{-# OPTIONS_GHC -XNoImplicitPrelude #-}
-#include "Typeable.h"
-
-----------------------------------------------------------------------------
-- |
-- 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
+#ifdef __HUGS__
+ SomeException,
+#else
SomeException(..),
- Exception(..), -- instance Eq, Ord, Show, Typeable
- IOException, -- instance Eq, Ord, Show, Typeable
- ArithException(..), -- instance Eq, Ord, Show, Typeable
- ArrayException(..), -- instance Eq, Ord, Show, Typeable
+#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
- NonTermination(..), nonTermination,
- BlockedOnDeadMVar(..),
- BlockedIndefinitely(..),
- NestedAtomically(..), nestedAtomically,
+ AsyncException(..), -- instance Eq, Ord, Show, Typeable, Exception
+
+#if __GLASGOW_HASKELL__ || __HUGS__
+ NonTermination(..),
+ NestedAtomically(..),
+#endif
+#ifdef __NHC__
+ System.ExitCode(), -- instance Exception
+#endif
+
+ 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
+ catch,
catches, Handler(..),
- 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,
- recSelError, recConError, irrefutPatError, runtimeError,
- nonExhaustiveGuardsError, patError, noMethodBindingError,
- assertError,
) 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 ( ThreadId(ThreadId) )
-#endif
-
-#ifdef __HUGS__
-import Hugs.Exception as ExceptionBase
-#endif
-
-import Data.Dynamic
-import Data.Either
import Data.Maybe
+#else
+import Prelude hiding (catch)
+#endif
#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())
-
--- minimum needed for nhc98 to pretend it has Exceptions
-data Exception = IOException IOException
- | ArithException ArithException
- | ArrayException ArrayException
- | AsyncException AsyncException
- | ExitException ExitCode
- deriving Show
-type IOException = IOError
-data ArithException
-data ArrayException
-data AsyncException
-instance Show ArithException
-instance Show ArrayException
-instance Show AsyncException
-
-catch :: IO a -> (Exception -> IO a) -> IO a
-a `catch` b = a `H'98.catch` (b . IOException)
-
-throwIO :: Exception -> IO a
-throwIO (IOException e) = ioError e
-throwIO _ = ioError (UserError "Control.Exception.throwIO"
- "unknown exception")
-throw :: Exception -> 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 (IOException (UserError "" "Assertion failed"))
+import System (ExitCode())
#endif
-#ifndef __GLASGOW_HASKELL__
--- Dummy definitions for implementations lacking asynchonous exceptions
+-- | You need this when using 'catches'.
+data Handler a = forall e . Exception e => Handler (e -> IO a)
-block :: IO a -> IO a
-block = id
-unblock :: IO a -> IO a
-unblock = id
-blocked :: IO Bool
-blocked = return False
-#endif
+{- |
+Sometimes you want to catch two different sorts of exception. You could
+do something like
------------------------------------------------------------------------------
--- Catching exceptions
+> f = expr `catch` \ (ex :: ArithException) -> handleArith ex
+> `catch` \ (ex :: IOException) -> handleIO ex
--- |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
+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
Just e' -> handler e'
Nothing -> res
-data Handler a = forall e . Exception e => Handler (e -> IO a)
-#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
+-- -----------------------------------------------------------------------------
+-- Catching exceptions
--- | 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)
+{- $catching
------------------------------------------------------------------------------
--- 'mapException'
+There are several functions for catching and examining
+exceptions; all of them may only be used from within the
+'IO' monad.
--- | This function maps one exception into another as proposed in the
--- paper \"A semantics for imprecise exceptions\".
+Here's a rule of thumb for deciding which catch-style function to
+use:
--- Notice that the usage of 'unsafePerformIO' is safe here.
+ * If you want to do some cleanup in the event that an exception
+ is raised, use 'finally', 'bracket' or 'onException'.
-mapException :: Exception e => (e -> e) -> a -> a
-mapException f v = unsafePerformIO (catch (evaluate v)
- (\x -> throw (f x)))
+ * To recover after an exception and do something else, the best
+ choice is to use one of the 'try' family.
------------------------------------------------------------------------------
--- 'try' and variations.
+ * ... unless you are recovering from an asynchronous exception, in which
+ case use 'catch' or 'catchJust'.
--- | 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 () -> IO a
-onException io what = io `catch` \e -> do what
- throw (e :: SomeException)
+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.
------------------------------------------------------------------------------
--- 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 typical use of 'tryJust' for recovery looks like this:
--- | 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
- )
+> 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
'System.IO.openFile'.
-}
-#if !(__GLASGOW_HASKELL__ || __NHC__)
-assert :: Bool -> a -> a
-assert True x = x
-assert False _ = throw (AssertionFailed "")
-#endif
-
-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 AssertionFailed = AssertionFailed String
-INSTANCE_TYPEABLE0(AssertionFailed,assertionFailedTc,"AssertionFailed")
-
-instance Exception AssertionFailed
-
-instance Show AssertionFailed where
- showsPrec _ (AssertionFailed 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
-
------
-
-assertError :: Addr# -> Bool -> a -> a
-assertError str pred v
- | pred = v
- | otherwise = throw (AssertionFailed (untangle str "Assertion failed"))
-
-{-
-(untangle coded message) expects "coded" to be of the form
- "location|details"
-It prints
- location message details
+{- $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.
-}
-untangle :: Addr# -> String -> String
-untangle coded message
- = location
- ++ ": "
- ++ message
- ++ details
- ++ "\n"
- where
- coded_str = unpackCStringUtf8# coded
-
- (location, details)
- = case (span not_bar coded_str) of { (loc, rest) ->
- case rest of
- ('|':det) -> (loc, ' ' : det)
- _ -> (loc, "")
- }
- not_bar c = c /= '|'
-
--- XXX From GHC.Conc
-throwTo :: Exception e => ThreadId -> e -> IO ()
-throwTo (ThreadId id) ex = IO $ \ s ->
- case (killThread# id (toException ex) s) of s1 -> (# s1, () #)
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