Copyright | (C) Edward Kmett 2013-2015 (c) Google Inc. 2012 |
---|---|
License | BSD-style (see the file LICENSE) |
Maintainer | Edward Kmett <ekmett@gmail.com> |
Stability | experimental |
Portability | non-portable |
Safe Haskell | Trustworthy |
Language | Haskell98 |
This module supports monads that can throw extensible exceptions. The
exceptions are the very same from Control.Exception, and the operations
offered very similar, but here they are not limited to IO
.
This code is in the style of both transformers and mtl, and is compatible with them, though doesn't mimic the module structure or offer the complete range of features in those packages.
This is very similar to ErrorT
and MonadError
, but based on features of
Control.Exception. In particular, it handles the complex case of
asynchronous exceptions by including mask
in the typeclass. Note that the
extensible exceptions feature relies on the RankNTypes language extension.
Synopsis
- class Monad m => MonadThrow m where
- class MonadThrow m => MonadCatch m where
- class MonadCatch m => MonadMask m 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)
- data ExitCase a
- mask_ :: MonadMask m => m a -> m a
- uninterruptibleMask_ :: MonadMask m => m a -> m a
- catchAll :: MonadCatch m => m a -> (SomeException -> m a) -> m a
- catchIOError :: MonadCatch m => m a -> (IOError -> m a) -> m a
- catchJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> m a -> (b -> m a) -> m a
- catchIf :: (MonadCatch m, Exception e) => (e -> Bool) -> m a -> (e -> m a) -> m a
- data Handler m a = Exception e => Handler (e -> m a)
- catches :: (Foldable f, MonadCatch m) => m a -> f (Handler m a) -> m a
- handle :: (MonadCatch m, Exception e) => (e -> m a) -> m a -> m a
- handleAll :: MonadCatch m => (SomeException -> m a) -> m a -> m a
- handleIOError :: MonadCatch m => (IOError -> m a) -> m a -> m a
- handleJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> (b -> m a) -> m a -> m a
- handleIf :: (MonadCatch m, Exception e) => (e -> Bool) -> (e -> m a) -> m a -> m a
- try :: (MonadCatch m, Exception e) => m a -> m (Either e a)
- tryJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> m a -> m (Either b a)
- onException :: MonadCatch m => m a -> m b -> m a
- onError :: MonadMask m => m a -> m b -> m a
- bracket :: MonadMask m => m a -> (a -> m c) -> (a -> m b) -> m b
- bracket_ :: MonadMask m => m a -> m c -> m b -> m b
- finally :: MonadMask m => m a -> m b -> m a
- bracketOnError :: MonadMask m => m a -> (a -> m c) -> (a -> m b) -> m b
- 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
Typeclass
The mtl style typeclass
class Monad m => MonadThrow m where #
A class for monads in which exceptions may be thrown.
Instances should obey the following law:
throwM e >> x = throwM e
In other words, throwing an exception short-circuits the rest of the monadic computation.
throwM :: Exception e => e -> m a #
Throw an exception. Note that this throws when this action is run in
the monad m
, not when it is applied. It is a generalization of
Control.Exception's throwIO
.
Should satisfy the law:
throwM e >> f = throwM e
Instances
class MonadThrow m => MonadCatch m where #
A class for monads which allow exceptions to be caught, in particular
exceptions which were thrown by throwM
.
Instances should obey the following law:
catch (throwM e) f = f e
Note that the ability to catch an exception does not guarantee that we can
deal with all possible exit points from a computation. Some monads, such as
continuation-based stacks, allow for more than just a success/failure
strategy, and therefore catch
cannot be used by those monads to properly
implement a function such as finally
. For more information, see
MonadMask
.
catch :: Exception e => m a -> (e -> m a) -> m a #
Provide a handler for exceptions thrown during execution of the first
action. Note that type of the type of the argument to the handler will
constrain which exceptions are caught. See Control.Exception's
catch
.
Instances
MonadCatch IO # | |
MonadCatch STM # | |
e ~ SomeException => MonadCatch (Either e) # | Since: 0.8.3 |
MonadCatch m => MonadCatch (ListT m) # | |
MonadCatch m => MonadCatch (MaybeT m) # | Catches exceptions from the base monad. |
Monad m => MonadCatch (CatchT m) # | |
MonadCatch m => MonadCatch (IdentityT m) # | |
(Error e, MonadCatch m) => MonadCatch (ErrorT e m) # | Catches exceptions from the base monad. |
MonadCatch m => MonadCatch (ExceptT e m) # | Catches exceptions from the base monad. |
MonadCatch m => MonadCatch (ReaderT r m) # | |
MonadCatch m => MonadCatch (StateT s m) # | |
MonadCatch m => MonadCatch (StateT s m) # | |
(MonadCatch m, Monoid w) => MonadCatch (WriterT w m) # | |
(MonadCatch m, Monoid w) => MonadCatch (WriterT w 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 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.
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.
:: 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 overridesuse
's state -- changes and the state reverts tos1
. 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 twoEither
effects determines -- which error will win if they are bothLeft
s. We want the error from --release
to win. c <- ec b <- eb return (b, c)
Since: 0.9.0
Instances
MonadMask IO # | |
e ~ SomeException => MonadMask (Either e) # | Since: 0.8.3 |
Defined in Control.Monad.Catch | |
MonadMask m => MonadMask (MaybeT m) # | Since: 0.10.0 |
Defined in Control.Monad.Catch | |
Monad m => MonadMask (CatchT m) # | Note: This instance is only valid if the underlying monad has a single exit point! For example, |
Defined in Control.Monad.Catch.Pure | |
MonadMask m => MonadMask (IdentityT m) # | |
Defined in Control.Monad.Catch 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 | |
MonadMask m => MonadMask (ExceptT e m) # | Since: 0.9.0 |
Defined in Control.Monad.Catch 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 (ReaderT r m) # | |
Defined in Control.Monad.Catch mask :: ((forall a. ReaderT r m a -> ReaderT r m a) -> ReaderT r m b) -> ReaderT r m b # uninterruptibleMask :: ((forall a. ReaderT r m a -> ReaderT r m a) -> ReaderT r m b) -> ReaderT r m b # generalBracket :: ReaderT r m a -> (a -> ExitCase b -> ReaderT r m c) -> (a -> ReaderT r m b) -> ReaderT r m (b, c) # | |
MonadMask m => MonadMask (StateT s m) # | |
Defined in Control.Monad.Catch | |
MonadMask m => MonadMask (StateT s m) # | |
Defined in Control.Monad.Catch | |
(MonadMask m, Monoid w) => MonadMask (WriterT w m) # | |
Defined in Control.Monad.Catch 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 mask :: ((forall a. WriterT w m a -> WriterT w m a) -> WriterT w m b) -> WriterT w m b # uninterruptibleMask :: ((forall a. WriterT w m a -> WriterT w m a) -> WriterT w m b) -> WriterT w m b # generalBracket :: WriterT w m a -> (a -> ExitCase b -> WriterT w m c) -> (a -> WriterT w m b) -> WriterT w m (b, c) # | |
(MonadMask m, Monoid w) => MonadMask (RWST r w s m) # | |
Defined in Control.Monad.Catch 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 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) # |
A MonadMask
computation may either succeed with a value, abort with an
exception, or abort for some other reason. For example, in ExceptT e IO
you can use throwM
to abort with an exception (ExitCaseException
) or
throwE
to abort with a value of type e
(ExitCaseAbort
).
Utilities
These functions follow those from Control.Exception, except that they are
based on methods from the MonadCatch
typeclass. See
Control.Exception for API usage.
uninterruptibleMask_ :: MonadMask m => m a -> m a #
Like uninterruptibleMask
, but does not pass a restore
action to the
argument.
catchAll :: MonadCatch m => m a -> (SomeException -> m a) -> m a #
Catches all exceptions, and somewhat defeats the purpose of the extensible exception system. Use sparingly.
NOTE This catches all exceptions, but if the monad supports other ways of aborting the computation, those other kinds of errors will not be caught.
catchIOError :: MonadCatch m => m a -> (IOError -> m a) -> m a #
Catch all IOError
(eqv. IOException
) exceptions. Still somewhat too
general, but better than using catchAll
. See catchIf
for an easy way
of catching specific IOError
s based on the predicates in System.IO.Error.
catchJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> m a -> (b -> m a) -> m a #
A more generalized way of determining which exceptions to catch at run time.
catchIf :: (MonadCatch m, Exception e) => (e -> Bool) -> m a -> (e -> m a) -> m a #
Catch exceptions only if they pass some predicate. Often useful with the
predicates for testing IOError
values in System.IO.Error.
Generalized version of Handler
catches :: (Foldable f, MonadCatch m) => m a -> f (Handler m a) -> m a #
Catches different sorts of exceptions. See Control.Exception's catches
handle :: (MonadCatch m, Exception e) => (e -> m a) -> m a -> m a #
Flipped catch
. See Control.Exception's handle
.
handleAll :: MonadCatch m => (SomeException -> m a) -> m a -> m a #
Flipped catchAll
handleIOError :: MonadCatch m => (IOError -> m a) -> m a -> m a #
Flipped catchIOError
handleJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> (b -> m a) -> m a -> m a #
Flipped catchJust
. See Control.Exception's handleJust
.
try :: (MonadCatch m, Exception e) => m a -> m (Either e a) #
Similar to catch
, but returns an Either
result. See Control.Exception's
try
.
tryJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> m a -> m (Either b a) #
A variant of try
that takes an exception predicate to select
which exceptions are caught. See Control.Exception's tryJust
onException :: MonadCatch m => m a -> m b -> m a #
Run an action only if an exception is thrown in the main action. The exception is not caught, simply rethrown.
NOTE The action is only run if an exception is thrown. If the monad
supports other ways of aborting the computation, the action won't run if
those other kinds of errors are thrown. See onError
.
onError :: MonadMask m => m a -> m b -> m a #
Run an action only if an error is thrown in the main action. Unlike
onException
, this works with every kind of error, not just exceptions. For
example, if f
is an ExceptT
computation which aborts with a Left
, the
computation onError f g
will execute g
, while onException f g
will not.
This distinction is only meaningful for monads which have multiple exit
points, such as Except
and MaybeT
. For monads that only have a single
exit point, there is no difference between onException
and onError
,
except that onError
has a more constrained type.
Since: 0.10.0
bracket :: MonadMask m => m a -> (a -> m c) -> (a -> m b) -> m b #
Generalized abstracted pattern of safe resource acquisition and release
in the face of errors. The first action "acquires" some value, which
is "released" by the second action at the end. The third action "uses"
the value and its result is the result of the bracket
.
If an error is thrown during the use, the release still happens before the error is rethrown.
Note that this is essentially a type-specialized version of
generalBracket
. This function has a more common signature (matching the
signature from Control.Exception), and is often more convenient to use. By
contrast, generalBracket
is more expressive, allowing us to implement
other functions like bracketOnError
.
bracket_ :: MonadMask m => m a -> m c -> m b -> m b #
Version of bracket
without any value being passed to the second and
third actions.
finally :: MonadMask m => m a -> m b -> m a #
Perform an action with a finalizer action that is run, even if an error occurs.
bracketOnError :: MonadMask m => m a -> (a -> m c) -> (a -> m b) -> m b #
Like bracket
, but only performs the final action if an error is
thrown by the in-between computation.
Re-exports from Control.Exception
class (Typeable e, Show e) => Exception e where #
Any type that you wish to throw or catch as an exception must be an
instance of the Exception
class. The simplest case is a new exception
type directly below the root:
data MyException = ThisException | ThatException deriving Show 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
Nothing
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
.
SomeException :: forall e. Exception e => e -> SomeException |
Instances
Show SomeException | Since: base-3.0 |
Defined in GHC.Exception.Type showsPrec :: Int -> SomeException -> ShowS # show :: SomeException -> String # showList :: [SomeException] -> ShowS # | |
Exception SomeException | Since: base-3.0 |
Defined in GHC.Exception.Type |