Control.Monad.Catch

This module supports monads that can throw extensible exceptions. The exceptions are the very same from Control.Exception, and the operations offered very similar, but here they are not limited to IO.

This code is in the style of both transformers and mtl, and is compatible with them, though doesn't mimic the module structure or offer the complete range of features in those packages.

This is very similar to ErrorT and MonadError, but based on features of Control.Exception. In particular, it handles the complex case of asynchronous exceptions by including mask in the typeclass. Note that the extensible exceptions feature relies on the RankNTypes language extension.

Typeclass

The mtl style typeclass

class Monad m => MonadThrow m where

A class for monads in which exceptions may be thrown.

Instances should obey the following law:

throwM e >> x = throwM e

In other words, throwing an exception short-circuits the rest of the monadic computation.

Minimal complete definition

throwM

Methods

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

Throw an exception. Note that this throws when this action is run in the monad m, not when it is applied. It is a generalization of Control.Exception's throwIO.

Should satisfy the law:

throwM e >> f = throwM e

Instances

MonadThrow []

Methods

throwM :: Exception e => e -> [a]

MonadThrow Maybe

Methods

throwM :: Exception e => e -> Maybe a

MonadThrow IO

Methods

throwM :: Exception e => e -> IO a

MonadThrow Q

Methods

throwM :: Exception e => e -> Q a

MonadThrow STM

Methods

throwM :: Exception e => e -> STM a

(~) * e SomeException => MonadThrow (Either e)

Methods

throwM :: Exception e => e -> Either e a

MonadThrow m => MonadThrow (ListT m)

Methods

throwM :: Exception e => e -> ListT m a

MonadThrow m => MonadThrow (MaybeT m)

Throws exceptions into the base monad.

Methods

throwM :: Exception e => e -> MaybeT m a

Monad m => MonadThrow (CatchT m)

Methods

throwM :: Exception e => e -> CatchT m a

MonadThrow m => MonadThrow (ExceptT e m)

Throws exceptions into the base monad.

Methods

throwM :: Exception e => e -> ExceptT e m a

(Error e, MonadThrow m) => MonadThrow (ErrorT e m)

Throws exceptions into the base monad.

Methods

throwM :: Exception e => e -> ErrorT e m a

MonadThrow m => MonadThrow (StateT s m)

Methods

throwM :: Exception e => e -> StateT s m a

MonadThrow m => MonadThrow (StateT s m)

Methods

throwM :: Exception e => e -> StateT s m a

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

Methods

throwM :: Exception e => e -> WriterT w m a

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

Methods

throwM :: Exception e => e -> WriterT w m a

MonadThrow m => MonadThrow (IdentityT * m)

Methods

throwM :: Exception e => e -> IdentityT * m a

MonadThrow m => MonadThrow (ContT * r m)

Methods

throwM :: Exception e => e -> ContT * r m a

MonadThrow m => MonadThrow (ReaderT * r m)

Methods

throwM :: Exception e => e -> ReaderT * r m a

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

Methods

throwM :: Exception e => e -> RWST r w s m a

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

Methods

throwM :: Exception e => e -> RWST r w s m a

class MonadThrow m => MonadCatch m where

A class for monads which allow exceptions to be caught, in particular exceptions which were thrown by throwM.

Instances should obey the following law:

catch (throwM e) f = f e

Note that the ability to catch an exception does not guarantee that we can deal with all possible exit points from a computation. Some monads, such as continuation-based stacks, allow for more than just a success/failure strategy, and therefore catch cannot be used by those monads to properly implement a function such as finally. For more information, see MonadMask.

Minimal complete definition

catch

Methods

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

Provide a handler for exceptions thrown during execution of the first action. Note that type of the type of the argument to the handler will constrain which exceptions are caught. See Control.Exception's catch.

Instances

MonadCatch IO

Methods

catch :: Exception e => IO a -> (e -> IO a) -> IO a

MonadCatch STM

Methods

catch :: Exception e => STM a -> (e -> STM a) -> STM a

(~) * e SomeException => MonadCatch (Either e)

Since: 0.8.3

Methods

catch :: Exception e => Either e a -> (e -> Either e a) -> Either e a

MonadCatch m => MonadCatch (ListT m)

Methods

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

MonadCatch m => MonadCatch (MaybeT m)

Catches exceptions from the base monad.

Methods

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

Monad m => MonadCatch (CatchT m)

Methods

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

MonadCatch m => MonadCatch (ExceptT e m)

Catches exceptions from the base monad.

Methods

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

(Error e, MonadCatch m) => MonadCatch (ErrorT e m)

Catches exceptions from the base monad.

Methods

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

MonadCatch m => MonadCatch (StateT s m)

Methods

catch :: Exception e => StateT s m a -> (e -> StateT s m a) -> StateT s m a

MonadCatch m => MonadCatch (StateT s m)

Methods

catch :: Exception e => StateT s m a -> (e -> StateT s m a) -> StateT s m a

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

Methods

catch :: Exception e => WriterT w m a -> (e -> WriterT w m a) -> WriterT w m a

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

Methods

catch :: Exception e => WriterT w m a -> (e -> WriterT w m a) -> WriterT w m a

MonadCatch m => MonadCatch (IdentityT * m)

Methods

catch :: Exception e => IdentityT * m a -> (e -> IdentityT * m a) -> IdentityT * m a

MonadCatch m => MonadCatch (ReaderT * r m)

Methods

catch :: Exception e => ReaderT * r m a -> (e -> ReaderT * r m a) -> ReaderT * r m a

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

Methods

catch :: Exception e => RWST r w s m a -> (e -> RWST r w s m a) -> RWST r w s m a

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

Methods

catch :: Exception e => RWST r w s m a -> (e -> RWST r w s m a) -> RWST r w s m a

class MonadCatch m => MonadMask m where

A class for monads which provide for the ability to account for all possible exit points from a computation, and to mask asynchronous exceptions. Continuation-based monads, and stacks such as ErrorT e IO which provide for multiple failure modes, are invalid instances of this class.

Note that this package does provide a MonadMask instance for CatchT. This instance is only valid if the base monad provides no ability to provide multiple exit. For example, IO or Either would be invalid base monads, but Reader or State would be acceptable.

Instances should ensure that, in the following code:

f `finally` g

The action g is called regardless of what occurs within f, including async exceptions.

Minimal complete definition

mask, uninterruptibleMask

Methods

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

Runs an action with asynchronous exceptions disabled. The action is provided a method for restoring the async. environment to what it was at the mask call. See Control.Exception's mask.

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

Like mask, but the masked computation is not interruptible (see Control.Exception's uninterruptibleMask. WARNING: Only use if you need to mask exceptions around an interruptible operation AND you can guarantee the interruptible operation will only block for a short period of time. Otherwise you render the program/thread unresponsive and/or unkillable.

Instances

MonadMask IO

Methods

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

uninterruptibleMask :: ((forall a. IO a -> IO a) -> IO b) -> IO b

(~) * e SomeException => MonadMask (Either e)

Since: 0.8.3

Methods

mask :: ((forall a. Either e a -> Either e a) -> Either e b) -> Either e b

uninterruptibleMask :: ((forall a. Either e a -> Either e a) -> Either e b) -> Either e b

Monad m => MonadMask (CatchT m)

Note: This instance is only valid if the underlying monad has a single exit point!

Methods

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

uninterruptibleMask :: ((forall a. CatchT m a -> CatchT m a) -> CatchT m b) -> CatchT m b

MonadMask m => MonadMask (StateT s m)

Methods

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

uninterruptibleMask :: ((forall a. StateT s m a -> StateT s m a) -> StateT s m b) -> StateT s m b

MonadMask m => MonadMask (StateT s m)

Methods

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

uninterruptibleMask :: ((forall a. StateT s m a -> StateT s m a) -> StateT s m b) -> StateT s m b

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

Methods

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

uninterruptibleMask :: ((forall a. WriterT w m a -> WriterT w m a) -> WriterT w m b) -> WriterT w m b

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

Methods

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

uninterruptibleMask :: ((forall a. WriterT w m a -> WriterT w m a) -> WriterT w m b) -> WriterT w m b

MonadMask m => MonadMask (IdentityT * m)

Methods

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

uninterruptibleMask :: ((forall a. IdentityT * m a -> IdentityT * m a) -> IdentityT * m b) -> IdentityT * m b

MonadMask m => MonadMask (ReaderT * r m)

Methods

mask :: ((forall a. ReaderT * r m a -> ReaderT * r m a) -> ReaderT * r m b) -> ReaderT * r m b

uninterruptibleMask :: ((forall a. ReaderT * r m a -> ReaderT * r m a) -> ReaderT * r m b) -> ReaderT * r m b

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

Methods

mask :: ((forall a. RWST r w s m a -> RWST r w s m a) -> RWST r w s m b) -> RWST r w s m b

uninterruptibleMask :: ((forall a. RWST r w s m a -> RWST r w s m a) -> RWST r w s m b) -> RWST r w s m b

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

Methods

mask :: ((forall a. RWST r w s m a -> RWST r w s m a) -> RWST r w s m b) -> RWST r w s m b

uninterruptibleMask :: ((forall a. RWST r w s m a -> RWST r w s m a) -> RWST r w s m b) -> RWST r w s m b

Utilities

These functions follow those from Control.Exception, except that they are based on methods from the MonadCatch typeclass. See Control.Exception for API usage.

mask_ :: MonadMask m => m a -> m a

Like mask, but does not pass a restore action to the argument.

uninterruptibleMask_ :: MonadMask m => m a -> m a

Like uninterruptibleMask, but does not pass a restore action to the argument.

catchAll :: MonadCatch m => m a -> (SomeException -> m a) -> m a

Catches all exceptions, and somewhat defeats the purpose of the extensible exception system. Use sparingly.

catchIOError :: MonadCatch m => m a -> (IOError -> m a) -> m a

Catch all IOError (eqv. IOException) exceptions. Still somewhat too general, but better than using catchAll. See catchIf for an easy way of catching specific IOErrors based on the predicates in System.IO.Error.

catchJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> m a -> (b -> m a) -> m a

A more generalized way of determining which exceptions to catch at run time.

catchIf :: (MonadCatch m, Exception e) => (e -> Bool) -> m a -> (e -> m a) -> m a

Catch exceptions only if they pass some predicate. Often useful with the predicates for testing IOError values in System.IO.Error.

data Handler m a

Generalized version of Handler

Constructors

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

Instances

Monad m => Functor (Handler m)

Methods

fmap :: (a -> b) -> Handler m a -> Handler m b

(<$) :: a -> Handler m b -> Handler m a

catches :: (Foldable f, MonadCatch m) => m a -> f (Handler m a) -> m a

Catches different sorts of exceptions. See Control.Exception's catches

handle :: (MonadCatch m, Exception e) => (e -> m a) -> m a -> m a

Flipped catch. See Control.Exception's handle.

handleAll :: MonadCatch m => (SomeException -> m a) -> m a -> m a

Flipped catchAll

handleIOError :: MonadCatch m => (IOError -> m a) -> m a -> m a

Flipped catchIOError

handleJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> (b -> m a) -> m a -> m a

Flipped catchJust. See Control.Exception's handleJust.

handleIf :: (MonadCatch m, Exception e) => (e -> Bool) -> (e -> m a) -> m a -> m a

Flipped catchIf

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

Similar to catch, but returns an Either result. See Control.Exception's try.

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

A variant of try that takes an exception predicate to select which exceptions are caught. See Control.Exception's tryJust

onException :: MonadCatch m => m a -> m b -> m a

Run an action only if an exception is thrown in the main action. The exception is not caught, simply rethrown.

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

Generalized abstracted pattern of safe resource acquisition and release in the face of exceptions. The first action "acquires" some value, which is "released" by the second action at the end. The third action "uses" the value and its result is the result of the bracket.

If an exception occurs during the use, the release still happens before the exception is rethrown.

bracket_ :: MonadMask m => m a -> m b -> m c -> m c

Version of bracket without any value being passed to the second and third actions.

finally :: MonadMask m => m a -> m b -> m a

Perform an action with a finalizer action that is run, even if an exception occurs.

bracketOnError :: MonadMask m => m a -> (a -> m b) -> (a -> m c) -> m c

Like bracket, but only performs the final action if there was an exception raised by the in-between computation.

Re-exports from Control.Exception

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

Minimal complete definition

Nothing

Instances

Exception Void

Methods

toException :: Void -> SomeException

fromException :: SomeException -> Maybe Void

displayException :: Void -> String

Exception PatternMatchFail

Methods

toException :: PatternMatchFail -> SomeException

fromException :: SomeException -> Maybe PatternMatchFail

displayException :: PatternMatchFail -> String

Exception RecSelError

Methods

toException :: RecSelError -> SomeException

fromException :: SomeException -> Maybe RecSelError

displayException :: RecSelError -> String

Exception RecConError

Methods

toException :: RecConError -> SomeException

fromException :: SomeException -> Maybe RecConError

displayException :: RecConError -> String

Exception RecUpdError

Methods

toException :: RecUpdError -> SomeException

fromException :: SomeException -> Maybe RecUpdError

displayException :: RecUpdError -> String

Exception NoMethodError

Methods

toException :: NoMethodError -> SomeException

fromException :: SomeException -> Maybe NoMethodError

displayException :: NoMethodError -> String

Exception TypeError

Methods

toException :: TypeError -> SomeException

fromException :: SomeException -> Maybe TypeError

displayException :: TypeError -> String

Exception NonTermination

Methods

toException :: NonTermination -> SomeException

fromException :: SomeException -> Maybe NonTermination

displayException :: NonTermination -> String

Exception NestedAtomically

Methods

toException :: NestedAtomically -> SomeException

fromException :: SomeException -> Maybe NestedAtomically

displayException :: NestedAtomically -> String

Exception BlockedIndefinitelyOnMVar

Methods

toException :: BlockedIndefinitelyOnMVar -> SomeException

fromException :: SomeException -> Maybe BlockedIndefinitelyOnMVar

displayException :: BlockedIndefinitelyOnMVar -> String

Exception BlockedIndefinitelyOnSTM

Methods

toException :: BlockedIndefinitelyOnSTM -> SomeException

fromException :: SomeException -> Maybe BlockedIndefinitelyOnSTM

displayException :: BlockedIndefinitelyOnSTM -> String

Exception Deadlock

Methods

toException :: Deadlock -> SomeException

fromException :: SomeException -> Maybe Deadlock

displayException :: Deadlock -> String

Exception AllocationLimitExceeded

Methods

toException :: AllocationLimitExceeded -> SomeException

fromException :: SomeException -> Maybe AllocationLimitExceeded

displayException :: AllocationLimitExceeded -> String

Exception AssertionFailed

Methods

toException :: AssertionFailed -> SomeException

fromException :: SomeException -> Maybe AssertionFailed

displayException :: AssertionFailed -> String

Exception SomeAsyncException

Methods

toException :: SomeAsyncException -> SomeException

fromException :: SomeException -> Maybe SomeAsyncException

displayException :: SomeAsyncException -> String

Exception AsyncException

Methods

toException :: AsyncException -> SomeException

fromException :: SomeException -> Maybe AsyncException

displayException :: AsyncException -> String

Exception ArrayException

Methods

toException :: ArrayException -> SomeException

fromException :: SomeException -> Maybe ArrayException

displayException :: ArrayException -> String

Exception ExitCode

Methods

toException :: ExitCode -> SomeException

fromException :: SomeException -> Maybe ExitCode

displayException :: ExitCode -> String

Exception IOException

Methods

toException :: IOException -> SomeException

fromException :: SomeException -> Maybe IOException

displayException :: IOException -> String

Exception ErrorCall

Methods

toException :: ErrorCall -> SomeException

fromException :: SomeException -> Maybe ErrorCall

displayException :: ErrorCall -> String

Exception ArithException

Methods

toException :: ArithException -> SomeException

fromException :: SomeException -> Maybe ArithException

displayException :: ArithException -> String

Exception SomeException

Methods

toException :: SomeException -> SomeException

fromException :: SomeException -> Maybe SomeException

displayException :: SomeException -> String

data SomeException :: * where

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.

Constructors

SomeException :: SomeException

Instances

Show SomeException

Methods

showsPrec :: Int -> SomeException -> ShowS

show :: SomeException -> String

showList :: [SomeException] -> ShowS

Exception SomeException

Methods

toException :: SomeException -> SomeException

fromException :: SomeException -> Maybe SomeException

displayException :: SomeException -> String