-- 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)
-- * /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 (
#else
SomeException(..),
#endif
- Exception(..), -- instance Eq, Ord, Show, Typeable
- IOException, -- instance Eq, Ord, Show, Typeable
- ArithException(..), -- instance Eq, Ord, Show, Typeable
- ArrayException(..), -- instance Eq, Ord, Show, Typeable
+ Exception(..), -- class
+ IOException, -- instance Eq, Ord, Show, Typeable, Exception
+ ArithException(..), -- instance Eq, Ord, Show, Typeable, Exception
+ ArrayException(..), -- instance Eq, Ord, Show, Typeable, Exception
AssertionFailed(..),
- AsyncException(..), -- instance Eq, Ord, Show, Typeable
+ AsyncException(..), -- instance Eq, Ord, Show, Typeable, Exception
#if __GLASGOW_HASKELL__ || __HUGS__
NonTermination(..),
NestedAtomically(..),
#endif
+#ifdef __NHC__
+ System.ExitCode(), -- instance Exception
+#endif
- BlockedOnDeadMVar(..),
- BlockedIndefinitely(..),
+ BlockedIndefinitelyOnMVar(..),
+ BlockedIndefinitelyOnSTM(..),
Deadlock(..),
NoMethodError(..),
PatternMatchFail(..),
ErrorCall(..),
-- * Throwing exceptions
- throwIO, -- :: Exception -> IO a
- throw, -- :: Exception -> a
- ioError, -- :: IOError -> IO a
+ throw,
+ throwIO,
+ ioError,
#ifdef __GLASGOW_HASKELL__
- throwTo, -- :: ThreadId -> Exception -> a
+ throwTo,
#endif
-- * Catching Exceptions
- -- |There are several functions for catching and examining
- -- exceptions; all of them may only be used from within the
- -- 'IO' monad.
+ -- $catching
+
+ -- ** Catching all exceptions
+
+ -- $catchall
-- ** The @catch@ functions
- catch, -- :: IO a -> (Exception -> IO a) -> IO a
-#if __GLASGOW_HASKELL__ || __HUGS__
+ catch,
catches, Handler(..),
-#endif
- catchJust, -- :: (Exception -> Maybe b) -> IO a -> (b -> IO a) -> IO a
+ catchJust,
-- ** The @handle@ functions
- handle, -- :: (Exception -> IO a) -> IO a -> IO a
- handleJust,-- :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a
+ handle,
+ handleJust,
-- ** The @try@ functions
- try, -- :: IO a -> IO (Either Exception a)
- tryJust, -- :: (Exception -> Maybe b) -> a -> IO (Either b a)
- onException,
+ try,
+ tryJust,
-- ** The @evaluate@ function
- evaluate, -- :: a -> IO a
+ evaluate,
-- ** The @mapException@ function
- mapException, -- :: (Exception -> Exception) -> a -> a
+ mapException,
-- * Asynchronous Exceptions
-- ** Asynchronous exception control
- -- |The following two functions allow a thread to control delivery of
+ -- |The following functions allow a thread to control delivery of
-- asynchronous exceptions during a critical region.
- block, -- :: IO a -> IO a
- unblock, -- :: IO a -> IO a
- blocked, -- :: IO Bool
+ mask,
+#ifndef __NHC__
+ mask_,
+ uninterruptibleMask,
+ uninterruptibleMask_,
+ MaskingState(..),
+ getMaskingState,
+#endif
+
+ -- ** (deprecated) Asynchronous exception control
+
+ block,
+ unblock,
+ blocked,
-- *** Applying @block@ to an exception handler
-- * Assertions
- assert, -- :: Bool -> a -> a
+ assert,
-- * Utilities
- bracket, -- :: IO a -> (a -> IO b) -> (a -> IO c) -> IO ()
- bracket_, -- :: IO a -> IO b -> IO c -> IO ()
+ bracket,
+ bracket_,
bracketOnError,
- finally, -- :: IO a -> IO b -> IO a
+ finally,
+ onException,
+
) where
import Control.Exception.Base
#ifdef __GLASGOW_HASKELL__
import GHC.Base
-import GHC.IOBase
import Data.Maybe
+#else
+import Prelude hiding (catch)
#endif
-#if __GLASGOW_HASKELL__ || __HUGS__
+#ifdef __NHC__
+import System (ExitCode())
+#endif
+
+-- | You need this when using 'catches'.
data Handler a = forall e . Exception e => Handler (e -> IO a)
+{- |
+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
= case fromException e of
Just e' -> handler e'
Nothing -> res
-#endif
+
+-- -----------------------------------------------------------------------------
+-- 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
-}
{- $block_handler
-There\'s an implied 'block' around every exception handler in a call
+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
before being interrupted. If this weren\'t the default, one would have
to write something like
-> block (
-> catch (unblock (...))
-> (\e -> handler)
-> )
+> block $ \restore ->
+> catch (restore (...))
+> (\e -> handler)
If you need to unblock asynchronous exceptions again in the exception
handler, just use 'unblock' as normal.
Note that 'try' and friends /do not/ have a similar default, because
-there is no exception handler in this case. If you want to use 'try'
-in an asynchronous-exception-safe way, you will need to use
-'block'.
+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 'block'. Any function
which may itself block is defined as interruptible; this includes
some I\/O with the outside world. The reason for having
interruptible operations is so that we can write things like
-> block (
+> mask $ \restore -> do
> a <- takeMVar m
-> catch (unblock (...))
+> catch (restore (...))
> (\e -> ...)
-> )
if the 'Control.Concurrent.MVar.takeMVar' was not interruptible,
then this particular
Similar arguments apply for other interruptible operations like
'System.IO.openFile'.
-}
+
+{- $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.
+-}
+
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