base- Basic libraries

Copyright(c) The University of Glasgow 2001
LicenseBSD-style (see the file libraries/base/LICENSE)
Portabilitynon-portable (extended exceptions)
Safe HaskellTrustworthy




Extensible exceptions, except for multiple handlers.


The Exception type

data SomeException Source

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.


forall e . Exception e => SomeException e 

class (Typeable e, Show e) => Exception e where Source

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, Typeable)

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
    deriving Typeable

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
    deriving Typeable

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 (Typeable, 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

Minimal complete definition



toException :: e -> SomeException Source

fromException :: SomeException -> Maybe e Source

displayException :: e -> String Source

Render this exception value in a human-friendly manner.

Default implementation: show.



Exception SomeException Source 
Exception ArithException Source 
Exception ErrorCall Source 
Exception IOException Source 
Exception Dynamic Source 
Exception ExitCode Source 
Exception ArrayException Source 
Exception AsyncException Source 
Exception SomeAsyncException Source 
Exception AssertionFailed Source 
Exception AllocationLimitExceeded Source 
Exception Deadlock Source 
Exception BlockedIndefinitelyOnSTM Source 
Exception BlockedIndefinitelyOnMVar Source 
Exception NestedAtomically Source 
Exception NonTermination Source 
Exception NoMethodError Source 
Exception RecUpdError Source 
Exception RecConError Source 
Exception RecSelError Source 
Exception PatternMatchFail Source 
Exception Void Source 

data IOException Source

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.

data AsyncException Source

Asynchronous exceptions.



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.


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.

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


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).

data NonTermination Source

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.



data Deadlock Source

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



data NoMethodError Source

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.


NoMethodError String 

data RecConError Source

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


RecConError String 

data RecSelError Source

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.


RecSelError String 

data RecUpdError Source

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.


RecUpdError String 

Throwing exceptions

throwIO :: Exception e => e -> IO a Source

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 :: Exception e => e -> a Source

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

ioError :: IOError -> IO a Source

Raise an IOError in the IO monad.

throwTo :: Exception e => ThreadId -> e -> IO () Source

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" ( 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.

Catching Exceptions

The catch functions

catch Source


:: Exception e 
=> IO a

The computation to run

-> (e -> 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 (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.

catchJust Source


:: Exception e 
=> (e -> Maybe b)

Predicate to select exceptions

-> IO a

Computation to run

-> (b -> IO a)


-> IO a 

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.

The handle functions

handle :: Exception e => (e -> IO a) -> IO a -> IO a Source

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)) $

handleJust :: Exception e => (e -> Maybe b) -> (b -> IO a) -> IO a -> IO a Source

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

The try functions

try :: Exception e => IO a -> IO (Either e a) Source

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)

tryJust :: Exception e => (e -> Maybe b) -> IO a -> IO (Either b a) Source

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.

onException :: IO a -> IO b -> IO a Source

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

The evaluate function

evaluate :: a -> IO a Source

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

The mapException function

mapException :: (Exception e1, Exception e2) => (e1 -> e2) -> a -> a Source

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

Asynchronous Exceptions

Asynchronous exception control

mask :: ((forall a. IO a -> IO a) -> IO b) -> IO b Source

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

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 a new thread in an unmasked state use forkIOUnmasked.

mask_ :: IO a -> IO a Source

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

uninterruptibleMask :: ((forall a. IO a -> IO a) -> IO b) -> IO b Source

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.

uninterruptibleMask_ :: IO a -> IO a Source

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

data MaskingState Source

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



asynchronous exceptions are unmasked (the normal state)


the state during mask: asynchronous exceptions are masked, but blocking operations may still be interrupted


the state during uninterruptibleMask: asynchronous exceptions are masked, and blocking operations may not be interrupted

getMaskingState :: IO MaskingState Source

Returns the MaskingState for the current thread.


assert :: Bool -> a -> a Source

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.


bracket Source


:: IO a

computation to run first ("acquire resource")

-> (a -> IO b)

computation to run last ("release resource")

-> (a -> IO c)

computation to run in-between

-> IO c 

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:

  (openFile "filename" ReadMode)
  (\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

bracket_ :: IO a -> IO b -> IO c -> IO c Source

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

bracketOnError Source


:: IO a

computation to run first ("acquire resource")

-> (a -> IO b)

computation to run last ("release resource")

-> (a -> IO c)

computation to run in-between

-> IO c 

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

finally Source


:: IO a

computation to run first

-> IO b

computation to run afterward (even if an exception was raised)

-> IO a 

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

Calls for GHC runtime