GHC.Exception

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.

data Exception

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).

For backwards compatibility with Haskell 98, IOError is a type synonym for Exception.

Constructors

ArithException ArithException	Exceptions raised by arithmetic operations. (NOTE: GHC currently does not throw `ArithException`s).
ArrayException ArrayException	Exceptions raised by array-related operations. (NOTE: GHC currently does not throw `ArrayException`s).
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).
BlockedOnDeadMVar	The current thread was executing a call to takeMVar that could never return, because there are no other references to this `MVar`.
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).
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 `exitWith` (and `exitFailure`). The `ExitCode` argument is the value passed to `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.

Instances

Typeable Exception

Show Exception

Eq Exception

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:

It is undefined which thread receives this exception.
GHC currently does not throw HeapOverflow exceptions.

ThreadKilled This exception is raised by another thread calling killThread, or by the system if it needs to terminate the thread for some reason.

Instances

Typeable AsyncException

Show AsyncException

data IOException

Constructors

IOError

ioe_handle :: (Maybe Handle)
ioe_type :: IOErrorType
ioe_location :: String
ioe_descr :: String
ioe_filename :: (Maybe FilePath)

Instances

Typeable IOException

Eq IOException

Show IOException

data ArithException

The type of arithmetic exceptions

Constructors

Overflow
Underflow
LossOfPrecision
DivideByZero
Denormal

Instances

Typeable ArithException

Show ArithException

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.

Instances

Typeable ArrayException

Show ArrayException

throw :: Exception -> a

Throw an exception. Exceptions may be thrown from purely functional code, but may only be caught within the IO monad.

ioError :: Exception -> IO a

A variant of throw that can be used within the IO monad.

Although ioError has a type that is an instance of the type of throw, the two functions are subtly different:

 throw e   `seq` return ()  ===> throw e
 ioError e `seq` return ()  ===> return ()

The first example will cause the exception e to be raised, whereas the second one won't. In fact, ioError will only cause an exception to be raised when it is used within the IO monad. The ioError 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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