Safe Haskell	Safe-Inferred
Language	Haskell2010

Control.Exception.Safe

Contents

Throwing
Catching (with recovery)
Cleanup (no recovery)
Coercion to sync and async
Check exception type
Reexports

Description

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

Synopsis

throw :: ( MonadThrow m, Exception e) => e -> m a
throwIO :: ( MonadThrow m, Exception e) => e -> m a
throwM :: ( MonadThrow m, Exception e) => e -> m a
throwString :: ( MonadThrow m, HasCallStack ) => String -> m a
data StringException = StringException String CallStack
throwTo :: ( Exception e, MonadIO m) => ThreadId -> e -> m ()
impureThrow :: Exception e => e -> a
catch :: ( MonadCatch m, Exception e) => m a -> (e -> m a) -> m a
catchIO :: MonadCatch m => m a -> ( IOException -> m a) -> m a
catchAny :: MonadCatch m => m a -> ( SomeException -> m a) -> m a
catchDeep :: ( MonadCatch m, MonadIO m, Exception e, NFData a) => m a -> (e -> m a) -> m a
catchAnyDeep :: ( MonadCatch m, MonadIO m, NFData a) => m a -> ( SomeException -> m a) -> m a
catchAsync :: ( MonadCatch m, Exception e) => m a -> (e -> m a) -> m a
catchJust :: ( MonadCatch m, Exception e) => (e -> Maybe b) -> m a -> (b -> m a) -> m a
handle :: ( MonadCatch m, Exception e) => (e -> m a) -> m a -> m a
handleIO :: MonadCatch m => ( IOException -> m a) -> m a -> m a
handleAny :: MonadCatch m => ( SomeException -> m a) -> m a -> m a
handleDeep :: ( MonadCatch m, Exception e, MonadIO m, NFData a) => (e -> m a) -> m a -> m a
handleAnyDeep :: ( MonadCatch m, MonadIO m, NFData a) => ( SomeException -> m a) -> m a -> m a
handleAsync :: ( MonadCatch m, Exception e) => (e -> m a) -> m a -> m a
handleJust :: ( MonadCatch m, Exception e) => (e -> Maybe b) -> (b -> m a) -> m a -> m a
try :: ( MonadCatch m, Exception e) => m a -> m ( Either e a)
tryIO :: MonadCatch m => m a -> m ( Either IOException a)
tryAny :: MonadCatch m => m a -> m ( Either SomeException a)
tryDeep :: ( MonadCatch m, MonadIO m, Exception e, NFData a) => m a -> m ( Either e a)
tryAnyDeep :: ( MonadCatch m, MonadIO m, NFData a) => m a -> m ( Either SomeException a)
tryAsync :: ( MonadCatch m, Exception e) => m a -> m ( Either e a)
tryJust :: ( MonadCatch m, Exception e) => (e -> Maybe b) -> m a -> m ( Either b a)
data Handler (m :: Type -> Type ) a = Exception e => Handler (e -> m a)
catches :: ( MonadCatch m, MonadThrow m) => m a -> [ Handler m a] -> m a
catchesDeep :: ( MonadCatch m, MonadThrow m, MonadIO m, NFData a) => m a -> [ Handler m a] -> m a
catchesAsync :: ( MonadCatch m, MonadThrow m) => m a -> [ Handler m a] -> m a
onException :: MonadMask m => m a -> m b -> m a
bracket :: forall m a b c. MonadMask m => m a -> (a -> m b) -> (a -> m c) -> m c
bracket_ :: MonadMask m => m a -> m b -> m c -> m c
finally :: MonadMask m => m a -> m b -> m a
withException :: ( MonadMask m, Exception e) => m a -> (e -> m b) -> m a
bracketOnError :: forall m a b c. MonadMask m => m a -> (a -> m b) -> (a -> m c) -> m c
bracketOnError_ :: MonadMask m => m a -> m b -> m c -> m c
bracketWithError :: forall m a b c. MonadMask m => m a -> ( Maybe SomeException -> a -> m b) -> (a -> m c) -> m c
data SyncExceptionWrapper = forall e. Exception e => SyncExceptionWrapper e
toSyncException :: Exception e => e -> SomeException
data AsyncExceptionWrapper = forall e. Exception e => AsyncExceptionWrapper e
toAsyncException :: Exception e => e -> SomeException
isSyncException :: Exception e => e -> Bool
isAsyncException :: Exception e => e -> Bool
class Monad m => MonadThrow (m :: Type -> Type )
class MonadThrow m => MonadCatch (m :: Type -> Type )
class MonadCatch m => MonadMask (m :: Type -> Type ) where
- mask :: (( forall a. m a -> m a) -> m b) -> m b
- uninterruptibleMask :: (( forall a. m a -> m a) -> m b) -> m b
- generalBracket :: m a -> (a -> ExitCase b -> m c) -> (a -> m b) -> m (b, c)
mask_ :: MonadMask m => m a -> m a
uninterruptibleMask_ :: MonadMask m => m a -> m a
catchIOError :: MonadCatch m => m a -> ( IOError -> m a) -> m a
handleIOError :: MonadCatch m => ( IOError -> m a) -> m a -> m a
class ( Typeable e, Show e) => Exception e where
- toException :: e -> SomeException
- fromException :: SomeException -> Maybe e
- displayException :: e -> String
class Typeable (a :: k)
data SomeException = Exception e => SomeException e
data SomeAsyncException = Exception e => SomeAsyncException e
data IOException
assert :: Bool -> a -> a

Throwing

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

Synchronously throw the given exception

Since: 0.1.0.0

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

Synonym for throw

Since: 0.1.0.0

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

Synonym for throw

Since: 0.1.0.0

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

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 Source #

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

Instances details

Show StringException Source #
Instance details Defined in Control.Exception.Safe Methods showsPrec :: Int -> StringException -> ShowS Source # show :: StringException -> String Source # showList :: [ StringException ] -> ShowS Source #
Exception StringException Source #
Instance details Defined in Control.Exception.Safe Methods toException :: StringException -> SomeException Source # fromException :: SomeException -> Maybe StringException Source # displayException :: StringException -> String Source #

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

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 Source #

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 Source #

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 Source #

catch specialized to only catching IOException s

Since: 0.1.3.0

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

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 Source #

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 Source #

catchDeep specialized to catch all synchronous exception

Since: 0.1.1.0

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

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 Source #

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 Source #

Flipped version of catch

Since: 0.1.0.0

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

handle specialized to only catching IOException s

Since: 0.1.3.0

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

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 Source #

Flipped version of catchDeep

Since: 0.1.1.0

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

Flipped version of catchAnyDeep

Since: 0.1.1.0

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

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 Source #

Flipped catchJust .

Since: 0.1.4.0

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

Same as upstream try , but will not catch asynchronous exceptions

Since: 0.1.0.0

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

try specialized to only catching IOException s

Since: 0.1.3.0

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

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) Source #

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) Source #

tryDeep specialized to catch all synchronous exceptions

Since: 0.1.1.0

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

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) Source #

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 Source #

Generalized version of Handler

Constructors

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

Instances

Instances details

Monad m => Functor ( Handler m)
Instance details Defined in Control.Monad.Catch Methods fmap :: (a -> b) -> Handler m a -> Handler m b Source # (<$) :: a -> Handler m b -> Handler m a Source #

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

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 Source #

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 Source #

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 Source #

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 Source #

Async safe version of bracket

Since: 0.1.0.0

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

Async safe version of bracket_

Since: 0.1.0.0

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

Async safe version of finally

Since: 0.1.0.0

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

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 Source #

Async safe version of bracketOnError

Since: 0.1.0.0

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

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 Source #

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 Source #

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

Instances details

Show SyncExceptionWrapper Source #
Instance details Defined in Control.Exception.Safe Methods showsPrec :: Int -> SyncExceptionWrapper -> ShowS Source # show :: SyncExceptionWrapper -> String Source # showList :: [ SyncExceptionWrapper ] -> ShowS Source #
Exception SyncExceptionWrapper Source #
Instance details Defined in Control.Exception.Safe Methods toException :: SyncExceptionWrapper -> SomeException Source # fromException :: SomeException -> Maybe SyncExceptionWrapper Source # displayException :: SyncExceptionWrapper -> String Source #

toSyncException :: Exception e => e -> SomeException Source #

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 Source #

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

Instances details

Show AsyncExceptionWrapper Source #
Instance details Defined in Control.Exception.Safe Methods showsPrec :: Int -> AsyncExceptionWrapper -> ShowS Source # show :: AsyncExceptionWrapper -> String Source # showList :: [ AsyncExceptionWrapper ] -> ShowS Source #
Exception AsyncExceptionWrapper Source #
Instance details Defined in Control.Exception.Safe Methods toException :: AsyncExceptionWrapper -> SomeException Source # fromException :: SomeException -> Maybe AsyncExceptionWrapper Source # displayException :: AsyncExceptionWrapper -> String Source #

toAsyncException :: Exception e => e -> SomeException Source #

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 Source #

Check if the given exception is synchronous

Since: 0.1.0.0

isAsyncException :: Exception e => e -> Bool Source #

Check if the given exception is asynchronous

Since: 0.1.0.0

Reexports

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

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

Instances details

MonadThrow []
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> [a] Source #
MonadThrow Maybe
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> Maybe a Source #
MonadThrow IO
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> IO a Source #
MonadThrow Q
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> Q a Source #
MonadThrow STM
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> STM a Source #
e ~ SomeException => MonadThrow ( Either e)
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e0 => e0 -> Either e a Source #
MonadThrow ( ST s)
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> ST s a Source #
MonadThrow m => MonadThrow ( MaybeT m)	Throws exceptions into the base monad.
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> MaybeT m a Source #
MonadThrow m => MonadThrow ( ListT m)
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> ListT m a Source #
MonadThrow m => MonadThrow ( ExceptT e m)	Throws exceptions into the base monad.
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e0 => e0 -> ExceptT e m a Source #
MonadThrow m => MonadThrow ( IdentityT m)
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> IdentityT m a Source #
( Error e, MonadThrow m) => MonadThrow ( ErrorT e m)	Throws exceptions into the base monad.
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e0 => e0 -> ErrorT e m a Source #
MonadThrow m => MonadThrow ( ReaderT r m)
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> ReaderT r m a Source #
MonadThrow m => MonadThrow ( StateT s m)
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> StateT s m a Source #
MonadThrow m => MonadThrow ( StateT s m)
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> StateT s m a Source #
( MonadThrow m, Monoid w) => MonadThrow ( WriterT w m)
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> WriterT w m a Source #
( MonadThrow m, Monoid w) => MonadThrow ( WriterT w m)
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> WriterT w m a Source #
MonadThrow m => MonadThrow ( ContT r m)
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> ContT r m a Source #
( MonadThrow m, Monoid w) => MonadThrow ( RWST r w s m)
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> RWST r w s m a Source #
( MonadThrow m, Monoid w) => MonadThrow ( RWST r w s m)
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> RWST r w s m a Source #

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

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

Instances details

MonadCatch IO
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => IO a -> (e -> IO a) -> IO a Source #
MonadCatch STM
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => STM a -> (e -> STM a) -> STM a Source #
e ~ SomeException => MonadCatch ( Either e)	Since: exceptions-0.8.3
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e0 => Either e a -> (e0 -> Either e a) -> Either e a Source #
MonadCatch m => MonadCatch ( MaybeT m)	Catches exceptions from the base monad.
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => MaybeT m a -> (e -> MaybeT m a) -> MaybeT m a Source #
MonadCatch m => MonadCatch ( ListT m)
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => ListT m a -> (e -> ListT m a) -> ListT m a Source #
MonadCatch m => MonadCatch ( ExceptT e m)	Catches exceptions from the base monad.
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e0 => ExceptT e m a -> (e0 -> ExceptT e m a) -> ExceptT e m a Source #
MonadCatch m => MonadCatch ( IdentityT m)
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => IdentityT m a -> (e -> IdentityT m a) -> IdentityT m a Source #
( Error e, MonadCatch m) => MonadCatch ( ErrorT e m)	Catches exceptions from the base monad.
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e0 => ErrorT e m a -> (e0 -> ErrorT e m a) -> ErrorT e m a Source #
MonadCatch m => MonadCatch ( ReaderT r m)
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => ReaderT r m a -> (e -> ReaderT r m a) -> ReaderT r m a Source #
MonadCatch m => MonadCatch ( StateT s m)
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => StateT s m a -> (e -> StateT s m a) -> StateT s m a Source #
MonadCatch m => MonadCatch ( StateT s m)
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => StateT s m a -> (e -> StateT s m a) -> StateT s m a Source #
( MonadCatch m, Monoid w) => MonadCatch ( WriterT w m)
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => WriterT w m a -> (e -> WriterT w m a) -> WriterT w m a Source #
( MonadCatch m, Monoid w) => MonadCatch ( WriterT w m)
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => WriterT w m a -> (e -> WriterT w m a) -> WriterT w m a Source #
( MonadCatch m, Monoid w) => MonadCatch ( RWST r w s m)
Instance details 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 Source #
( MonadCatch m, Monoid w) => MonadCatch ( RWST r w s m)
Instance details 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 Source #

class MonadCatch m => MonadMask (m :: Type -> Type ) where Source #

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 Source #

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 Source #

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 Source #

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

Instances details

MonadMask IO
Instance details Defined in Control.Monad.Catch Methods mask :: (( forall a. IO a -> IO a) -> IO b) -> IO b Source # uninterruptibleMask :: (( forall a. IO a -> IO a) -> IO b) -> IO b Source # generalBracket :: IO a -> (a -> ExitCase b -> IO c) -> (a -> IO b) -> IO (b, c) Source #
e ~ SomeException => MonadMask ( Either e)	Since: exceptions-0.8.3
Instance details Defined in Control.Monad.Catch Methods mask :: (( forall a. Either e a -> Either e a) -> Either e b) -> Either e b Source # uninterruptibleMask :: (( forall a. Either e a -> Either e a) -> Either e b) -> Either e b Source # generalBracket :: Either e a -> (a -> ExitCase b -> Either e c) -> (a -> Either e b) -> Either e (b, c) Source #
MonadMask m => MonadMask ( MaybeT m)	Since: exceptions-0.10.0
Instance details Defined in Control.Monad.Catch Methods mask :: (( forall a. MaybeT m a -> MaybeT m a) -> MaybeT m b) -> MaybeT m b Source # uninterruptibleMask :: (( forall a. MaybeT m a -> MaybeT m a) -> MaybeT m b) -> MaybeT m b Source # generalBracket :: MaybeT m a -> (a -> ExitCase b -> MaybeT m c) -> (a -> MaybeT m b) -> MaybeT m (b, c) Source #
MonadMask m => MonadMask ( ExceptT e m)	Since: exceptions-0.9.0
Instance details 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 Source # uninterruptibleMask :: (( forall a. ExceptT e m a -> ExceptT e m a) -> ExceptT e m b) -> ExceptT e m b Source # generalBracket :: ExceptT e m a -> (a -> ExitCase b -> ExceptT e m c) -> (a -> ExceptT e m b) -> ExceptT e m (b, c) Source #
MonadMask m => MonadMask ( IdentityT m)
Instance details Defined in Control.Monad.Catch Methods mask :: (( forall a. IdentityT m a -> IdentityT m a) -> IdentityT m b) -> IdentityT m b Source # uninterruptibleMask :: (( forall a. IdentityT m a -> IdentityT m a) -> IdentityT m b) -> IdentityT m b Source # generalBracket :: IdentityT m a -> (a -> ExitCase b -> IdentityT m c) -> (a -> IdentityT m b) -> IdentityT m (b, c) Source #
( Error e, MonadMask m) => MonadMask ( ErrorT e m)
Instance details 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 Source # uninterruptibleMask :: (( forall a. ErrorT e m a -> ErrorT e m a) -> ErrorT e m b) -> ErrorT e m b Source # generalBracket :: ErrorT e m a -> (a -> ExitCase b -> ErrorT e m c) -> (a -> ErrorT e m b) -> ErrorT e m (b, c) Source #
MonadMask m => MonadMask ( ReaderT r m)
Instance details 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 Source # uninterruptibleMask :: (( forall a. ReaderT r m a -> ReaderT r m a) -> ReaderT r m b) -> ReaderT r m b Source # generalBracket :: ReaderT r m a -> (a -> ExitCase b -> ReaderT r m c) -> (a -> ReaderT r m b) -> ReaderT r m (b, c) Source #
MonadMask m => MonadMask ( StateT s m)
Instance details 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 Source # uninterruptibleMask :: (( forall a. StateT s m a -> StateT s m a) -> StateT s m b) -> StateT s m b Source # generalBracket :: StateT s m a -> (a -> ExitCase b -> StateT s m c) -> (a -> StateT s m b) -> StateT s m (b, c) Source #
MonadMask m => MonadMask ( StateT s m)
Instance details 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 Source # uninterruptibleMask :: (( forall a. StateT s m a -> StateT s m a) -> StateT s m b) -> StateT s m b Source # generalBracket :: StateT s m a -> (a -> ExitCase b -> StateT s m c) -> (a -> StateT s m b) -> StateT s m (b, c) Source #
( MonadMask m, Monoid w) => MonadMask ( WriterT w m)
Instance details 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 Source # uninterruptibleMask :: (( forall a. WriterT w m a -> WriterT w m a) -> WriterT w m b) -> WriterT w m b Source # generalBracket :: WriterT w m a -> (a -> ExitCase b -> WriterT w m c) -> (a -> WriterT w m b) -> WriterT w m (b, c) Source #
( MonadMask m, Monoid w) => MonadMask ( WriterT w m)
Instance details 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 Source # uninterruptibleMask :: (( forall a. WriterT w m a -> WriterT w m a) -> WriterT w m b) -> WriterT w m b Source # generalBracket :: WriterT w m a -> (a -> ExitCase b -> WriterT w m c) -> (a -> WriterT w m b) -> WriterT w m (b, c) Source #
( MonadMask m, Monoid w) => MonadMask ( RWST r w s m)
Instance details 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 Source # 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 Source # 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) Source #
( MonadMask m, Monoid w) => MonadMask ( RWST r w s m)
Instance details 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 Source # 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 Source # 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) Source #

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

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

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

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

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

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

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

Flipped catchIOError

class ( Typeable e, Show e) => Exception e where Source #

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 Source #

fromException :: SomeException -> Maybe e Source #

displayException :: e -> String Source #

Render this exception value in a human-friendly manner.

Default implementation: show .

Since: base-4.8.0.0

Instances

Instances details

Exception Void	Since: base-4.8.0.0
Instance details Defined in Data.Void Methods toException :: Void -> SomeException Source # fromException :: SomeException -> Maybe Void Source # displayException :: Void -> String Source #
Exception BlockedIndefinitelyOnMVar	Since: base-4.1.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: BlockedIndefinitelyOnMVar -> SomeException Source # fromException :: SomeException -> Maybe BlockedIndefinitelyOnMVar Source # displayException :: BlockedIndefinitelyOnMVar -> String Source #
Exception BlockedIndefinitelyOnSTM	Since: base-4.1.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: BlockedIndefinitelyOnSTM -> SomeException Source # fromException :: SomeException -> Maybe BlockedIndefinitelyOnSTM Source # displayException :: BlockedIndefinitelyOnSTM -> String Source #
Exception Deadlock	Since: base-4.1.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: Deadlock -> SomeException Source # fromException :: SomeException -> Maybe Deadlock Source # displayException :: Deadlock -> String Source #
Exception AllocationLimitExceeded	Since: base-4.8.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: AllocationLimitExceeded -> SomeException Source # fromException :: SomeException -> Maybe AllocationLimitExceeded Source # displayException :: AllocationLimitExceeded -> String Source #
Exception CompactionFailed	Since: base-4.10.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: CompactionFailed -> SomeException Source # fromException :: SomeException -> Maybe CompactionFailed Source # displayException :: CompactionFailed -> String Source #
Exception AssertionFailed	Since: base-4.1.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: AssertionFailed -> SomeException Source # fromException :: SomeException -> Maybe AssertionFailed Source # displayException :: AssertionFailed -> String Source #
Exception SomeAsyncException	Since: base-4.7.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: SomeAsyncException -> SomeException Source # fromException :: SomeException -> Maybe SomeAsyncException Source # displayException :: SomeAsyncException -> String Source #
Exception AsyncException	Since: base-4.7.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: AsyncException -> SomeException Source # fromException :: SomeException -> Maybe AsyncException Source # displayException :: AsyncException -> String Source #
Exception ArrayException	Since: base-4.1.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: ArrayException -> SomeException Source # fromException :: SomeException -> Maybe ArrayException Source # displayException :: ArrayException -> String Source #
Exception FixIOException	Since: base-4.11.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: FixIOException -> SomeException Source # fromException :: SomeException -> Maybe FixIOException Source # displayException :: FixIOException -> String Source #
Exception ExitCode	Since: base-4.1.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: ExitCode -> SomeException Source # fromException :: SomeException -> Maybe ExitCode Source # displayException :: ExitCode -> String Source #
Exception IOException	Since: base-4.1.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: IOException -> SomeException Source # fromException :: SomeException -> Maybe IOException Source # displayException :: IOException -> String Source #
Exception ErrorCall	Since: base-4.0.0.0
Instance details Defined in GHC.Exception Methods toException :: ErrorCall -> SomeException Source # fromException :: SomeException -> Maybe ErrorCall Source # displayException :: ErrorCall -> String Source #
Exception ArithException	Since: base-4.0.0.0
Instance details Defined in GHC.Exception.Type Methods toException :: ArithException -> SomeException Source # fromException :: SomeException -> Maybe ArithException Source # displayException :: ArithException -> String Source #
Exception SomeException	Since: base-3.0
Instance details Defined in GHC.Exception.Type Methods toException :: SomeException -> SomeException Source # fromException :: SomeException -> Maybe SomeException Source # displayException :: SomeException -> String Source #
Exception AsyncExceptionWrapper Source #
Instance details Defined in Control.Exception.Safe Methods toException :: AsyncExceptionWrapper -> SomeException Source # fromException :: SomeException -> Maybe AsyncExceptionWrapper Source # displayException :: AsyncExceptionWrapper -> String Source #
Exception SyncExceptionWrapper Source #
Instance details Defined in Control.Exception.Safe Methods toException :: SyncExceptionWrapper -> SomeException Source # fromException :: SomeException -> Maybe SyncExceptionWrapper Source # displayException :: SyncExceptionWrapper -> String Source #
Exception StringException Source #
Instance details Defined in Control.Exception.Safe Methods toException :: StringException -> SomeException Source # fromException :: SomeException -> Maybe StringException Source # displayException :: StringException -> String Source #

class Typeable (a :: k) Source #

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

Minimal complete definition

typeRep#

data SomeException Source #

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

Instances details

Show SomeException	Since: base-3.0
Instance details Defined in GHC.Exception.Type Methods showsPrec :: Int -> SomeException -> ShowS Source # show :: SomeException -> String Source # showList :: [ SomeException ] -> ShowS Source #
Exception SomeException	Since: base-3.0
Instance details Defined in GHC.Exception.Type Methods toException :: SomeException -> SomeException Source # fromException :: SomeException -> Maybe SomeException Source # displayException :: SomeException -> String Source #

data SomeAsyncException Source #

Superclass for asynchronous exceptions.

Since: base-4.7.0.0

Constructors

Exception e => SomeAsyncException e

Instances

Instances details

Show SomeAsyncException	Since: base-4.7.0.0
Instance details Defined in GHC.IO.Exception Methods showsPrec :: Int -> SomeAsyncException -> ShowS Source # show :: SomeAsyncException -> String Source # showList :: [ SomeAsyncException ] -> ShowS Source #
Exception SomeAsyncException	Since: base-4.7.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: SomeAsyncException -> SomeException Source # fromException :: SomeException -> Maybe SomeAsyncException Source # displayException :: SomeAsyncException -> String Source #

data IOException Source #

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

Instances details

Eq IOException	Since: base-4.1.0.0
Instance details Defined in GHC.IO.Exception Methods (==) :: IOException -> IOException -> Bool Source # (/=) :: IOException -> IOException -> Bool Source #
Show IOException	Since: base-4.1.0.0
Instance details Defined in GHC.IO.Exception Methods showsPrec :: Int -> IOException -> ShowS Source # show :: IOException -> String Source # showList :: [ IOException ] -> ShowS Source #
Exception IOException	Since: base-4.1.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: IOException -> SomeException Source # fromException :: SomeException -> Maybe IOException Source # displayException :: IOException -> String Source #
Error IOException
Instance details Defined in Control.Monad.Trans.Error Methods noMsg :: IOException Source # strMsg :: String -> IOException Source #

assert :: Bool -> a -> a Source #

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.