+{-# OPTIONS_GHC -XNoImplicitPrelude #-}
+
-----------------------------------------------------------------------------
-- |
-- 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 (
- -- * The Exception type
- Exception(..), -- instance Eq, Ord, Show, Typeable
- IOException, -- instance Eq, Ord, Show, Typeable
- ArithException(..), -- instance Eq, Ord, Show, Typeable
- ArrayException(..), -- instance Eq, Ord, Show, Typeable
- AsyncException(..), -- instance Eq, Ord, Show, Typeable
-
- -- * Throwing exceptions
- throwIO, -- :: Exception -> IO a
- throw, -- :: Exception -> a
- ioError, -- :: IOError -> IO a
+ -- * The Exception type
+#ifdef __HUGS__
+ SomeException,
+#else
+ SomeException(..),
+#endif
+ 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, Exception
+
+#if __GLASGOW_HASKELL__ || __HUGS__
+ NonTermination(..),
+ NestedAtomically(..),
+#endif
+#ifdef __NHC__
+ System.ExitCode(), -- instance Exception
+#endif
+
+ BlockedOnDeadMVar(..),
+ BlockedIndefinitely(..),
+ Deadlock(..),
+ NoMethodError(..),
+ PatternMatchFail(..),
+ RecConError(..),
+ RecSelError(..),
+ RecUpdError(..),
+ ErrorCall(..),
+
+ -- * Throwing exceptions
+ throw,
+ throwIO,
+ ioError,
#ifdef __GLASGOW_HASKELL__
- throwTo, -- :: ThreadId -> Exception -> a
+ throwTo,
#endif
- -- * Catching Exceptions
+ -- * Catching Exceptions
- -- |There are several functions for catching and examining
- -- exceptions; all of them may only be used from within the
- -- 'IO' monad.
+ -- $catching
- -- ** The @catch@ functions
- catch, -- :: IO a -> (Exception -> IO a) -> IO a
- catchJust, -- :: (Exception -> Maybe b) -> IO a -> (b -> IO a) -> IO a
+ -- ** Catching all exceptions
- -- ** The @handle@ functions
- handle, -- :: (Exception -> IO a) -> IO a -> IO a
- handleJust,-- :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a
+ -- $catchall
- -- ** The @try@ functions
- try, -- :: IO a -> IO (Either Exception a)
- tryJust, -- :: (Exception -> Maybe b) -> a -> IO (Either b a)
+ -- ** The @catch@ functions
+ catch,
+ catches, Handler(..),
+ catchJust,
- -- ** The @evaluate@ function
- evaluate, -- :: a -> IO a
+ -- ** The @handle@ functions
+ handle,
+ handleJust,
- -- ** The @mapException@ function
- mapException, -- :: (Exception -> Exception) -> a -> a
+ -- ** The @try@ functions
+ try,
+ tryJust,
- -- ** Exception predicates
-
- -- $preds
+ -- ** The @evaluate@ function
+ evaluate,
- ioErrors, -- :: Exception -> Maybe IOError
- arithExceptions, -- :: Exception -> Maybe ArithException
- errorCalls, -- :: Exception -> Maybe String
- dynExceptions, -- :: Exception -> Maybe Dynamic
- assertions, -- :: Exception -> Maybe String
- asyncExceptions, -- :: Exception -> Maybe AsyncException
- userErrors, -- :: Exception -> Maybe String
+ -- ** The @mapException@ function
+ mapException,
- -- * Dynamic exceptions
+ -- * Asynchronous Exceptions
- -- $dynamic
- throwDyn, -- :: Typeable ex => ex -> b
-#ifdef __GLASGOW_HASKELL__
- throwDynTo, -- :: Typeable ex => ThreadId -> ex -> b
-#endif
- catchDyn, -- :: Typeable ex => IO a -> (ex -> IO a) -> IO a
-
- -- * Asynchronous Exceptions
+ -- $async
- -- $async
+ -- ** Asynchronous exception control
- -- ** Asynchronous exception control
+ -- |The following two functions allow a thread to control delivery of
+ -- asynchronous exceptions during a critical region.
- -- |The following two functions allow a thread to control delivery of
- -- asynchronous exceptions during a critical region.
+ block,
+ unblock,
+ blocked,
- block, -- :: IO a -> IO a
- unblock, -- :: IO a -> IO a
+ -- *** Applying @block@ to an exception handler
- -- *** Applying @block@ to an exception handler
+ -- $block_handler
- -- $block_handler
+ -- *** Interruptible operations
- -- *** Interruptible operations
+ -- $interruptible
- -- $interruptible
+ -- * Assertions
- -- * Assertions
+ assert,
- assert, -- :: Bool -> a -> a
+ -- * Utilities
- -- * Utilities
+ bracket,
+ bracket_,
+ bracketOnError,
- bracket, -- :: IO a -> (a -> IO b) -> (a -> IO c) -> IO ()
- bracket_, -- :: IO a -> IO b -> IO c -> IO ()
- bracketOnError,
+ finally,
+ onException,
- finally, -- :: IO a -> IO b -> IO a
-
-#ifdef __GLASGOW_HASKELL__
- setUncaughtExceptionHandler, -- :: (Exception -> IO ()) -> IO ()
- getUncaughtExceptionHandler -- :: IO (Exception -> IO ())
-#endif
) where
+import Control.Exception.Base
+
#ifdef __GLASGOW_HASKELL__
-import GHC.Base ( assert )
-import GHC.Exception as ExceptionBase hiding (catch)
-import GHC.Conc ( throwTo, ThreadId )
-import Data.IORef ( IORef, newIORef, readIORef, writeIORef )
-import Foreign.C.String ( CString, withCString )
-import System.IO ( stdout, hFlush )
+import GHC.Base
+-- import GHC.IO hiding ( onException, finally )
+import Data.Maybe
+#else
+import Prelude hiding (catch)
#endif
-#ifdef __HUGS__
-import Hugs.Exception as ExceptionBase
+#ifdef __NHC__
+import System (ExitCode())
#endif
-import Prelude hiding ( catch )
-import System.IO.Error hiding ( catch, try )
-import System.IO.Unsafe (unsafePerformIO)
-import Data.Dynamic
-
------------------------------------------------------------------------------
--- Catching exceptions
-
--- |This is the simplest of the exception-catching functions. It
--- takes a single argument, runs it, and if an exception is raised
--- the \"handler\" is executed, with the value of the exception passed as an
--- argument. Otherwise, the result is returned as normal. For example:
---
--- > catch (openFile f ReadMode)
--- > (\e -> hPutStr stderr ("Couldn't open "++f++": " ++ show e))
---
--- For catching exceptions in pure (non-'IO') expressions, see the
--- function 'evaluate'.
---
--- Note that due to Haskell\'s unspecified evaluation order, an
--- expression may return one of several possible exceptions: consider
--- the expression @error \"urk\" + 1 \`div\` 0@. Does
--- 'catch' execute the handler passing
--- @ErrorCall \"urk\"@, or @ArithError DivideByZero@?
---
--- The answer is \"either\": 'catch' makes a
--- non-deterministic choice about which exception to catch. If you
--- call it again, you might get a different exception back. This is
--- ok, because 'catch' is an 'IO' computation.
---
--- Note that 'catch' catches all types of exceptions, and is generally
--- used for \"cleaning up\" before passing on the exception using
--- 'throwIO'. It is not good practice to discard the exception and
--- continue, without first checking the type of the exception (it
--- might be a 'ThreadKilled', for example). In this case it is usually better
--- to use 'catchJust' and select the kinds of exceptions to catch.
---
--- Also note that the "Prelude" also exports a function called
--- 'Prelude.catch' with a similar type to 'Control.Exception.catch',
--- except that the "Prelude" version only catches the IO and user
--- families of exceptions (as required by Haskell 98).
---
--- We recommend either hiding the "Prelude" version of 'Prelude.catch'
--- when importing "Control.Exception":
---
--- > import Prelude hiding (catch)
---
--- or importing "Control.Exception" qualified, to avoid name-clashes:
---
--- > import qualified Control.Exception as C
---
--- and then using @C.catch@
---
-
-catch :: IO a -- ^ The computation to run
- -> (Exception -> IO a) -- ^ Handler to invoke if an exception is raised
- -> IO a
-catch = ExceptionBase.catchException
-
--- | The function 'catchJust' is like 'catch', but it takes an extra
--- argument which is an /exception predicate/, a function which
--- selects which type of exceptions we\'re interested in. There are
--- some predefined exception predicates for useful subsets of
--- exceptions: 'ioErrors', 'arithExceptions', and so on. For example,
--- to catch just calls to the 'error' function, we could use
---
--- > result <- catchJust errorCalls thing_to_try handler
---
--- Any other exceptions which are not matched by the predicate
--- are re-raised, and may be caught by an enclosing
--- 'catch' or 'catchJust'.
-catchJust
- :: (Exception -> Maybe b) -- ^ Predicate to select exceptions
- -> IO a -- ^ Computation to run
- -> (b -> IO a) -- ^ Handler
- -> IO a
-catchJust p a handler = catch a handler'
- where handler' e = case p e of
- Nothing -> throw e
- Just b -> handler b
-
--- | A version of 'catch' with the arguments swapped around; useful in
--- situations where the code for the handler is shorter. For example:
---
--- > do handle (\e -> exitWith (ExitFailure 1)) $
--- > ...
-handle :: (Exception -> IO a) -> IO a -> IO a
-handle = flip catch
-
--- | A version of 'catchJust' with the arguments swapped around (see
--- 'handle').
-handleJust :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a
-handleJust p = flip (catchJust p)
-
------------------------------------------------------------------------------
--- 'mapException'
-
--- | This function maps one exception into another as proposed in the
--- paper \"A semantics for imprecise exceptions\".
-
--- Notice that the usage of 'unsafePerformIO' is safe here.
-
-mapException :: (Exception -> Exception) -> a -> a
-mapException f v = unsafePerformIO (catch (evaluate v)
- (\x -> throw (f x)))
+-- | You need this when using 'catches'.
+data Handler a = forall e . Exception e => Handler (e -> IO a)
------------------------------------------------------------------------------
--- 'try' and variations.
+{- |
+Sometimes you want to catch two different sorts of exception. You could
+do something like
--- | Similar to 'catch', but returns an 'Either' result which is
--- @('Right' a)@ if no exception was raised, or @('Left' e)@ if an
--- exception was raised and its value is @e@.
---
--- > try a = catch (Right `liftM` a) (return . Left)
---
--- Note: as with 'catch', it is only polite to use this variant if you intend
--- to re-throw the exception after performing whatever cleanup is needed.
--- Otherwise, 'tryJust' is generally considered to be better.
---
--- Also note that "System.IO.Error" also exports a function called
--- 'System.IO.Error.try' with a similar type to 'Control.Exception.try',
--- except that it catches only the IO and user families of exceptions
--- (as required by the Haskell 98 @IO@ module).
-
-try :: IO a -> IO (Either Exception a)
-try a = catch (a >>= \ v -> return (Right v)) (\e -> return (Left e))
-
--- | A variant of 'try' that takes an exception predicate to select
--- which exceptions are caught (c.f. 'catchJust'). If the exception
--- does not match the predicate, it is re-thrown.
-tryJust :: (Exception -> Maybe b) -> IO a -> IO (Either b a)
-tryJust p a = do
- r <- try a
- case r of
- Right v -> return (Right v)
- Left e -> case p e of
- Nothing -> throw e
- Just b -> return (Left b)
+> f = expr `catch` \ (ex :: ArithException) -> handleArith ex
+> `catch` \ (ex :: IOException) -> handleIO ex
------------------------------------------------------------------------------
--- Dynamic exceptions
+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.
--- $dynamic
--- #DynamicExceptions# Because the 'Exception' datatype is not extensible, there is an
--- interface for throwing and catching exceptions of type 'Dynamic'
--- (see "Data.Dynamic") which allows exception values of any type in
--- the 'Typeable' class to be thrown and caught.
+Instead, we provide a function 'catches', which would be used thus:
--- | Raise any value as an exception, provided it is in the
--- 'Typeable' class.
-throwDyn :: Typeable exception => exception -> b
-throwDyn exception = throw (DynException (toDyn exception))
+> 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
-#ifdef __GLASGOW_HASKELL__
--- | A variant of 'throwDyn' that throws the dynamic exception to an
--- arbitrary thread (GHC only: c.f. 'throwTo').
-throwDynTo :: Typeable exception => ThreadId -> exception -> IO ()
-throwDynTo t exception = throwTo t (DynException (toDyn exception))
-#endif /* __GLASGOW_HASKELL__ */
-
--- | Catch dynamic exceptions of the required type. All other
--- exceptions are re-thrown, including dynamic exceptions of the wrong
--- type.
---
--- When using dynamic exceptions it is advisable to define a new
--- datatype to use for your exception type, to avoid possible clashes
--- with dynamic exceptions used in other libraries.
---
-catchDyn :: Typeable exception => IO a -> (exception -> IO a) -> IO a
-catchDyn m k = catchException m handle
- where handle ex = case ex of
- (DynException dyn) ->
- case fromDynamic dyn of
- Just exception -> k exception
- Nothing -> throw ex
- _ -> throw ex
+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
------------------------------------------------------------------------------
--- Exception Predicates
+-- -----------------------------------------------------------------------------
+-- Catching exceptions
--- $preds
--- These pre-defined predicates may be used as the first argument to
--- 'catchJust', 'tryJust', or 'handleJust' to select certain common
--- classes of exceptions.
+{- $catching
-ioErrors :: Exception -> Maybe IOError
-arithExceptions :: Exception -> Maybe ArithException
-errorCalls :: Exception -> Maybe String
-assertions :: Exception -> Maybe String
-dynExceptions :: Exception -> Maybe Dynamic
-asyncExceptions :: Exception -> Maybe AsyncException
-userErrors :: Exception -> Maybe String
+There are several functions for catching and examining
+exceptions; all of them may only be used from within the
+'IO' monad.
-ioErrors (IOException e) = Just e
-ioErrors _ = Nothing
+Here's a rule of thumb for deciding which catch-style function to
+use:
-arithExceptions (ArithException e) = Just e
-arithExceptions _ = Nothing
+ * If you want to do some cleanup in the event that an exception
+ is raised, use 'finally', 'bracket' or 'onException'.
-errorCalls (ErrorCall e) = Just e
-errorCalls _ = Nothing
+ * To recover after an exception and do something else, the best
+ choice is to use one of the 'try' family.
-assertions (AssertionFailed e) = Just e
-assertions _ = Nothing
+ * ... unless you are recovering from an asynchronous exception, in which
+ case use 'catch' or 'catchJust'.
-dynExceptions (DynException e) = Just e
-dynExceptions _ = Nothing
+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.
-asyncExceptions (AsyncException e) = Just e
-asyncExceptions _ = Nothing
+A typical use of 'tryJust' for recovery looks like this:
-userErrors (IOException e) | isUserError e = Just (ioeGetErrorString e)
-userErrors _ = Nothing
+> do r <- tryJust (guard . isDoesNotExistError) $ getEnv "HOME"
+> case r of
+> Left e -> ...
+> Right home -> ...
------------------------------------------------------------------------------
--- Some Useful Functions
-
--- | When you want to acquire a resource, do some work with it, and
--- then release the resource, it is a good idea to use 'bracket',
--- because 'bracket' will install the necessary exception handler to
--- release the resource in the event that an exception is raised
--- during the computation. If an exception is raised, then 'bracket' will
--- re-raise the exception (after performing the release).
---
--- A common example is opening a file:
---
--- > bracket
--- > (openFile "filename" ReadMode)
--- > (hClose)
--- > (\handle -> do { ... })
---
--- The arguments to 'bracket' are in this order so that we can partially apply
--- it, e.g.:
---
--- > withFile name mode = bracket (openFile name mode) hClose
---
-bracket
- :: IO a -- ^ computation to run first (\"acquire resource\")
- -> (a -> IO b) -- ^ computation to run last (\"release resource\")
- -> (a -> IO c) -- ^ computation to run in-between
- -> IO c -- returns the value from the in-between computation
-bracket before after thing =
- block (do
- a <- before
- r <- catch
- (unblock (thing a))
- (\e -> do { after a; throw e })
- after a
- return r
- )
-
-
--- | A specialised variant of 'bracket' with just a computation to run
--- afterward.
---
-finally :: IO a -- ^ computation to run first
- -> IO b -- ^ computation to run afterward (even if an exception
- -- was raised)
- -> IO a -- returns the value from the first computation
-a `finally` sequel =
- block (do
- r <- catch
- (unblock a)
- (\e -> do { sequel; throw e })
- sequel
- return r
- )
-
--- | A variant of 'bracket' where the return value from the first computation
--- is not required.
-bracket_ :: IO a -> IO b -> IO c -> IO c
-bracket_ before after thing = bracket before (const after) (const thing)
-
--- | Like bracket, but only performs the final action if there was an
--- exception raised by the in-between computation.
-bracketOnError
- :: IO a -- ^ computation to run first (\"acquire resource\")
- -> (a -> IO b) -- ^ computation to run last (\"release resource\")
- -> (a -> IO c) -- ^ computation to run in-between
- -> IO c -- returns the value from the in-between computation
-bracketOnError before after thing =
- block (do
- a <- before
- catch
- (unblock (thing a))
- (\e -> do { after a; throw e })
- )
+-}
-- -----------------------------------------------------------------------------
-- Asynchronous exceptions
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
'System.IO.openFile'.
-}
-#ifndef __GLASGOW_HASKELL__
-assert :: Bool -> a -> a
-assert True x = x
-assert False _ = throw (AssertionFailed "")
-#endif
-
+{- $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.
+-}
-#ifdef __GLASGOW_HASKELL__
-{-# NOINLINE uncaughtExceptionHandler #-}
-uncaughtExceptionHandler :: IORef (Exception -> IO ())
-uncaughtExceptionHandler = unsafePerformIO (newIORef defaultHandler)
- where
- defaultHandler :: Exception -> IO ()
- defaultHandler ex = do
- (hFlush stdout) `catchException` (\ _ -> return ())
- let msg = case ex of
- Deadlock -> "no threads to run: infinite loop or deadlock?"
- ErrorCall s -> s
- other -> showsPrec 0 other "\n"
- withCString "%s" $ \cfmt ->
- withCString msg $ \cmsg ->
- errorBelch cfmt cmsg
-
-foreign import ccall unsafe "RtsMessages.h errorBelch"
- errorBelch :: CString -> CString -> IO ()
-
-setUncaughtExceptionHandler :: (Exception -> IO ()) -> IO ()
-setUncaughtExceptionHandler = writeIORef uncaughtExceptionHandler
-
-getUncaughtExceptionHandler :: IO (Exception -> IO ())
-getUncaughtExceptionHandler = readIORef uncaughtExceptionHandler
-#endif
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