Control.Exception.Base

base-4.9.0.0: Basic libraries

Copyright | (c) The University of Glasgow 2001
License | BSD-style (see the file libraries/base/LICENSE)
Maintainer | libraries@haskell.org
Stability | experimental
Portability | non-portable (extended exceptions)
Safe Haskell | Trustworthy
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 => SomeException e
- class (Typeable e, Show e) => Exception e where
- 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 BlockedIndefinitelyOnSTM = BlockedIndefinitelyOnSTM
- data AllocationLimitExceeded = AllocationLimitExceeded
- 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 = ErrorCallWithLocation String String
- newtype TypeError = TypeError String
- throwIO :: Exception e => e -> IO a
- throw :: 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
- irrefutPatError :: Addr# -> a
- runtimeError :: Addr# -> a
- nonExhaustiveGuardsError :: Addr# -> a
- patError :: Addr# -> a
- noMethodBindingError :: Addr# -> a
- absentError :: Addr# -> a
- typeError :: Addr# -> a
- nonTermination :: SomeException
- nestedAtomically :: 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 => SomeException e |

Instances

Show SomeException # |
Methods showsPrec :: Int -> SomeException -> ShowS Source # show :: SomeException -> String Source # showList :: [SomeException] -> ShowS Source #
Exception SomeException # |
Methods toException :: SomeException -> SomeException Source # fromException :: SomeException -> Maybe SomeException Source # displayException :: SomeException -> String 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, 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 Source #

fromException :: SomeException -> Maybe e Source #

displayException :: e -> String Source #

Render this exception value in a human-friendly manner.

Default implementation: ``show``.

Since: 4.8.0.0

Instances

Exception SomeException # |
Methods toException :: SomeException -> SomeException Source # fromException :: SomeException -> Maybe SomeException Source # displayException :: SomeException -> String Source #
Exception ArithException # |
Methods toException :: ArithException -> SomeException Source # fromException :: SomeException -> Maybe ArithException Source # displayException :: ArithException -> String Source #
Exception ErrorCall # |
Methods toException :: ErrorCall -> SomeException Source # fromException :: SomeException -> Maybe ErrorCall Source # displayException :: ErrorCall -> String Source #
Exception Dynamic # |
Methods toException :: Dynamic -> SomeException Source # fromException :: SomeException -> Maybe Dynamic Source # displayException :: Dynamic -> String Source #
Exception IOException # |
Methods toException :: IOException -> SomeException Source # fromException :: SomeException -> Maybe IOException Source # displayException :: IOException -> String Source #
Exception ExitCode # |
Methods toException :: ExitCode -> SomeException Source # fromException :: SomeException -> Maybe ExitCode Source # displayException :: ExitCode -> String Source #
Exception ArrayException # |
Methods toException :: ArrayException -> SomeException Source # fromException :: SomeException -> Maybe ArrayException Source # displayException :: ArrayException -> String Source #
Exception AsyncException # |
Methods toException :: AsyncException -> SomeException Source # fromException :: SomeException -> Maybe AsyncException Source # displayException :: AsyncException -> String Source #
Exception SomeAsyncException # |
Methods toException :: SomeAsyncException -> SomeException Source # fromException :: SomeException -> Maybe SomeAsyncException Source # displayException :: SomeAsyncException -> String Source #
Exception AssertionFailed # |
Methods toException :: AssertionFailed -> SomeException Source # fromException :: SomeException -> Maybe AssertionFailed Source # displayException :: AssertionFailed -> String Source #
Exception AllocationLimitExceeded # |
Methods toException :: AllocationLimitExceeded -> SomeException Source # fromException :: SomeException -> Maybe AllocationLimitExceeded Source # displayException :: AllocationLimitExceeded -> String Source #
Exception Deadlock # |
Methods toException :: Deadlock -> SomeException Source # fromException :: SomeException -> Maybe Deadlock Source # displayException :: Deadlock -> String Source #
Exception BlockedIndefinitelyOnSTM # |
Methods toException :: BlockedIndefinitelyOnSTM -> SomeException Source # fromException :: SomeException -> Maybe BlockedIndefinitelyOnSTM Source # displayException :: BlockedIndefinitelyOnSTM -> String Source #
Exception BlockedIndefinitelyOnMVar # |
Methods toException :: BlockedIndefinitelyOnMVar -> SomeException Source # fromException :: SomeException -> Maybe BlockedIndefinitelyOnMVar Source # displayException :: BlockedIndefinitelyOnMVar -> String Source #
Exception NestedAtomically # |
Methods toException :: NestedAtomically -> SomeException Source # fromException :: SomeException -> Maybe NestedAtomically Source # displayException :: NestedAtomically -> String Source #
Exception NonTermination # |
Methods toException :: NonTermination -> SomeException Source # fromException :: SomeException -> Maybe NonTermination Source # displayException :: NonTermination -> String Source #
Exception TypeError # |
Methods toException :: TypeError -> SomeException Source # fromException :: SomeException -> Maybe TypeError Source # displayException :: TypeError -> String Source #
Exception NoMethodError # |
Methods toException :: NoMethodError -> SomeException Source # fromException :: SomeException -> Maybe NoMethodError Source # displayException :: NoMethodError -> String Source #
Exception RecUpdError # |
Methods toException :: RecUpdError -> SomeException Source # fromException :: SomeException -> Maybe RecUpdError Source # displayException :: RecUpdError -> String Source #
Exception RecConError # |
Methods toException :: RecConError -> SomeException Source # fromException :: SomeException -> Maybe RecConError Source # displayException :: RecConError -> String Source #
Exception RecSelError # |
Methods toException :: RecSelError -> SomeException Source # fromException :: SomeException -> Maybe RecSelError Source # displayException :: RecSelError -> String Source #
Exception PatternMatchFail # |
Methods toException :: PatternMatchFail -> SomeException Source # fromException :: SomeException -> Maybe PatternMatchFail Source # displayException :: PatternMatchFail -> String Source #
Exception Void # |
Methods toException :: Void -> SomeException Source # fromException :: SomeException -> Maybe Void Source # displayException :: Void -> String 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

Eq IOException # |
Methods (==) :: IOException -> IOException -> Bool Source # (/=) :: IOException -> IOException -> Bool Source #
Show IOException # |
Methods showsPrec :: Int -> IOException -> ShowS Source # show :: IOException -> String Source # showList :: [IOException] -> ShowS Source #
Exception IOException # |
Methods toException :: IOException -> SomeException Source # fromException :: SomeException -> Maybe IOException Source # displayException :: IOException -> String Source #

data ArithException Source #

Arithmetic exceptions.

Constructors

Overflow |
Underflow |
LossOfPrecision |
DivideByZero |
Denormal |
RatioZeroDenominator | Since: 4.6.0.0

Instances

Eq ArithException # |
Methods (==) :: ArithException -> ArithException -> Bool Source # (/=) :: ArithException -> ArithException -> Bool Source #
Ord ArithException # |
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 #
Show ArithException # |
Methods showsPrec :: Int -> ArithException -> ShowS Source # show :: ArithException -> String Source # showList :: [ArithException] -> ShowS Source #
Exception ArithException # |
Methods toException :: ArithException -> SomeException Source # fromException :: SomeException -> Maybe ArithException Source # displayException :: ArithException -> String 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

Eq ArrayException # |
Methods (==) :: ArrayException -> ArrayException -> Bool Source # (/=) :: ArrayException -> ArrayException -> Bool Source #
Ord ArrayException # |
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 #
Show ArrayException # |
Methods showsPrec :: Int -> ArrayException -> ShowS Source # show :: ArrayException -> String Source # showList :: [ArrayException] -> ShowS Source #
Exception ArrayException # |
Methods toException :: ArrayException -> SomeException Source # fromException :: SomeException -> Maybe ArrayException Source # displayException :: ArrayException -> String Source #

newtype AssertionFailed Source #

`assert` was applied to `False`.

Constructors

AssertionFailed String |

Instances

Show AssertionFailed # |
Methods showsPrec :: Int -> AssertionFailed -> ShowS Source # show :: AssertionFailed -> String Source # showList :: [AssertionFailed] -> ShowS Source #
Exception AssertionFailed # |
Methods toException :: AssertionFailed -> SomeException Source # fromException :: SomeException -> Maybe AssertionFailed Source # displayException :: AssertionFailed -> String Source #

data SomeAsyncException Source #

Superclass for asynchronous exceptions.

Since: 4.7.0.0

Constructors

Exception e => SomeAsyncException e |

Instances

Show SomeAsyncException # |
Methods showsPrec :: Int -> SomeAsyncException -> ShowS Source # show :: SomeAsyncException -> String Source # showList :: [SomeAsyncException] -> ShowS Source #
Exception SomeAsyncException # |
Methods toException :: SomeAsyncException -> SomeException Source # fromException :: SomeException -> Maybe SomeAsyncException Source # displayException :: SomeAsyncException -> String 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 does not throw `HeapOverflow` exceptions.
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

Eq AsyncException # |
Methods (==) :: AsyncException -> AsyncException -> Bool Source # (/=) :: AsyncException -> AsyncException -> Bool Source #
Ord AsyncException # |
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 #
Show AsyncException # |
Methods showsPrec :: Int -> AsyncException -> ShowS Source # show :: AsyncException -> String Source # showList :: [AsyncException] -> ShowS Source #
Exception AsyncException # |
Methods toException :: AsyncException -> SomeException Source # fromException :: SomeException -> Maybe AsyncException Source # displayException :: AsyncException -> String Source #

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

Since: 4.7.0.0

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

Since: 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

Show NonTermination # |
Methods showsPrec :: Int -> NonTermination -> ShowS Source # show :: NonTermination -> String Source # showList :: [NonTermination] -> ShowS Source #
Exception NonTermination # |
Methods toException :: NonTermination -> SomeException Source # fromException :: SomeException -> Maybe NonTermination Source # displayException :: NonTermination -> String Source #

data NestedAtomically Source #

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

Constructors

NestedAtomically |

Instances

Show NestedAtomically # |
Methods showsPrec :: Int -> NestedAtomically -> ShowS Source # show :: NestedAtomically -> String Source # showList :: [NestedAtomically] -> ShowS Source #
Exception NestedAtomically # |
Methods toException :: NestedAtomically -> SomeException Source # fromException :: SomeException -> Maybe NestedAtomically Source # displayException :: NestedAtomically -> String 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

Show BlockedIndefinitelyOnMVar # |
Methods showsPrec :: Int -> BlockedIndefinitelyOnMVar -> ShowS Source # show :: BlockedIndefinitelyOnMVar -> String Source # showList :: [BlockedIndefinitelyOnMVar] -> ShowS Source #
Exception BlockedIndefinitelyOnMVar # |
Methods toException :: BlockedIndefinitelyOnMVar -> SomeException Source # fromException :: SomeException -> Maybe BlockedIndefinitelyOnMVar Source # displayException :: BlockedIndefinitelyOnMVar -> String 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

Show BlockedIndefinitelyOnSTM # |
Methods showsPrec :: Int -> BlockedIndefinitelyOnSTM -> ShowS Source # show :: BlockedIndefinitelyOnSTM -> String Source # showList :: [BlockedIndefinitelyOnSTM] -> ShowS Source #
Exception BlockedIndefinitelyOnSTM # |
Methods toException :: BlockedIndefinitelyOnSTM -> SomeException Source # fromException :: SomeException -> Maybe BlockedIndefinitelyOnSTM Source # displayException :: BlockedIndefinitelyOnSTM -> String Source #

data AllocationLimitExceeded Source #

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

Since: 4.8.0.0

Constructors

AllocationLimitExceeded |

Instances

Show AllocationLimitExceeded # |
Methods showsPrec :: Int -> AllocationLimitExceeded -> ShowS Source # show :: AllocationLimitExceeded -> String Source # showList :: [AllocationLimitExceeded] -> ShowS Source #
Exception AllocationLimitExceeded # |
Methods toException :: AllocationLimitExceeded -> SomeException Source # fromException :: SomeException -> Maybe AllocationLimitExceeded Source # displayException :: AllocationLimitExceeded -> String 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

Show Deadlock # |
Methods showsPrec :: Int -> Deadlock -> ShowS Source # show :: Deadlock -> String Source # showList :: [Deadlock] -> ShowS Source #
Exception Deadlock # |
Methods toException :: Deadlock -> SomeException Source # fromException :: SomeException -> Maybe Deadlock Source # displayException :: Deadlock -> String 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

Show NoMethodError # |
Methods showsPrec :: Int -> NoMethodError -> ShowS Source # show :: NoMethodError -> String Source # showList :: [NoMethodError] -> ShowS Source #
Exception NoMethodError # |
Methods toException :: NoMethodError -> SomeException Source # fromException :: SomeException -> Maybe NoMethodError Source # displayException :: NoMethodError -> String Source #

newtype PatternMatchFail Source #

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

Constructors

PatternMatchFail String |

Instances

Show PatternMatchFail # |
Methods showsPrec :: Int -> PatternMatchFail -> ShowS Source # show :: PatternMatchFail -> String Source # showList :: [PatternMatchFail] -> ShowS Source #
Exception PatternMatchFail # |
Methods toException :: PatternMatchFail -> SomeException Source # fromException :: SomeException -> Maybe PatternMatchFail Source # displayException :: PatternMatchFail -> String 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

Show RecConError # |
Methods showsPrec :: Int -> RecConError -> ShowS Source # show :: RecConError -> String Source # showList :: [RecConError] -> ShowS Source #
Exception RecConError # |
Methods toException :: RecConError -> SomeException Source # fromException :: SomeException -> Maybe RecConError Source # displayException :: RecConError -> String 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

Show RecSelError # |
Methods showsPrec :: Int -> RecSelError -> ShowS Source # show :: RecSelError -> String Source # showList :: [RecSelError] -> ShowS Source #
Exception RecSelError # |
Methods toException :: RecSelError -> SomeException Source # fromException :: SomeException -> Maybe RecSelError Source # displayException :: RecSelError -> String 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

Show RecUpdError # |
Methods showsPrec :: Int -> RecUpdError -> ShowS Source # show :: RecUpdError -> String Source # showList :: [RecUpdError] -> ShowS Source #
Exception RecUpdError # |
Methods toException :: RecUpdError -> SomeException Source # fromException :: SomeException -> Maybe RecUpdError Source # displayException :: RecUpdError -> String Source #

data ErrorCall Source #

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

Constructors

ErrorCallWithLocation String String |

Instances

Eq ErrorCall # |
Methods (==) :: ErrorCall -> ErrorCall -> Bool Source # (/=) :: ErrorCall -> ErrorCall -> Bool Source #
Ord ErrorCall # |
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 #
Show ErrorCall # |
Methods showsPrec :: Int -> ErrorCall -> ShowS Source # show :: ErrorCall -> String Source # showList :: [ErrorCall] -> ShowS Source #
Exception ErrorCall # |
Methods toException :: ErrorCall -> SomeException Source # fromException :: SomeException -> Maybe ErrorCall Source # displayException :: ErrorCall -> String 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: 4.9.0.0

Constructors

TypeError String |

Instances

Show TypeError # |
Methods showsPrec :: Int -> TypeError -> ShowS Source # show :: TypeError -> String Source # showList :: [TypeError] -> ShowS Source #
Exception TypeError # |
Methods toException :: TypeError -> SomeException Source # fromException :: SomeException -> Maybe TypeError Source # displayException :: TypeError -> String Source #

# Throwing exceptions

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

A variant of `throw` that can only be used within the `IO` monad.

Although `throwIO` has a type that is an instance of the type of `throw`, the two functions are subtly different:

    throw e   `seq` x  ===> throw e
    throwIO e `seq` x  ===> x

The first example will cause the exception `e` to be raised, whereas the second one won't. In fact, `throwIO` will only cause an exception to be raised when it is used within the `IO` monad. The `throwIO` variant should be used in preference to `throw` to raise an exception within the `IO` monad because it guarantees ordering with respect to other `IO` operations, whereas `throw` does not.

throw :: Exception e => e -> a Source #

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

ioError :: IOError -> IO a Source #

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

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

`throwTo` raises an arbitrary exception in the target thread (GHC only).

Exception delivery synchronizes between the source and target thread: `throwTo` does not return until the exception has been raised in the target thread. The calling thread can thus be certain that the target thread has received the exception. Exception delivery is also atomic with respect to other exceptions. Atomicity is a useful property to have when dealing with race conditions: e.g. if there are two threads that can kill each other, it is guaranteed that only one of the threads will get to kill the other.

Whatever work the target thread was doing when the exception was raised is not lost: the computation is suspended until required by another thread.

If the target thread is currently making a foreign call, then the exception will not be raised (and hence `throwTo` will not return) until the call has completed. This is the case regardless of whether the call is inside a `mask` or not. However, in GHC a foreign call can be annotated as `interruptible`, in which case a `throwTo` will cause the RTS to attempt to cause the call to return; see the GHC documentation for more details.

Important note: the behaviour of `throwTo` differs from that described in the paper "Asynchronous exceptions in Haskell" (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 propogated further up. If you call it again, you might get a the opposite behaviour. This is ok, because `catch` is an `IO` computation.

catchJust Source #

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 than it will be propogated up to the next enclosing exception handler.

     try a = catch (Right `liftM` a) (return . Left)

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

A variant of `try` that takes an exception predicate to select which exceptions are caught (c.f. `catchJust`). If the exception does not match the predicate, it is re-thrown.

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

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

## The `evaluate` function

evaluate :: a -> IO a Source #

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 a new thread in an unmasked state use `forkIOUnmasked`.

mask_ :: IO a -> IO a Source #

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

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

Like `mask`, but the masked computation is not interruptible (see Control.Exception). THIS SHOULD BE USED WITH GREAT CARE, because if a thread executing in `uninterruptibleMask` blocks for any reason, then the thread (and possibly the program, if this is the main thread) will be unresponsive and unkillable. This function should only be necessary if you need to mask exceptions around an interruptible operation, and you can guarantee that the interruptible operation will only block for a short period of time.

uninterruptibleMask_ :: IO a -> IO a Source #

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

data MaskingState Source #

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

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 # |
Methods (==) :: MaskingState -> MaskingState -> Bool Source # (/=) :: MaskingState -> MaskingState -> Bool Source #
Show MaskingState # |
Methods showsPrec :: Int -> MaskingState -> ShowS Source # show :: MaskingState -> String Source # showList :: [MaskingState] -> ShowS 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_ :: 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 #

irrefutPatError :: Addr# -> a Source #

runtimeError :: Addr# -> a Source #

nonExhaustiveGuardsError :: Addr# -> a Source #

patError :: Addr# -> a Source #

noMethodBindingError :: Addr# -> a Source #

absentError :: Addr# -> a Source #

typeError :: Addr# -> a Source #

nonTermination :: SomeException Source #

nestedAtomically :: SomeException Source #