Control.Monad.CatchIO

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

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fromException :: SomeException -> Maybe e

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Instances

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

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Generalized version of throwIO

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

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Generalized version of try

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

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Generalized version of tryJust

data Handler m a

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Generalized version of Handler

Constructors

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

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

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Generalized version of catches

Utilities

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

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Generalized version of bracket

bracket_

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:: 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

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:: 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

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:: 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

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Generalized version of onException

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