Exception +Control
This module provides support for raising and catching both built-in and user-defined exceptions.
In addition to exceptions thrown by IO operations, exceptions may be thrown by pure code (imprecise exceptions) or by external events (asynchronous exceptions), but may only be caught in the IO monad. For more details, see:
* A semantics for imprecise exceptions, by Simon Peyton Jones, Alastair Reid, Tony Hoare, Simon Marlow, Fergus Henderson, in PLDI'99.
* Asynchronous exceptions in Haskell, by Simon Marlow, Simon Peyton Jones, Andy Moran and John Reppy, in PLDI'01.
* An Extensible Dynamically-Typed Hierarchy of Exceptions, by Simon Marlow, in Haskell '06.
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
> 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
> show (SomeFrontendException e) = show e
>
> instance Exception SomeFrontendException
> 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
> 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
This package provides exception monad transformer instances for the classes defined by monads-fd.
Version 0.2
This package provides exception monad transformer instances for the classes defined by monads-tf.
Version 0.3.0.1
This package provides exception monad transformer instances for the classes defined by mtl2.
Version 0.3.0.2
This package provides type classes, a monad and a monad transformer that support unchecked extensible exceptions as well as asynchronous exceptions. It is compatible with the transformers package.
Version 0.3.0.2
Exceptions raised by arithmetic operations. (NOTE: GHC currently does not throw ArithExceptions except for DivideByZero).
Arithmetic exceptions.
Exceptions raised by array-related operations. (NOTE: GHC currently does not throw ArrayExceptions).
Exceptions generated by array operations
Asynchronous exceptions.
This module provides support for raising and catching both built-in and user-defined exceptions.
In addition to exceptions thrown by IO operations, exceptions may be thrown by pure code (imprecise exceptions) or by external events (asynchronous exceptions), but may only be caught in the IO monad. For more details, see:
* A semantics for imprecise exceptions, by Simon Peyton Jones, Alastair Reid, Tony Hoare, Simon Marlow, Fergus Henderson, in PLDI'99.
* Asynchronous exceptions in Haskell, by Simon Marlow, Simon Peyton Jones, Andy Moran and John Reppy, in PLDI'01.
The ExitException exception is thrown by System.Exit.exitWith (and System.Exit.exitFailure). The ExitCode argument is the value passed to System.Exit.exitWith. An unhandled ExitException exception in the main thread will cause the program to be terminated with the given exit code.
Show more results