UnliftIO.Exception

Unlifted Control.Exception, with extra async exception safety and more helper functions.

This module works best when your cleanup functions adhere to certain expectations around exception safety and interruptible actions. For more details, see this exception safety tutorial.

Throwing

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

Synchronously throw the given exception.

Since: 0.1.0.0

throwString :: (MonadIO 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: 0.1.0.0

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: 0.1.0.0

Constructors

StringException String CallStack

Instances

Show StringException

Since: 0.1.0.0

Defined in UnliftIO.Exception

Methods

showsPrec :: Int -> StringException -> ShowS

show :: StringException -> String

showList :: [StringException] -> ShowS

Exception StringException

Since: 0.1.0.0

Defined in UnliftIO.Exception

Methods

toException :: StringException -> SomeException

fromException :: SomeException -> Maybe StringException

displayException :: StringException -> String

stringException :: HasCallStack => String -> StringException

Smart constructor for a StringException that deals with the call stack.

Since: 0.1.0.0

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 UnliftIO.Async module, 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 throwIO, see https://github.com/fpco/safe-exceptions#quickstart.

Since: 0.1.0.0

fromEither :: (Exception e, MonadIO m) => Either e a -> m a

Unwrap an Either value, throwing its Left value as a runtime exception via throwIO if present.

Since: 0.1.0.0

fromEitherIO :: (Exception e, MonadIO m) => IO (Either e a) -> m a

Same as fromEither, but works on an IO-wrapped Either.

Since: 0.1.0.0

fromEitherM :: (Exception e, MonadIO m) => m (Either e a) -> m a

Same as fromEither, but works on an m-wrapped Either.

Since: 0.1.0.0

Catching (with recovery)

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

Unlifted catch, but will not catch asynchronous exceptions.

Since: 0.1.0.0

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

catch specialized to only catching IOExceptions.

Since: 0.1.0.0

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

catch specialized to catch all synchronous exception.

Since: 0.1.0.0

catchDeep :: (MonadUnliftIO 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: 0.1.0.0

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

catchDeep specialized to catch all synchronous exception.

Since: 0.1.0.0

catchJust :: (MonadUnliftIO 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: 0.1.0.0

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

Flipped version of catch.

Since: 0.1.0.0

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

handle specialized to only catching IOExceptions.

Since: 0.1.0.0

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

Flipped version of catchAny.

Since: 0.1.0.0

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

Flipped version of catchDeep.

Since: 0.1.0.0

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

Flipped version of catchAnyDeep.

Since: 0.1.0.0

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

Flipped catchJust.

Since: 0.1.0.0

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

Unlifted try, but will not catch asynchronous exceptions.

Since: 0.1.0.0

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

try specialized to only catching IOExceptions.

Since: 0.1.0.0

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

try specialized to catch all synchronous exceptions.

Since: 0.1.0.0

tryDeep :: (MonadUnliftIO 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: 0.1.0.0

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

tryDeep specialized to catch all synchronous exceptions.

Since: 0.1.0.0

tryJust :: (MonadUnliftIO 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: 0.1.0.0

pureTry :: a -> Either SomeException a

Evaluate the value to WHNF and catch any synchronous exceptions.

The expression may still have bottom values within it; you may instead want to use pureTryDeep.

Since: 0.2.2.0

pureTryDeep :: NFData a => a -> Either SomeException a

Evaluate the value to NF and catch any synchronous exceptions.

Since: 0.2.2.0

data Handler m a

Generalized version of Handler.

Since: 0.1.0.0

Constructors

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

catches :: MonadUnliftIO m => m a -> [Handler m a] -> m a

Same as upstream catches, but will not catch asynchronous exceptions.

Since: 0.1.0.0

catchesDeep :: (MonadUnliftIO 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: 0.1.0.0

Cleanup (no recovery)

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

Async safe version of onException.

Since: 0.1.0.0

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

Async safe version of bracket.

Since: 0.1.0.0

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

Async safe version of bracket_.

Since: 0.1.0.0

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

Async safe version of finally.

Since: 0.1.0.0

withException :: (MonadUnliftIO 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 :: MonadUnliftIO m => m a -> (a -> m b) -> (a -> m c) -> m c

Async safe version of bracketOnError.

Since: 0.1.0.0

bracketOnError_ :: MonadUnliftIO 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

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

Exception e => SyncExceptionWrapper e

Instances

Show SyncExceptionWrapper

Since: 0.1.0.0

Defined in UnliftIO.Exception

Methods

showsPrec :: Int -> SyncExceptionWrapper -> ShowS

show :: SyncExceptionWrapper -> String

showList :: [SyncExceptionWrapper] -> ShowS

Exception SyncExceptionWrapper

Since: 0.1.0.0

Defined in UnliftIO.Exception

Methods

toException :: SyncExceptionWrapper -> SomeException

fromException :: SomeException -> Maybe SyncExceptionWrapper

displayException :: SyncExceptionWrapper -> String

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

Exception e => AsyncExceptionWrapper e

Instances

Show AsyncExceptionWrapper

Since: 0.1.0.0

Defined in UnliftIO.Exception

Methods

showsPrec :: Int -> AsyncExceptionWrapper -> ShowS

show :: AsyncExceptionWrapper -> String

showList :: [AsyncExceptionWrapper] -> ShowS

Exception AsyncExceptionWrapper

Since: 0.1.0.0

Defined in UnliftIO.Exception

Methods

toException :: AsyncExceptionWrapper -> SomeException

fromException :: SomeException -> Maybe AsyncExceptionWrapper

displayException :: AsyncExceptionWrapper -> String

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

Masking

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

Unlifted version of mask.

Since: 0.1.0.0

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

Unlifted version of uninterruptibleMask.

Since: 0.1.0.0

mask_ :: MonadUnliftIO m => m a -> m a

Unlifted version of mask_.

Since: 0.1.0.0

uninterruptibleMask_ :: MonadUnliftIO m => m a -> m a

Unlifted version of uninterruptibleMask_.

Since: 0.1.0.0

Evaluation

evaluate :: MonadIO m => a -> m a

Lifted version of evaluate.

Since: 0.1.0.0

evaluateDeep :: (MonadIO m, NFData a) => a -> m a

Deeply evaluate a value using evaluate and NFData.

Since: 0.1.0.0

Reexports

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 AsyncCancelled

Defined in Control.Concurrent.Async

Methods

toException :: AsyncCancelled -> SomeException

fromException :: SomeException -> Maybe AsyncCancelled

displayException :: AsyncCancelled -> String

Exception ExceptionInLinkedThread

Defined in Control.Concurrent.Async

Methods

toException :: ExceptionInLinkedThread -> SomeException

fromException :: SomeException -> Maybe ExceptionInLinkedThread

displayException :: ExceptionInLinkedThread -> String

Exception Void

Since: base-4.8.0.0

Defined in Data.Void

Methods

toException :: Void -> SomeException

fromException :: SomeException -> Maybe Void

displayException :: Void -> String

Exception PatternMatchFail

Since: base-4.0

Defined in Control.Exception.Base

Methods

toException :: PatternMatchFail -> SomeException

fromException :: SomeException -> Maybe PatternMatchFail

displayException :: PatternMatchFail -> String

Exception RecSelError

Since: base-4.0

Defined in Control.Exception.Base

Methods

toException :: RecSelError -> SomeException

fromException :: SomeException -> Maybe RecSelError

displayException :: RecSelError -> String

Exception RecConError

Since: base-4.0

Defined in Control.Exception.Base

Methods

toException :: RecConError -> SomeException

fromException :: SomeException -> Maybe RecConError

displayException :: RecConError -> String

Exception RecUpdError

Since: base-4.0

Defined in Control.Exception.Base

Methods

toException :: RecUpdError -> SomeException

fromException :: SomeException -> Maybe RecUpdError

displayException :: RecUpdError -> String

Exception NoMethodError

Since: base-4.0

Defined in Control.Exception.Base

Methods

toException :: NoMethodError -> SomeException

fromException :: SomeException -> Maybe NoMethodError

displayException :: NoMethodError -> String

Exception TypeError

Since: base-4.9.0.0

Defined in Control.Exception.Base

Methods

toException :: TypeError -> SomeException

fromException :: SomeException -> Maybe TypeError

displayException :: TypeError -> String

Exception NonTermination

Since: base-4.0

Defined in Control.Exception.Base

Methods

toException :: NonTermination -> SomeException

fromException :: SomeException -> Maybe NonTermination

displayException :: NonTermination -> String

Exception NestedAtomically

Since: base-4.0

Defined in Control.Exception.Base

Methods

toException :: NestedAtomically -> SomeException

fromException :: SomeException -> Maybe NestedAtomically

displayException :: NestedAtomically -> 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 Deadlock

Since: base-4.1.0.0

Defined in GHC.IO.Exception

Methods

toException :: Deadlock -> SomeException

fromException :: SomeException -> Maybe Deadlock

displayException :: Deadlock -> 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 CompactionFailed

Since: base-4.10.0.0

Defined in GHC.IO.Exception

Methods

toException :: CompactionFailed -> SomeException

fromException :: SomeException -> Maybe CompactionFailed

displayException :: CompactionFailed -> 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 SomeAsyncException

Since: base-4.7.0.0

Defined in GHC.IO.Exception

Methods

toException :: SomeAsyncException -> SomeException

fromException :: SomeException -> Maybe SomeAsyncException

displayException :: SomeAsyncException -> 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 ArrayException

Since: base-4.1.0.0

Defined in GHC.IO.Exception

Methods

toException :: ArrayException -> SomeException

fromException :: SomeException -> Maybe ArrayException

displayException :: ArrayException -> 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 ExitCode

Since: base-4.1.0.0

Defined in GHC.IO.Exception

Methods

toException :: ExitCode -> SomeException

fromException :: SomeException -> Maybe ExitCode

displayException :: ExitCode -> 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 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 StringException

Since: 0.1.0.0

Defined in UnliftIO.Exception

Methods

toException :: StringException -> SomeException

fromException :: SomeException -> Maybe StringException

displayException :: StringException -> String

Exception AsyncExceptionWrapper

Since: 0.1.0.0

Defined in UnliftIO.Exception

Methods

toException :: AsyncExceptionWrapper -> SomeException

fromException :: SomeException -> Maybe AsyncExceptionWrapper

displayException :: AsyncExceptionWrapper -> String

Exception SyncExceptionWrapper

Since: 0.1.0.0

Defined in UnliftIO.Exception

Methods

toException :: SyncExceptionWrapper -> SomeException

fromException :: SomeException -> Maybe SyncExceptionWrapper

displayException :: SyncExceptionWrapper -> String

Exception ConcException

Defined in UnliftIO.Internals.Async

Methods

toException :: ConcException -> SomeException

fromException :: SomeException -> Maybe ConcException

displayException :: ConcException -> String

Exception Timeout

Since: base-4.7.0.0

Defined in System.Timeout

Methods

toException :: Timeout -> SomeException

fromException :: SomeException -> Maybe Timeout

displayException :: Timeout -> String

class Typeable (a :: k)

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

Minimal complete definition

typeRep#

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 :: forall e. Exception e => e -> SomeException

Instances

Show SomeException

Since: base-3.0

Defined in GHC.Exception.Type

Methods

showsPrec :: Int -> SomeException -> ShowS

show :: SomeException -> String

showList :: [SomeException] -> ShowS

Exception SomeException

Since: base-3.0

Defined in GHC.Exception.Type

Methods

toException :: SomeException -> SomeException

fromException :: SomeException -> Maybe SomeException

displayException :: SomeException -> String

data SomeAsyncException where

Superclass for asynchronous exceptions.

Since: base-4.7.0.0

Constructors

SomeAsyncException :: forall e. Exception e => e -> SomeAsyncException

Instances

Show SomeAsyncException

Since: base-4.7.0.0

Defined in GHC.IO.Exception

Methods

showsPrec :: Int -> SomeAsyncException -> ShowS

show :: SomeAsyncException -> String

showList :: [SomeAsyncException] -> ShowS

Exception SomeAsyncException

Since: base-4.7.0.0

Defined in GHC.IO.Exception

Methods

toException :: SomeAsyncException -> SomeException

fromException :: SomeException -> Maybe SomeAsyncException

displayException :: SomeAsyncException -> String

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

Eq IOException

Since: base-4.1.0.0

Defined in GHC.IO.Exception

Methods

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

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

Show IOException

Since: base-4.1.0.0

Defined in GHC.IO.Exception

Methods

showsPrec :: Int -> IOException -> ShowS

show :: IOException -> String

showList :: [IOException] -> ShowS

Exception IOException

Since: base-4.1.0.0

Defined in GHC.IO.Exception

Methods

toException :: IOException -> SomeException

fromException :: SomeException -> Maybe IOException

displayException :: IOException -> String

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.