MonadCatchIO-transformers-0.3.1.0: Monad-transformer compatible version of the Control.Exception module

Safe Haskell | None
Language | Haskell98

Control.Monad.CatchIO

Contents

- Utilities

Description

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.

Synopsis

- class MonadIO m => MonadCatchIO m where
  - catch :: Exception e => m a -> (e -> m a) -> m a
  - block :: m a -> m a
  - unblock :: m a -> m a
- 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 where

Methods

catch :: Exception e => m a -> (e -> m a) -> m a

Generalized version of `catch`

block :: m a -> m a

unblock :: m a -> m a

Instances

MonadCatchIO IO |
MonadCatchIO m => MonadCatchIO (ListT m) |
MonadCatchIO m => MonadCatchIO (MaybeT m) |
MonadCatchIO m => MonadCatchIO (IdentityT m) |
MonadCatchIO m => MonadCatchIO (ContT r m) | Warning: this instance is somewhat contentious. In the same way that the `ErrorT e` instance may fail to perform the final action, due to the "early exit" behaviour of the monad, this instance may perform the final action any number of times, due to the nonlinear nature of the continuation monad. See the mailing list message http://web.archiveorange.com/archive/v/nDNOvaYx1poDHZNlmlgh for an example of what can go wrong (freeing memory twice).
(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 `ErrorT e IO`), there are errors of two sorts: 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, `bracket` will not perform its second argument, if its third argument decides to "exit early" by throwing an error of type 2. 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 (ReaderT r m) |
MonadCatchIO m => MonadCatchIO (StateT s m) |
MonadCatchIO m => MonadCatchIO (StateT s m) |
(Monoid w, MonadCatchIO m) => MonadCatchIO (WriterT w m) |
(Monoid w, MonadCatchIO m) => MonadCatchIO (WriterT w m) |
(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

Minimal complete definition

Nothing

Methods

toException :: e -> SomeException

fromException :: SomeException -> Maybe e

Instances

Exception PatternMatchFail |
Exception RecSelError |
Exception RecConError |
Exception RecUpdError |
Exception NoMethodError |
Exception NonTermination |
Exception NestedAtomically |
Exception BlockedIndefinitelyOnMVar |
Exception BlockedIndefinitelyOnSTM |
Exception Deadlock |
Exception AssertionFailed |
Exception SomeAsyncException |
Exception AsyncException |
Exception ArrayException |
Exception ExitCode |
Exception IOException |
Exception ErrorCall |
Exception ArithException |
Exception SomeException |

throw :: (MonadIO m, Exception e) => e -> m a

Generalized version of `throwIO`

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

Generalized version of `try`

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

Generalized version of `tryJust`

data Handler m a

Generalized version of `Handler`

Constructors

forall e . Exception e => Handler (e -> m a) |

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

Generalized version of `catches`

# Utilities

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

Generalized version of `bracket`

bracket_

Arguments

:: 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.

bracketOnError

Arguments

:: 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.

finally

Arguments

:: 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 a

Generalized version of `onException`