base-4.3.1.0: Basic libraries

Portability | non-portable (extended exceptions)
Stability | experimental
Maintainer | libraries@haskell.org

Control.Exception

Contents

- 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
  - (deprecated) Asynchronous exception control
    - Applying `block` to an exception handler
    - Interruptible operations
- Assertions
- Utilities

Description

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.

Synopsis

- 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
- mask :: ((forall a. IO a -> IO a) -> IO b) -> IO b
- mask_ :: IO a -> IO a
- uninterruptibleMask :: ((forall a. IO a -> IO a) -> IO b) -> IO b
- uninterruptibleMask_ :: IO a -> IO a
- data MaskingState
  - = Unmasked
  - | MaskedInterruptible
  - | MaskedUninterruptible
- getMaskingState :: IO MaskingState
- 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

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`.

Constructors

forall e . Exception e => SomeException e |

Instances

Show SomeException |
Typeable SomeException |
Exception SomeException |

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

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

Methods

toException :: e -> SomeException

fromException :: SomeException -> Maybe e

Instances

Exception ArithException |
Exception ErrorCall |
Exception SomeException |
Exception IOException |
Exception Dynamic |
Exception ExitCode |
Exception ArrayException |
Exception AsyncException |
Exception AssertionFailed |
Exception Deadlock |
Exception BlockedIndefinitelyOnSTM |
Exception BlockedIndefinitelyOnMVar |
Exception BlockedIndefinitely |
Exception BlockedOnDeadMVar |
Exception NestedAtomically |
Exception NonTermination |
Exception NoMethodError |
Exception RecUpdError |
Exception RecConError |
Exception RecSelError |
Exception PatternMatchFail |
Exception Timeout |
Exception Exception |

data IOException

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.

Instances

Eq IOException |
Show IOException |
Typeable IOException |
Exception IOException |

data ArithException

Arithmetic exceptions.

Constructors

Overflow |
Underflow |
LossOfPrecision |
DivideByZero |
Denormal |

Instances

Eq ArithException |
Ord ArithException |
Show ArithException |
Typeable ArithException |
Exception 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

Eq ArrayException |
Ord ArrayException |
Show ArrayException |
Typeable ArrayException |
Exception ArrayException |

data AssertionFailed

`assert` was applied to `False`.

Constructors

AssertionFailed String |

Instances

Show AssertionFailed |
Typeable AssertionFailed |
Exception AssertionFailed |

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 `Control.Concurrent.killThread`, or by the system if it needs to terminate the thread for some reason.
UserInterrupt | 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).

Instances

Eq AsyncException |
Ord AsyncException |
Show AsyncException |
Typeable AsyncException |
Exception AsyncException |

data NonTermination

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.

Constructors

NonTermination |

Instances

Show NonTermination |
Typeable NonTermination |
Exception NonTermination |

data NestedAtomically

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

Constructors

NestedAtomically |

Instances

Show NestedAtomically |
Typeable NestedAtomically |
Exception NestedAtomically |

data BlockedIndefinitelyOnMVar

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

Constructors

BlockedIndefinitelyOnMVar |

Instances

Show BlockedIndefinitelyOnMVar |
Typeable BlockedIndefinitelyOnMVar |
Exception BlockedIndefinitelyOnMVar |

data BlockedIndefinitelyOnSTM

The thread is waiting to retry an STM transaction, but there are no other references to any `TVar`s involved, so it can't ever continue.

Constructors

BlockedIndefinitelyOnSTM |

Instances

Show BlockedIndefinitelyOnSTM |
Typeable BlockedIndefinitelyOnSTM |
Exception BlockedIndefinitelyOnSTM |

data Deadlock

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

Constructors

Deadlock |

Instances

Show Deadlock |
Typeable Deadlock |
Exception Deadlock |

data NoMethodError

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.

Constructors

NoMethodError String |

Instances

Show NoMethodError |
Typeable NoMethodError |
Exception NoMethodError |

data PatternMatchFail

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

Constructors

PatternMatchFail String |

Instances

Show PatternMatchFail |
Typeable PatternMatchFail |
Exception PatternMatchFail |

data RecConError

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

Constructors

RecConError String |

Instances

Show RecConError |
Typeable RecConError |
Exception RecConError |

data RecSelError

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.

Constructors

RecSelError String |

Instances

Show RecSelError |
Typeable RecSelError |
Exception RecSelError |

data RecUpdError

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.

Constructors

RecUpdError String |

Instances

Show RecUpdError |
Typeable RecUpdError |
Exception RecUpdError |

data ErrorCall

This is thrown when the user calls `error`. The `String` is the argument given to `error`.

Constructors

ErrorCall String |

Instances

Show ErrorCall |
Typeable ErrorCall |
Exception ErrorCall |

# Throwing exceptions

throw :: Exception e => e -> a

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 a

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 a

Raise an `IOError` in the `IO` monad.

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

`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.

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`.

Blocked `throwTo` is fair: if multiple threads are trying to throw an exception to the same target thread, they will succeed in FIFO order.

# 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

catch

Arguments

:: 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 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 a

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

You need this when using `catches`.

Constructors

forall e . Exception e => Handler (e -> IO a) |

catchJust

Arguments

:: Exception e |
=> (e -> Maybe b) | Predicate to select exceptions
-> IO a | Computation to run
-> (b -> IO a) | Handler
-> 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

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

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

## The `try` functions

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

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)

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 a

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

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 `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 functions allow a thread to control delivery of asynchronous exceptions during a critical region.

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

Executes an IO computation with asynchronous exceptions masked. 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 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 `Control.Exception.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 `Control.Concurrent.forkIOUnmasked`.

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 `Control.Concurrent.forkIO` inherit the masked state from the parent; that is, to start a thread in blocked mode, 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 `Control.Concurrent.forkIOUnmasked`.

mask_ :: IO a -> IO a

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

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

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

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

data MaskingState

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

Constructors

Unmasked | asynchronous exceptions are unmasked (the normal state)
MaskedInterruptible | the state during `mask`: asynchronous exceptions are masked, but blocking operations may still be interrupted
MaskedUninterruptible | the state during `uninterruptibleMask`: asynchronous exceptions are masked, and blocking operations may not be interrupted

Instances

Eq MaskingState |
Show MaskingState |

getMaskingState :: IO MaskingState

Returns the `MaskingState` for the current thread.

## (deprecated) Asynchronous exception control

block :: IO a -> IO a

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 `Control.Exception.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 `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 a

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.

blocked :: IO Bool

returns True if asynchronous exceptions are blocked in the current thread.

### Applying `block` to an exception handler

There's an implied `mask` 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 $ \restore ->
               catch (restore (...))
                     (\e -> handler)

If you need to unblock asynchronous exceptions again in the exception handler, `restore` can be used there too.

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

          mask $ \restore -> do
             a <- takeMVar m
             catch (restore (...))
                   (\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`.

It is useful to think of `mask` not as a way to completely prevent asynchronous exceptions, but as a filter that allows them to be raised only at certain places. The main difficulty with asynchronous exceptions is that they normally can occur anywhere, but within a `mask` an asynchronous exception is only raised by operations that are interruptible (or call other interruptible operations). In many cases these operations may themselves raise exceptions, such as I/O errors, so the caller should be prepared to handle exceptions arising from the operation anyway.

Sometimes it is too onerous to handle exceptions in the middle of a critical piece of stateful code. There are three ways to handle this kind of situation:

- Use STM. Since a transaction is always either completely executed or not at all, transactions are a good way to maintain invariants over state in the presence of asynchronous (and indeed synchronous) exceptions.
- Use `mask`, and avoid interruptible operations. In order to do this, we have to know which operations are interruptible. It is impossible to know for any given library function whether it might invoke an interruptible operation internally; so instead we give a list of guaranteed-not-to-be-interruptible operations below.
- Use `uninterruptibleMask`. This is generally not recommended, unless you can guarantee that any interruptible operations invoked during the scope of `uninterruptibleMask` can only ever block for a short time. Otherwise, `uninterruptibleMask` is a good way to make your program deadlock and be unresponsive to user interrupts.

The following operations are guaranteed not to be interruptible:

- operations on `IORef` from Data.IORef * STM transactions that do not use `retry` * everything from the `Foreign` modules * everything from `Control.Exception` * `tryTakeMVar`, `tryPutMVar`, `isEmptyMVar` * `takeMVar` if the `MVar` is definitely full, and conversely `putMVar` if the `MVar` is definitely empty * `newEmptyMVar`, `newMVar` * `forkIO`, `forkIOUnmasked`, `myThreadId`

# Assertions

assert :: Bool -> a -> a

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.

# Utilities

bracket

Arguments

:: 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:

     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

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

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

bracketOnError

Arguments

:: 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

Arguments

:: 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.

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

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