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

Catch an exception.

Instances

MonadException IO

MonadException STM

Monad m => MonadException (ExceptionT m)

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

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

MonadException m => MonadException (StateT s m)

MonadException m => MonadException (StateT s m)

MonadException m => MonadException (ReaderT r m)

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

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

class (MonadIO m, MonadException m) => MonadAsyncException m where

Methods

block :: m a -> m a

Applying block to a computation will execute that computation with asynchronous exceptions blocked.

unblock :: m a -> m a

To re-enable asynchronous exceptions inside the scope of block, unblock can be used.

Instances

MonadAsyncException IO

MonadAsyncException m => MonadAsyncException (ExceptionT m)

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

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

MonadAsyncException m => MonadAsyncException (StateT s m)

MonadAsyncException m => MonadAsyncException (StateT s m)

MonadAsyncException m => MonadAsyncException (ReaderT r m)

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

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

newtype ExceptionT m a

Constructors

ExceptionT

Fields

runExceptionT :: m (Either SomeException a)

Instances

MonadTrans ExceptionT

Monad m => Monad (ExceptionT m)

Monad m => Functor (ExceptionT m)

MonadFix m => MonadFix (ExceptionT m)

Monad m => MonadPlus (ExceptionT m)

MonadIO m => MonadIO (ExceptionT m)

MonadAsyncException m => MonadAsyncException (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.