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Within LTS Haskell 11.10 (ghc-8.2.2)

  1. module Control.Exception

    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.

  2. class (Typeable e, Show e) => Exception e

    base Control.Exception

    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
    

  3. class (Typeable e, Show e) => Exception e

    base Control.Exception.Base

    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
    

  4. module Control.Lens.Internal.Exception

    This module uses dirty tricks to generate a Handler from an arbitrary Fold.

  5. module TextShow.Control.Exception

    TextShow instances for Exception data types. Since: 2

  6. Exception :: e -> Exceptional e a

    explicit-exception Control.Monad.Exception.Synchronous

    No documentation available.

  7. module Control.Monad.Exception

    No documentation available.

  8. module Control.Eff.Exception

    Exception-producing and exception-handling effects

  9. module Web.Simple.Controller.Exception

    No documentation available.

  10. exception :: Exception a => Prism' SomeException a

    lens Control.Exception.Lens

    Traverse the strongly typed Exception contained in SomeException where the type of your function matches the desired Exception.

    exception :: (Applicative f, Exception a)
    => (a -> f a) -> SomeException -> f SomeException
    

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