| Copyright | (c) Harvard University 2008-2011 (c) Geoffrey Mainland 2011-2021 |
|---|---|
| License | BSD-style |
| Maintainer | mainland@cs.drexel.edu |
| Safe Haskell | Safe-Inferred |
| Language | Haskell98 |
Control.Monad.Exception
Description
Synopsis
- class (Typeable e, Show e) => Exception e where
- toException :: e -> SomeException
- fromException :: SomeException -> Maybe e
- displayException :: e -> String
- data SomeException
- class Monad m => MonadException m where
- onException :: MonadException m => m a -> m b -> m a
- class (MonadIO m, MonadException m) => MonadAsyncException m where
- mask :: ((forall a. m a -> m a) -> m b) -> m b
- bracket :: MonadAsyncException m => m a -> (a -> m b) -> (a -> m c) -> m c
- bracket_ :: MonadAsyncException m => m a -> m b -> m c -> m c
- newtype ExceptionT m a = ExceptionT {
- runExceptionT :: m (Either SomeException a)
- mapExceptionT :: (m (Either SomeException a) -> n (Either SomeException b)) -> ExceptionT m a -> ExceptionT n b
- liftException :: MonadException m => Either SomeException a -> m a
Documentation
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 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 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 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 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 #
displayException :: e -> String #
Render this exception value in a human-friendly manner.
Default implementation: show
Since: base-4.8.0.0
Instances
data SomeException #
The SomeException type is the root of the exception type hierarchy. When an exception of type e is thrown, behind the scenes it is encapsulated in a SomeException.
Instances
| Exception SomeException | Since: base-3.0 |
Defined in GHC.Exception.Type Methods toException :: SomeException -> SomeException # fromException :: SomeException -> Maybe SomeException # displayException :: SomeException -> String # | |
| Show SomeException | Since: base-3.0 |
Defined in GHC.Exception.Type Methods showsPrec :: Int -> SomeException -> ShowS # show :: SomeException -> String # showList :: [SomeException] -> ShowS # | |
class Monad m => MonadException m where Source #
Methods
throw :: Exception e => e -> m a Source #
Throw an exception.
Arguments
| :: Exception e | |
| => m a | The computation to run |
| -> (e -> m a) | Handler to invoke if an exception is raised |
| -> m a |
Catch an exception.
Arguments
| :: m a | The computation to run |
| -> m b | Computation to run afterward (even if an exception was raised) |
| -> m a |
Run a computation and always perform a second, final computation even if an exception is raised. If a short-circuiting monad transformer such as ErrorT or MaybeT is used to transform a MonadException monad, then the implementation of finally for the transformed monad must guarantee that the final action is also always performed when any short-circuiting occurs.
Instances
Arguments
| :: MonadException m | |
| => m a | The computation to run |
| -> m b | Computation to run if an exception is raised |
| -> m a |
If an exception is raised by the computation, then perform a final action and re-raise the exception.
class (MonadIO m, MonadException m) => MonadAsyncException m where Source #
Methods
mask :: ((forall a. m a -> m a) -> m b) -> m b Source #
Executes a computation with asynchronous exceptions masked. The argument passed to mask is a function that takes as its argument another function, which can be used to restore the prevailing masking state within the context of the masked computation.
Instances
Arguments
| :: MonadAsyncException 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 |
When you want to acquire a resource, do some work with it, and then release the resource, it is a good idea to use bracket, because bracket will install the necessary exception handler to release the resource in the event that an exception is raised during the computation. If an exception is raised, then bracket will re-raise the exception (after performing the release).
bracket_ :: MonadAsyncException m => m a -> m b -> m c -> m c Source #
A variant of bracket where the return value from the first computation is not required.
newtype ExceptionT m a Source #
Constructors
| ExceptionT | |
Fields
| |
Instances
mapExceptionT :: (m (Either SomeException a) -> n (Either SomeException b)) -> ExceptionT m a -> ExceptionT n b Source #
liftException :: MonadException m => Either SomeException a -> m a Source #
Lift the result of running a computation in a monad transformed by ExceptionT into another monad that supports exceptions.