| Portability | non-portable (extended exceptions) | 
|---|---|
| Stability | experimental | 
| Maintainer | libraries@haskell.org | 
| Safe Haskell | Trustworthy | 
Control.Exception.Base
Contents
Description
Extensible exceptions, except for multiple handlers.
- data SomeException = forall e . Exception e => SomeException e
- class (Typeable e, Show e) => Exception e  where- toException :: e -> SomeException
- fromException :: SomeException -> Maybe e
 
- data IOException
- data ArithException
- data ArrayException
- data AssertionFailed = AssertionFailed String
- data AsyncException
- data NonTermination = NonTermination
- data NestedAtomically = NestedAtomically
- data BlockedIndefinitelyOnMVar = BlockedIndefinitelyOnMVar
- data BlockedIndefinitelyOnSTM = BlockedIndefinitelyOnSTM
- data Deadlock = Deadlock
- data NoMethodError = NoMethodError String
- data PatternMatchFail = PatternMatchFail String
- data RecConError = RecConError String
- data RecSelError = RecSelError String
- data RecUpdError = RecUpdError String
- data ErrorCall = ErrorCall String
- throwIO :: Exception e => e -> IO a
- throw :: Exception e => e -> a
- ioError :: IOError -> IO a
- throwTo :: Exception e => ThreadId -> e -> IO ()
- catch :: Exception e => IO a -> (e -> IO a) -> IO a
- catchJust :: Exception e => (e -> Maybe b) -> IO a -> (b -> IO a) -> IO a
- handle :: Exception e => (e -> IO a) -> IO a -> IO a
- handleJust :: Exception e => (e -> Maybe b) -> (b -> IO a) -> IO a -> IO a
- try :: Exception e => IO a -> IO (Either e a)
- tryJust :: Exception e => (e -> Maybe b) -> IO a -> IO (Either b a)
- onException :: IO a -> IO b -> IO a
- evaluate :: a -> IO a
- mapException :: (Exception e1, Exception e2) => (e1 -> e2) -> a -> a
- mask :: ((forall a. IO a -> IO a) -> IO b) -> IO b
- mask_ :: IO a -> IO a
- uninterruptibleMask :: ((forall a. IO a -> IO a) -> IO b) -> IO b
- uninterruptibleMask_ :: IO a -> IO a
- data MaskingState
- getMaskingState :: IO MaskingState
- block :: IO a -> IO a
- unblock :: IO a -> IO a
- blocked :: IO Bool
- assert :: Bool -> a -> a
- bracket :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c
- bracket_ :: IO a -> IO b -> IO c -> IO c
- bracketOnError :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c
- finally :: IO a -> IO b -> IO a
- recSelError :: Addr# -> a
- recConError :: Addr# -> a
- irrefutPatError :: Addr# -> a
- runtimeError :: Addr# -> a
- nonExhaustiveGuardsError :: Addr# -> a
- patError :: Addr# -> a
- noMethodBindingError :: Addr# -> a
- absentError :: Addr# -> a
- nonTermination :: SomeException
- nestedAtomically :: SomeException
The Exception type
data SomeException
The SomeException type is the root of the exception type hierarchy.
When an exception of type e is thrown, behind the scenes it is
encapsulated in a SomeException.
Constructors
| forall e . Exception e => SomeException e | 
class (Typeable e, Show e) => Exception e where
Any type that you wish to throw or catch as an exception must be an
instance of the Exception class. The simplest case is a new exception
type directly below the root:
 data MyException = ThisException | ThatException
     deriving (Show, Typeable)
 instance Exception MyException
The default method definitions in the Exception class do what we need
in this case. You can now throw and catch ThisException and
ThatException as exceptions:
*Main> throw ThisException `catch` \e -> putStrLn ("Caught " ++ show (e :: MyException))
Caught ThisException
In more complicated examples, you may wish to define a whole hierarchy of exceptions:
 ---------------------------------------------------------------------
 -- Make the root exception type for all the exceptions in a compiler
 data SomeCompilerException = forall e . Exception e => SomeCompilerException e
     deriving Typeable
 instance Show SomeCompilerException where
     show (SomeCompilerException e) = show e
 instance Exception SomeCompilerException
 compilerExceptionToException :: Exception e => e -> SomeException
 compilerExceptionToException = toException . SomeCompilerException
 compilerExceptionFromException :: Exception e => SomeException -> Maybe e
 compilerExceptionFromException x = do
     SomeCompilerException a <- fromException x
     cast a
 ---------------------------------------------------------------------
 -- Make a subhierarchy for exceptions in the frontend of the compiler
 data SomeFrontendException = forall e . Exception e => SomeFrontendException e
     deriving Typeable
 instance Show SomeFrontendException where
     show (SomeFrontendException e) = show e
 instance Exception SomeFrontendException where
     toException = compilerExceptionToException
     fromException = compilerExceptionFromException
 frontendExceptionToException :: Exception e => e -> SomeException
 frontendExceptionToException = toException . SomeFrontendException
 frontendExceptionFromException :: Exception e => SomeException -> Maybe e
 frontendExceptionFromException x = do
     SomeFrontendException a <- fromException x
     cast a
 ---------------------------------------------------------------------
 -- Make an exception type for a particular frontend compiler exception
 data MismatchedParentheses = MismatchedParentheses
     deriving (Typeable, Show)
 instance Exception MismatchedParentheses where
     toException   = frontendExceptionToException
     fromException = frontendExceptionFromException
We can now catch a MismatchedParentheses exception as
MismatchedParentheses, SomeFrontendException or
SomeCompilerException, but not other types, e.g. IOException:
*Main> throw MismatchedParenthesescatche -> putStrLn ("Caught " ++ show (e :: MismatchedParentheses)) Caught MismatchedParentheses *Main> throw MismatchedParenthesescatche -> putStrLn ("Caught " ++ show (e :: SomeFrontendException)) Caught MismatchedParentheses *Main> throw MismatchedParenthesescatche -> putStrLn ("Caught " ++ show (e :: SomeCompilerException)) Caught MismatchedParentheses *Main> throw MismatchedParenthesescatche -> putStrLn ("Caught " ++ show (e :: IOException)) *** Exception: MismatchedParentheses
Instances
data IOException
Exceptions that occur in the IO monad.
 An IOException records a more specific error type, a descriptive
 string and maybe the handle that was used when the error was
 flagged.
data ArithException
Arithmetic exceptions.
data ArrayException
Exceptions generated by array operations
Constructors
| IndexOutOfBounds String | An attempt was made to index an array outside its declared bounds. | 
| UndefinedElement String | An attempt was made to evaluate an element of an array that had not been initialized. | 
data AssertionFailed
Constructors
| AssertionFailed String | 
data AsyncException
Asynchronous exceptions.
Constructors
| StackOverflow | The current thread's stack exceeded its limit. Since an exception has been raised, the thread's stack will certainly be below its limit again, but the programmer should take remedial action immediately. | 
| HeapOverflow | The program's heap is reaching its limit, and the program should take action to reduce the amount of live data it has. Notes: 
 | 
| ThreadKilled | This exception is raised by another thread
 calling  | 
| UserInterrupt | This exception is raised by default in the main thread of the program when the user requests to terminate the program via the usual mechanism(s) (e.g. Control-C in the console). | 
data NonTermination
Thrown when the runtime system detects that the computation is guaranteed not to terminate. Note that there is no guarantee that the runtime system will notice whether any given computation is guaranteed to terminate or not.
Constructors
| NonTermination | 
data NestedAtomically
Thrown when the program attempts to call atomically, from the stm
 package, inside another call to atomically.
Constructors
| NestedAtomically | 
data BlockedIndefinitelyOnMVar
The thread is blocked on an MVar, but there are no other references
 to the MVar so it can't ever continue.
Constructors
| BlockedIndefinitelyOnMVar | 
The thread is waiting to retry an STM transaction, but there are no
 other references to any TVars involved, so it can't ever continue.
Constructors
| BlockedIndefinitelyOnSTM | 
data Deadlock
There are no runnable threads, so the program is deadlocked.
 The Deadlock exception is raised in the main thread only.
Constructors
| Deadlock | 
data NoMethodError
A class method without a definition (neither a default definition,
 nor a definition in the appropriate instance) was called. The
 String gives information about which method it was.
Constructors
| NoMethodError String | 
data PatternMatchFail
A pattern match failed. The String gives information about the
 source location of the pattern.
Constructors
| PatternMatchFail String | 
data RecConError
An uninitialised record field was used. The String gives
 information about the source location where the record was
 constructed.
Constructors
| RecConError String | 
Instances
data RecSelError
A record selector was applied to a constructor without the
 appropriate field. This can only happen with a datatype with
 multiple constructors, where some fields are in one constructor
 but not another. The String gives information about the source
 location of the record selector.
Constructors
| RecSelError String | 
Instances
data RecUpdError
A record update was performed on a constructor without the
 appropriate field. This can only happen with a datatype with
 multiple constructors, where some fields are in one constructor
 but not another. The String gives information about the source
 location of the record update.
Constructors
| RecUpdError String | 
Instances
data ErrorCall
Throwing exceptions
throwIO :: Exception e => e -> IO a
A variant of throw that can only be used within the IO monad.
Although throwIO has a type that is an instance of the type of throw, the
 two functions are subtly different:
throw e `seq` x ===> throw e throwIO e `seq` x ===> x
The first example will cause the exception e to be raised,
 whereas the second one won't.  In fact, throwIO will only cause
 an exception to be raised when it is used within the IO monad.
 The throwIO variant should be used in preference to throw to
 raise an exception within the IO monad because it guarantees
 ordering with respect to other IO operations, whereas throw
 does not.
throw :: Exception e => e -> a
Throw an exception.  Exceptions may be thrown from purely
 functional code, but may only be caught within the IO monad.
throwTo :: Exception e => ThreadId -> e -> IO ()
throwTo raises an arbitrary exception in the target thread (GHC only).
throwTo does not return until the exception has been raised in the
target thread.
The calling thread can thus be certain that the target
thread has received the exception.  This is a useful property to know
when dealing with race conditions: eg. if there are two threads that
can kill each other, it is guaranteed that only one of the threads
will get to kill the other.
Whatever work the target thread was doing when the exception was raised is not lost: the computation is suspended until required by another thread.
If the target thread is currently making a foreign call, then the
exception will not be raised (and hence throwTo will not return)
until the call has completed.  This is the case regardless of whether
the call is inside a mask or not.  However, in GHC a foreign call
can be annotated as interruptible, in which case a throwTo will
cause the RTS to attempt to cause the call to return; see the GHC
documentation for more details.
Important note: the behaviour of throwTo differs from that described in
the paper "Asynchronous exceptions in Haskell"
(http://research.microsoft.com/~simonpj/Papers/asynch-exns.htm).
In the paper, throwTo is non-blocking; but the library implementation adopts
a more synchronous design in which throwTo does not return until the exception
is received by the target thread.  The trade-off is discussed in Section 9 of the paper.
Like any blocking operation, throwTo is therefore interruptible (see Section 5.3 of
the paper).  Unlike other interruptible operations, however, throwTo
is always interruptible, even if it does not actually block.
There is no guarantee that the exception will be delivered promptly,
although the runtime will endeavour to ensure that arbitrary
delays don't occur.  In GHC, an exception can only be raised when a
thread reaches a safe point, where a safe point is where memory
allocation occurs.  Some loops do not perform any memory allocation
inside the loop and therefore cannot be interrupted by a throwTo.
If the target of throwTo is the calling thread, then the behaviour
is the same as throwIO, except that the exception
is thrown as an asynchronous exception.  This means that if there is
an enclosing pure computation, which would be the case if the current
IO operation is inside unsafePerformIO or unsafeInterleaveIO, that
computation is not permanently replaced by the exception, but is
suspended as if it had received an asynchronous exception.
Note that if throwTo is called with the current thread as the
target, the exception will be thrown even if the thread is currently
inside mask or uninterruptibleMask.
Catching Exceptions
The catch functions
Arguments
| :: Exception e | |
| => IO a | The computation to run | 
| -> (e -> IO a) | Handler to invoke if an exception is raised | 
| -> IO a | 
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 (readFile f)
         (\e -> do let err = show (e :: IOException)
                   hPutStr stderr ("Warning: Couldn't open " ++ f ++ ": " ++ err)
                   return "")
Note that we have to give a type signature to e, or the program
 will not typecheck as the type is ambiguous. While it is possible
 to catch exceptions of any type, see the section "Catching all
 exceptions" (in Control.Exception) for an explanation of the problems with doing so.
For catching exceptions in pure (non-IO) expressions, see the
 function evaluate.
Note that due to Haskell's unspecified evaluation order, an
 expression may throw one of several possible exceptions: consider
 the expression (error "urk") + (1 `div` 0).  Does
 the expression throw
 ErrorCall "urk", or DivideByZero?
The answer is "it might throw either"; the choice is
 non-deterministic. If you are catching any type of exception then you
 might catch either. If you are calling catch with type
 IO Int -> (ArithException -> IO Int) -> IO Int then the handler may
 get run with DivideByZero as an argument, or an ErrorCall "urk"
 exception may be propogated further up. If you call it again, you
 might get a the opposite behaviour. This is ok, because catch is an
 IO computation.
Arguments
| :: Exception e | |
| => (e -> Maybe b) | Predicate to select exceptions | 
| -> IO a | Computation to run | 
| -> (b -> IO a) | Handler | 
| -> 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.
 catchJust (\e -> if isDoesNotExistErrorType (ioeGetErrorType e) then Just () else Nothing)
           (readFile f)
           (\_ -> do hPutStrLn stderr ("No such file: " ++ show f)
                     return "")
Any other exceptions which are not matched by the predicate
 are re-raised, and may be caught by an enclosing
 catch, catchJust, etc.
The handle functions
handle :: Exception e => (e -> IO a) -> IO a -> IO a
A version of catch with the arguments swapped around; useful in
 situations where the code for the handler is shorter.  For example:
   do handle (\NonTermination -> exitWith (ExitFailure 1)) $
      ...
The try functions
try :: Exception e => IO a -> IO (Either e a)
Similar to catch, but returns an Either result which is
 ( if no exception of type Right a)e was raised, or (
 if an exception of type Left ex)e was raised and its value is ex.
 If any other type of exception is raised than it will be propogated
 up to the next enclosing exception handler.
try a = catch (Right `liftM` a) (return . Left)
Note that System.IO.Error also exports a function called
 try with a similar type to try,
 except that it catches only the IO and user families of exceptions
 (as required by the Haskell 98 IO module).
onException :: IO a -> IO b -> IO a
Like finally, but only performs the final action if there was an
 exception raised by the computation.
The evaluate function
Forces its argument to be evaluated to weak head normal form when
 the resultant IO action is executed. It can be used to order
 evaluation with respect to other IO operations; its semantics are
 given by
evaluate x `seq` y ==> y evaluate x `catch` f ==> (return $! x) `catch` f evaluate x >>= f ==> (return $! x) >>= f
Note: the first equation implies that (evaluate x) is not the
 same as (return $! x).  A correct definition is
evaluate x = (return $! x) >>= return
The mapException function
mapException :: (Exception e1, Exception e2) => (e1 -> e2) -> a -> a
This function maps one exception into another as proposed in the paper "A semantics for imprecise exceptions".
Asynchronous Exceptions
Asynchronous exception control
mask :: ((forall a. IO a -> IO a) -> IO b) -> IO b
Executes an IO computation with asynchronous
 exceptions masked.  That is, any thread which attempts to raise
 an exception in the current thread with throwTo
 will be blocked until asynchronous exceptions are unmasked again.
The argument passed to mask is a function that takes as its
 argument another function, which can be used to restore the
 prevailing masking state within the context of the masked
 computation.  For example, a common way to use mask is to protect
 the acquisition of a resource:
 mask $ \restore -> do
     x <- acquire
     restore (do_something_with x) `onException` release
     release
This code guarantees that acquire is paired with release, by masking
 asynchronous exceptions for the critical parts. (Rather than write
 this code yourself, it would be better to use
 bracket which abstracts the general pattern).
Note that the restore action passed to the argument to mask
 does not necessarily unmask asynchronous exceptions, it just
 restores the masking state to that of the enclosing context.  Thus
 if asynchronous exceptions are already masked, mask cannot be used
 to unmask exceptions again.  This is so that if you call a library function
 with exceptions masked, you can be sure that the library call will not be
 able to unmask exceptions again.  If you are writing library code and need
 to use asynchronous exceptions, the only way is to create a new thread;
 see forkIOWithUnmask.
Asynchronous exceptions may still be received while in the masked state if the masked thread blocks in certain ways; see Control.Exception.
Threads created by forkIO inherit the masked
 state from the parent; that is, to start a thread in blocked mode,
 use mask_ $ forkIO ....  This is particularly useful if you need
 to establish an exception handler in the forked thread before any
 asynchronous exceptions are received.  To create a a new thread in
 an unmasked state use forkIOUnmasked.
uninterruptibleMask :: ((forall a. IO a -> IO a) -> IO b) -> IO b
Like mask, but the masked computation is not interruptible (see
 Control.Exception).  THIS SHOULD BE USED WITH
 GREAT CARE, because if a thread executing in uninterruptibleMask
 blocks for any reason, then the thread (and possibly the program,
 if this is the main thread) will be unresponsive and unkillable.
 This function should only be necessary if you need to mask
 exceptions around an interruptible operation, and you can guarantee
 that the interruptible operation will only block for a short period
 of time.
uninterruptibleMask_ :: IO a -> IO a
Like uninterruptibleMask, but does not pass a restore action
 to the argument.
data MaskingState
Describes the behaviour of a thread when an asynchronous exception is received.
Constructors
| Unmasked | asynchronous exceptions are unmasked (the normal state) | 
| MaskedInterruptible | the state during  | 
| MaskedUninterruptible | the state during  | 
Instances
getMaskingState :: IO MaskingState
Returns the MaskingState for the current thread.
(deprecated) Asynchronous exception control
Deprecated: use Control.Exception.mask instead
Note: this function is deprecated, please use mask instead.
Applying block to a computation will
 execute that computation with asynchronous exceptions
 blocked.  That is, any thread which
 attempts to raise an exception in the current thread with throwTo will be
 blocked until asynchronous exceptions are unblocked again.  There's
 no need to worry about re-enabling asynchronous exceptions; that is
 done automatically on exiting the scope of
 block.
Threads created by forkIO inherit the blocked
 state from the parent; that is, to start a thread in blocked mode,
 use block $ forkIO ....  This is particularly useful if you need to
 establish an exception handler in the forked thread before any
 asynchronous exceptions are received.
Deprecated: use Control.Exception.mask instead
Note: this function is deprecated, please use mask instead.
To re-enable asynchronous exceptions inside the scope of
 block, unblock can be
 used.  It scopes in exactly the same way, so on exit from
 unblock asynchronous exception delivery will
 be disabled again.
Deprecated: use Control.Exception.getMaskingState instead
returns True if asynchronous exceptions are blocked in the current thread.
Assertions
If the first argument evaluates to True, then the result is the
 second argument.  Otherwise an AssertionFailed exception is raised,
 containing a String with the source file and line number of the
 call to assert.
Assertions can normally be turned on or off with a compiler flag
 (for GHC, assertions are normally on unless optimisation is turned on 
 with -O or the -fignore-asserts
 option is given).  When assertions are turned off, the first
 argument to assert is ignored, and the second argument is
 returned as the result.
Utilities
Arguments
| :: 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 | 
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)
   (\fileHandle -> 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 -> IO b -> IO c -> IO c
A variant of bracket where the return value from the first computation
 is not required.
Arguments
| :: 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 | 
Like bracket, but only performs the final action if there was an
 exception raised by the in-between computation.
Arguments
| :: IO a | computation to run first | 
| -> IO b | computation to run afterward (even if an exception was raised) | 
| -> IO a | 
A specialised variant of bracket with just a computation to run
 afterward.
Calls for GHC runtime
recSelError :: Addr# -> a
recConError :: Addr# -> a
irrefutPatError :: Addr# -> a
runtimeError :: Addr# -> a
nonExhaustiveGuardsError :: Addr# -> a
noMethodBindingError :: Addr# -> a
absentError :: Addr# -> a