------------------------- -- | -- Module : Control.Exception -- Copyright : (c) The University of Glasgow 2001 -- License : BSD-style (see the file LICENSE) -- -- Maintainer : libraries@haskell.org -- Stability : experimental -- Portability : non-portable (uses ExistentialQuantification and DeriveDataTypeable) -- -- This module provides the extensible exceptions API for raising and catching both -- built-in and user-defined exceptions. -- -- For newer versions of GHC (>=6.9), this package re-exports 'Control.Exception'. -- Otherwise, it provides a compatibility layer around the previous version of the -- extensions API. #ifdef USE_NEW_EXCEPTIONS module Control.Exception.Extensible (module Control.Exception) where import Control.Exception hiding (blocked) #else module Control.Exception.Extensible ( -- * The Exception type SomeException(..), Exception(..), E.IOException, E.ArithException(..), E.ArrayException(..), AssertionFailed(..), E.AsyncException(..), NonTermination(..), NestedAtomically(..), ExitCode(..), BlockedOnDeadMVar(..), BlockedIndefinitely(..), Deadlock(..), NoMethodError(..), PatternMatchFail(..), RecConError(..), RecSelError(..), RecUpdError(..), ErrorCall(..), -- * Throwing exceptions throwIO, throw, ioError, throwTo, -- * Catching Exceptions -- |There are several functions for catching and examining -- exceptions; all of them may only be used from within the -- 'IO' monad. -- ** The @catch@ functions catch, -- :: IO a -> (Exception -> IO a) -> IO a catches, Handler(..), catchJust, -- :: (Exception -> Maybe b) -> IO a -> (b -> IO a) -> IO a -- ** The @handle@ functions handle, -- :: (Exception -> IO a) -> IO a -> IO a handleJust,-- :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a -- ** The @try@ functions try, -- :: IO a -> IO (Either Exception a) tryJust, -- :: (Exception -> Maybe b) -> a -> IO (Either b a) onException, -- ** The @evaluate@ function E.evaluate, -- :: a -> IO a -- ** The @mapException@ function mapException, -- :: (Exception -> Exception) -> a -> a -- * Asynchronous Exceptions -- $async -- ** Asynchronous exception control -- |The following two functions allow a thread to control delivery of -- asynchronous exceptions during a critical region. E.block, E.unblock, -- *** Applying @block@ to an exception handler -- $block_handler -- *** Interruptible operations -- $interruptible -- * Assertions assert, -- * Utilities bracket, bracket_, bracketOnError, finally ) where import Prelude hiding (catch) import Control.Concurrent hiding (throwTo) import qualified Control.Exception as E import Data.Dynamic import Data.Typeable import System.Exit import System.IO.Unsafe(unsafePerformIO) class (Typeable e, Show e) => Exception e where toException :: e -> SomeException fromException :: SomeException -> Maybe e toException = SomeException fromException (SomeException e) = cast e data SomeException = forall e . Exception e => SomeException e deriving Typeable instance Show SomeException where showsPrec p (SomeException e) = showsPrec p e instance Exception SomeException where toException se = se fromException = Just mkOldException :: Exception e => e -> E.Exception mkOldException e = let e' = toException e in case fromException e' of Just e'' -> -- If the exception is actually a legacy exception -- then throw it directly so the legacy functions -- catch it as they expect e'' Nothing -> -- Otherwise, throw it as a dynamic E.DynException (toDyn e') throw :: Exception e => e -> a throw e = E.throw (mkOldException e) throwIO :: Exception e => e -> IO a throwIO e = E.throwIO (mkOldException e) throwTo :: Exception e => ThreadId -> e -> IO () throwTo tid e = E.throwTo tid (mkOldException e) ----------------------------------------------------------------------------- -- 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 :: Exception e => IO a -- ^ The computation to run -> (e -> IO a) -- ^ Handler to invoke if an exception is raised -> IO a catch io poly_handler = io `E.catch` handler' where -- First look for "new style" exceptions, which are thrown -- as E.DynException (SomeException e) -- needs scoped TVs: handler' :: E.Exception -> IO a handler' e = case e of E.DynException dyn -> case fromDynamic dyn of Just se@(SomeException _) -> case fromException se of Just e' -> poly_handler e' Nothing -> E.throw e Nothing -> try_old e _ -> try_old e -- If it's a legacy exception (E.Exception or one of the -- types that make up E.Exception), check for a handler than -- can handle them: -- needs scoped TVs: try_old :: E.Exception -> IO a try_old e = case fromException (toException e) of Just e' -> poly_handler e' Nothing -> E.throw e -- | 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 = E.block (do a <- before r <- E.unblock (thing a) `onException` after a after a return r ) onException :: IO a -> IO b -> IO a onException io what = io `catch` \e -> do what throw (e :: SomeException) block, unblock :: IO a -> IO a block = E.block unblock = E.unblock -- | 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 = E.block (do r <- E.unblock a `onException` sequel 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 unblock (thing a) `onException` after a ) assert :: Bool -> a -> a assert True x = x assert False _ = throw (AssertionFailed "") -- | 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. -- -- > 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 e => (e -> 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 e => (e -> IO a) -> IO a -> IO a handle = flip catch -- | A version of 'catchJust' with the arguments swapped around (see -- 'handle'). handleJust :: Exception e => (e -> 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 e1, Exception e2) => (e1 -> e2) -> a -> a mapException f v = unsafePerformIO (catch (E.evaluate v) (\x -> throw (f x))) ----------------------------------------------------------------------------- -- 'try' and variations. -- | 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 :: Exception e => IO a -> IO (Either e 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 e => (e -> 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) ------------- data Handler a = forall e . Exception e => Handler (e -> IO a) 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 -- ----------------------------------------------------------------------------- -- Asynchronous exceptions {- $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 'throwDynTo' and '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 'block' 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 blocked 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 > block ( > catch (unblock (...)) > (\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'. -} {- $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 '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 > block ( > a <- takeMVar m > catch (unblock (...)) > (\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'. -} ---------------------------------------------------------------------- -- Exception instance for the legacy Exception type instance Exception E.Exception ---------------------------------------------------------------------- -- The new Exception types. These need to map to/from E.Exception so -- that uses of legacy catch/throw functions work. ---- instance Exception E.ArithException where toException ae = toException (E.ArithException ae) fromException (SomeException e) = case cast e of Just (E.ArithException ae) -> Just ae _ -> Nothing ---- instance Exception E.ArrayException where toException ae = toException (E.ArrayException ae) fromException (SomeException e) = case cast e of Just (E.ArrayException ae) -> Just ae _ -> Nothing ---- data AssertionFailed = AssertionFailed String deriving Typeable instance Exception AssertionFailed where toException (AssertionFailed str) = toException (E.AssertionFailed str) fromException (SomeException e) = case cast e of Just (E.AssertionFailed str) -> Just (AssertionFailed str) _ -> Nothing instance Show AssertionFailed where showsPrec _ (AssertionFailed err) = showString err ----- instance Exception E.AsyncException where toException ae = toException (E.AsyncException ae) fromException (SomeException e) = case cast e of Just (E.AsyncException ae) -> Just ae _ -> Nothing ---- data BlockedOnDeadMVar = BlockedOnDeadMVar deriving Typeable instance Exception BlockedOnDeadMVar where toException BlockedOnDeadMVar = toException (E.BlockedOnDeadMVar) fromException (SomeException e) = case cast e of Just E.BlockedOnDeadMVar -> Just BlockedOnDeadMVar _ -> Nothing instance Show BlockedOnDeadMVar where showsPrec n BlockedOnDeadMVar = showsPrec n E.BlockedOnDeadMVar ---- data BlockedIndefinitely = BlockedIndefinitely deriving Typeable instance Exception BlockedIndefinitely where toException BlockedIndefinitely = toException E.BlockedIndefinitely fromException (SomeException e) = case cast e of Just E.BlockedIndefinitely -> Just BlockedIndefinitely _ -> Nothing instance Show BlockedIndefinitely where showsPrec n BlockedIndefinitely = showsPrec n E.BlockedIndefinitely ---- data NestedAtomically = NestedAtomically deriving Typeable instance Exception NestedAtomically where toException NestedAtomically = toException E.NestedAtomically fromException (SomeException e) = case cast e of Just E.NestedAtomically -> Just NestedAtomically _ -> Nothing instance Show NestedAtomically where showsPrec n NestedAtomically = showsPrec n E.NestedAtomically ---- data Deadlock = Deadlock deriving Typeable instance Exception Deadlock where toException Deadlock = toException E.Deadlock fromException (SomeException e) = case cast e of Just E.Deadlock -> Just Deadlock _ -> Nothing instance Show Deadlock where showsPrec n Deadlock = showsPrec n E.Deadlock ----- data ErrorCall = ErrorCall String deriving Typeable instance Exception ErrorCall where toException (ErrorCall str) = toException (E.ErrorCall str) fromException (SomeException e) = case cast e of Just (E.ErrorCall str) -> Just (ErrorCall str) _ -> Nothing instance Show ErrorCall where showsPrec _ (ErrorCall err) = showString err ----- instance Typeable ExitCode where typeOf _ = mkTyConApp (mkTyCon "ExitCode") [] instance Exception ExitCode where toException ee = toException (E.ExitException ee) fromException (SomeException e) = case cast e of Just (E.ExitException ee) -> Just ee _ -> Nothing ----- instance Exception E.IOException where toException ioe = toException (E.IOException ioe) fromException (SomeException e) = case cast e of Just (E.IOException ioe) -> Just ioe _ -> Nothing ---- data NoMethodError = NoMethodError String deriving Typeable instance Exception NoMethodError where toException (NoMethodError str) = toException (E.NoMethodError str) fromException (SomeException e) = case cast e of Just (E.NoMethodError str) -> Just (NoMethodError str) _ -> Nothing instance Show NoMethodError where showsPrec _ (NoMethodError str) = showString str ---- data NonTermination = NonTermination deriving Typeable instance Exception NonTermination where toException NonTermination = toException E.NonTermination fromException (SomeException e) = case cast e of Just E.NonTermination -> Just NonTermination _ -> Nothing instance Show NonTermination where showsPrec n NonTermination = showsPrec n E.NonTermination ---- data PatternMatchFail = PatternMatchFail String deriving Typeable instance Exception PatternMatchFail where toException (PatternMatchFail str) = toException (E.PatternMatchFail str) fromException (SomeException e) = case cast e of Just (E.PatternMatchFail str) -> Just (PatternMatchFail str) _ -> Nothing instance Show PatternMatchFail where showsPrec _ (PatternMatchFail str) = showString str ---- data RecConError = RecConError String deriving Typeable instance Exception RecConError where toException (RecConError str) = toException (E.RecConError str) fromException (SomeException e) = case cast e of Just (E.RecConError str) -> Just (RecConError str) _ -> Nothing instance Show RecConError where showsPrec _ (RecConError str) = showString str ---- data RecSelError = RecSelError String deriving Typeable instance Exception RecSelError where toException (RecSelError str) = toException (E.RecSelError str) fromException (SomeException e) = case cast e of Just (E.RecSelError str) -> Just (RecSelError str) _ -> Nothing instance Show RecSelError where showsPrec _ (RecSelError str) = showString str ---- data RecUpdError = RecUpdError String deriving Typeable instance Exception RecUpdError where toException (RecUpdError str) = toException (E.RecUpdError str) fromException (SomeException e) = case cast e of Just (E.RecUpdError str) -> Just (RecUpdError str) _ -> Nothing instance Show RecUpdError where showsPrec _ (RecUpdError str) = showString str #endif