| Copyright | (c) Harvard University 2008-2011 (c) Geoffrey Mainland 2011-2021 |
|---|---|
| License | BSD-style |
| Maintainer | mainland@cs.drexel.edu |
| Safe Haskell | None |
| 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 MyExceptionThe 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 = frontendExceptionFromExceptionWe 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
| Show SomeException | Since: base-3.0 |
Defined in GHC.Exception.Type Methods showsPrec :: Int -> SomeException -> ShowS # show :: SomeException -> String # showList :: [SomeException] -> ShowS # | |
| Exception SomeException | Since: base-3.0 |
Defined in GHC.Exception.Type Methods toException :: SomeException -> SomeException # fromException :: SomeException -> Maybe SomeException # displayException :: SomeException -> String # | |
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.