Safe Haskell | Trustworthy |
---|---|
Language | Haskell98 |
Pipes.Safe
Description
This module provides an orphan MonadCatch
instance for Proxy
of the
form:
instance (MonadCatch m, MonadIO m) => MonadCatch (Proxy a' a b' b m) where
... so you can throw and catch exceptions within pipes using all
MonadCatch
operations.
This module also provides generalized versions of some MonadCatch
operations so that you can also protect against premature termination of
connected components. For example, if you protect a readFile
computation
using bracket
from this module:
-- readFile.hs import Pipes import qualified Pipes.Prelude as P import Pipes.Safe import qualified System.IO as IO import Prelude hiding (readFile) readFile :: FilePath -> Producer' String (SafeT IO) () readFile file = bracket (do h <- IO.openFile file IO.ReadMode putStrLn $ "{" ++ file ++ " open}" return h ) (\h -> do IO.hClose h putStrLn $ "{" ++ file ++ " closed}" ) P.fromHandle
... then this generalized bracket
will guard against both exceptions and
premature termination of other pipes:
>>>
runSafeT $ runEffect $ readFile "readFile.hs" >-> P.take 4 >-> P.stdoutLn
{readFile.hs open} -- readFile.hs import Pipes import qualified Pipes.Prelude as P import Pipes.Safe {readFile.hs closed}
Note that the MonadCatch
instance for Proxy
provides weaker versions of
mask
and uninterruptibleMask
that do not completely prevent asynchronous
exceptions. Instead, they provide a weaker guarantee that asynchronous
exceptions will only occur during await
s or yield
s and
nowhere else. For example, if you write:
mask_ $ do x <- await lift $ print x lift $ print x
... then you may receive an asynchronous exception during the await
,
but you will not receive an asynchronous exception during or in between the
two print
statements. This weaker guarantee suffices to provide
asynchronous exception safety.
Synopsis
- data SafeT m r
- runSafeT :: (MonadMask m, MonadIO m) => SafeT m r -> m r
- runSafeP :: (MonadMask m, MonadIO m) => Effect (SafeT m) r -> Effect' m r
- data ReleaseKey
- class (MonadCatch m, MonadMask m, MonadIO m, MonadIO (Base m)) => MonadSafe m where
- onException :: MonadSafe m => m a -> Base m b -> m a
- tryP :: (MonadSafe m, Exception e) => Proxy a' a b' b m r -> Proxy a' a b' b m (Either e r)
- catchP :: (MonadSafe m, Exception e) => Proxy a' a b' b m r -> (e -> Proxy a' a b' b m r) -> Proxy a' a b' b m r
- finally :: MonadSafe m => m a -> Base m b -> m a
- bracket :: MonadSafe m => Base m a -> (a -> Base m b) -> (a -> m c) -> m c
- bracket_ :: MonadSafe m => Base m a -> Base m b -> m c -> m c
- bracketOnError :: MonadSafe m => Base m a -> (a -> Base m b) -> (a -> m c) -> m c
- class (Typeable e, Show e) => Exception e where
- data SomeException
- catches :: (Foldable f, MonadCatch m) => m a -> f (Handler m a) -> m a
- tryJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> m a -> m (Either b 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
- handleAll :: MonadCatch m => (SomeException -> m a) -> m a -> m a
- handleIOError :: MonadCatch m => (IOError -> m a) -> m a -> m a
- handle :: (MonadCatch m, Exception e) => (e -> m a) -> 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
- catchIOError :: MonadCatch m => m a -> (IOError -> m a) -> m a
- catchAll :: MonadCatch m => m a -> (SomeException -> m a) -> m a
- uninterruptibleMask_ :: MonadMask m => m a -> m a
- mask_ :: MonadMask m => m a -> m a
- class Monad m => MonadThrow (m :: * -> *) where
- class MonadThrow m => MonadCatch (m :: * -> *) where
- class MonadCatch m => MonadMask (m :: * -> *) where
- data Handler (m :: * -> *) a where
- class (Typeable e, Show e) => Exception e where
- data SomeException where
SafeT
SafeT
is a monad transformer that extends the base monad with the ability
to register
and release
finalizers.
All unreleased finalizers are called at the end of the SafeT
block, even
in the event of exceptions.
Instances
runSafeT :: (MonadMask m, MonadIO m) => SafeT m r -> m r Source #
Run the SafeT
monad transformer, executing all unreleased finalizers at
the end of the computation
MonadSafe
data ReleaseKey Source #
class (MonadCatch m, MonadMask m, MonadIO m, MonadIO (Base m)) => MonadSafe m where Source #
Associated Types
type Base (m :: * -> *) :: * -> * Source #
The monad used to run resource management actions, corresponding to the
monad directly beneath SafeT
Methods
liftBase :: Base m r -> m r Source #
Lift an action from the Base
monad
register :: Base m () -> m ReleaseKey Source #
register
a finalizer, ensuring that the finalizer gets called if the
finalizer is not release
d before the end of the surrounding SafeT
block.
release :: ReleaseKey -> m () Source #
release
a registered finalizer
You can safely call release
more than once on the same ReleaseKey
.
Every release
after the first one does nothing.
Instances
MonadSafe m => MonadSafe (CatchT m) Source # | |
(MonadIO m, MonadCatch m, MonadMask m) => MonadSafe (SafeT m) Source # | |
(MonadSafe m, Monoid w) => MonadSafe (WriterT w m) Source # | |
MonadSafe m => MonadSafe (StateT s m) Source # | |
MonadSafe m => MonadSafe (IdentityT m) Source # | |
MonadSafe m => MonadSafe (StateT s m) Source # | |
(MonadSafe m, Monoid w) => MonadSafe (WriterT w m) Source # | |
MonadSafe m => MonadSafe (ReaderT i m) Source # | |
(MonadSafe m, Monoid w) => MonadSafe (RWST i w s m) Source # | |
(MonadSafe m, Monoid w) => MonadSafe (RWST i w s m) Source # | |
MonadSafe m => MonadSafe (Proxy a' a b' b m) Source # | |
Utilities
These utilities let you supply a finalizer that runs in the Base
monad
(i.e. the monad directly beneath SafeT
). If you don't need to use the
full power of the Base
monad and you only need to use to use IO
, then
just wrap the finalizer in liftIO
, like this:
myAction `finally` (liftIO myFinalizer)
This will lead to a simple inferred type with a single MonadSafe
constraint:
(MonadSafe m) => ...
For examples of this, see the utilities in Pipes.Safe.Prelude.
If you omit the liftIO
, the compiler will infer the following constraint
instead:
(MonadSafe m, Base m ~ IO) => ...
This means that this function would require IO
directly beneath the
SafeT
monad transformer, which might not be what you want.
onException :: MonadSafe m => m a -> Base m b -> m a Source #
Analogous to onException
from Control.Monad.Catch
, except this also
protects against premature termination
(`onException` io)
is a monad morphism.
tryP :: (MonadSafe m, Exception e) => Proxy a' a b' b m r -> Proxy a' a b' b m (Either e r) Source #
Transform a Proxy
into one that catches any exceptions caused by its
effects, and returns the resulting exception.
catchP :: (MonadSafe m, Exception e) => Proxy a' a b' b m r -> (e -> Proxy a' a b' b m r) -> Proxy a' a b' b m r Source #
Allows direct handling of exceptions raised by the effects in a Proxy
.
finally :: MonadSafe m => m a -> Base m b -> m a Source #
Analogous to finally
from Control.Monad.Catch
, except this also
protects against premature termination
bracket :: MonadSafe m => Base m a -> (a -> Base m b) -> (a -> m c) -> m c Source #
Analogous to bracket
from Control.Monad.Catch
, except this also
protects against premature termination
bracket_ :: MonadSafe m => Base m a -> Base m b -> m c -> m c Source #
Analogous to bracket_
from Control.Monad.Catch
, except this also
protects against premature termination
bracketOnError :: MonadSafe m => Base m a -> (a -> Base m b) -> (a -> m c) -> m c Source #
Analogous to bracketOnError
from Control.Monad.Catch
, except this also
protects against premature termination
Re-exports
Control.Monad.Catch
re-exports all functions except for the ones that
conflict with the generalized versions provided here (i.e. bracket
,
finally
, etc.).
Control.Exception
re-exports Exception
and SomeException
.
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
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 #
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
.
Instances
Show SomeException | Since: base-3.0 |
Defined in GHC.Exception Methods showsPrec :: Int -> SomeException -> ShowS # show :: SomeException -> String # showList :: [SomeException] -> ShowS # | |
Exception SomeException | Since: base-3.0 |
Defined in GHC.Exception Methods toException :: SomeException -> SomeException # fromException :: SomeException -> Maybe SomeException # displayException :: SomeException -> String # |
catches :: (Foldable f, MonadCatch m) => m a -> f (Handler m a) -> m a #
Catches different sorts of exceptions. See Control.Exception's catches
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
handleJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> (b -> m a) -> m a -> m a #
Flipped catchJust
. See Control.Exception's handleJust
.
handleAll :: MonadCatch m => (SomeException -> m a) -> m a -> m a #
Flipped catchAll
handleIOError :: MonadCatch m => (IOError -> m a) -> m a -> m a #
Flipped catchIOError
handle :: (MonadCatch m, Exception e) => (e -> m a) -> m a -> m a #
Flipped catch
. See Control.Exception's handle
.
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.
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.
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.
uninterruptibleMask_ :: MonadMask m => m a -> m a #
Like uninterruptibleMask
, but does not pass a restore
action to the
argument.
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.
Minimal complete definition
Methods
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
.
Minimal complete definition
Methods
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: exceptions-0.8.3 |
Monad m => MonadCatch (CatchT m) | |
MonadCatch m => MonadCatch (ListT m) | |
MonadCatch m => MonadCatch (MaybeT m) | Catches exceptions from the base monad. |
MonadCatch m => MonadCatch (ListT m) | |
MonadCatch m => MonadCatch (SafeT m) # | |
(MonadCatch m, Monoid w) => MonadCatch (WriterT w m) | |
MonadCatch m => MonadCatch (StateT s m) | |
MonadCatch m => MonadCatch (ExceptT e m) | Catches exceptions from the base monad. |
(Error e, MonadCatch m) => MonadCatch (ErrorT e m) | Catches exceptions from the base monad. |
MonadCatch m => MonadCatch (IdentityT m) | |
MonadCatch m => MonadCatch (StateT s 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) | |
MonadCatch m => MonadCatch (Proxy a' a b' b 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, 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.
Minimal complete definition
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.
Instances
MonadMask IO | |
e ~ SomeException => MonadMask (Either e) | Since: exceptions-0.8.3 |
Monad m => MonadMask (CatchT m) | Note: This instance is only valid if the underlying monad has a single exit point! |
MonadMask m => MonadMask (SafeT m) # | |
(MonadMask m, Monoid w) => MonadMask (WriterT w m) | |
MonadMask m => MonadMask (StateT s m) | |
MonadMask m => MonadMask (IdentityT m) | |
MonadMask m => MonadMask (StateT s m) | |
(MonadMask m, Monoid w) => MonadMask (WriterT w m) | |
MonadMask m => MonadMask (ReaderT r m) | |
(MonadMask m, Monoid w) => MonadMask (RWST r w s m) | |
(MonadMask m, Monoid w) => MonadMask (RWST r w s m) | |
(MonadMask m, MonadIO m) => MonadMask (Proxy a' a b' b m) # | |
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
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 :: SomeException |
Instances
Show SomeException | Since: base-3.0 |
Defined in GHC.Exception Methods showsPrec :: Int -> SomeException -> ShowS # show :: SomeException -> String # showList :: [SomeException] -> ShowS # | |
Exception SomeException | Since: base-3.0 |
Defined in GHC.Exception Methods toException :: SomeException -> SomeException # fromException :: SomeException -> Maybe SomeException # displayException :: SomeException -> String # |