Safe Haskell | None |
---|

Warning: this module is *deprecated*.

Please consider using the package exceptions instead, if possible.

The functions `block`

and `unblock`

, which are part of the `MonadCatchIO`

class, have known problems. The IO instances of these functions, which are
provided by the base library, have been deprecated for some time, and have
been removed in base-4.7.

- class MonadIO m => MonadCatchIO m where
- class (Typeable e, Show e) => Exception e where
- toException :: e -> SomeException
- fromException :: SomeException -> Maybe e

- throw :: (MonadIO m, Exception e) => e -> m a
- try :: (MonadCatchIO m, Functor m, Exception e) => m a -> m (Either e a)
- tryJust :: (MonadCatchIO m, Functor m, Exception e) => (e -> Maybe b) -> m a -> m (Either b a)
- data Handler m a = forall e . Exception e => Handler (e -> m a)
- catches :: MonadCatchIO m => m a -> [Handler m a] -> m a
- bracket :: MonadCatchIO m => m a -> (a -> m b) -> (a -> m c) -> m c
- bracket_ :: MonadCatchIO m => m a -> m b -> m c -> m c
- bracketOnError :: MonadCatchIO m => m a -> (a -> m b) -> (a -> m c) -> m c
- finally :: MonadCatchIO m => m a -> m b -> m a
- onException :: MonadCatchIO m => m a -> m b -> m a

# Documentation

class MonadIO m => MonadCatchIO m whereSource

MonadCatchIO IO | |

MonadCatchIO m => MonadCatchIO (ListT m) | |

MonadCatchIO m => MonadCatchIO (IdentityT m) | |

MonadCatchIO m => MonadCatchIO (MaybeT m) | |

(Monoid w, MonadCatchIO m) => MonadCatchIO (WriterT w m) | |

(Monoid w, MonadCatchIO m) => MonadCatchIO (WriterT w m) | |

MonadCatchIO m => MonadCatchIO (StateT s m) | |

MonadCatchIO m => MonadCatchIO (StateT s m) | |

MonadCatchIO m => MonadCatchIO (ReaderT r m) | |

(MonadCatchIO m, Error e) => MonadCatchIO (ErrorT e m) | Warning: this instance is somewhat contentious. Note that in monads that fall under this instance (the most basic example
is - exceptions, (i.e., exceptional values in the underlying
`IO` monad); - error values of type
`e` , introduced by the`ErrorT e` part of the monad.
The instance takes no special action to deal with errors of type 2.
In particular, This may or may not be what you want. See the mailing list thread starting with http://www.mail-archive.com/haskell-cafe@haskell.org/msg82859.html for some details. |

MonadCatchIO m => MonadCatchIO (ContT r m) | Warning: this instance is somewhat contentious. In the same way that the See the mailing list message http://web.archiveorange.com/archive/v/nDNOvaYx1poDHZNlmlgh for an example of what can go wrong (freeing memory twice). |

(Monoid w, MonadCatchIO m) => MonadCatchIO (RWST r w s m) | |

(Monoid w, MonadCatchIO m) => MonadCatchIO (RWST r w s m) |

class (Typeable e, Show e) => Exception e where

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 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 deriving Typeable 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 (Typeable, 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

toException :: e -> SomeException

fromException :: SomeException -> Maybe e

try :: (MonadCatchIO m, Functor m, Exception e) => m a -> m (Either e a)Source

Generalized version of `try`

tryJust :: (MonadCatchIO m, Functor m, Exception e) => (e -> Maybe b) -> m a -> m (Either b a)Source

Generalized version of `tryJust`

catches :: MonadCatchIO m => m a -> [Handler m a] -> m aSource

Generalized version of `catches`

# Utilities

bracket :: MonadCatchIO m => m a -> (a -> m b) -> (a -> m c) -> m cSource

Generalized version of `bracket`

:: MonadCatchIO m | |

=> m a | computation to run first ("acquire resource") |

-> m b | computation to run last ("release resource") |

-> m c | computation to run in-between |

-> m c |

A variant of `bracket`

where the return value from the first computation
is not required.

:: MonadCatchIO m | |

=> m a | computation to run first ("acquire resource") |

-> (a -> m b) | computation to run last ("release resource") |

-> (a -> m c) | computation to run in-between |

-> m c |

Like `bracket`

, but only performs the final action if there was an
exception raised by the in-between computation.

:: MonadCatchIO m | |

=> m a | computation to run first |

-> m b | computation to run afterward (even if an exception was raised) |

-> m a |

A specialised variant of `bracket`

with just a computation to run
afterward.

onException :: MonadCatchIO m => m a -> m b -> m aSource

Generalized version of `onException`