base- Basic librariesSource codeContentsIndex
Portabilitynon-portable (extended exceptions)
The Exception type
Throwing exceptions
Catching Exceptions
Catching all exceptions
The catch functions
The handle functions
The try functions
The evaluate function
The mapException function
Asynchronous Exceptions
Asynchronous exception control
Applying block to an exception handler
Interruptible operations

This module provides support for raising and catching both built-in and user-defined exceptions.

In addition to exceptions thrown by IO operations, exceptions may be thrown by pure code (imprecise exceptions) or by external events (asynchronous exceptions), but may only be caught in the IO monad. For more details, see:

  • A semantics for imprecise exceptions, by Simon Peyton Jones, Alastair Reid, Tony Hoare, Simon Marlow, Fergus Henderson, in PLDI'99.
  • Asynchronous exceptions in Haskell, by Simon Marlow, Simon Peyton Jones, Andy Moran and John Reppy, in PLDI'01.
  • An Extensible Dynamically-Typed Hierarchy of Exceptions, by Simon Marlow, in Haskell '06.
data SomeException = forall e . Exception e => SomeException e
class (Typeable e, Show e) => Exception e where
toException :: e -> SomeException
fromException :: SomeException -> Maybe e
data IOException
data ArithException
= Overflow
| Underflow
| LossOfPrecision
| DivideByZero
| Denormal
data ArrayException
= IndexOutOfBounds String
| UndefinedElement String
data AssertionFailed = AssertionFailed String
data AsyncException
= StackOverflow
| HeapOverflow
| ThreadKilled
| UserInterrupt
data NonTermination = NonTermination
data NestedAtomically = NestedAtomically
data BlockedIndefinitelyOnMVar = BlockedIndefinitelyOnMVar
data BlockedIndefinitelyOnSTM = BlockedIndefinitelyOnSTM
data Deadlock = Deadlock
data NoMethodError = NoMethodError String
data PatternMatchFail = PatternMatchFail String
data RecConError = RecConError String
data RecSelError = RecSelError String
data RecUpdError = RecUpdError String
data ErrorCall = ErrorCall String
throw :: Exception e => e -> a
throwIO :: Exception e => e -> IO a
ioError :: IOError -> IO a
throwTo :: Exception e => ThreadId -> e -> IO ()
catch :: Exception e => IO a -> (e -> IO a) -> IO a
catches :: IO a -> [Handler a] -> IO a
data Handler a = forall e . Exception e => Handler (e -> IO a)
catchJust :: Exception e => (e -> Maybe b) -> IO a -> (b -> IO a) -> IO a
handle :: Exception e => (e -> IO a) -> IO a -> IO a
handleJust :: Exception e => (e -> Maybe b) -> (b -> IO a) -> IO a -> IO a
try :: Exception e => IO a -> IO (Either e a)
tryJust :: Exception e => (e -> Maybe b) -> IO a -> IO (Either b a)
evaluate :: a -> IO a
mapException :: (Exception e1, Exception e2) => (e1 -> e2) -> a -> a
block :: IO a -> IO a
unblock :: IO a -> IO a
blocked :: IO Bool
assert :: Bool -> a -> a
bracket :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c
bracket_ :: IO a -> IO b -> IO c -> IO c
bracketOnError :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c
finally :: IO a -> IO b -> IO a
onException :: IO a -> IO b -> IO a
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
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class (Typeable e, Show e) => Exception e whereSource

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
toException :: e -> SomeExceptionSource
fromException :: SomeException -> Maybe eSource
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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.
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data ArithException Source
Arithmetic exceptions.
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data ArrayException Source
Exceptions generated by array operations
IndexOutOfBounds StringAn attempt was made to index an array outside its declared bounds.
UndefinedElement StringAn attempt was made to evaluate an element of an array that had not been initialized.
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data AssertionFailed Source
assert was applied to False.
AssertionFailed String
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data AsyncException Source
Asynchronous exceptions.
StackOverflowThe 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.
ThreadKilledThis exception is raised by another thread calling Control.Concurrent.killThread, or by the system if it needs to terminate the thread for some reason.
UserInterruptThis 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).
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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.
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data NestedAtomically Source
Thrown when the program attempts to call atomically, from the stm package, inside another call to atomically.
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data BlockedIndefinitelyOnMVar Source
The thread is blocked on an MVar, but there are no other references to the MVar so it can't ever continue.
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data BlockedIndefinitelyOnSTM Source
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.
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data Deadlock Source
There are no runnable threads, so the program is deadlocked. The Deadlock exception is raised in the main thread only.
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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
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data PatternMatchFail Source
A pattern match failed. The String gives information about the source location of the pattern.
PatternMatchFail String
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data RecConError Source
An uninitialised record field was used. The String gives information about the source location where the record was constructed.
RecConError String
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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
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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
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data ErrorCall Source
This is thrown when the user calls error. The String is the argument given to error.
ErrorCall String
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Throwing exceptions
throw :: Exception e => e -> aSource
Throw an exception. Exceptions may be thrown from purely functional code, but may only be caught within the IO monad.
throwIO :: Exception e => e -> IO aSource

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.

ioError :: IOError -> IO aSource
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).

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.

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 block or not.

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).

There is currently no guarantee that the exception delivered by throwTo will be delivered at the first possible opportunity. In particular, a thread may unblock and then re-block exceptions (using unblock and block) without receiving a pending throwTo. This is arguably undesirable behaviour.

Catching Exceptions

There are several functions for catching and examining exceptions; all of them may only be used from within the IO monad.

Here's a rule of thumb for deciding which catch-style function to use:

  • If you want to do some cleanup in the event that an exception is raised, use finally, bracket or onException.
  • To recover after an exception and do something else, the best choice is to use one of the try family.
  • ... unless you are recovering from an asynchronous exception, in which case use catch or catchJust.

The difference between using try and catch for recovery is that in catch the handler is inside an implicit block (see "Asynchronous Exceptions") which is important when catching asynchronous exceptions, but when catching other kinds of exception it is unnecessary. Furthermore it is possible to accidentally stay inside the implicit block by tail-calling rather than returning from the handler, which is why we recommend using try rather than catch for ordinary exception recovery.

A typical use of tryJust for recovery looks like this:

  do r <- tryJust (guard . isDoesNotExistError) $ getEnv "HOME"
     case r of
       Left  e    -> ...
       Right home -> ...
Catching all exceptions

It is possible to catch all exceptions, by using the type SomeException:

 catch f (\e -> ... (e :: SomeException) ...)

HOWEVER, this is normally not what you want to do!

For example, suppose you want to read a file, but if it doesn't exist then continue as if it contained "". You might be tempted to just catch all exceptions and return "" in the handler. However, this has all sorts of undesirable consequences. For example, if the user presses control-C at just the right moment then the UserInterrupt exception will be caught, and the program will continue running under the belief that the file contains "". Similarly, if another thread tries to kill the thread reading the file then the ThreadKilled exception will be ignored.

Instead, you should only catch exactly the exceptions that you really want. In this case, this would likely be more specific than even "any IO exception"; a permissions error would likely also want to be handled differently. Instead, you would probably want something like:

 e <- tryJust (guard . isDoesNotExistError) (readFile f)
 let str = either (const "") id e

There are occassions when you really do need to catch any sort of exception. However, in most cases this is just so you can do some cleaning up; you aren't actually interested in the exception itself. For example, if you open a file then you want to close it again, whether processing the file executes normally or throws an exception. However, in these cases you can use functions like bracket, finally and onException, which never actually pass you the exception, but just call the cleanup functions at the appropriate points.

But sometimes you really do need to catch any exception, and actually see what the exception is. One example is at the very top-level of a program, you may wish to catch any exception, print it to a logfile or the screen, and then exit gracefully. For these cases, you can use catch (or one of the other exception-catching functions) with the SomeException type.

The catch functions
:: Exception e
=> IO aThe computation to run
-> e -> IO aHandler 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 previous section "Catching all exceptions" 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.

Note that the Prelude also exports a function called Prelude.catch with a similar type to Control.Exception.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 Prelude.catch when importing Control.Exception:

 import Prelude hiding (catch)

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

 import qualified Control.Exception as C

and then using C.catch

catches :: IO a -> [Handler a] -> IO aSource

Sometimes you want to catch two different sorts of exception. You could do something like

 f = expr `catch` \ (ex :: ArithException) -> handleArith ex
          `catch` \ (ex :: IOException)    -> handleIO    ex

However, there are a couple of problems with this approach. The first is that having two exception handlers is inefficient. However, the more serious issue is that the second exception handler will catch exceptions in the first, e.g. in the example above, if handleArith throws an IOException then the second exception handler will catch it.

Instead, we provide a function catches, which would be used thus:

 f = expr `catches` [Handler (\ (ex :: ArithException) -> handleArith ex),
                     Handler (\ (ex :: IOException)    -> handleIO    ex)]
data Handler a Source
You need this when using catches.
forall e . Exception e => Handler (e -> IO a)
:: Exception e
=> e -> Maybe bPredicate to select exceptions
-> IO aComputation to run
-> b -> IO aHandler
-> 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 aSource

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 aSource
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)

Note that System.IO.Error also exports a function called System.IO.Error.try with a similar type to Control.Exception.try, except that it catches only the IO and user families of exceptions (as required by the Haskell 98 IO module).

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.
The evaluate function
evaluate :: a -> IO aSource

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 -> aSource
This function maps one exception into another as proposed in the paper "A semantics for imprecise exceptions".
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 Control.Concurrent.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 aSource

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 Control.Exception.throwTo 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.

Threads created by Control.Concurrent.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.

unblock :: IO a -> IO aSource
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.
blocked :: IO BoolSource
returns True if asynchronous exceptions are blocked in the current thread.
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. Don't use try for recovering from an asynchronous exception.

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 Control.Concurrent.MVar.takeMVar (but not Control.Concurrent.MVar.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 Control.Concurrent.MVar.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 Control.Concurrent.MVar.takeMVar interruptible, however, we can be safe in the knowledge that the thread can receive exceptions right up until the point when the Control.Concurrent.MVar.takeMVar succeeds. Similar arguments apply for other interruptible operations like System.IO.openFile.

assert :: Bool -> a -> aSource

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.

:: IO acomputation to run first ("acquire resource")
-> a -> IO bcomputation to run last ("release resource")
-> a -> IO ccomputation 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 cSource
A variant of bracket where the return value from the first computation is not required.
:: IO acomputation to run first ("acquire resource")
-> a -> IO bcomputation to run last ("release resource")
-> a -> IO ccomputation 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.
:: IO acomputation to run first
-> IO bcomputation to run afterward (even if an exception was raised)
-> IO a
A specialised variant of bracket with just a computation to run afterward.
onException :: IO a -> IO b -> IO aSource
Like finally, but only performs the final action if there was an exception raised by the computation.
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