Safe Haskell	None

Control.Monad.CatchIO

Synopsis

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

Methods

toException :: e -> SomeException

fromException :: SomeException -> Maybe e

Instances

Exception SomeException
Exception ErrorCall
Exception ArithException

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

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

Generalized version of onException

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.

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.

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.

data Handler m a where

Generalized version of Handler

Constructors

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

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

Generalized version of catches