{-# OPTIONS_GHC -XNoImplicitPrelude #-}
-#include "Typeable.h"
-
-----------------------------------------------------------------------------
-- |
-- Module : Control.Exception
module Control.Exception (
-- * The Exception type
+#ifdef __HUGS__
+ SomeException,
+#else
SomeException(..),
+#endif
Exception(..), -- instance Eq, Ord, Show, Typeable
IOException, -- instance Eq, Ord, Show, Typeable
ArithException(..), -- instance Eq, Ord, Show, Typeable
AssertionFailed(..),
AsyncException(..), -- instance Eq, Ord, Show, Typeable
-#ifdef __GLASGOW_HASKELL__
- NonTermination(..), nonTermination,
- NestedAtomically(..), nestedAtomically,
+#if __GLASGOW_HASKELL__ || __HUGS__
+ NonTermination(..),
+ NestedAtomically(..),
#endif
BlockedOnDeadMVar(..),
-- ** The @catch@ functions
catch, -- :: IO a -> (Exception -> IO a) -> IO a
-#ifdef __GLASGOW_HASKELL__
+#if __GLASGOW_HASKELL__ || __HUGS__
catches, Handler(..),
- catchJust, -- :: (Exception -> Maybe b) -> IO a -> (b -> IO a) -> IO a
#endif
+ catchJust, -- :: (Exception -> Maybe b) -> IO a -> (b -> IO a) -> IO a
-- ** The @handle@ functions
handle, -- :: (Exception -> IO a) -> IO a -> IO a
bracketOnError,
finally, -- :: IO a -> IO b -> IO a
-
-#ifdef __GLASGOW_HASKELL__
- recSelError, recConError, irrefutPatError, runtimeError,
- nonExhaustiveGuardsError, patError, noMethodBindingError,
-#endif
) where
+import Control.Exception.Base
+
#ifdef __GLASGOW_HASKELL__
import GHC.Base
import GHC.IOBase
-import GHC.List
-import GHC.Show
-import GHC.IOBase as ExceptionBase
-import GHC.Exception hiding ( Exception )
-import GHC.Conc
-#endif
-
-#ifdef __HUGS__
-import Hugs.Exception as ExceptionBase
-#endif
-
-import Data.Dynamic
-import Data.Either
import Data.Maybe
-
-#ifdef __NHC__
-import qualified System.IO.Error as H'98 (catch)
-import System.IO.Error (ioError)
-import IO (bracket)
-import DIOError -- defn of IOError type
-import System (ExitCode())
-import System.IO.Unsafe (unsafePerformIO)
-import Unsafe.Coerce (unsafeCoerce)
-
--- minimum needed for nhc98 to pretend it has Exceptions
-
-{-
-data Exception = IOException IOException
- | ArithException ArithException
- | ArrayException ArrayException
- | AsyncException AsyncException
- | ExitException ExitCode
- deriving Show
--}
-class ({-Typeable e,-} Show e) => Exception e where
- toException :: e -> SomeException
- fromException :: SomeException -> Maybe e
-
-data SomeException = forall e . Exception e => SomeException e
-
-INSTANCE_TYPEABLE0(SomeException,someExceptionTc,"SomeException")
-
-instance Show SomeException where
- showsPrec p (SomeException e) = showsPrec p e
-instance Exception SomeException where
- toException se = se
- fromException = Just
-
-type IOException = IOError
-instance Exception IOError where
- toException = SomeException
- fromException (SomeException e) = Just (unsafeCoerce e)
-
-instance Exception ExitCode where
- toException = SomeException
- fromException (SomeException e) = Just (unsafeCoerce e)
-
-data ArithException
-data ArrayException
-data AsyncException
-data AssertionFailed
-data PatternMatchFail
-data NoMethodError
-data Deadlock
-data BlockedOnDeadMVar
-data BlockedIndefinitely
-data ErrorCall
-data RecConError
-data RecSelError
-data RecUpdError
-instance Show ArithException
-instance Show ArrayException
-instance Show AsyncException
-instance Show AssertionFailed
-instance Show PatternMatchFail
-instance Show NoMethodError
-instance Show Deadlock
-instance Show BlockedOnDeadMVar
-instance Show BlockedIndefinitely
-instance Show ErrorCall
-instance Show RecConError
-instance Show RecSelError
-instance Show RecUpdError
-
-catch :: Exception e
- => IO a -- ^ The computation to run
- -> (e -> IO a) -- ^ Handler to invoke if an exception is raised
- -> IO a
-catch io h = H'98.catch io (h . fromJust . fromException . toException)
-
-throwIO :: Exception e => e -> IO a
-throwIO = ioError . fromJust . fromException . toException
-
-throw :: Exception e => e -> a
-throw = unsafePerformIO . throwIO
-
-evaluate :: a -> IO a
-evaluate x = x `seq` return x
-
-assert :: Bool -> a -> a
-assert True x = x
-assert False _ = throw (toException (UserError "" "Assertion failed"))
-
#endif
-#ifndef __GLASGOW_HASKELL__
--- Dummy definitions for implementations lacking asynchonous exceptions
-
-block :: IO a -> IO a
-block = id
-unblock :: IO a -> IO a
-unblock = id
-blocked :: IO Bool
-blocked = return False
-#endif
-
------------------------------------------------------------------------------
--- 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@
---
-#ifndef __NHC__
-catch :: Exception e
- => IO a -- ^ The computation to run
- -> (e -> IO a) -- ^ Handler to invoke if an exception is raised
- -> IO a
-catch = ExceptionBase.catchException
-#endif
+#if __GLASGOW_HASKELL__ || __HUGS__
+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
= case fromException e of
Just e' -> handler e'
Nothing -> res
-
-data Handler a = forall e . Exception e => Handler (e -> IO a)
--- | 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 (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)
-
-onException :: IO a -> IO b -> IO a
-onException io what = io `catch` \e -> do what
- throw (e :: SomeException)
-
------------------------------------------------------------------------------
--- 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
---
-#ifndef __NHC__
-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 <- unblock (thing a) `onException` after a
- after a
- return r
- )
#endif
--- | 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 <- 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
- )
-
-- -----------------------------------------------------------------------------
-- Asynchronous exceptions
Similar arguments apply for other interruptible operations like
'System.IO.openFile'.
-}
-
-#if !(__GLASGOW_HASKELL__ || __NHC__)
-assert :: Bool -> a -> a
-assert True x = x
-assert False _ = throw (AssertionFailed "")
-#endif
-
-#ifndef __NHC__
-recSelError, recConError, irrefutPatError, runtimeError,
- nonExhaustiveGuardsError, patError, noMethodBindingError
- :: Addr# -> a -- All take a UTF8-encoded C string
-
-recSelError s = throw (RecSelError (unpackCStringUtf8# s)) -- No location info unfortunately
-runtimeError s = error (unpackCStringUtf8# s) -- No location info unfortunately
-
-nonExhaustiveGuardsError s = throw (PatternMatchFail (untangle s "Non-exhaustive guards in"))
-irrefutPatError s = throw (PatternMatchFail (untangle s "Irrefutable pattern failed for pattern"))
-recConError s = throw (RecConError (untangle s "Missing field in record construction"))
-noMethodBindingError s = throw (NoMethodError (untangle s "No instance nor default method for class operation"))
-patError s = throw (PatternMatchFail (untangle s "Non-exhaustive patterns in"))
-
------
-
-data PatternMatchFail = PatternMatchFail String
-INSTANCE_TYPEABLE0(PatternMatchFail,patternMatchFailTc,"PatternMatchFail")
-
-instance Exception PatternMatchFail
-
-instance Show PatternMatchFail where
- showsPrec _ (PatternMatchFail err) = showString err
-
------
-
-data RecSelError = RecSelError String
-INSTANCE_TYPEABLE0(RecSelError,recSelErrorTc,"RecSelError")
-
-instance Exception RecSelError
-
-instance Show RecSelError where
- showsPrec _ (RecSelError err) = showString err
-
------
-
-data RecConError = RecConError String
-INSTANCE_TYPEABLE0(RecConError,recConErrorTc,"RecConError")
-
-instance Exception RecConError
-
-instance Show RecConError where
- showsPrec _ (RecConError err) = showString err
-
------
-
-data RecUpdError = RecUpdError String
-INSTANCE_TYPEABLE0(RecUpdError,recUpdErrorTc,"RecUpdError")
-
-instance Exception RecUpdError
-
-instance Show RecUpdError where
- showsPrec _ (RecUpdError err) = showString err
-
------
-
-data NoMethodError = NoMethodError String
-INSTANCE_TYPEABLE0(NoMethodError,noMethodErrorTc,"NoMethodError")
-
-instance Exception NoMethodError
-
-instance Show NoMethodError where
- showsPrec _ (NoMethodError err) = showString err
-
------
-
-data NonTermination = NonTermination
-INSTANCE_TYPEABLE0(NonTermination,nonTerminationTc,"NonTermination")
-
-instance Exception NonTermination
-
-instance Show NonTermination where
- showsPrec _ NonTermination = showString "<<loop>>"
-
--- GHC's RTS calls this
-nonTermination :: SomeException
-nonTermination = toException NonTermination
-
------
-
-data NestedAtomically = NestedAtomically
-INSTANCE_TYPEABLE0(NestedAtomically,nestedAtomicallyTc,"NestedAtomically")
-
-instance Exception NestedAtomically
-
-instance Show NestedAtomically where
- showsPrec _ NestedAtomically = showString "Control.Concurrent.STM.atomically was nested"
-
--- GHC's RTS calls this
-nestedAtomically :: SomeException
-nestedAtomically = toException NestedAtomically
-
------
-
-instance Exception Dynamic
-
-#endif
-
--- /dev/null
+{-# OPTIONS_GHC -XNoImplicitPrelude #-}
+
+#include "Typeable.h"
+
+-----------------------------------------------------------------------------
+-- |
+-- Module : Control.Exception.Base
+-- 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)
+--
+-- Extensible exceptions, except for multiple handlers.
+--
+-----------------------------------------------------------------------------
+
+module Control.Exception.Base (
+
+ -- * The Exception type
+#ifdef __HUGS__
+ SomeException,
+#else
+ SomeException(..),
+#endif
+ Exception(..),
+ IOException,
+ ArithException(..),
+ ArrayException(..),
+ AssertionFailed(..),
+ AsyncException(..),
+
+#if __GLASGOW_HASKELL__ || __HUGS__
+ NonTermination(..),
+ NestedAtomically(..),
+#endif
+
+ BlockedOnDeadMVar(..),
+ BlockedIndefinitely(..),
+ Deadlock(..),
+ NoMethodError(..),
+ PatternMatchFail(..),
+ RecConError(..),
+ RecSelError(..),
+ RecUpdError(..),
+ ErrorCall(..),
+
+ -- * Throwing exceptions
+ throwIO,
+ throw,
+ ioError,
+#ifdef __GLASGOW_HASKELL__
+ throwTo,
+#endif
+
+ -- * Catching Exceptions
+
+ -- ** The @catch@ functions
+ catch,
+ catchJust,
+
+ -- ** The @handle@ functions
+ handle,
+ handleJust,
+
+ -- ** The @try@ functions
+ try,
+ tryJust,
+ onException,
+
+ -- ** The @evaluate@ function
+ evaluate,
+
+ -- ** The @mapException@ function
+ mapException,
+
+ -- * Asynchronous Exceptions
+
+ -- ** Asynchronous exception control
+
+ block,
+ unblock,
+ blocked,
+
+ -- * Assertions
+
+ assert,
+
+ -- * Utilities
+
+ bracket,
+ bracket_,
+ bracketOnError,
+
+ finally,
+
+#ifdef __GLASGOW_HASKELL__
+ -- * Calls for GHC runtime
+ recSelError, recConError, irrefutPatError, runtimeError,
+ nonExhaustiveGuardsError, patError, noMethodBindingError,
+ nonTermination, nestedAtomically,
+#endif
+ ) where
+
+#ifdef __GLASGOW_HASKELL__
+import GHC.Base
+import GHC.IOBase
+import GHC.List
+import GHC.Show
+import GHC.IOBase
+import GHC.Exception hiding ( Exception )
+import GHC.Conc
+#endif
+
+#ifdef __HUGS__
+import Prelude hiding (catch)
+import Hugs.Prelude (ExitCode(..))
+import Hugs.IOExts (unsafePerformIO)
+import Hugs.Exception (SomeException(DynamicException, IOException,
+ ArithException, ArrayException, ExitException),
+ evaluate, IOException, ArithException, ArrayException)
+import qualified Hugs.Exception
+#endif
+
+import Data.Dynamic
+import Data.Either
+import Data.Maybe
+
+#ifdef __NHC__
+import qualified System.IO.Error as H'98 (catch)
+import System.IO.Error (ioError)
+import IO (bracket)
+import DIOError -- defn of IOError type
+import System (ExitCode())
+import System.IO.Unsafe (unsafePerformIO)
+import Unsafe.Coerce (unsafeCoerce)
+
+-- minimum needed for nhc98 to pretend it has Exceptions
+
+{-
+data Exception = IOException IOException
+ | ArithException ArithException
+ | ArrayException ArrayException
+ | AsyncException AsyncException
+ | ExitException ExitCode
+ deriving Show
+-}
+class ({-Typeable e,-} Show e) => Exception e where
+ toException :: e -> SomeException
+ fromException :: SomeException -> Maybe e
+
+data SomeException = forall e . Exception e => SomeException e
+
+INSTANCE_TYPEABLE0(SomeException,someExceptionTc,"SomeException")
+
+instance Show SomeException where
+ showsPrec p (SomeException e) = showsPrec p e
+instance Exception SomeException where
+ toException se = se
+ fromException = Just
+
+type IOException = IOError
+instance Exception IOError where
+ toException = SomeException
+ fromException (SomeException e) = Just (unsafeCoerce e)
+
+instance Exception ExitCode where
+ toException = SomeException
+ fromException (SomeException e) = Just (unsafeCoerce e)
+
+data ArithException
+data ArrayException
+data AsyncException
+data AssertionFailed
+data PatternMatchFail
+data NoMethodError
+data Deadlock
+data BlockedOnDeadMVar
+data BlockedIndefinitely
+data ErrorCall
+data RecConError
+data RecSelError
+data RecUpdError
+instance Show ArithException
+instance Show ArrayException
+instance Show AsyncException
+instance Show AssertionFailed
+instance Show PatternMatchFail
+instance Show NoMethodError
+instance Show Deadlock
+instance Show BlockedOnDeadMVar
+instance Show BlockedIndefinitely
+instance Show ErrorCall
+instance Show RecConError
+instance Show RecSelError
+instance Show RecUpdError
+
+catch :: Exception e
+ => IO a -- ^ The computation to run
+ -> (e -> IO a) -- ^ Handler to invoke if an exception is raised
+ -> IO a
+catch io h = H'98.catch io (h . fromJust . fromException . toException)
+
+throwIO :: Exception e => e -> IO a
+throwIO = ioError . fromJust . fromException . toException
+
+throw :: Exception e => e -> a
+throw = unsafePerformIO . throwIO
+
+evaluate :: a -> IO a
+evaluate x = x `seq` return x
+
+assert :: Bool -> a -> a
+assert True x = x
+assert False _ = throw (toException (UserError "" "Assertion failed"))
+
+#endif
+
+#ifdef __HUGS__
+class (Typeable e, Show e) => Exception e where
+ toException :: e -> SomeException
+ fromException :: SomeException -> Maybe e
+
+ toException e = DynamicException (toDyn e) (flip showsPrec e)
+ fromException (DynamicException dyn _) = fromDynamic dyn
+ fromException _ = Nothing
+
+INSTANCE_TYPEABLE0(SomeException,someExceptionTc,"SomeException")
+INSTANCE_TYPEABLE0(IOException,iOExceptionTc,"IOException")
+INSTANCE_TYPEABLE0(ArithException,arithExceptionTc,"ArithException")
+INSTANCE_TYPEABLE0(ArrayException,arrayExceptionTc,"ArrayException")
+INSTANCE_TYPEABLE0(ExitCode,exitCodeTc,"ExitCode")
+INSTANCE_TYPEABLE0(ErrorCall,errorCallTc,"ErrorCall")
+INSTANCE_TYPEABLE0(AssertionFailed,assertionFailedTc,"AssertionFailed")
+INSTANCE_TYPEABLE0(AsyncException,asyncExceptionTc,"AsyncException")
+INSTANCE_TYPEABLE0(BlockedOnDeadMVar,blockedOnDeadMVarTc,"BlockedOnDeadMVar")
+INSTANCE_TYPEABLE0(BlockedIndefinitely,blockedIndefinitelyTc,"BlockedIndefinitely")
+INSTANCE_TYPEABLE0(Deadlock,deadlockTc,"Deadlock")
+
+instance Exception SomeException where
+ toException se = se
+ fromException = Just
+
+instance Exception IOException where
+ toException = IOException
+ fromException (IOException e) = Just e
+ fromException _ = Nothing
+
+instance Exception ArrayException where
+ toException = ArrayException
+ fromException (ArrayException e) = Just e
+ fromException _ = Nothing
+
+instance Exception ArithException where
+ toException = ArithException
+ fromException (ArithException e) = Just e
+ fromException _ = Nothing
+
+instance Exception ExitCode where
+ toException = ExitException
+ fromException (ExitException e) = Just e
+ fromException _ = Nothing
+
+data ErrorCall = ErrorCall String
+
+instance Show ErrorCall where
+ showsPrec _ (ErrorCall err) = showString err
+
+instance Exception ErrorCall where
+ toException (ErrorCall s) = Hugs.Exception.ErrorCall s
+ fromException (Hugs.Exception.ErrorCall s) = Just (ErrorCall s)
+ fromException _ = Nothing
+
+data BlockedOnDeadMVar = BlockedOnDeadMVar
+data BlockedIndefinitely = BlockedIndefinitely
+data Deadlock = Deadlock
+data AssertionFailed = AssertionFailed String
+data AsyncException
+ = StackOverflow
+ | HeapOverflow
+ | ThreadKilled
+ | UserInterrupt
+ deriving (Eq, Ord)
+
+instance Show BlockedOnDeadMVar where
+ showsPrec _ BlockedOnDeadMVar = showString "thread blocked indefinitely"
+
+instance Show BlockedIndefinitely where
+ showsPrec _ BlockedIndefinitely = showString "thread blocked indefinitely"
+
+instance Show Deadlock where
+ showsPrec _ Deadlock = showString "<<deadlock>>"
+
+instance Show AssertionFailed where
+ showsPrec _ (AssertionFailed err) = showString err
+
+instance Show AsyncException where
+ showsPrec _ StackOverflow = showString "stack overflow"
+ showsPrec _ HeapOverflow = showString "heap overflow"
+ showsPrec _ ThreadKilled = showString "thread killed"
+ showsPrec _ UserInterrupt = showString "user interrupt"
+
+instance Exception BlockedOnDeadMVar
+instance Exception BlockedIndefinitely
+instance Exception Deadlock
+instance Exception AssertionFailed
+instance Exception AsyncException
+
+throw :: Exception e => e -> a
+throw e = Hugs.Exception.throw (toException e)
+
+throwIO :: Exception e => e -> IO a
+throwIO e = Hugs.Exception.throwIO (toException e)
+#endif
+
+#ifndef __GLASGOW_HASKELL__
+-- Dummy definitions for implementations lacking asynchonous exceptions
+
+block :: IO a -> IO a
+block = id
+unblock :: IO a -> IO a
+unblock = id
+blocked :: IO Bool
+blocked = return False
+#endif
+
+-----------------------------------------------------------------------------
+-- 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@
+--
+#ifndef __NHC__
+catch :: Exception e
+ => IO a -- ^ The computation to run
+ -> (e -> IO a) -- ^ Handler to invoke if an exception is raised
+ -> IO a
+#if __GLASGOW_HASKELL__
+catch = GHC.IOBase.catchException
+#elif __HUGS__
+catch m h = Hugs.Exception.catchException m h'
+ where h' e = case fromException e of
+ Just e' -> h e'
+ Nothing -> throwIO e
+#endif
+#endif
+
+-- | 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 (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)
+
+onException :: IO a -> IO b -> IO a
+onException io what = io `catch` \e -> do what
+ throw (e :: SomeException)
+
+-----------------------------------------------------------------------------
+-- 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
+--
+#ifndef __NHC__
+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 <- unblock (thing a) `onException` after a
+ after a
+ return r
+ )
+#endif
+
+-- | 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 <- 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
+ )
+
+#if !(__GLASGOW_HASKELL__ || __NHC__)
+assert :: Bool -> a -> a
+assert True x = x
+assert False _ = throw (AssertionFailed "")
+#endif
+
+-----
+
+#if __GLASGOW_HASKELL__ || __HUGS__
+data PatternMatchFail = PatternMatchFail String
+INSTANCE_TYPEABLE0(PatternMatchFail,patternMatchFailTc,"PatternMatchFail")
+
+instance Show PatternMatchFail where
+ showsPrec _ (PatternMatchFail err) = showString err
+
+#ifdef __HUGS__
+instance Exception PatternMatchFail where
+ toException (PatternMatchFail err) = Hugs.Exception.PatternMatchFail err
+ fromException (Hugs.Exception.PatternMatchFail err) = Just (PatternMatchFail err)
+ fromException _ = Nothing
+#else
+instance Exception PatternMatchFail
+#endif
+
+-----
+
+data RecSelError = RecSelError String
+INSTANCE_TYPEABLE0(RecSelError,recSelErrorTc,"RecSelError")
+
+instance Show RecSelError where
+ showsPrec _ (RecSelError err) = showString err
+
+#ifdef __HUGS__
+instance Exception RecSelError where
+ toException (RecSelError err) = Hugs.Exception.RecSelError err
+ fromException (Hugs.Exception.RecSelError err) = Just (RecSelError err)
+ fromException _ = Nothing
+#else
+instance Exception RecSelError
+#endif
+
+-----
+
+data RecConError = RecConError String
+INSTANCE_TYPEABLE0(RecConError,recConErrorTc,"RecConError")
+
+instance Show RecConError where
+ showsPrec _ (RecConError err) = showString err
+
+#ifdef __HUGS__
+instance Exception RecConError where
+ toException (RecConError err) = Hugs.Exception.RecConError err
+ fromException (Hugs.Exception.RecConError err) = Just (RecConError err)
+ fromException _ = Nothing
+#else
+instance Exception RecConError
+#endif
+
+-----
+
+data RecUpdError = RecUpdError String
+INSTANCE_TYPEABLE0(RecUpdError,recUpdErrorTc,"RecUpdError")
+
+instance Show RecUpdError where
+ showsPrec _ (RecUpdError err) = showString err
+
+#ifdef __HUGS__
+instance Exception RecUpdError where
+ toException (RecUpdError err) = Hugs.Exception.RecUpdError err
+ fromException (Hugs.Exception.RecUpdError err) = Just (RecUpdError err)
+ fromException _ = Nothing
+#else
+instance Exception RecUpdError
+#endif
+
+-----
+
+data NoMethodError = NoMethodError String
+INSTANCE_TYPEABLE0(NoMethodError,noMethodErrorTc,"NoMethodError")
+
+instance Show NoMethodError where
+ showsPrec _ (NoMethodError err) = showString err
+
+#ifdef __HUGS__
+instance Exception NoMethodError where
+ toException (NoMethodError err) = Hugs.Exception.NoMethodError err
+ fromException (Hugs.Exception.NoMethodError err) = Just (NoMethodError err)
+ fromException _ = Nothing
+#else
+instance Exception NoMethodError
+#endif
+
+-----
+
+data NonTermination = NonTermination
+INSTANCE_TYPEABLE0(NonTermination,nonTerminationTc,"NonTermination")
+
+instance Show NonTermination where
+ showsPrec _ NonTermination = showString "<<loop>>"
+
+#ifdef __HUGS__
+instance Exception NonTermination where
+ toException NonTermination = Hugs.Exception.NonTermination
+ fromException Hugs.Exception.NonTermination = Just NonTermination
+ fromException _ = Nothing
+#else
+instance Exception NonTermination
+#endif
+
+-----
+
+data NestedAtomically = NestedAtomically
+INSTANCE_TYPEABLE0(NestedAtomically,nestedAtomicallyTc,"NestedAtomically")
+
+instance Show NestedAtomically where
+ showsPrec _ NestedAtomically = showString "Control.Concurrent.STM.atomically was nested"
+
+instance Exception NestedAtomically
+
+-----
+
+instance Exception Dynamic
+
+#endif /* __GLASGOW_HASKELL__ || __HUGS__ */
+
+#ifdef __GLASGOW_HASKELL__
+recSelError, recConError, irrefutPatError, runtimeError,
+ nonExhaustiveGuardsError, patError, noMethodBindingError
+ :: Addr# -> a -- All take a UTF8-encoded C string
+
+recSelError s = throw (RecSelError (unpackCStringUtf8# s)) -- No location info unfortunately
+runtimeError s = error (unpackCStringUtf8# s) -- No location info unfortunately
+
+nonExhaustiveGuardsError s = throw (PatternMatchFail (untangle s "Non-exhaustive guards in"))
+irrefutPatError s = throw (PatternMatchFail (untangle s "Irrefutable pattern failed for pattern"))
+recConError s = throw (RecConError (untangle s "Missing field in record construction"))
+noMethodBindingError s = throw (NoMethodError (untangle s "No instance nor default method for class operation"))
+patError s = throw (PatternMatchFail (untangle s "Non-exhaustive patterns in"))
+
+-- GHC's RTS calls this
+nonTermination :: SomeException
+nonTermination = toException NonTermination
+
+-- GHC's RTS calls this
+nestedAtomically :: SomeException
+nestedAtomically = toException NestedAtomically
+#endif
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