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| Control.OldException | | Portability | non-portable (extended exceptions) | | Stability | experimental | | Maintainer | libraries@haskell.org |
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| Description |
This module provides support for raising and catching both built-in
and user-defined exceptions.
In addition to exceptions thrown by IO operations, exceptions may
be thrown by pure code (imprecise exceptions) or by external events
(asynchronous exceptions), but may only be caught in the IO monad.
For more details, see:
- A semantics for imprecise exceptions, by Simon Peyton Jones,
Alastair Reid, Tony Hoare, Simon Marlow, Fergus Henderson,
in PLDI'99.
- Asynchronous exceptions in Haskell, by Simon Marlow, Simon Peyton
Jones, Andy Moran and John Reppy, in PLDI'01.
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| Synopsis |
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| The Exception type
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| The type of exceptions. Every kind of system-generated exception
has a constructor in the Exception type, and values of other
types may be injected into Exception by coercing them to
Dynamic (see the section on Dynamic Exceptions:
Control.OldException).
| | Constructors | | ArithException ArithException | Exceptions raised by arithmetic
operations. (NOTE: GHC currently does not throw
ArithExceptions except for DivideByZero).
| | ArrayException ArrayException | Exceptions raised by array-related
operations. (NOTE: GHC currently does not throw
ArrayExceptions).
| | AssertionFailed String | This exception is thrown by the
assert operation when the condition
fails. The String argument contains the
location of the assertion in the source program.
| | AsyncException AsyncException | Asynchronous exceptions (see section on Asynchronous Exceptions: Control.OldException).
| | BlockedOnDeadMVar | The current thread was executing a call to
Control.Concurrent.MVar.takeMVar that could never return,
because there are no other references to this MVar.
| | BlockedIndefinitely | The current thread was waiting to retry an atomic memory transaction
that could never become possible to complete because there are no other
threads referring to any of the TVars involved.
| | NestedAtomically | The runtime detected an attempt to nest one STM transaction
inside another one, presumably due to the use of
unsafePeformIO with atomically.
| | Deadlock | There are no runnable threads, so the program is
deadlocked. The Deadlock exception is
raised in the main thread only (see also: Control.Concurrent).
| | DynException Dynamic | Dynamically typed exceptions (see section on Dynamic Exceptions: Control.OldException).
| | ErrorCall String | The ErrorCall exception is thrown by error. The String
argument of ErrorCall is the string passed to error when it was
called.
| | ExitException ExitCode | The ExitException exception is thrown by System.Exit.exitWith (and
System.Exit.exitFailure). The ExitCode argument is the value passed
to System.Exit.exitWith. An unhandled ExitException exception in the
main thread will cause the program to be terminated with the given
exit code.
| | IOException IOException | These are the standard IO exceptions generated by
Haskell's IO operations. See also System.IO.Error.
| | NoMethodError String | An attempt was made to invoke a class method which has
no definition in this instance, and there was no default
definition given in the class declaration. GHC issues a
warning when you compile an instance which has missing
methods.
| | NonTermination | The current thread is stuck in an infinite loop. This
exception may or may not be thrown when the program is
non-terminating.
| | PatternMatchFail String | A pattern matching failure. The String argument should contain a
descriptive message including the function name, source file
and line number.
| | RecConError String | An attempt was made to evaluate a field of a record
for which no value was given at construction time. The
String argument gives the location of the
record construction in the source program.
| | RecSelError String | A field selection was attempted on a constructor that
doesn't have the requested field. This can happen with
multi-constructor records when one or more fields are
missing from some of the constructors. The
String argument gives the location of the
record selection in the source program.
| | RecUpdError String | An attempt was made to update a field in a record,
where the record doesn't have the requested field. This can
only occur with multi-constructor records, when one or more
fields are missing from some of the constructors. The
String argument gives the location of the
record update in the source program.
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|  Instances | |
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| 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.
| | Instances | |
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| Arithmetic exceptions.
| | Constructors | | Overflow | | | Underflow | | | LossOfPrecision | | | DivideByZero | | | Denormal | |
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| 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.
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| 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:
- It is undefined which thread receives this exception.
- GHC currently does not throw HeapOverflow exceptions.
| | ThreadKilled | This exception is raised by another thread
calling Control.Concurrent.killThread, or by the system
if it needs to terminate the thread for some
reason.
| | 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).
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| Throwing exceptions
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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.
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| Throw an exception. Exceptions may be thrown from purely
functional code, but may only be caught within the IO monad.
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| Raise an IOError in the IO monad.
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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 block or not.
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).
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.
Blocked throwTo is fair: if multiple threads are trying to throw an
exception to the same target thread, they will succeed in FIFO order.
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| Catching Exceptions
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| There are several functions for catching and examining
exceptions; all of them may only be used from within the
IO monad.
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| The catch functions
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| :: IO a | The computation to run
| | -> Exception -> 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 (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 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 |
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