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.

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

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

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

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 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 MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: MismatchedParentheses))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: SomeFrontendException))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: SomeCompilerException))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: IOException))
*** Exception: MismatchedParentheses

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

assert was applied to False.

Superclass for asynchronous exceptions.

Since: base-4.7.0.0

Asynchronous exceptions.

Since: base-4.7.0.0

Since: base-4.7.0.0

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.

Thrown when the program attempts to call atomically, from the stm package, inside another call to atomically.

The thread is blocked on an MVar, but there are no other references to the MVar so it can't ever continue.

The exception thrown when an infinite cycle is detected in fixIO.

Since: base-4.11.0.0

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.

This thread has exceeded its allocation limit. See setAllocationCounter and enableAllocationLimit.

Since: base-4.8.0.0

Compaction found an object that cannot be compacted. Functions cannot be compacted, nor can mutable objects or pinned objects. See compact.

Since: base-4.10.0.0

There are no runnable threads, so the program is deadlocked. The Deadlock exception is raised in the main thread only.

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.

A pattern match failed. The String gives information about the source location of the pattern.

An uninitialised record field was used. The String gives information about the source location where the record was constructed.

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.

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.

This is thrown when the user calls error. The first String is the argument given to error, second String is the location.

An expression that didn't typecheck during compile time was called. This is only possible with -fdefer-type-errors. The String gives details about the failed type check.

Since: base-4.9.0.0

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).

Exception delivery synchronizes between the source and target thread: 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. Exception delivery is also atomic with respect to other exceptions. Atomicity is a useful property to have when dealing with race conditions: e.g. 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 (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.

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.

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)) $
     ...

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 of type e was raised, or (Left ex) if an exception of type 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)

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.

Like finally, but only performs the final action if there was an exception raised by the computation.

Evaluate the argument to weak head normal form.

evaluate is typically used to uncover any exceptions that a lazy value may contain, and possibly handle them.

evaluate only evaluates to weak head normal form. If deeper evaluation is needed, the force function from Control.DeepSeq may be handy:

evaluate $ force x

There is a subtle difference between evaluate x and return $! x, analogous to the difference between throwIO and throw. If the lazy value x throws an exception, return $! x will fail to return an IO action and will throw an exception instead. evaluate x, on the other hand, always produces an IO action; that action will throw an exception upon execution iff x throws an exception upon evaluation.

The practical implication of this difference is that due to the imprecise exceptions semantics,

(return $! error "foo") >> error "bar"

may throw either "foo" or "bar", depending on the optimizations performed by the compiler. On the other hand,

evaluate (error "foo") >> error "bar"

is guaranteed to throw "foo".

The rule of thumb is to use evaluate to force or handle exceptions in lazy values. If, on the other hand, you are forcing a lazy value for efficiency reasons only and do not care about exceptions, you may use return $! x.

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

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 MaskingState from the parent; that is, to start a thread in the MaskedInterruptible state, 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 new thread in an unmasked state use forkIOWithUnmask.

Like mask, but does not pass a restore action to the argument.

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.

Like uninterruptibleMask, but does not pass a restore action to the argument.

Describes the behaviour of a thread when an asynchronous exception is received.

Returns the MaskingState for the current thread.

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)
  (\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

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.