Control.Exception.Base

base-4.20.0.0: Core data structures and operations

Copyright | (c) The University of Glasgow 2001
License | BSD-style (see the file libraries/base/LICENSE)
Maintainer | libraries@haskell.org
Stability | stable
Portability | non-portable (extended exceptions)
Safe Haskell | Safe
Language | Haskell2010

Control.Exception.Base

Contents

- The Exception type
- Throwing exceptions
- Catching Exceptions
  - The `catch` functions
  - The `handle` functions
  - The `try` functions
  - The `evaluate` function
  - The `mapException` function
- Asynchronous Exceptions
  - Asynchronous exception control
- Assertions
- Utilities
- Calls for GHC runtime

Description

Extensible exceptions, except for multiple handlers.

Synopsis

- data SomeException = (Exception e, HasExceptionContext) => SomeException e
- class (Typeable e, Show e) => Exception e where
  - toException :: e -> SomeException
  - fromException :: SomeException -> Maybe e
  - displayException :: e -> String
  - backtraceDesired :: e -> Bool
- data IOException
- data ArithException
  - = Overflow
  - | Underflow
  - | LossOfPrecision
  - | DivideByZero
  - | Denormal
  - | RatioZeroDenominator
- data ArrayException
  - = IndexOutOfBounds String
  - | UndefinedElement String
- newtype AssertionFailed = AssertionFailed String
- data SomeAsyncException = Exception e => SomeAsyncException e
- data AsyncException
  - = StackOverflow
  - | HeapOverflow
  - | ThreadKilled
  - | UserInterrupt
- asyncExceptionToException :: Exception e => e -> SomeException
- asyncExceptionFromException :: Exception e => SomeException -> Maybe e
- data NonTermination = NonTermination
- data NestedAtomically = NestedAtomically
- data BlockedIndefinitelyOnMVar = BlockedIndefinitelyOnMVar
- data FixIOException = FixIOException
- data BlockedIndefinitelyOnSTM = BlockedIndefinitelyOnSTM
- data AllocationLimitExceeded = AllocationLimitExceeded
- newtype CompactionFailed = CompactionFailed String
- data Deadlock = Deadlock
- newtype NoMethodError = NoMethodError String
- newtype PatternMatchFail = PatternMatchFail String
- newtype RecConError = RecConError String
- newtype RecSelError = RecSelError String
- newtype RecUpdError = RecUpdError String
- data ErrorCall where
  - ErrorCallWithLocation String String
  - pattern ErrorCall :: String -> ErrorCall
- newtype TypeError = TypeError String
- data NoMatchingContinuationPrompt = NoMatchingContinuationPrompt
- throwIO :: (HasCallStack, Exception e) => e -> IO a
- throw :: forall a e. (?callStack :: CallStack, Exception e) => e -> a
- ioError :: IOError -> IO a
- throwTo :: Exception e => ThreadId -> e -> IO ()
- catch :: Exception e => IO a -> (e -> IO a) -> 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)
- onException :: IO a -> IO b -> IO 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
- 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
- recSelError :: Addr# -> a
- recConError :: Addr# -> a
- impossibleError :: Addr# -> a
- impossibleConstraintError :: Addr# -> a
- nonExhaustiveGuardsError :: Addr# -> a
- patError :: Addr# -> a
- noMethodBindingError :: Addr# -> a
- typeError :: Addr# -> a
- nonTermination :: SomeException
- nestedAtomically :: SomeException
- noMatchingContinuationPrompt :: SomeException

# 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`.

Constructors

(Exception e, HasExceptionContext) => SomeException e |

#### Instances

Instances details

Exception SomeException | This drops any attached `ExceptionContext`. @since base-3.0
Instance details Defined in GHC.Internal.Exception.Type Methods toException :: SomeException -> SomeException Source # fromException :: SomeException -> Maybe SomeException Source # displayException :: SomeException -> String Source # backtraceDesired :: SomeException -> Bool Source #
Show SomeException | Since: ghc-internal-3.0
Instance details Defined in GHC.Internal.Exception.Type Methods showsPrec :: Int -> SomeException -> ShowS Source # show :: SomeException -> String Source # showList :: [SomeException] -> ShowS Source #

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

    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

Minimal complete definition

Nothing

Methods

toException :: e -> SomeException Source #

`toException` should produce a `SomeException` with no attached `ExceptionContext`.

fromException :: SomeException -> Maybe e Source #

displayException :: e -> String Source #

Render this exception value in a human-friendly manner.

Default implementation: ``show``.

@since base-4.8.0.0

backtraceDesired :: e -> Bool Source #

#### Instances

Instances details

Exception Timeout Source # | Since: base-4.7.0.0
Instance details Defined in System.Timeout Methods toException :: Timeout -> SomeException Source # fromException :: SomeException -> Maybe Timeout Source # displayException :: Timeout -> String Source # backtraceDesired :: Timeout -> Bool Source #
Exception Void | @since base-4.8.0.0
Instance details Defined in GHC.Internal.Exception.Type Methods toException :: Void -> SomeException Source # fromException :: SomeException -> Maybe Void Source # displayException :: Void -> String Source # backtraceDesired :: Void -> Bool Source #
Exception NestedAtomically | @since base-4.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods toException :: NestedAtomically -> SomeException Source # fromException :: SomeException -> Maybe NestedAtomically Source # displayException :: NestedAtomically -> String Source # backtraceDesired :: NestedAtomically -> Bool Source #
Exception NoMatchingContinuationPrompt | @since base-4.18
Instance details Defined in GHC.Internal.Control.Exception.Base Methods toException :: NoMatchingContinuationPrompt -> SomeException Source # fromException :: SomeException -> Maybe NoMatchingContinuationPrompt Source # displayException :: NoMatchingContinuationPrompt -> String Source # backtraceDesired :: NoMatchingContinuationPrompt -> Bool Source #
Exception NoMethodError | @since base-4.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods toException :: NoMethodError -> SomeException Source # fromException :: SomeException -> Maybe NoMethodError Source # displayException :: NoMethodError -> String Source # backtraceDesired :: NoMethodError -> Bool Source #
Exception NonTermination | @since base-4.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods toException :: NonTermination -> SomeException Source # fromException :: SomeException -> Maybe NonTermination Source # displayException :: NonTermination -> String Source # backtraceDesired :: NonTermination -> Bool Source #
Exception PatternMatchFail | @since base-4.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods toException :: PatternMatchFail -> SomeException Source # fromException :: SomeException -> Maybe PatternMatchFail Source # displayException :: PatternMatchFail -> String Source # backtraceDesired :: PatternMatchFail -> Bool Source #
Exception RecConError | @since base-4.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods toException :: RecConError -> SomeException Source # fromException :: SomeException -> Maybe RecConError Source # displayException :: RecConError -> String Source # backtraceDesired :: RecConError -> Bool Source #
Exception RecSelError | @since base-4.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods toException :: RecSelError -> SomeException Source # fromException :: SomeException -> Maybe RecSelError Source # displayException :: RecSelError -> String Source # backtraceDesired :: RecSelError -> Bool Source #
Exception RecUpdError | @since base-4.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods toException :: RecUpdError -> SomeException Source # fromException :: SomeException -> Maybe RecUpdError Source # displayException :: RecUpdError -> String Source # backtraceDesired :: RecUpdError -> Bool Source #
Exception TypeError | @since base-4.9.0.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods toException :: TypeError -> SomeException Source # fromException :: SomeException -> Maybe TypeError Source # displayException :: TypeError -> String Source # backtraceDesired :: TypeError -> Bool Source #
Exception Dynamic | @since base-4.0.0.0
Instance details Defined in GHC.Internal.Data.Dynamic Methods toException :: Dynamic -> SomeException Source # fromException :: SomeException -> Maybe Dynamic Source # displayException :: Dynamic -> String Source # backtraceDesired :: Dynamic -> Bool Source #
Exception ErrorCall | @since base-4.0.0.0
Instance details Defined in GHC.Internal.Exception Methods toException :: ErrorCall -> SomeException Source # fromException :: SomeException -> Maybe ErrorCall Source # displayException :: ErrorCall -> String Source # backtraceDesired :: ErrorCall -> Bool Source #
Exception ArithException | @since base-4.0.0.0
Instance details Defined in GHC.Internal.Exception.Type Methods toException :: ArithException -> SomeException Source # fromException :: SomeException -> Maybe ArithException Source # displayException :: ArithException -> String Source # backtraceDesired :: ArithException -> Bool Source #
Exception SomeException | This drops any attached `ExceptionContext`. @since base-3.0
Instance details Defined in GHC.Internal.Exception.Type Methods toException :: SomeException -> SomeException Source # fromException :: SomeException -> Maybe SomeException Source # displayException :: SomeException -> String Source # backtraceDesired :: SomeException -> Bool Source #
Exception AllocationLimitExceeded | @since base-4.8.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: AllocationLimitExceeded -> SomeException Source # fromException :: SomeException -> Maybe AllocationLimitExceeded Source # displayException :: AllocationLimitExceeded -> String Source # backtraceDesired :: AllocationLimitExceeded -> Bool Source #
Exception ArrayException | @since base-4.1.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: ArrayException -> SomeException Source # fromException :: SomeException -> Maybe ArrayException Source # displayException :: ArrayException -> String Source # backtraceDesired :: ArrayException -> Bool Source #
Exception AssertionFailed | @since base-4.1.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: AssertionFailed -> SomeException Source # fromException :: SomeException -> Maybe AssertionFailed Source # displayException :: AssertionFailed -> String Source # backtraceDesired :: AssertionFailed -> Bool Source #
Exception AsyncException | @since base-4.7.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: AsyncException -> SomeException Source # fromException :: SomeException -> Maybe AsyncException Source # displayException :: AsyncException -> String Source # backtraceDesired :: AsyncException -> Bool Source #
Exception BlockedIndefinitelyOnMVar | @since base-4.1.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: BlockedIndefinitelyOnMVar -> SomeException Source # fromException :: SomeException -> Maybe BlockedIndefinitelyOnMVar Source # displayException :: BlockedIndefinitelyOnMVar -> String Source # backtraceDesired :: BlockedIndefinitelyOnMVar -> Bool Source #
Exception BlockedIndefinitelyOnSTM | @since base-4.1.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: BlockedIndefinitelyOnSTM -> SomeException Source # fromException :: SomeException -> Maybe BlockedIndefinitelyOnSTM Source # displayException :: BlockedIndefinitelyOnSTM -> String Source # backtraceDesired :: BlockedIndefinitelyOnSTM -> Bool Source #
Exception CompactionFailed | @since base-4.10.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: CompactionFailed -> SomeException Source # fromException :: SomeException -> Maybe CompactionFailed Source # displayException :: CompactionFailed -> String Source # backtraceDesired :: CompactionFailed -> Bool Source #
Exception Deadlock | @since base-4.1.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: Deadlock -> SomeException Source # fromException :: SomeException -> Maybe Deadlock Source # displayException :: Deadlock -> String Source # backtraceDesired :: Deadlock -> Bool Source #
Exception ExitCode | @since base-4.1.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: ExitCode -> SomeException Source # fromException :: SomeException -> Maybe ExitCode Source # displayException :: ExitCode -> String Source # backtraceDesired :: ExitCode -> Bool Source #
Exception FixIOException | @since base-4.11.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: FixIOException -> SomeException Source # fromException :: SomeException -> Maybe FixIOException Source # displayException :: FixIOException -> String Source # backtraceDesired :: FixIOException -> Bool Source #
Exception IOException | @since base-4.1.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: IOException -> SomeException Source # fromException :: SomeException -> Maybe IOException Source # displayException :: IOException -> String Source # backtraceDesired :: IOException -> Bool Source #
Exception SomeAsyncException | @since base-4.7.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: SomeAsyncException -> SomeException Source # fromException :: SomeException -> Maybe SomeAsyncException Source # displayException :: SomeAsyncException -> String Source # backtraceDesired :: SomeAsyncException -> Bool Source #
Exception FileLockingNotSupported |
Instance details Defined in GHC.Internal.IO.Handle.Lock.Common Methods toException :: FileLockingNotSupported -> SomeException Source # fromException :: SomeException -> Maybe FileLockingNotSupported Source # displayException :: FileLockingNotSupported -> String Source # backtraceDesired :: FileLockingNotSupported -> Bool Source #
Exception IOPortException |
Instance details Defined in GHC.Internal.IOPort Methods toException :: IOPortException -> SomeException Source # fromException :: SomeException -> Maybe IOPortException Source # displayException :: IOPortException -> String Source # backtraceDesired :: IOPortException -> Bool Source #
Exception a => Exception (ExceptionWithContext a) |
Instance details Defined in GHC.Internal.Exception.Type Methods toException :: ExceptionWithContext a -> SomeException Source # fromException :: SomeException -> Maybe (ExceptionWithContext a) Source # displayException :: ExceptionWithContext a -> String Source # backtraceDesired :: ExceptionWithContext a -> Bool Source #
Exception e => Exception (NoBacktrace e) |
Instance details Defined in GHC.Internal.Exception.Type Methods toException :: NoBacktrace e -> SomeException Source # fromException :: SomeException -> Maybe (NoBacktrace e) Source # displayException :: NoBacktrace e -> String Source # backtraceDesired :: NoBacktrace e -> Bool 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.

#### Instances

Instances details

Exception IOException | @since base-4.1.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: IOException -> SomeException Source # fromException :: SomeException -> Maybe IOException Source # displayException :: IOException -> String Source # backtraceDesired :: IOException -> Bool Source #
Show IOException | @since base-4.1.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods showsPrec :: Int -> IOException -> ShowS Source # show :: IOException -> String Source # showList :: [IOException] -> ShowS Source #
Eq IOException | @since base-4.1.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods (==) :: IOException -> IOException -> Bool Source # (/=) :: IOException -> IOException -> Bool Source #

data ArithException Source #

Arithmetic exceptions.

Constructors

Overflow |
Underflow |
LossOfPrecision |
DivideByZero |
Denormal |
RatioZeroDenominator | @since base-4.6.0.0

#### Instances

Instances details

Exception ArithException | @since base-4.0.0.0
Instance details Defined in GHC.Internal.Exception.Type Methods toException :: ArithException -> SomeException Source # fromException :: SomeException -> Maybe ArithException Source # displayException :: ArithException -> String Source # backtraceDesired :: ArithException -> Bool Source #
Show ArithException | @since base-4.0.0.0
Instance details Defined in GHC.Internal.Exception.Type Methods showsPrec :: Int -> ArithException -> ShowS Source # show :: ArithException -> String Source # showList :: [ArithException] -> ShowS Source #
Eq ArithException | @since base-3.0
Instance details Defined in GHC.Internal.Exception.Type Methods (==) :: ArithException -> ArithException -> Bool Source # (/=) :: ArithException -> ArithException -> Bool Source #
Ord ArithException | @since base-3.0
Instance details Defined in GHC.Internal.Exception.Type Methods compare :: ArithException -> ArithException -> Ordering Source # (<) :: ArithException -> ArithException -> Bool Source # (<=) :: ArithException -> ArithException -> Bool Source # (>) :: ArithException -> ArithException -> Bool Source # (>=) :: ArithException -> ArithException -> Bool Source # max :: ArithException -> ArithException -> ArithException Source # min :: ArithException -> ArithException -> ArithException Source #

data ArrayException Source #

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

Instances details

Exception ArrayException | @since base-4.1.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: ArrayException -> SomeException Source # fromException :: SomeException -> Maybe ArrayException Source # displayException :: ArrayException -> String Source # backtraceDesired :: ArrayException -> Bool Source #
Show ArrayException | @since base-4.1.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods showsPrec :: Int -> ArrayException -> ShowS Source # show :: ArrayException -> String Source # showList :: [ArrayException] -> ShowS Source #
Eq ArrayException | @since base-4.2.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods (==) :: ArrayException -> ArrayException -> Bool Source # (/=) :: ArrayException -> ArrayException -> Bool Source #
Ord ArrayException | @since base-4.2.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods compare :: ArrayException -> ArrayException -> Ordering Source # (<) :: ArrayException -> ArrayException -> Bool Source # (<=) :: ArrayException -> ArrayException -> Bool Source # (>) :: ArrayException -> ArrayException -> Bool Source # (>=) :: ArrayException -> ArrayException -> Bool Source # max :: ArrayException -> ArrayException -> ArrayException Source # min :: ArrayException -> ArrayException -> ArrayException Source #

newtype AssertionFailed Source #

`assert` was applied to `False`.

Constructors

AssertionFailed String |

#### Instances

Instances details

Exception AssertionFailed | @since base-4.1.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: AssertionFailed -> SomeException Source # fromException :: SomeException -> Maybe AssertionFailed Source # displayException :: AssertionFailed -> String Source # backtraceDesired :: AssertionFailed -> Bool Source #
Show AssertionFailed | @since base-4.1.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods showsPrec :: Int -> AssertionFailed -> ShowS Source # show :: AssertionFailed -> String Source # showList :: [AssertionFailed] -> ShowS Source #

data SomeAsyncException Source #

Superclass for asynchronous exceptions.

@since base-4.7.0.0

Constructors

Exception e => SomeAsyncException e |

#### Instances

Instances details

Exception SomeAsyncException | @since base-4.7.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: SomeAsyncException -> SomeException Source # fromException :: SomeException -> Maybe SomeAsyncException Source # displayException :: SomeAsyncException -> String Source # backtraceDesired :: SomeAsyncException -> Bool Source #
Show SomeAsyncException | @since base-4.7.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods showsPrec :: Int -> SomeAsyncException -> ShowS Source # show :: SomeAsyncException -> String Source # showList :: [SomeAsyncException] -> ShowS Source #

data AsyncException Source #

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 throws this to the same thread that receives `UserInterrupt`, but this may change in the future. The GHC RTS currently can only recover from heap overflow if it detects that an explicit memory limit (set via RTS flags). has been exceeded. Currently, failure to allocate memory from the operating system results in immediate termination of the program.
ThreadKilled | This exception is raised by another thread calling `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

Instances details

Exception AsyncException | @since base-4.7.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: AsyncException -> SomeException Source # fromException :: SomeException -> Maybe AsyncException Source # displayException :: AsyncException -> String Source # backtraceDesired :: AsyncException -> Bool Source #
Show AsyncException | @since base-4.1.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods showsPrec :: Int -> AsyncException -> ShowS Source # show :: AsyncException -> String Source # showList :: [AsyncException] -> ShowS Source #
Eq AsyncException | @since base-4.2.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods (==) :: AsyncException -> AsyncException -> Bool Source # (/=) :: AsyncException -> AsyncException -> Bool Source #
Ord AsyncException | @since base-4.2.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods compare :: AsyncException -> AsyncException -> Ordering Source # (<) :: AsyncException -> AsyncException -> Bool Source # (<=) :: AsyncException -> AsyncException -> Bool Source # (>) :: AsyncException -> AsyncException -> Bool Source # (>=) :: AsyncException -> AsyncException -> Bool Source # max :: AsyncException -> AsyncException -> AsyncException Source # min :: AsyncException -> AsyncException -> AsyncException Source #

asyncExceptionToException :: Exception e => e -> SomeException Source #

@since base-4.7.0.0

asyncExceptionFromException :: Exception e => SomeException -> Maybe e Source #

@since base-4.7.0.0

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.

Constructors

NonTermination |

#### Instances

Instances details

Exception NonTermination | @since base-4.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods toException :: NonTermination -> SomeException Source # fromException :: SomeException -> Maybe NonTermination Source # displayException :: NonTermination -> String Source # backtraceDesired :: NonTermination -> Bool Source #
Show NonTermination | @since base-4.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods showsPrec :: Int -> NonTermination -> ShowS Source # show :: NonTermination -> String Source # showList :: [NonTermination] -> ShowS Source #

data NestedAtomically Source #

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

Constructors

NestedAtomically |

#### Instances

Instances details

Exception NestedAtomically | @since base-4.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods toException :: NestedAtomically -> SomeException Source # fromException :: SomeException -> Maybe NestedAtomically Source # displayException :: NestedAtomically -> String Source # backtraceDesired :: NestedAtomically -> Bool Source #
Show NestedAtomically | @since base-4.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods showsPrec :: Int -> NestedAtomically -> ShowS Source # show :: NestedAtomically -> String Source # showList :: [NestedAtomically] -> ShowS Source #

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.

Constructors

BlockedIndefinitelyOnMVar |

#### Instances

Instances details

Exception BlockedIndefinitelyOnMVar | @since base-4.1.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: BlockedIndefinitelyOnMVar -> SomeException Source # fromException :: SomeException -> Maybe BlockedIndefinitelyOnMVar Source # displayException :: BlockedIndefinitelyOnMVar -> String Source # backtraceDesired :: BlockedIndefinitelyOnMVar -> Bool Source #
Show BlockedIndefinitelyOnMVar | @since base-4.1.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods showsPrec :: Int -> BlockedIndefinitelyOnMVar -> ShowS Source # show :: BlockedIndefinitelyOnMVar -> String Source # showList :: [BlockedIndefinitelyOnMVar] -> ShowS Source #

data FixIOException Source #

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

@since base-4.11.0.0

Constructors

FixIOException |

#### Instances

Instances details

Exception FixIOException | @since base-4.11.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: FixIOException -> SomeException Source # fromException :: SomeException -> Maybe FixIOException Source # displayException :: FixIOException -> String Source # backtraceDesired :: FixIOException -> Bool Source #
Show FixIOException | @since base-4.11.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods showsPrec :: Int -> FixIOException -> ShowS Source # show :: FixIOException -> String Source # showList :: [FixIOException] -> ShowS Source #

data BlockedIndefinitelyOnSTM Source #

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

Instances details

Exception BlockedIndefinitelyOnSTM | @since base-4.1.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: BlockedIndefinitelyOnSTM -> SomeException Source # fromException :: SomeException -> Maybe BlockedIndefinitelyOnSTM Source # displayException :: BlockedIndefinitelyOnSTM -> String Source # backtraceDesired :: BlockedIndefinitelyOnSTM -> Bool Source #
Show BlockedIndefinitelyOnSTM | @since base-4.1.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods showsPrec :: Int -> BlockedIndefinitelyOnSTM -> ShowS Source # show :: BlockedIndefinitelyOnSTM -> String Source # showList :: [BlockedIndefinitelyOnSTM] -> ShowS Source #

data AllocationLimitExceeded Source #

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

@since base-4.8.0.0

Constructors

AllocationLimitExceeded |

#### Instances

Instances details

Exception AllocationLimitExceeded | @since base-4.8.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: AllocationLimitExceeded -> SomeException Source # fromException :: SomeException -> Maybe AllocationLimitExceeded Source # displayException :: AllocationLimitExceeded -> String Source # backtraceDesired :: AllocationLimitExceeded -> Bool Source #
Show AllocationLimitExceeded | @since base-4.7.1.0
Instance details Defined in GHC.Internal.IO.Exception Methods showsPrec :: Int -> AllocationLimitExceeded -> ShowS Source # show :: AllocationLimitExceeded -> String Source # showList :: [AllocationLimitExceeded] -> ShowS Source #

newtype CompactionFailed Source #

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

Constructors

CompactionFailed String |

#### Instances

Instances details

Exception CompactionFailed | @since base-4.10.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: CompactionFailed -> SomeException Source # fromException :: SomeException -> Maybe CompactionFailed Source # displayException :: CompactionFailed -> String Source # backtraceDesired :: CompactionFailed -> Bool Source #
Show CompactionFailed | @since base-4.10.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods showsPrec :: Int -> CompactionFailed -> ShowS Source # show :: CompactionFailed -> String Source # showList :: [CompactionFailed] -> ShowS Source #

data Deadlock Source #

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

Constructors

Deadlock |

#### Instances

Instances details

Exception Deadlock | @since base-4.1.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods toException :: Deadlock -> SomeException Source # fromException :: SomeException -> Maybe Deadlock Source # displayException :: Deadlock -> String Source # backtraceDesired :: Deadlock -> Bool Source #
Show Deadlock | @since base-4.1.0.0
Instance details Defined in GHC.Internal.IO.Exception Methods showsPrec :: Int -> Deadlock -> ShowS Source # show :: Deadlock -> String Source # showList :: [Deadlock] -> ShowS Source #

newtype 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.

Constructors

NoMethodError String |

#### Instances

Instances details

Exception NoMethodError | @since base-4.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods toException :: NoMethodError -> SomeException Source # fromException :: SomeException -> Maybe NoMethodError Source # displayException :: NoMethodError -> String Source # backtraceDesired :: NoMethodError -> Bool Source #
Show NoMethodError | @since base-4.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods showsPrec :: Int -> NoMethodError -> ShowS Source # show :: NoMethodError -> String Source # showList :: [NoMethodError] -> ShowS Source #

newtype PatternMatchFail Source #

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

Constructors

PatternMatchFail String |

#### Instances

Instances details

Exception PatternMatchFail | @since base-4.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods toException :: PatternMatchFail -> SomeException Source # fromException :: SomeException -> Maybe PatternMatchFail Source # displayException :: PatternMatchFail -> String Source # backtraceDesired :: PatternMatchFail -> Bool Source #
Show PatternMatchFail | @since base-4.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods showsPrec :: Int -> PatternMatchFail -> ShowS Source # show :: PatternMatchFail -> String Source # showList :: [PatternMatchFail] -> ShowS Source #

newtype RecConError Source #

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

Constructors

RecConError String |

#### Instances

Instances details

Exception RecConError | @since base-4.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods toException :: RecConError -> SomeException Source # fromException :: SomeException -> Maybe RecConError Source # displayException :: RecConError -> String Source # backtraceDesired :: RecConError -> Bool Source #
Show RecConError | @since base-4.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods showsPrec :: Int -> RecConError -> ShowS Source # show :: RecConError -> String Source # showList :: [RecConError] -> ShowS Source #

newtype 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.

Constructors

RecSelError String |

#### Instances

Instances details

Exception RecSelError | @since base-4.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods toException :: RecSelError -> SomeException Source # fromException :: SomeException -> Maybe RecSelError Source # displayException :: RecSelError -> String Source # backtraceDesired :: RecSelError -> Bool Source #
Show RecSelError | @since base-4.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods showsPrec :: Int -> RecSelError -> ShowS Source # show :: RecSelError -> String Source # showList :: [RecSelError] -> ShowS Source #

newtype 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.

Constructors

RecUpdError String |

#### Instances

Instances details

Exception RecUpdError | @since base-4.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods toException :: RecUpdError -> SomeException Source # fromException :: SomeException -> Maybe RecUpdError Source # displayException :: RecUpdError -> String Source # backtraceDesired :: RecUpdError -> Bool Source #
Show RecUpdError | @since base-4.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods showsPrec :: Int -> RecUpdError -> ShowS Source # show :: RecUpdError -> String Source # showList :: [RecUpdError] -> ShowS Source #

data ErrorCall Source #

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

Constructors

ErrorCallWithLocation String String |

Bundled Patterns

pattern ErrorCall :: String -> ErrorCall |

#### Instances

Instances details

Exception ErrorCall | @since base-4.0.0.0
Instance details Defined in GHC.Internal.Exception Methods toException :: ErrorCall -> SomeException Source # fromException :: SomeException -> Maybe ErrorCall Source # displayException :: ErrorCall -> String Source # backtraceDesired :: ErrorCall -> Bool Source #
Show ErrorCall | @since base-4.0.0.0
Instance details Defined in GHC.Internal.Exception Methods showsPrec :: Int -> ErrorCall -> ShowS Source # show :: ErrorCall -> String Source # showList :: [ErrorCall] -> ShowS Source #
Eq ErrorCall | @since base-4.7.0.0
Instance details Defined in GHC.Internal.Exception Methods (==) :: ErrorCall -> ErrorCall -> Bool Source # (/=) :: ErrorCall -> ErrorCall -> Bool Source #
Ord ErrorCall | @since base-4.7.0.0
Instance details Defined in GHC.Internal.Exception Methods compare :: ErrorCall -> ErrorCall -> Ordering Source # (<) :: ErrorCall -> ErrorCall -> Bool Source # (<=) :: ErrorCall -> ErrorCall -> Bool Source # (>) :: ErrorCall -> ErrorCall -> Bool Source # (>=) :: ErrorCall -> ErrorCall -> Bool Source # max :: ErrorCall -> ErrorCall -> ErrorCall Source # min :: ErrorCall -> ErrorCall -> ErrorCall Source #

newtype TypeError Source #

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

Constructors

TypeError String |

#### Instances

Instances details

Exception TypeError | @since base-4.9.0.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods toException :: TypeError -> SomeException Source # fromException :: SomeException -> Maybe TypeError Source # displayException :: TypeError -> String Source # backtraceDesired :: TypeError -> Bool Source #
Show TypeError | @since base-4.9.0.0
Instance details Defined in GHC.Internal.Control.Exception.Base Methods showsPrec :: Int -> TypeError -> ShowS Source # show :: TypeError -> String Source # showList :: [TypeError] -> ShowS Source #

data NoMatchingContinuationPrompt Source #

Thrown when the program attempts a continuation capture, but no prompt with the given prompt tag exists in the current continuation.

@since base-4.18

Constructors

NoMatchingContinuationPrompt |

#### Instances

Instances details

Exception NoMatchingContinuationPrompt | @since base-4.18
Instance details Defined in GHC.Internal.Control.Exception.Base Methods toException :: NoMatchingContinuationPrompt -> SomeException Source # fromException :: SomeException -> Maybe NoMatchingContinuationPrompt Source # displayException :: NoMatchingContinuationPrompt -> String Source # backtraceDesired :: NoMatchingContinuationPrompt -> Bool Source #
Show NoMatchingContinuationPrompt | @since base-4.18
Instance details Defined in GHC.Internal.Control.Exception.Base Methods showsPrec :: Int -> NoMatchingContinuationPrompt -> ShowS Source # show :: NoMatchingContinuationPrompt -> String Source # showList :: [NoMatchingContinuationPrompt] -> ShowS Source #

# Throwing exceptions

throwIO :: (HasCallStack, 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` ()  ===> throw e
    throwIO e `seq` ()  ===> ()

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 operations, whereas `throw` does not. We say that `throwIO` throws *precise* exceptions and `throw`, `error`, etc. all throw *imprecise* exceptions. For example

    throw e + error "boom" ===> error "boom"
    throw e + error "boom" ===> throw e

are both valid reductions and the compiler may pick any (loop, even), whereas

    throwIO e >> error "boom" ===> throwIO e

will always throw `e` when executed.

See also the GHC wiki page on precise exceptions for a more technical introduction to how GHC optimises around precise vs. imprecise exceptions.

throw :: forall a e. (?callStack :: CallStack, 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.

WARNING: You may want to use `throwIO` instead so that your pure code stays exception-free.

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" (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`.

# Catching Exceptions

## The `catch` functions

catch Source #

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 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 propagated further up. If you call it again, you might get the opposite behaviour. This is ok, because `catch` is an `IO` computation.

catchJust Source #

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 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 then it will be propagated 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 #

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`.

## 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
        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`.

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.

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

Instances details

Show MaskingState | @since base-4.3.0.0
Instance details Defined in GHC.Internal.IO Methods showsPrec :: Int -> MaskingState -> ShowS Source # show :: MaskingState -> String Source # showList :: [MaskingState] -> ShowS Source #
Eq MaskingState | @since base-4.3.0.0
Instance details Defined in GHC.Internal.IO Methods (==) :: MaskingState -> MaskingState -> Bool Source # (/=) :: MaskingState -> MaskingState -> Bool Source #

getMaskingState :: IO MaskingState Source #

Returns the `MaskingState` for the current thread.

# Assertions

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.

# Utilities

bracket Source #

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 wraps the release action with `mask`, which is sufficient to ensure that the release action executes to completion when it does not invoke any interruptible actions, even in the presence of asynchronous exceptions. For example, `hClose` is uninterruptible when it is not racing other uses of the handle. Similarly, closing a socket (from "network" package) is also uninterruptible under similar conditions. An example of an interruptible action is `killThread`. Completion of interruptible release actions can be ensured by wrapping them in `uninterruptibleMask_`, but this risks making the program non-responsive to `Control-C`, or timeouts. Another option is to run the release action asynchronously in its own thread:

    void $ uninterruptibleMask_ $ forkIO $ do { ... }

The resource will be released as soon as possible, but the thread that invoked bracket will not block in an uninterruptible state.

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 #

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 Source #

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.

# Calls for GHC runtime

recSelError :: Addr# -> a Source #

recConError :: Addr# -> a Source #

impossibleError :: Addr# -> a Source #

impossibleConstraintError :: Addr# -> a Source #

nonExhaustiveGuardsError :: Addr# -> a Source #

patError :: Addr# -> a Source #

noMethodBindingError :: Addr# -> a Source #

typeError :: Addr# -> a Source #

nonTermination :: SomeException Source #

nestedAtomically :: SomeException Source #

noMatchingContinuationPrompt :: SomeException Source #