Control.Exception.Safe

Please see the README.md file in the safe-exceptions repo for information on how to use this module. Relevant links:

• https://github.com/fpco/safe-exceptions#readme
• https://www.stackage.org/package/safe-exceptions

Throwing

throw :: (MonadThrow m, Exception e) => e -> m a

Synchronously throw the given exception

Since: 0.1.0.0

throwIO :: (MonadThrow m, Exception e) => e -> m a

Synonym for throw

Since: 0.1.0.0

throwM :: (MonadThrow m, Exception e) => e -> m a

Synonym for throw

Since: 0.1.0.0

throwString :: (MonadThrow m, HasCallStack) => String -> m a

A convenience function for throwing a user error. This is useful for cases where it would be too high a burden to define your own exception type.

This throws an exception of type StringException. When GHC supports it (base 4.9 and GHC 8.0 and onward), it includes a call stack.

Since: 0.1.5.0

data StringException

Exception type thrown by throwString.

Note that the second field of the data constructor depends on GHC/base version. For base 4.9 and GHC 8.0 and later, the second field is a call stack. Previous versions of GHC and base do not support call stacks, and the field is simply unit (provided to make pattern matching across GHC versions easier).

Since: 0.1.5.0

Constructors

StringException String CallStack

Instances

Exception StringException

Defined in Control.Exception.Safe

Methods

toException :: StringException -> SomeException

fromException :: SomeException -> Maybe StringException

displayException :: StringException -> String

Show StringException

Defined in Control.Exception.Safe

Methods

showsPrec :: Int -> StringException -> ShowS

show :: StringException -> String

showList :: [StringException] -> ShowS

throwTo :: (Exception e, MonadIO m) => ThreadId -> e -> m ()

Throw an asynchronous exception to another thread.

Synchronously typed exceptions will be wrapped into an AsyncExceptionWrapper, see https://github.com/fpco/safe-exceptions#determining-sync-vs-async

It's usually a better idea to use the async package, see https://github.com/fpco/safe-exceptions#quickstart

Since: 0.1.0.0

impureThrow :: Exception e => e -> a

Generate a pure value which, when forced, will synchronously throw the given exception

Generally it's better to avoid using this function and instead use throw, see https://github.com/fpco/safe-exceptions#quickstart

Since: 0.1.0.0

Catching (with recovery)

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

Same as upstream catch, but will not catch asynchronous exceptions

Since: 0.1.0.0

catchIO :: MonadCatch m => m a -> (IOException -> m a) -> m a

catch specialized to only catching IOExceptions

Since: 0.1.3.0

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

catch specialized to catch all synchronous exception

Since: 0.1.0.0

catchDeep :: (MonadCatch m, MonadIO m, Exception e, NFData a) => m a -> (e -> m a) -> m a

Same as catch, but fully force evaluation of the result value to find all impure exceptions.

Since: 0.1.1.0

catchAnyDeep :: (MonadCatch m, MonadIO m, NFData a) => m a -> (SomeException -> m a) -> m a

catchDeep specialized to catch all synchronous exception

Since: 0.1.1.0

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

catch without async exception safety

Generally it's better to avoid using this function since we do not want to recover from async exceptions, see https://github.com/fpco/safe-exceptions#quickstart

Since: 0.1.0.0

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

catchJust is like catch but it takes an extra argument which is an exception predicate, a function which selects which type of exceptions we're interested in.

Since: 0.1.4.0

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

Flipped version of catch

Since: 0.1.0.0

handleIO :: MonadCatch m => (IOException -> m a) -> m a -> m a

handle specialized to only catching IOExceptions

Since: 0.1.3.0

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

Flipped version of catchAny

Since: 0.1.0.0

handleDeep :: (MonadCatch m, Exception e, MonadIO m, NFData a) => (e -> m a) -> m a -> m a

Flipped version of catchDeep

Since: 0.1.1.0

handleAnyDeep :: (MonadCatch m, MonadIO m, NFData a) => (SomeException -> m a) -> m a -> m a

Flipped version of catchAnyDeep

Since: 0.1.1.0

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

Flipped version of catchAsync

Generally it's better to avoid using this function since we do not want to recover from async exceptions, see https://github.com/fpco/safe-exceptions#quickstart

Since: 0.1.0.0

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

Flipped catchJust.

Since: 0.1.4.0

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

Same as upstream try, but will not catch asynchronous exceptions

Since: 0.1.0.0

tryIO :: MonadCatch m => m a -> m (Either IOException a)

try specialized to only catching IOExceptions

Since: 0.1.3.0

tryAny :: MonadCatch m => m a -> m (Either SomeException a)

try specialized to catch all synchronous exceptions

Since: 0.1.0.0

tryDeep :: (MonadCatch m, MonadIO m, Exception e, NFData a) => m a -> m (Either e a)

Same as try, but fully force evaluation of the result value to find all impure exceptions.

Since: 0.1.1.0

tryAnyDeep :: (MonadCatch m, MonadIO m, NFData a) => m a -> m (Either SomeException a)

tryDeep specialized to catch all synchronous exceptions

Since: 0.1.1.0

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

try without async exception safety

Generally it's better to avoid using this function since we do not want to recover from async exceptions, see https://github.com/fpco/safe-exceptions#quickstart

Since: 0.1.0.0

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.

Since: 0.1.4.0

data Handler (m :: Type -> Type) a

Generalized version of Handler

Constructors

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

Instances

Monad m => Functor (Handler m)

Defined in Control.Monad.Catch

Methods

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

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

catches :: (MonadCatch m, MonadThrow m) => m a -> [Handler m a] -> m a

Same as upstream catches, but will not catch asynchronous exceptions

Since: 0.1.2.0

catchesDeep :: (MonadCatch m, MonadThrow m, MonadIO m, NFData a) => m a -> [Handler m a] -> m a

Same as catches, but fully force evaluation of the result value to find all impure exceptions.

Since: 0.1.2.0

catchesAsync :: (MonadCatch m, MonadThrow m) => m a -> [Handler m a] -> m a

catches without async exception safety

Generally it's better to avoid using this function since we do not want to recover from async exceptions, see https://github.com/fpco/safe-exceptions#quickstart

Since: 0.1.2.0

Cleanup (no recovery)

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

Async safe version of onException

Since: 0.1.0.0

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

Async safe version of bracket

Since: 0.1.0.0

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

Async safe version of bracket_

Since: 0.1.0.0

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

Async safe version of finally

Since: 0.1.0.0

withException :: (MonadMask m, Exception e) => m a -> (e -> m b) -> m a

Like onException, but provides the handler the thrown exception.

Since: 0.1.0.0

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

Async safe version of bracketOnError

Since: 0.1.0.0

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

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

Since: 0.1.0.0

bracketWithError :: forall m a b c. MonadMask m => m a -> (Maybe SomeException -> a -> m b) -> (a -> m c) -> m c

Async safe version of bracket with access to the exception in the cleanup action.

Since: 0.1.7.0

Coercion to sync and async

data SyncExceptionWrapper

Wrap up an asynchronous exception to be treated as a synchronous exception

This is intended to be created via toSyncException

Since: 0.1.0.0

Constructors

forall e.Exception e => SyncExceptionWrapper e

Instances

Exception SyncExceptionWrapper

Defined in Control.Exception.Safe

Methods

toException :: SyncExceptionWrapper -> SomeException

fromException :: SomeException -> Maybe SyncExceptionWrapper

displayException :: SyncExceptionWrapper -> String

Show SyncExceptionWrapper

Defined in Control.Exception.Safe

Methods

showsPrec :: Int -> SyncExceptionWrapper -> ShowS

show :: SyncExceptionWrapper -> String

showList :: [SyncExceptionWrapper] -> ShowS

toSyncException :: Exception e => e -> SomeException

Convert an exception into a synchronous exception

For synchronous exceptions, this is the same as toException. For asynchronous exceptions, this will wrap up the exception with SyncExceptionWrapper

Since: 0.1.0.0

data AsyncExceptionWrapper

Wrap up a synchronous exception to be treated as an asynchronous exception

This is intended to be created via toAsyncException

Since: 0.1.0.0

Constructors

forall e.Exception e => AsyncExceptionWrapper e

Instances

Exception AsyncExceptionWrapper

Defined in Control.Exception.Safe

Methods

toException :: AsyncExceptionWrapper -> SomeException

fromException :: SomeException -> Maybe AsyncExceptionWrapper

displayException :: AsyncExceptionWrapper -> String

Show AsyncExceptionWrapper

Defined in Control.Exception.Safe

Methods

showsPrec :: Int -> AsyncExceptionWrapper -> ShowS

show :: AsyncExceptionWrapper -> String

showList :: [AsyncExceptionWrapper] -> ShowS

toAsyncException :: Exception e => e -> SomeException

Convert an exception into an asynchronous exception

For asynchronous exceptions, this is the same as toException. For synchronous exceptions, this will wrap up the exception with AsyncExceptionWrapper

Since: 0.1.0.0

Check exception type

isSyncException :: Exception e => e -> Bool

Check if the given exception is synchronous

Since: 0.1.0.0

isAsyncException :: Exception e => e -> Bool

Check if the given exception is asynchronous

Since: 0.1.0.0

Reexports

class Monad m => MonadThrow (m :: Type -> Type)

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

Instances

MonadThrow STM

Defined in Control.Monad.Catch

Methods

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

MonadThrow IO

Defined in Control.Monad.Catch

Methods

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

MonadThrow Q

Defined in Control.Monad.Catch

Methods

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

MonadThrow Maybe

Defined in Control.Monad.Catch

Methods

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

MonadThrow []

Defined in Control.Monad.Catch

Methods

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

e ~ SomeException => MonadThrow (Either e)

Defined in Control.Monad.Catch

Methods

throwM :: Exception e0 => e0 -> Either e a

MonadThrow (ST s)

Defined in Control.Monad.Catch

Methods

throwM :: Exception e => e -> ST s a

MonadThrow m => MonadThrow (ListT m)

Defined in Control.Monad.Catch

Methods

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

MonadThrow m => MonadThrow (MaybeT m)

Throws exceptions into the base monad.

Defined in Control.Monad.Catch

Methods

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

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

Throws exceptions into the base monad.

Defined in Control.Monad.Catch

Methods

throwM :: Exception e0 => e0 -> ErrorT e m a

MonadThrow m => MonadThrow (ExceptT e m)

Throws exceptions into the base monad.

Defined in Control.Monad.Catch

Methods

throwM :: Exception e0 => e0 -> ExceptT e m a

MonadThrow m => MonadThrow (IdentityT m)

Defined in Control.Monad.Catch

Methods

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

MonadThrow m => MonadThrow (ReaderT r m)

Defined in Control.Monad.Catch

Methods

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

MonadThrow m => MonadThrow (StateT s m)

Defined in Control.Monad.Catch

Methods

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

MonadThrow m => MonadThrow (StateT s m)

Defined in Control.Monad.Catch

Methods

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

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

Defined in Control.Monad.Catch

Methods

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

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

Defined in Control.Monad.Catch

Methods

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

MonadThrow m => MonadThrow (ContT r m)

Defined in Control.Monad.Catch

Methods

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

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

Defined in Control.Monad.Catch

Methods

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

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

Defined in Control.Monad.Catch

Methods

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

class MonadThrow m => MonadCatch (m :: Type -> Type)

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

Instances

MonadCatch STM

Defined in Control.Monad.Catch

Methods

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

MonadCatch IO

Defined in Control.Monad.Catch

Methods

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

e ~ SomeException => MonadCatch (Either e)

Since: exceptions-0.8.3

Defined in Control.Monad.Catch

Methods

catch :: Exception e0 => Either e a -> (e0 -> Either e a) -> Either e a

MonadCatch m => MonadCatch (ListT m)

Defined in Control.Monad.Catch

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.

Defined in Control.Monad.Catch

Methods

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

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

Catches exceptions from the base monad.

Defined in Control.Monad.Catch

Methods

catch :: Exception e0 => ErrorT e m a -> (e0 -> ErrorT e m a) -> ErrorT e m a

MonadCatch m => MonadCatch (ExceptT e m)

Catches exceptions from the base monad.

Defined in Control.Monad.Catch

Methods

catch :: Exception e0 => ExceptT e m a -> (e0 -> ExceptT e m a) -> ExceptT e m a

MonadCatch m => MonadCatch (IdentityT m)

Defined in Control.Monad.Catch

Methods

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

MonadCatch m => MonadCatch (ReaderT r m)

Defined in Control.Monad.Catch

Methods

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

MonadCatch m => MonadCatch (StateT s m)

Defined in Control.Monad.Catch

Methods

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

MonadCatch m => MonadCatch (StateT s m)

Defined in Control.Monad.Catch

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)

Defined in Control.Monad.Catch

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)

Defined in Control.Monad.Catch

Methods

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

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

Defined in Control.Monad.Catch

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)

Defined in Control.Monad.Catch

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 :: Type -> Type) 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 are invalid instances of this class.

Instances should ensure that, in the following code:

fg = f `finally` g

The action g is called regardless of what occurs within f, including async exceptions. Some monads allow f to abort the computation via other effects than throwing an exception. For simplicity, we will consider aborting and throwing an exception to be two forms of "throwing an error".

If f and g both throw an error, the error thrown by fg depends on which errors we're talking about. In a monad transformer stack, the deeper layers override the effects of the inner layers; for example, ExceptT e1 (Except e2) a represents a value of type Either e2 (Either e1 a), so throwing both an e1 and an e2 will result in Left e2. If f and g both throw an error from the same layer, instances should ensure that the error from g wins.

Effects other than throwing an error are also overriden by the deeper layers. For example, StateT s Maybe a represents a value of type s -> Maybe (a, s), so if an error thrown from f causes this function to return Nothing, any changes to the state which f also performed will be erased. As a result, g will see the state as it was before f. Once g completes, f's error will be rethrown, so g' state changes will be erased as well. This is the normal interaction between effects in a monad transformer stack.

By contrast, lifted-base's version of finally always discards all of g's non-IO effects, and g never sees any of f's non-IO effects, regardless of the layer ordering and regardless of whether f throws an error. This is not the result of interacting effects, but a consequence of MonadBaseControl's approach.

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.

generalBracket

Arguments

:: m a

acquire some resource

-> (a -> ExitCase b -> m c)

release the resource, observing the outcome of the inner action

-> (a -> m b)

inner action to perform with the resource

-> m (b, c)

A generalized version of bracket which uses ExitCase to distinguish the different exit cases, and returns the values of both the use and release actions. In practice, this extra information is rarely needed, so it is often more convenient to use one of the simpler functions which are defined in terms of this one, such as bracket, finally, onError, and bracketOnError.

This function exists because in order to thread their effects through the execution of bracket, monad transformers need values to be threaded from use to release and from release to the output value.

NOTE This method was added in version 0.9.0 of this library. Previously, implementation of functions like bracket and finally in this module were based on the mask and uninterruptibleMask functions only, disallowing some classes of tranformers from having MonadMask instances (notably multi-exit-point transformers like ExceptT). If you are a library author, you'll now need to provide an implementation for this method. The StateT implementation demonstrates most of the subtleties:

generalBracket acquire release use = StateT $ s0 -> do
  ((b, _s2), (c, s3)) <- generalBracket
    (runStateT acquire s0)
    ((resource, s1) exitCase -> case exitCase of
      ExitCaseSuccess (b, s2) -> runStateT (release resource (ExitCaseSuccess b)) s2

      -- In the two other cases, the base monad overrides use's state
      -- changes and the state reverts to s1.
      ExitCaseException e     -> runStateT (release resource (ExitCaseException e)) s1
      ExitCaseAbort           -> runStateT (release resource ExitCaseAbort) s1
    )
    ((resource, s1) -> runStateT (use resource) s1)
  return ((b, c), s3)

The StateT s m implementation of generalBracket delegates to the m implementation of generalBracket. The acquire, use, and release arguments given to StateT's implementation produce actions of type StateT s m a, StateT s m b, and StateT s m c. In order to run those actions in the base monad, we need to call runStateT, from which we obtain actions of type m (a, s), m (b, s), and m (c, s). Since each action produces the next state, it is important to feed the state produced by the previous action to the next action.

In the ExitCaseSuccess case, the state starts at s0, flows through acquire to become s1, flows through use to become s2, and finally flows through release to become s3. In the other two cases, release does not receive the value s2, so its action cannot see the state changes performed by use. This is fine, because in those two cases, an error was thrown in the base monad, so as per the usual interaction between effects in a monad transformer stack, those state changes get reverted. So we start from s1 instead.

Finally, the m implementation of generalBracket returns the pairs (b, s) and (c, s). For monad transformers other than StateT, this will be some other type representing the effects and values performed and returned by the use and release actions. The effect part of the use result, in this case _s2, usually needs to be discarded, since those effects have already been incorporated in the release action.

The only effect which is intentionally not incorporated in the release action is the effect of throwing an error. In that case, the error must be re-thrown. One subtlety which is easy to miss is that in the case in which use and release both throw an error, the error from release should take priority. Here is an implementation for ExceptT which demonstrates how to do this.

generalBracket acquire release use = ExceptT $ do
  (eb, ec) <- generalBracket
    (runExceptT acquire)
    (eresource exitCase -> case eresource of
      Left e -> return (Left e) -- nothing to release, acquire didn't succeed
      Right resource -> case exitCase of
        ExitCaseSuccess (Right b) -> runExceptT (release resource (ExitCaseSuccess b))
        ExitCaseException e       -> runExceptT (release resource (ExitCaseException e))
        _                         -> runExceptT (release resource ExitCaseAbort))
    (either (return . Left) (runExceptT . use))
  return $ do
    -- The order in which we perform those two Either effects determines
    -- which error will win if they are both Lefts. We want the error from
    -- release to win.
    c <- ec
    b <- eb
    return (b, c)

Since: exceptions-0.9.0

Instances

MonadMask IO

Defined in Control.Monad.Catch

Methods

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

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

generalBracket :: IO a -> (a -> ExitCase b -> IO c) -> (a -> IO b) -> IO (b, c)

e ~ SomeException => MonadMask (Either e)

Since: exceptions-0.8.3

Defined in Control.Monad.Catch

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

generalBracket :: Either e a -> (a -> ExitCase b -> Either e c) -> (a -> Either e b) -> Either e (b, c)

MonadMask m => MonadMask (MaybeT m)

Since: exceptions-0.10.0

Defined in Control.Monad.Catch

Methods

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

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

generalBracket :: MaybeT m a -> (a -> ExitCase b -> MaybeT m c) -> (a -> MaybeT m b) -> MaybeT m (b, c)

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

Defined in Control.Monad.Catch

Methods

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

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

generalBracket :: ErrorT e m a -> (a -> ExitCase b -> ErrorT e m c) -> (a -> ErrorT e m b) -> ErrorT e m (b, c)

MonadMask m => MonadMask (ExceptT e m)

Since: exceptions-0.9.0

Defined in Control.Monad.Catch

Methods

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

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

generalBracket :: ExceptT e m a -> (a -> ExitCase b -> ExceptT e m c) -> (a -> ExceptT e m b) -> ExceptT e m (b, c)

MonadMask m => MonadMask (IdentityT m)

Defined in Control.Monad.Catch

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

generalBracket :: IdentityT m a -> (a -> ExitCase b -> IdentityT m c) -> (a -> IdentityT m b) -> IdentityT m (b, c)

MonadMask m => MonadMask (ReaderT r m)

Defined in Control.Monad.Catch

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

generalBracket :: ReaderT r m a -> (a -> ExitCase b -> ReaderT r m c) -> (a -> ReaderT r m b) -> ReaderT r m (b, c)

MonadMask m => MonadMask (StateT s m)

Defined in Control.Monad.Catch

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

generalBracket :: StateT s m a -> (a -> ExitCase b -> StateT s m c) -> (a -> StateT s m b) -> StateT s m (b, c)

MonadMask m => MonadMask (StateT s m)

Defined in Control.Monad.Catch

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

generalBracket :: StateT s m a -> (a -> ExitCase b -> StateT s m c) -> (a -> StateT s m b) -> StateT s m (b, c)

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

Defined in Control.Monad.Catch

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

generalBracket :: WriterT w m a -> (a -> ExitCase b -> WriterT w m c) -> (a -> WriterT w m b) -> WriterT w m (b, c)

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

Defined in Control.Monad.Catch

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

generalBracket :: WriterT w m a -> (a -> ExitCase b -> WriterT w m c) -> (a -> WriterT w m b) -> WriterT w m (b, c)

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

Defined in Control.Monad.Catch

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

generalBracket :: RWST r w s m a -> (a -> ExitCase b -> RWST r w s m c) -> (a -> RWST r w s m b) -> RWST r w s m (b, c)

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

Defined in Control.Monad.Catch

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

generalBracket :: RWST r w s m a -> (a -> ExitCase b -> RWST r w s m c) -> (a -> RWST r w s m b) -> RWST r w s m (b, c)

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.

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.

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

Flipped catchIOError

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

Exception Void

Since: base-4.8.0.0

Defined in Data.Void

Methods

toException :: Void -> SomeException

fromException :: SomeException -> Maybe Void

displayException :: Void -> String

Exception ErrorCall

Since: base-4.0.0.0

Defined in GHC.Exception

Methods

toException :: ErrorCall -> SomeException

fromException :: SomeException -> Maybe ErrorCall

displayException :: ErrorCall -> String

Exception ArithException

Since: base-4.0.0.0

Defined in GHC.Exception.Type

Methods

toException :: ArithException -> SomeException

fromException :: SomeException -> Maybe ArithException

displayException :: ArithException -> String

Exception SomeException

Since: base-3.0

Defined in GHC.Exception.Type

Methods

toException :: SomeException -> SomeException

fromException :: SomeException -> Maybe SomeException

displayException :: SomeException -> String

Exception AllocationLimitExceeded

Since: base-4.8.0.0

Defined in GHC.IO.Exception

Methods

toException :: AllocationLimitExceeded -> SomeException

fromException :: SomeException -> Maybe AllocationLimitExceeded

displayException :: AllocationLimitExceeded -> String

Exception ArrayException

Since: base-4.1.0.0

Defined in GHC.IO.Exception

Methods

toException :: ArrayException -> SomeException

fromException :: SomeException -> Maybe ArrayException

displayException :: ArrayException -> String

Exception AssertionFailed

Since: base-4.1.0.0

Defined in GHC.IO.Exception

Methods

toException :: AssertionFailed -> SomeException

fromException :: SomeException -> Maybe AssertionFailed

displayException :: AssertionFailed -> String

Exception AsyncException

Since: base-4.7.0.0

Defined in GHC.IO.Exception

Methods

toException :: AsyncException -> SomeException

fromException :: SomeException -> Maybe AsyncException

displayException :: AsyncException -> String

Exception BlockedIndefinitelyOnMVar

Since: base-4.1.0.0

Defined in GHC.IO.Exception

Methods

toException :: BlockedIndefinitelyOnMVar -> SomeException

fromException :: SomeException -> Maybe BlockedIndefinitelyOnMVar

displayException :: BlockedIndefinitelyOnMVar -> String

Exception BlockedIndefinitelyOnSTM

Since: base-4.1.0.0

Defined in GHC.IO.Exception

Methods

toException :: BlockedIndefinitelyOnSTM -> SomeException

fromException :: SomeException -> Maybe BlockedIndefinitelyOnSTM

displayException :: BlockedIndefinitelyOnSTM -> String

Exception CompactionFailed

Since: base-4.10.0.0

Defined in GHC.IO.Exception

Methods

toException :: CompactionFailed -> SomeException

fromException :: SomeException -> Maybe CompactionFailed

displayException :: CompactionFailed -> String

Exception Deadlock

Since: base-4.1.0.0

Defined in GHC.IO.Exception

Methods

toException :: Deadlock -> SomeException

fromException :: SomeException -> Maybe Deadlock

displayException :: Deadlock -> String

Exception ExitCode

Since: base-4.1.0.0

Defined in GHC.IO.Exception

Methods

toException :: ExitCode -> SomeException

fromException :: SomeException -> Maybe ExitCode

displayException :: ExitCode -> String

Exception FixIOException

Since: base-4.11.0.0

Defined in GHC.IO.Exception

Methods

toException :: FixIOException -> SomeException

fromException :: SomeException -> Maybe FixIOException

displayException :: FixIOException -> String

Exception IOException

Since: base-4.1.0.0

Defined in GHC.IO.Exception

Methods

toException :: IOException -> SomeException

fromException :: SomeException -> Maybe IOException

displayException :: IOException -> String

Exception SomeAsyncException

Since: base-4.7.0.0

Defined in GHC.IO.Exception

Methods

toException :: SomeAsyncException -> SomeException

fromException :: SomeException -> Maybe SomeAsyncException

displayException :: SomeAsyncException -> String

Exception AsyncExceptionWrapper

Defined in Control.Exception.Safe

Methods

toException :: AsyncExceptionWrapper -> SomeException

fromException :: SomeException -> Maybe AsyncExceptionWrapper

displayException :: AsyncExceptionWrapper -> String

Exception StringException

Defined in Control.Exception.Safe

Methods

toException :: StringException -> SomeException

fromException :: SomeException -> Maybe StringException

displayException :: StringException -> String

Exception SyncExceptionWrapper

Defined in Control.Exception.Safe

Methods

toException :: SyncExceptionWrapper -> SomeException

fromException :: SomeException -> Maybe SyncExceptionWrapper

displayException :: SyncExceptionWrapper -> String

class Typeable (a :: k)

The class Typeable allows a concrete representation of a type to be calculated.

Minimal complete definition

typeRep#

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.

Constructors

Exception e => SomeException e

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

data SomeAsyncException

Superclass for asynchronous exceptions.

Since: base-4.7.0.0

Constructors

Exception e => SomeAsyncException e

Instances

Exception SomeAsyncException

Since: base-4.7.0.0

Defined in GHC.IO.Exception

Methods

toException :: SomeAsyncException -> SomeException

fromException :: SomeException -> Maybe SomeAsyncException

displayException :: SomeAsyncException -> String

Show SomeAsyncException

Since: base-4.7.0.0

Defined in GHC.IO.Exception

Methods

showsPrec :: Int -> SomeAsyncException -> ShowS

show :: SomeAsyncException -> String

showList :: [SomeAsyncException] -> ShowS

data IOException

Exceptions that occur in the IO monad. An IOException records a more specific error type, a descriptive string and maybe the handle that was used when the error was flagged.

Instances

Exception IOException

Since: base-4.1.0.0

Defined in GHC.IO.Exception

Methods

toException :: IOException -> SomeException

fromException :: SomeException -> Maybe IOException

displayException :: IOException -> String

Show IOException

Since: base-4.1.0.0

Defined in GHC.IO.Exception

Methods

showsPrec :: Int -> IOException -> ShowS

show :: IOException -> String

showList :: [IOException] -> ShowS

Eq IOException

Since: base-4.1.0.0

Defined in GHC.IO.Exception

Methods

(==) :: IOException -> IOException -> Bool

(/=) :: IOException -> IOException -> Bool

Error IOException

Defined in Control.Monad.Trans.Error

Methods

noMsg :: IOException

strMsg :: String -> IOException

assert :: Bool -> a -> a

If the first argument evaluates to True, then the result is the second argument. Otherwise an AssertionFailed exception is raised, containing a String with the source file and line number of the call to assert.

Assertions can normally be turned on or off with a compiler flag (for GHC, assertions are normally on unless optimisation is turned on with -O or the -fignore-asserts option is given). When assertions are turned off, the first argument to assert is ignored, and the second argument is returned as the result.