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Contents | ||||||||||||||||||||||||||||||||||||||||
Description | ||||||||||||||||||||||||||||||||||||||||
This module provides support for raising and catching both built-in and user-defined exceptions. | ||||||||||||||||||||||||||||||||||||||||
Synopsis | ||||||||||||||||||||||||||||||||||||||||
The Exception type | ||||||||||||||||||||||||||||||||||||||||
data Exception | ||||||||||||||||||||||||||||||||||||||||
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data IOException | ||||||||||||||||||||||||||||||||||||||||
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data ArithException | ||||||||||||||||||||||||||||||||||||||||
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data ArrayException | ||||||||||||||||||||||||||||||||||||||||
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data AsyncException | ||||||||||||||||||||||||||||||||||||||||
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Throwing exceptions | ||||||||||||||||||||||||||||||||||||||||
throwIO :: Exception -> IO a | ||||||||||||||||||||||||||||||||||||||||
A variant of throw that can 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` return () ===> throw e throwIO e `seq` return () ===> return () 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 -> a | ||||||||||||||||||||||||||||||||||||||||
Throw an exception. Exceptions may be thrown from purely functional code, but may only be caught within the IO monad. | ||||||||||||||||||||||||||||||||||||||||
ioError :: IOError -> IO a | ||||||||||||||||||||||||||||||||||||||||
throwTo :: ThreadId -> Exception -> 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. | ||||||||||||||||||||||||||||||||||||||||
Catching Exceptions | ||||||||||||||||||||||||||||||||||||||||
There are several functions for catching and examining exceptions; all of them may only be used from within the IO monad. | ||||||||||||||||||||||||||||||||||||||||
The catch functions | ||||||||||||||||||||||||||||||||||||||||
catch | ||||||||||||||||||||||||||||||||||||||||
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catchJust | ||||||||||||||||||||||||||||||||||||||||
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The handle functions | ||||||||||||||||||||||||||||||||||||||||
handle :: (Exception -> 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 (\e -> exitWith (ExitFailure 1)) $ ... | ||||||||||||||||||||||||||||||||||||||||
handleJust :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a | ||||||||||||||||||||||||||||||||||||||||
A version of catchJust with the arguments swapped around (see handle). | ||||||||||||||||||||||||||||||||||||||||
The try functions | ||||||||||||||||||||||||||||||||||||||||
try :: IO a -> IO (Either Exception a) | ||||||||||||||||||||||||||||||||||||||||
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. | ||||||||||||||||||||||||||||||||||||||||
tryJust :: (Exception -> Maybe b) -> IO a -> IO (Either b a) | ||||||||||||||||||||||||||||||||||||||||
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. | ||||||||||||||||||||||||||||||||||||||||
The evaluate function | ||||||||||||||||||||||||||||||||||||||||
evaluate :: a -> IO a | ||||||||||||||||||||||||||||||||||||||||
Forces its argument to be evaluated, and returns the result in the IO monad. It can be used to order evaluation with respect to other IO operations; its semantics are given by evaluate undefined `seq` return () ==> return () catch (evaluate undefined) (\e -> return ()) ==> return () NOTE: (evaluate a) is not the same as (a `seq` return a). | ||||||||||||||||||||||||||||||||||||||||
The mapException function | ||||||||||||||||||||||||||||||||||||||||
mapException :: (Exception -> Exception) -> a -> a | ||||||||||||||||||||||||||||||||||||||||
This function maps one exception into another as proposed in the paper "A semantics for imprecise exceptions". | ||||||||||||||||||||||||||||||||||||||||
Exception predicates | ||||||||||||||||||||||||||||||||||||||||
These pre-defined predicates may be used as the first argument to catchJust, tryJust, or handleJust to select certain common classes of exceptions. | ||||||||||||||||||||||||||||||||||||||||
ioErrors :: Exception -> Maybe IOError | ||||||||||||||||||||||||||||||||||||||||
arithExceptions :: Exception -> Maybe ArithException | ||||||||||||||||||||||||||||||||||||||||
errorCalls :: Exception -> Maybe String | ||||||||||||||||||||||||||||||||||||||||
dynExceptions :: Exception -> Maybe Dynamic | ||||||||||||||||||||||||||||||||||||||||
assertions :: Exception -> Maybe String | ||||||||||||||||||||||||||||||||||||||||
asyncExceptions :: Exception -> Maybe AsyncException | ||||||||||||||||||||||||||||||||||||||||
userErrors :: Exception -> Maybe String | ||||||||||||||||||||||||||||||||||||||||
Dynamic exceptions | ||||||||||||||||||||||||||||||||||||||||
Because the Exception datatype is not extensible, there is an interface for throwing and catching exceptions of type Dynamic (see Data.Dynamic) which allows exception values of any type in the Typeable class to be thrown and caught. | ||||||||||||||||||||||||||||||||||||||||
throwDyn :: (Typeable exception) => exception -> b | ||||||||||||||||||||||||||||||||||||||||
Raise any value as an exception, provided it is in the Typeable class. | ||||||||||||||||||||||||||||||||||||||||
throwDynTo :: (Typeable exception) => ThreadId -> exception -> IO () | ||||||||||||||||||||||||||||||||||||||||
A variant of throwDyn that throws the dynamic exception to an arbitrary thread (GHC only: c.f. throwTo). | ||||||||||||||||||||||||||||||||||||||||
catchDyn :: (Typeable exception) => IO a -> (exception -> IO a) -> IO a | ||||||||||||||||||||||||||||||||||||||||
Catch dynamic exceptions of the required type. All other exceptions are re-thrown, including dynamic exceptions of the wrong type. When using dynamic exceptions it is advisable to define a new datatype to use for your exception type, to avoid possible clashes with dynamic exceptions used in other libraries. | ||||||||||||||||||||||||||||||||||||||||
Asynchronous Exceptions | ||||||||||||||||||||||||||||||||||||||||
Asynchronous exceptions are so-called because they arise due to external influences, and can be raised at any point during execution. StackOverflow and HeapOverflow are two examples of system-generated asynchronous exceptions. The primary source of asynchronous exceptions, however, is throwTo: throwTo :: ThreadId -> Exception -> IO () throwTo (also throwDynTo and killThread) allows one running thread to raise an arbitrary exception in another thread. The exception is therefore asynchronous with respect to the target thread, which could be doing anything at the time it receives the exception. Great care should be taken with asynchronous exceptions; it is all too easy to introduce race conditions by the over zealous use of throwTo. | ||||||||||||||||||||||||||||||||||||||||
Asynchronous exception control | ||||||||||||||||||||||||||||||||||||||||
The following two functions allow a thread to control delivery of asynchronous exceptions during a critical region. | ||||||||||||||||||||||||||||||||||||||||
block :: IO a -> IO a | ||||||||||||||||||||||||||||||||||||||||
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 will be blocked until asynchronous exceptions are enabled again. There's no need to worry about re-enabling asynchronous exceptions; that is done automatically on exiting the scope of block. | ||||||||||||||||||||||||||||||||||||||||
unblock :: IO a -> IO a | ||||||||||||||||||||||||||||||||||||||||
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. | ||||||||||||||||||||||||||||||||||||||||
Applying block to an exception handler | ||||||||||||||||||||||||||||||||||||||||
There's an implied block around every exception handler in a call to one of the catch family of functions. This is because that is what you want most of the time - it eliminates a common race condition in starting an exception handler, because there may be no exception handler on the stack to handle another exception if one arrives immediately. If asynchronous exceptions are blocked on entering the handler, though, we have time to install a new exception handler before being interrupted. If this weren't the default, one would have to write something like block ( catch (unblock (...)) (\e -> handler) ) If you need to unblock asynchronous exceptions again in the exception handler, just use unblock as normal. Note that try and friends do not have a similar default, because there is no exception handler in this case. If you want to use try in an asynchronous-exception-safe way, you will need to use block. | ||||||||||||||||||||||||||||||||||||||||
Interruptible operations | ||||||||||||||||||||||||||||||||||||||||
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 takeMVar (but not tryTakeMVar), and most operations which perform some I/O with the outside world.. The reason for having interruptible operations is so that we can write things like block ( a <- takeMVar m catch (unblock (...)) (\e -> ...) ) if the takeMVar was not interruptible, then this particular combination could lead to deadlock, because the thread itself would be blocked in a state where it can't receive any asynchronous exceptions. With takeMVar interruptible, however, we can be safe in the knowledge that the thread can receive exceptions right up until the point when the takeMVar succeeds. Similar arguments apply for other interruptible operations like openFile. | ||||||||||||||||||||||||||||||||||||||||
Assertions | ||||||||||||||||||||||||||||||||||||||||
assert :: Bool -> a -> a | ||||||||||||||||||||||||||||||||||||||||
If the first argument evaluates to True, then the result is the second argument. Otherwise an Assertion 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 the -fignore-asserts option is give). When assertions are turned off, the first argument to assert is ignored, and the second argument is returned as the result. | ||||||||||||||||||||||||||||||||||||||||
Utilities | ||||||||||||||||||||||||||||||||||||||||
bracket | ||||||||||||||||||||||||||||||||||||||||
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bracket_ :: IO a -> IO b -> IO c -> IO c | ||||||||||||||||||||||||||||||||||||||||
A variant of bracket where the return value from the first computation is not required. | ||||||||||||||||||||||||||||||||||||||||
finally | ||||||||||||||||||||||||||||||||||||||||
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Produced by Haddock version 0.4 |