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.
- class Monad m => MonadThrow m where
- class MonadThrow m => MonadCatch m where
- class MonadCatch m => MonadMask m where
- 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
- bracket :: 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
- bracketOnError :: MonadMask m => m a -> (a -> m b) -> (a -> m c) -> m c
- class (Typeable * e, Show e) => Exception e where
- toException :: e -> SomeException
- fromException :: SomeException -> Maybe e
- displayException :: e -> String
- data SomeException :: * where
Typeclass
The mtl style typeclass
class Monad m => MonadThrow m where 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.
throwM :: Exception e => e -> m a Source #
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
class MonadThrow m => MonadCatch m where 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
.
catch :: Exception e => m a -> (e -> m a) -> m a Source #
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
.
MonadCatch IO Source # | |
MonadCatch STM Source # | |
(~) * e SomeException => MonadCatch (Either e) Source # | Since: 0.8.3 |
MonadCatch m => MonadCatch (ListT m) Source # | |
MonadCatch m => MonadCatch (MaybeT m) Source # | Catches exceptions from the base monad. |
Monad m => MonadCatch (CatchT m) Source # | |
MonadCatch m => MonadCatch (ExceptT e m) Source # | Catches exceptions from the base monad. |
(Error e, MonadCatch m) => MonadCatch (ErrorT e m) Source # | Catches exceptions from the base monad. |
MonadCatch m => MonadCatch (StateT s m) Source # | |
MonadCatch m => MonadCatch (StateT s m) Source # | |
(MonadCatch m, Monoid w) => MonadCatch (WriterT w m) Source # | |
(MonadCatch m, Monoid w) => MonadCatch (WriterT w m) Source # | |
MonadCatch m => MonadCatch (IdentityT * m) Source # | |
MonadCatch m => MonadCatch (ReaderT * r m) Source # | |
(MonadCatch m, Monoid w) => MonadCatch (RWST r w s m) Source # | |
(MonadCatch m, Monoid w) => MonadCatch (RWST r w s m) Source # | |
class MonadCatch m => MonadMask m 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, and stacks such as ErrorT e IO
which provide for multiple failure modes, are invalid instances of this
class.
Note that this package does provide a MonadMask
instance for CatchT
.
This instance is only valid if the base monad provides no ability to
provide multiple exit. For example, IO
or Either
would be invalid base
monads, but Reader
or State
would be acceptable.
Instances should ensure that, in the following code:
f `finally` g
The action g
is called regardless of what occurs within f
, including
async exceptions.
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.
MonadMask IO Source # | |
(~) * e SomeException => MonadMask (Either e) Source # | Since: 0.8.3 |
Monad m => MonadMask (CatchT m) Source # | Note: This instance is only valid if the underlying monad has a single exit point! |
MonadMask m => MonadMask (StateT s m) Source # | |
MonadMask m => MonadMask (StateT s m) Source # | |
(MonadMask m, Monoid w) => MonadMask (WriterT w m) Source # | |
(MonadMask m, Monoid w) => MonadMask (WriterT w m) Source # | |
MonadMask m => MonadMask (IdentityT * m) Source # | |
MonadMask m => MonadMask (ReaderT * r m) Source # | |
(MonadMask m, Monoid w) => MonadMask (RWST r w s m) Source # | |
(MonadMask m, Monoid w) => MonadMask (RWST r w s m) Source # | |
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.
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.
catchAll :: MonadCatch m => m a -> (SomeException -> m a) -> m a Source #
Catches all exceptions, and somewhat defeats the purpose of the extensible exception system. Use sparingly.
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.
catchJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> m a -> (b -> m a) -> m a Source #
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 Source #
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 Source #
Catches different sorts of exceptions. See Control.Exception's catches
handle :: (MonadCatch m, Exception e) => (e -> m a) -> m a -> m a Source #
Flipped catch
. See Control.Exception's handle
.
handleAll :: MonadCatch m => (SomeException -> m a) -> m a -> m a Source #
Flipped catchAll
handleIOError :: MonadCatch m => (IOError -> m a) -> m a -> m a Source #
Flipped catchIOError
handleJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> (b -> m a) -> m a -> m a Source #
Flipped catchJust
. See Control.Exception's handleJust
.
handleIf :: (MonadCatch m, Exception e) => (e -> Bool) -> (e -> m a) -> m a -> m a Source #
Flipped catchIf
try :: (MonadCatch m, Exception e) => m a -> m (Either e a) Source #
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) Source #
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 Source #
Run an action only if an exception is thrown in the main action. The exception is not caught, simply rethrown.
bracket :: MonadMask m => m a -> (a -> m b) -> (a -> m c) -> m c Source #
Generalized abstracted pattern of safe resource acquisition and release
in the face of exceptions. 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 exception occurs during the use, the release still happens before the exception is rethrown.
bracket_ :: MonadMask m => m a -> m b -> m c -> m c Source #
Version of bracket
without any value being passed to the second and
third actions.
finally :: MonadMask m => m a -> m b -> m a Source #
Perform an action with a finalizer action that is run, even if an exception occurs.
bracketOnError :: MonadMask m => m a -> (a -> m b) -> (a -> m c) -> m c Source #
Like bracket
, but only performs the final action if there was an
exception raised 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, Typeable) 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 deriving Typeable 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 deriving Typeable 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 (Typeable, 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 MismatchedParenthesescatch
e -> putStrLn ("Caught " ++ show (e :: MismatchedParentheses)) Caught MismatchedParentheses *Main> throw MismatchedParenthesescatch
e -> putStrLn ("Caught " ++ show (e :: SomeFrontendException)) Caught MismatchedParentheses *Main> throw MismatchedParenthesescatch
e -> putStrLn ("Caught " ++ show (e :: SomeCompilerException)) Caught MismatchedParentheses *Main> throw MismatchedParenthesescatch
e -> putStrLn ("Caught " ++ show (e :: IOException)) *** Exception: MismatchedParentheses
Nothing
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
.