Control.Monad.Exception

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, 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 PatternMatchFail

Exception RecSelError

Exception RecConError

Exception RecUpdError

Exception NoMethodError

Exception NonTermination

Exception NestedAtomically

Exception BlockedIndefinitelyOnMVar

Exception BlockedIndefinitelyOnSTM

Exception Deadlock

Exception AssertionFailed

Exception AsyncException

Exception ArrayException

Exception ExitCode

Exception IOException

Exception SomeException

Exception ErrorCall

Exception ArithException

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

Typeable SomeException

Exception SomeException

class Monad m => MonadException m where

Methods

throw :: Exception e => e -> m a

Throw an exception.

catch

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.

finally

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

MonadException IO

MonadException STM

(Monad (MaybeT m), MonadException m) => MonadException (MaybeT m)

(Monad (ListT m), MonadException m) => MonadException (ListT m)

(Monad (IdentityT m), MonadException m) => MonadException (IdentityT m)

(Monad (ExceptionT m), Monad m) => MonadException (ExceptionT m)

(Monad (WriterT w m), Monoid w, MonadException m) => MonadException (WriterT w m)

(Monad (WriterT w m), Monoid w, MonadException m) => MonadException (WriterT w m)

(Monad (StateT s m), MonadException m) => MonadException (StateT s m)

(Monad (StateT s m), MonadException m) => MonadException (StateT s m)

(Monad (ReaderT r m), MonadException m) => MonadException (ReaderT r m)

(Monad (ErrorT e m), MonadException m, Error e) => MonadException (ErrorT e m)

(Monad (RWST r w s m), Monoid w, MonadException m) => MonadException (RWST r w s m)

(Monad (RWST r w s m), Monoid w, MonadException m) => MonadException (RWST r w s m)

onException

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

Methods

mask :: ((forall a. m a -> m a) -> m b) -> m b

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

MonadAsyncException IO

(MonadIO (MaybeT m), MonadException (MaybeT m), MonadAsyncException m) => MonadAsyncException (MaybeT m)

(MonadIO (ListT m), MonadException (ListT m), MonadAsyncException m) => MonadAsyncException (ListT m)

(MonadIO (IdentityT m), MonadException (IdentityT m), MonadAsyncException m) => MonadAsyncException (IdentityT m)

(MonadIO (ExceptionT m), MonadException (ExceptionT m), MonadAsyncException m) => MonadAsyncException (ExceptionT m)

(MonadIO (WriterT w m), MonadException (WriterT w m), Monoid w, MonadAsyncException m) => MonadAsyncException (WriterT w m)

(MonadIO (WriterT w m), MonadException (WriterT w m), Monoid w, MonadAsyncException m) => MonadAsyncException (WriterT w m)

(MonadIO (StateT s m), MonadException (StateT s m), MonadAsyncException m) => MonadAsyncException (StateT s m)

(MonadIO (StateT s m), MonadException (StateT s m), MonadAsyncException m) => MonadAsyncException (StateT s m)

(MonadIO (ReaderT r m), MonadException (ReaderT r m), MonadAsyncException m) => MonadAsyncException (ReaderT r m)

(MonadIO (ErrorT e m), MonadException (ErrorT e m), MonadAsyncException m, Error e) => MonadAsyncException (ErrorT e m)

(MonadIO (RWST r w s m), MonadException (RWST r w s m), Monoid w, MonadAsyncException m) => MonadAsyncException (RWST r w s m)

(MonadIO (RWST r w s m), MonadException (RWST r w s m), Monoid w, MonadAsyncException m) => MonadAsyncException (RWST r w s m)

bracket

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

A variant of bracket where the return value from the first computation is not required.

newtype ExceptionT m a

Constructors

ExceptionT

Fields

runExceptionT :: m (Either SomeException a)

Instances

MonadTrans ExceptionT

Monad m => Monad (ExceptionT m)

Functor m => Functor (ExceptionT m)

(Monad (ExceptionT m), MonadFix m) => MonadFix (ExceptionT m)

(Monad (ExceptionT m), Monad m) => MonadPlus (ExceptionT m)

(Functor (ExceptionT m), Functor m, Monad m) => Applicative (ExceptionT m)

(Applicative (ExceptionT m), Functor m, Monad m) => Alternative (ExceptionT m)

(Monad (ExceptionT m), MonadIO m) => MonadIO (ExceptionT m)

(MonadIO (ExceptionT m), MonadException (ExceptionT m), MonadAsyncException m) => MonadAsyncException (ExceptionT m)

(Monad (ExceptionT m), Monad m) => MonadException (ExceptionT m)

mapExceptionT :: (m (Either SomeException a) -> n (Either SomeException b)) -> ExceptionT m a -> ExceptionT n b

liftException :: MonadException m => Either SomeException a -> m a

Lift the result of running a computation in a monad transformed by ExceptionT into another monad that supports exceptions.