MonadCatchIO-transformers-0.3.0.0: Monad-transformer compatible version of the Control.Exception module

Safe HaskellSafe-Inferred

Control.Monad.CatchIO

Contents

Synopsis

Documentation

class MonadIO m => MonadCatchIO m whereSource

Methods

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

Generalized version of catch

block :: m a -> m aSource

Generalized version of block

unblock :: m a -> m aSource

Generalized version of unblock

Instances

MonadCatchIO IO 
MonadCatchIO m => MonadCatchIO (MaybeT m) 
MonadCatchIO m => MonadCatchIO (ListT m) 
MonadCatchIO m => MonadCatchIO (IdentityT 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 ErrorT e IO), there are errors of two sorts:

  1. exceptions, (i.e., exceptional values in the underlying IO monad);
  2. 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 (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).

(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

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

Generalized version of throwIO

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

data Handler m a Source

Generalized version of Handler

Constructors

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

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

bracket_Source

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.

bracketOnErrorSource

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.

finallySource

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 aSource

Generalized version of onException