Exception +Control.Exception

module Control.Exception
base 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.
class (Typeable e, Show e) => Exception e
base Control.Exception.Base, 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, 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
package exception-mailer
package
This module is designed to be used in production code when no runtime exceptions are expected.  This module will catch any unexpected runtime exception and send a notification email Version 0.3
package exception-monads-fd
package
This package provides exception monad transformer instances for the classes defined by monads-fd. Version 0.2
package exception-monads-tf
package
This package provides exception monad transformer instances for the classes defined by monads-tf. Version 0.3.0.2
package exception-mtl
package
This package provides exception monad transformer instances for the classes defined by mtl2. Version 0.3.0.3
package exception-transformers
package
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 0.3.0.3
package exceptions
package
Extensible optionally-pure exceptions Version 0.3.2
data ArithException
base Control.Exception.Base, base Control.Exception, base Control.OldException
Arithmetic exceptions.
data ArrayException
base Control.Exception.Base, base Control.Exception, base Control.OldException
Exceptions generated by array operations
data AsyncException
base Control.Exception.Base, base Control.Exception, base Control.OldException
Asynchronous exceptions.
fromException :: Exception e => SomeException -> Maybe e
base Control.Exception.Base, base Control.Exception
data IOException
base Control.Exception.Base, base Control.Exception, base Control.OldException
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.
mapException :: (Exception e1, Exception e2) => (e1 -> e2) -> a -> a
base Control.Exception.Base, base Control.Exception
This function maps one exception into another as proposed in the paper "A semantics for imprecise exceptions".
onException :: IO a -> IO b -> IO a
base Control.Exception.Base, base Control.Exception
Like finally, but only performs the final action if there was an exception raised by the computation.
SomeException :: e -> SomeException
base Control.Exception.Base, base Control.Exception
data SomeException
base Control.Exception.Base, base Control.Exception
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.
toException :: Exception e => e -> SomeException
base Control.Exception.Base, base Control.Exception
package control-monad-exception
package
This package provides explicitly typed, checked exceptions as a library. Computations throwing different types of exception can be combined seamlessly. Example > data Expr = Add Expr Expr | Div Expr Expr | Val Double > eval (Val x)     = return x > eval (Add a1 a2) = do > v1 <- eval a1 > v2 <- eval a2 > let sum = v1 + v2 > if sum < v1 || sum < v2 then throw SumOverflow else return sum > eval (Div a1 a2) = do > v1 <- eval a1 > v2 <- eval a2 > if v2 == 0 then throw DivideByZero else return (v1 / v2) > data DivideByZero = DivideByZero deriving (Show, Typeable) > data SumOverflow  = SumOverflow  deriving (Show, Typeable) > instance Exception DivideByZero > instance Exception SumOverflow GHCi infers the following types > eval :: (Throws DivideByZero l, Throws SumOverflow l) => > Expr -> EM l Double > eval `catch` \ (e::DivideByZero) -> return (-1)  :: Throws > SumOverflow l => Expr -> EM l Double > runEM(eval `catch` \ (e::SomeException) -> return (-1)) > :: Expr -> Double In addition to explicitly typed exceptions his package provides: * Support for explicitly documented, unchecked exceptions (via Control.Monad.Exception.tryEMT). * Support for selective unchecked exceptions (via Control.Monad.Exception.UncaughtException). * Support for exception call traces via Control.Monad.Loc.MonadLoc. Example: > f () = do throw MyException > g a  = do f a > main = runEMT $ do g () `catchWithSrcLoc` > \loc (e::MyException) -> lift(putStrLn$ > showExceptionWithTrace loc e) > -- Running main produces the output: > *Main> main > MyException > in f, Main(example.hs): (1,6) > g, Main(example.hs): (2,6) > main, Main(example.hs): (5,9) > main, Main(example.hs): (4,16) Changes: * 0.9.0 - Moved to transformers (finally leaving the whole mtl-transformers cisma behind) * 0.10.0 - removed the dependency on safe-failure * 0.10.1 - compatibility with failure 0.2 (thanks to Bas van Dijk) * 0.10.2 - use the identity type from mtl (thanks to Bas), require minimum cabal-install version (thanks to Bernhard Urban) * 0.10.3 - Moved the MonadIO instance to the main package * 0.10.3.1 - Compatibility with base 4.6 Version 0.10.3.1
package control-monad-exception-monadsfd
package
Monads-fd instances for the EMT exceptions monad transformer Version 0.10.3

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