base-4.20.0.1: Core data structures and operations
Copyright(c) The University of Glasgow 1998-2002
Licensesee libraries/base/LICENSE
Maintainercvs-ghc@haskell.org
Stabilityinternal
Portabilitynon-portable (GHC extensions)
Safe HaskellSafe
LanguageHaskell2010

GHC.Exception.Type

Description

Exceptions and exception-handling functions.

Synopsis

Documentation

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

Exception Void

Since: base-4.8.0.0

Instance details

Defined in GHC.Internal.Exception.Type

Exception NestedAtomically

Since: base-4.0

Instance details

Defined in GHC.Internal.Control.Exception.Base

Exception NoMatchingContinuationPrompt

Since: base-4.18

Instance details

Defined in GHC.Internal.Control.Exception.Base

Exception NoMethodError

Since: base-4.0

Instance details

Defined in GHC.Internal.Control.Exception.Base

Exception NonTermination

Since: base-4.0

Instance details

Defined in GHC.Internal.Control.Exception.Base

Exception PatternMatchFail

Since: base-4.0

Instance details

Defined in GHC.Internal.Control.Exception.Base

Exception RecConError

Since: base-4.0

Instance details

Defined in GHC.Internal.Control.Exception.Base

Exception RecSelError

Since: base-4.0

Instance details

Defined in GHC.Internal.Control.Exception.Base

Exception RecUpdError

Since: base-4.0

Instance details

Defined in GHC.Internal.Control.Exception.Base

Exception TypeError

Since: base-4.9.0.0

Instance details

Defined in GHC.Internal.Control.Exception.Base

Exception Dynamic

Since: base-4.0.0.0

Instance details

Defined in GHC.Internal.Data.Dynamic

Exception ErrorCall

Since: base-4.0.0.0

Instance details

Defined in GHC.Internal.Exception

Exception ArithException

Since: base-4.0.0.0

Instance details

Defined in GHC.Internal.Exception.Type

Exception SomeException

This drops any attached ExceptionContext.

Since: base-3.0

Instance details

Defined in GHC.Internal.Exception.Type

Exception AllocationLimitExceeded

Since: base-4.8.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception ArrayException

Since: base-4.1.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception AssertionFailed

Since: base-4.1.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception AsyncException

Since: base-4.7.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception BlockedIndefinitelyOnMVar

Since: base-4.1.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception BlockedIndefinitelyOnSTM

Since: base-4.1.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception CompactionFailed

Since: base-4.10.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception Deadlock

Since: base-4.1.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception ExitCode

Since: base-4.1.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception FixIOException

Since: base-4.11.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception IOException

Since: base-4.1.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception SomeAsyncException

Since: base-4.7.0.0

Instance details

Defined in GHC.Internal.IO.Exception

Exception FileLockingNotSupported 
Instance details

Defined in GHC.Internal.IO.Handle.Lock.Common

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

Exception e => Exception (NoBacktrace e) 
Instance details

Defined in GHC.Internal.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.