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).

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.

Arithmetic exceptions.

Exceptions generated by array operations

Asynchronous exceptions.

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

Raise an IOError in the IO monad.

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.

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 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 catch when importing Control.OldException:

 import Prelude hiding (catch)

or importing Control.OldException qualified, to avoid name-clashes:

 import qualified Control.OldException as C

and then using C.catch

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. There are some predefined exception predicates for useful subsets of exceptions: ioErrors, arithExceptions, and so on. For example, to catch just calls to the error function, we could use

   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.

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)) $
      ...

A version of catchJust with the arguments swapped around (see handle).

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 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).

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.

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

This function maps one exception into another as proposed in the paper "A semantics for imprecise exceptions".

Raise any value as an exception, provided it is in the Typeable class.

A variant of throwDyn that throws the dynamic exception to an arbitrary thread (GHC only: c.f. throwTo).

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.

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.

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.

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.

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

A variant of bracket where the return value from the first computation is not required.

Like bracket, but only performs the final action if there was an exception raised by the in-between computation.

A specialised variant of bracket with just a computation to run afterward.