base-4.16.0.0: Basic libraries
Copyright(c) The University of Glasgow 1998-2002
Licensesee libraries/base/LICENSE
Maintainercvs-ghc@haskell.org
Stabilityinternal
Portabilitynon-portable (GHC extensions)
Safe HaskellTrustworthy
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 #

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

Instances

Instances details
Exception SomeException #

Since: base-3.0

Instance details

Defined in GHC.Exception.Type

Exception ArithException #

Since: base-4.0.0.0

Instance details

Defined in GHC.Exception.Type

Exception ErrorCall #

Since: base-4.0.0.0

Instance details

Defined in GHC.Exception

Exception FileLockingNotSupported # 
Instance details

Defined in GHC.IO.Handle.Lock.Common

Exception IOException #

Since: base-4.1.0.0

Instance details

Defined in GHC.IO.Exception

Exception AllocationLimitExceeded #

Since: base-4.8.0.0

Instance details

Defined in GHC.IO.Exception

Exception ArrayException #

Since: base-4.1.0.0

Instance details

Defined in GHC.IO.Exception

Exception AssertionFailed #

Since: base-4.1.0.0

Instance details

Defined in GHC.IO.Exception

Exception AsyncException #

Since: base-4.7.0.0

Instance details

Defined in GHC.IO.Exception

Exception BlockedIndefinitelyOnMVar #

Since: base-4.1.0.0

Instance details

Defined in GHC.IO.Exception

Exception BlockedIndefinitelyOnSTM #

Since: base-4.1.0.0

Instance details

Defined in GHC.IO.Exception

Exception CompactionFailed #

Since: base-4.10.0.0

Instance details

Defined in GHC.IO.Exception

Exception Deadlock #

Since: base-4.1.0.0

Instance details

Defined in GHC.IO.Exception

Exception ExitCode #

Since: base-4.1.0.0

Instance details

Defined in GHC.IO.Exception

Exception FixIOException #

Since: base-4.11.0.0

Instance details

Defined in GHC.IO.Exception

Exception SomeAsyncException #

Since: base-4.7.0.0

Instance details

Defined in GHC.IO.Exception

Exception Dynamic #

Since: base-4.0.0.0

Instance details

Defined in Data.Dynamic

Exception NestedAtomically #

Since: base-4.0

Instance details

Defined in Control.Exception.Base

Exception NoMethodError #

Since: base-4.0

Instance details

Defined in Control.Exception.Base

Exception NonTermination #

Since: base-4.0

Instance details

Defined in Control.Exception.Base

Exception PatternMatchFail #

Since: base-4.0

Instance details

Defined in Control.Exception.Base

Exception RecConError #

Since: base-4.0

Instance details

Defined in Control.Exception.Base

Exception RecSelError #

Since: base-4.0

Instance details

Defined in Control.Exception.Base

Exception RecUpdError #

Since: base-4.0

Instance details

Defined in Control.Exception.Base

Exception TypeError #

Since: base-4.9.0.0

Instance details

Defined in Control.Exception.Base

Exception Timeout #

Since: base-4.7.0.0

Instance details

Defined in System.Timeout

Exception Void #

Since: base-4.8.0.0

Instance details

Defined in Data.Void

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