The @evaluate@ function evaluate, -- ** The @mapException@ function mapException, -- * Asynchronous Exceptions -- $async -- ** Asynchronous exception control -- |The following functions allow a thread to control delivery of -- asynchronous exceptions during a critical region. mask, mask_, uninterruptibleMask, uninterruptibleMask_, MaskingState(..), getMaskingState, allowInterrupt, -- *** Applying @mask@ to an exception handler -- $block_handler -- *** Interruptible operations -- $interruptible -- * Assertions assert, -- * Utilities bracket, bracket_, bracketOnError, finally, onException, ) where import Control.Exception.Base import GHC.Base import GHC.IO (unsafeUnmask) import Data.Maybe -- | You need this when using 'catches'. data Handler a = forall e . Exception e => Handler (e -> IO a) instance Functor Handler where fmap f (Handler h) = Handler (fmap f . h) {- | Sometimes you want to catch two different sorts of exception. You could do something like > f = expr `catch` \ (ex :: ArithException) -> handleArith ex > `catch` \ (ex :: IOException) -> handleIO ex However, there are a couple of problems with this approach. The first is that having two exception handlers is inefficient. However, the more serious issue is that the second exception handler will catch exceptions in the first, e.g. in the example above, if @handleArith@ throws an @IOException@ then the second exception handler will catch it. Instead, we provide a function 'catches', which would be used thus: > f = expr `catches` [Handler (\ (ex :: ArithException) -> handleArith ex), > Handler (\ (ex :: IOException) -> handleIO ex)] -} catches :: IO a -> [Handler a] -> IO a catches io handlers = io `catch` catchesHandler handlers catchesHandler :: [Handler a] -> SomeException -> IO a catchesHandler handlers e = foldr tryHandler (throw e) handlers where tryHandler (Handler handler) res = case fromException e of Just e' -> handler e' Nothing -> res -- ----------------------------------------------------------------------------- -- Catching exceptions {- $catching There are several functions for catching and examining exceptions; all of them may only be used from within the 'IO' monad. Here's a rule of thumb for deciding which catch-style function to use: * If you want to do some cleanup in the event that an exception is raised, use 'finally', 'bracket' or 'onException'. * To recover after an exception and do something else, the best choice is to use one of the 'try' family. * ... unless you are recovering from an asynchronous exception, in which case use 'catch' or 'catchJust'. The difference between using 'try' and 'catch' for recovery is that in 'catch' the handler is inside an implicit 'block' (see \"Asynchronous Exceptions\") which is important when catching asynchronous exceptions, but when catching other kinds of exception it is unnecessary. Furthermore it is possible to accidentally stay inside the implicit 'block' by tail-calling rather than returning from the handler, which is why we recommend using 'try' rather than 'catch' for ordinary exception recovery. A typical use of 'tryJust' for recovery looks like this: > do r <- tryJust (guard . isDoesNotExistError) $ getEnv "HOME" > case r of > Left e -> ... > Right home -> ... -} -- ----------------------------------------------------------------------------- -- Asynchronous exceptions -- | When invoked inside 'mask', this function allows a blocked -- asynchronous exception to be raised, if one exists. It is -- equivalent to performing an interruptible operation (see -- #interruptible#), but does not involve any actual blocking. -- -- When called outside 'mask', or inside 'uninterruptibleMask', this -- function has no effect. -- -- /Since: 4.4.0.0/ allowInterrupt :: IO () allowInterrupt = unsafeUnmask $ return () {- $async #AsynchronousExceptions# Asynchronous exceptions are so-called because they arise due to external influences, and can be raised at any point during execution. 'StackOverflow' and 'HeapOverflow' are two examples of system-generated asynchronous exceptions. The primary source of asynchronous exceptions, however, is 'throwTo': > throwTo :: ThreadId -> Exception -> IO () 'throwTo' (also 'Control.Concurrent.killThread') allows one running thread to raise an arbitrary exception in another thread. The exception is therefore asynchronous with respect to the target thread, which could be doing anything at the time it receives the exception. Great care should be taken with asynchronous exceptions; it is all too easy to introduce race conditions by the over zealous use of 'throwTo'. -} {- $block_handler There\'s an implied 'mask' around every exception handler in a call to one of the 'catch' family of functions. This is because that is what you want most of the time - it eliminates a common race condition in starting an exception handler, because there may be no exception handler on the stack to handle another exception if one arrives immediately. If asynchronous exceptions are masked on entering the handler, though, we have time to install a new exception handler before being interrupted. If this weren\'t the default, one would have to write something like > mask $ \restore -> > catch (restore (...)) > (\e -> handler) If you need to unblock asynchronous exceptions again in the exception handler, 'restore' can be used there too. Note that 'try' and friends /do not/ have a similar default, because there is no exception handler in this case. Don't use 'try' for recovering from an asynchronous exception. -} {- $interruptible #interruptible# Some operations are /interruptible/, which means that they can receive asynchronous exceptions even in the scope of a 'mask'. Any function which may itself block is defined as interruptible; this includes 'Control.Concurrent.MVar.takeMVar' (but not 'Control.Concurrent.MVar.tryTakeMVar'), and most operations which perform some I\/O with the outside world. The reason for having interruptible operations is so that we can write things like > mask $ \restore -> do > a <- takeMVar m > catch (restore (...)) > (\e -> ...) if the 'Control.Concurrent.MVar.takeMVar' was not interruptible, then this particular combination could lead to deadlock, because the thread itself would be blocked in a state where it can\'t receive any asynchronous exceptions. With 'Control.Concurrent.MVar.takeMVar' interruptible, however, we can be safe in the knowledge that the thread can receive exceptions right up until the point when the 'Control.Concurrent.MVar.takeMVar' succeeds. Similar arguments apply for other interruptible operations like 'System.IO.openFile'. It is useful to think of 'mask' not as a way to completely prevent asynchronous exceptions, but as a way to switch from asynchronous mode to polling mode. The main difficulty with asynchronous exceptions is that they normally can occur anywhere, but within a 'mask' an asynchronous exception is only raised by operations that are interruptible (or call other interruptible operations). In many cases these operations may themselves raise exceptions, such as I\/O errors, so the caller will usually be prepared to handle exceptions arising from the operation anyway. To perfom an explicit poll for asynchronous exceptions inside 'mask', use 'allowInterrupt'. Sometimes it is too onerous to handle exceptions in the middle of a critical piece of stateful code. There are three ways to handle this kind of situation: * Use STM. Since a transaction is always either completely executed or not at all, transactions are a good way to maintain invariants over state in the presence of asynchronous (and indeed synchronous) exceptions. * Use 'mask', and avoid interruptible operations. In order to do this, we have to know which operations are interruptible. It is impossible to know for any given library function whether it might invoke an interruptible operation internally; so instead we give a list of guaranteed-not-to-be-interruptible operations below. * Use 'uninterruptibleMask'. This is generally not recommended, unless you can guarantee that any interruptible operations invoked during the scope of 'uninterruptibleMask' can only ever block for a short time. Otherwise, 'uninterruptibleMask' is a good way to make your program deadlock and be unresponsive to user interrupts. The following operations are guaranteed not to be interruptible: * operations on 'IORef' from "Data.IORef" * STM transactions that do not use 'retry' * everything from the @Foreign@ modules * everything from @Control.Exception@ except for 'throwTo' * @tryTakeMVar@, @tryPutMVar@, @isEmptyMVar@ * @takeMVar@ if the @MVar@ is definitely full, and conversely @putMVar@ if the @MVar@ is definitely empty * @newEmptyMVar@, @newMVar@ * @forkIO@, @forkIOUnmasked@, @myThreadId@ -} {- $catchall It is possible to catch all exceptions, by using the type 'SomeException': > catch f (\e -> ... (e :: SomeException) ...) HOWEVER, this is normally not what you want to do! For example, suppose you want to read a file, but if it doesn't exist then continue as if it contained \"\". You might be tempted to just catch all exceptions and return \"\" in the handler. However, this has all sorts of undesirable consequences. For example, if the user presses control-C at just the right moment then the 'UserInterrupt' exception will be caught, and the program will continue running under the belief that the file contains \"\". Similarly, if another thread tries to kill the thread reading the file then the 'ThreadKilled' exception will be ignored. Instead, you should only catch exactly the exceptions that you really want. In this case, this would likely be more specific than even \"any IO exception\"; a permissions error would likely also want to be handled differently. Instead, you would probably want something like: > e <- tryJust (guard . isDoesNotExistError) (readFile f) > let str = either (const "") id e There are occassions when you really do need to catch any sort of exception. However, in most cases this is just so you can do some cleaning up; you aren't actually interested in the exception itself. For example, if you open a file then you want to close it again, whether processing the file executes normally or throws an exception. However, in these cases you can use functions like 'bracket', 'finally' and 'onException', which never actually pass you the exception, but just call the cleanup functions at the appropriate points. But sometimes you really do need to catch any exception, and actually see what the exception is. One example is at the very top-level of a program, you may wish to catch any exception, print it to a logfile or the screen, and then exit gracefully. For these cases, you can use 'catch' (or one of the other exception-catching functions) with the 'SomeException' type. -}