| Safe Haskell | Safe |
|---|---|
| Language | Haskell2010 |
Universum.Exception
Description
Re-exports most useful functionality from 'safe-exceptions'. Also
provides some functions to work with exceptions over MonadError.
Synopsis
- class (Typeable e, Show e) => Exception e where
- toException :: e -> SomeException
- fromException :: SomeException -> Maybe e
- displayException :: e -> String
- data SomeException where
- SomeException :: forall e. Exception e => e -> SomeException
- 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)
- bracketOnError :: MonadMask m => m a -> (a -> m b) -> (a -> m c) -> m c
- finally :: MonadMask m => m a -> m b -> m a
- bracket_ :: MonadMask m => m a -> m b -> m c -> m c
- bracket :: MonadMask m => m a -> (a -> m b) -> (a -> m c) -> m c
- onException :: MonadMask m => m a -> m b -> m a
- tryAny :: MonadCatch m => m a -> m (Either SomeException a)
- try :: (MonadCatch m, Exception e) => m a -> m (Either e a)
- handleAny :: MonadCatch m => (SomeException -> m a) -> m a -> m a
- catchAny :: MonadCatch m => m a -> (SomeException -> m a) -> m a
- catch :: (MonadCatch m, Exception e) => m a -> (e -> m a) -> m a
- throwM :: (MonadThrow m, Exception e) => e -> m a
- data Bug = Bug SomeException CallStack
- bug :: (HasCallStack, Exception e) => e -> a
- pattern Exc :: Exception e => e -> SomeException
- note :: MonadError e m => e -> Maybe a -> m a
Documentation
class (Typeable e, Show e) => Exception e where #
Any type that you wish to throw or catch as an exception must be an
instance of the Exception class. The simplest case is a new exception
type directly below the root:
data MyException = ThisException | ThatException
deriving Show
instance Exception MyExceptionThe 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 = frontendExceptionFromExceptionWe 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
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 # | |
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
Instances
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
Instances
| MonadCatch IO | |
| MonadCatch STM | |
| e ~ SomeException => MonadCatch (Either e) | Since: exceptions-0.8.3 |
| MonadCatch m => MonadCatch (MaybeT m) | Catches exceptions from the base monad. |
| MonadCatch m => MonadCatch (ListT m) | |
| MonadCatch m => MonadCatch (ExceptT e m) | Catches exceptions from the base monad. |
| MonadCatch m => MonadCatch (StateT s m) | |
| MonadCatch m => MonadCatch (IdentityT m) | |
| (Error e, MonadCatch m) => MonadCatch (ErrorT e m) | Catches exceptions from the base monad. |
| MonadCatch m => MonadCatch (StateT s m) | |
| (MonadCatch m, Monoid w) => MonadCatch (WriterT w m) | |
| (MonadCatch m, Monoid w) => MonadCatch (WriterT w m) | |
| MonadCatch m => MonadCatch (ReaderT r m) | |
| (MonadCatch m, Monoid w) => MonadCatch (RWST r w s m) | |
| (MonadCatch m, Monoid w) => MonadCatch (RWST r w s m) | |
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.
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 | |
| e ~ SomeException => MonadMask (Either e) | Since: exceptions-0.8.3 |
Defined in Control.Monad.Catch | |
| MonadMask m => MonadMask (MaybeT m) | Since: exceptions-0.10.0 |
Defined in Control.Monad.Catch | |
| 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 (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 (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) # | |
| (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 (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 => 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, 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) # | |
bracketOnError :: MonadMask m => m a -> (a -> m b) -> (a -> m c) -> m c #
Async safe version of bracketOnError
Since: safe-exceptions-0.1.0.0
finally :: MonadMask m => m a -> m b -> m a #
Async safe version of finally
Since: safe-exceptions-0.1.0.0
bracket_ :: MonadMask m => m a -> m b -> m c -> m c #
Async safe version of bracket_
Since: safe-exceptions-0.1.0.0
bracket :: MonadMask m => m a -> (a -> m b) -> (a -> m c) -> m c #
Async safe version of bracket
Since: safe-exceptions-0.1.7.0
onException :: MonadMask m => m a -> m b -> m a #
Async safe version of onException
Since: safe-exceptions-0.1.0.0
tryAny :: MonadCatch m => m a -> m (Either SomeException a) #
try specialized to catch all synchronous exceptions
Since: safe-exceptions-0.1.0.0
try :: (MonadCatch m, Exception e) => m a -> m (Either e a) #
Same as upstream try, but will not catch asynchronous
exceptions
Since: safe-exceptions-0.1.0.0
handleAny :: MonadCatch m => (SomeException -> m a) -> m a -> m a #
Flipped version of catchAny
Since: safe-exceptions-0.1.0.0
catchAny :: MonadCatch m => m a -> (SomeException -> m a) -> m a #
catch specialized to catch all synchronous exception
Since: safe-exceptions-0.1.0.0
catch :: (MonadCatch m, Exception e) => m a -> (e -> m a) -> m a #
Same as upstream catch, but will not catch asynchronous
exceptions
Since: safe-exceptions-0.1.0.0
throwM :: (MonadThrow m, Exception e) => e -> m a #
Synonym for throw
Since: safe-exceptions-0.1.0.0
Type that represents exceptions used in cases when a particular codepath is not meant to be ever executed, but happens to be executed anyway.
Constructors
| Bug SomeException CallStack |
Instances
| Show Bug Source # | |
| Exception Bug Source # | |
Defined in Universum.Exception Methods toException :: Bug -> SomeException # fromException :: SomeException -> Maybe Bug # displayException :: Bug -> String # | |
bug :: (HasCallStack, Exception e) => e -> a Source #
Generate a pure value which, when forced, will synchronously
throw the exception wrapped into Bug data type.
pattern Exc :: Exception e => e -> SomeException Source #
Pattern synonym to easy pattern matching on exceptions. So intead of writing something like this:
isNonCriticalExc e
| Just (_ :: NodeAttackedError) <- fromException e = True
| Just DialogUnexpected{} <- fromException e = True
| otherwise = False
you can use Exc pattern synonym:
isNonCriticalExc = case
Exc (_ :: NodeAttackedError) -> True -- matching all exceptions of type NodeAttackedError
Exc DialogUnexpected{} -> True
_ -> False
This pattern is bidirectional. You can use Exc e instead of toException e.
note :: MonadError e m => e -> Maybe a -> m a Source #
Throws error for Maybe if Nothing is given.
Operates over MonadError.