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Language  Haskell98 
Allocate resources which are guaranteed to be released.
For more information, see https://www.fpcomplete.com/user/snoyberg/librarydocumentation/resourcet.
One point to note: all register cleanup actions live in the IO
monad, not
the main monad. This allows both more efficient code, and for monads to be
transformed.
 data ResourceT m a
 type ResIO = ResourceT IO
 data ReleaseKey
 runResourceT :: MonadBaseControl IO m => ResourceT m a > m a
 runResourceTChecked :: MonadUnliftIO m => ResourceT m a > m a
 data ResourceCleanupException = ResourceCleanupException {}
 resourceForkWith :: MonadBaseControl IO m => (IO () > IO a) > ResourceT m () > ResourceT m a
 resourceForkIO :: MonadBaseControl IO m => ResourceT m () > ResourceT m ThreadId
 transResourceT :: (m a > n b) > ResourceT m a > ResourceT n b
 joinResourceT :: ResourceT (ResourceT m) a > ResourceT m a
 allocate :: MonadResource m => IO a > (a > IO ()) > m (ReleaseKey, a)
 register :: MonadResource m => IO () > m ReleaseKey
 release :: MonadIO m => ReleaseKey > m ()
 unprotect :: MonadIO m => ReleaseKey > m (Maybe (IO ()))
 resourceMask :: MonadResource m => ((forall a. ResourceT IO a > ResourceT IO a) > ResourceT IO b) > m b
 class (MonadThrow m, MonadIO m, Applicative m, MonadBase IO m) => MonadResource m where
 type MonadResourceBase m = (MonadBaseControl IO m, MonadThrow m, MonadBase IO m, MonadIO m, Applicative m)
 data InvalidAccess = InvalidAccess {}
 class MonadBase b m => MonadBaseControl (b :: * > *) (m :: * > *)  m > b
 type InternalState = IORef ReleaseMap
 getInternalState :: Monad m => ResourceT m InternalState
 runInternalState :: ResourceT m a > InternalState > m a
 withInternalState :: (InternalState > m a) > ResourceT m a
 createInternalState :: MonadBase IO m => m InternalState
 closeInternalState :: MonadBase IO m => InternalState > m ()
 type ExceptionT = CatchT
 runExceptionT :: ExceptionT m a > m (Either SomeException a)
 runExceptionT_ :: Monad m => ExceptionT m a > m a
 runException :: ExceptionT Identity a > Either SomeException a
 runException_ :: ExceptionT Identity a > a
 class Monad m => MonadThrow (m :: * > *) where
 monadThrow :: (Exception e, MonadThrow m) => e > m a
Data types
The Resource transformer. This transformer keeps track of all registered
actions, and calls them upon exit (via runResourceT
). Actions may be
registered via register
, or resources may be allocated atomically via
allocate
. allocate
corresponds closely to bracket
.
Releasing may be performed before exit via the release
function. This is a
highly recommended optimization, as it will ensure that scarce resources are
freed early. Note that calling release
will deregister the action, so that
a release action will only ever be called once.
Since 0.3.0
data ReleaseKey Source #
A lookup key for a specific release action. This value is returned by
register
and allocate
, and is passed to release
.
Since 0.3.0
Unwrap
runResourceT :: MonadBaseControl IO m => ResourceT m a > m a Source #
Unwrap a ResourceT
transformer, and call all registered release actions.
Note that there is some reference counting involved due to resourceForkIO
.
If multiple threads are sharing the same collection of resources, only the
last call to runResourceT
will deallocate the resources.
NOTE Since version 1.1.11, this module has also provided
runResourceTChecked
, which is a safer version of this
function. In the next major release of this library, it will become
the behavior of this function.
Since 0.3.0
Check cleanup exceptions
runResourceTChecked :: MonadUnliftIO m => ResourceT m a > m a Source #
Like runResourceT
, but checks whether the cleanup functions
throw any exceptions. If they do, they are rethrown inside a
ResourceCleanupException
.
Since: 1.1.11
data ResourceCleanupException Source #
Thrown when one or more cleanup functions themselves throw an exception during cleanup.
Since: 1.1.11
ResourceCleanupException  

Special actions
resourceForkWith :: MonadBaseControl IO m => (IO () > IO a) > ResourceT m () > ResourceT m a Source #
Introduce a referencecounting scheme to allow a resource context to be shared by multiple threads. Once the last thread exits, all remaining resources will be released.
The first parameter is a function which will be used to create the
thread, such as forkIO
or async
.
Note that abuse of this function will greatly delay the deallocation of registered resources. This function should be used with care. A general guideline:
If you are allocating a resource that should be shared by multiple threads,
and will be held for a long time, you should allocate it at the beginning of
a new ResourceT
block and then call resourceForkWith
from there.
Since: 1.1.9
resourceForkIO :: MonadBaseControl IO m => ResourceT m () > ResourceT m ThreadId Source #
Launch a new reference counted resource context using forkIO
.
This is defined as resourceForkWith forkIO
.
Since: 0.3.0
Monad transformation
transResourceT :: (m a > n b) > ResourceT m a > ResourceT n b Source #
Transform the monad a ResourceT
lives in. This is most often used to
strip or add new transformers to a stack, e.g. to run a ReaderT
.
Note that this function is a slight generalization of hoist
.
Since 0.3.0
joinResourceT :: ResourceT (ResourceT m) a > ResourceT m a Source #
This function mirrors join
at the transformer level: it will collapse
two levels of ResourceT
into a single ResourceT
.
Since 0.4.6
Registering/releasing
:: MonadResource m  
=> IO a  allocate 
> (a > IO ())  free resource 
> m (ReleaseKey, a) 
Perform some allocation, and automatically register a cleanup action.
This is almost identical to calling the allocation and then
register
ing the release action, but this properly handles masking of
asynchronous exceptions.
Since 0.3.0
register :: MonadResource m => IO () > m ReleaseKey Source #
Register some action that will be called precisely once, either when
runResourceT
is called, or when the ReleaseKey
is passed to release
.
Since 0.3.0
release :: MonadIO m => ReleaseKey > m () Source #
Call a release action early, and deregister it from the list of cleanup actions to be performed.
Since 0.3.0
unprotect :: MonadIO m => ReleaseKey > m (Maybe (IO ())) Source #
Unprotect resource from cleanup actions; this allows you to send resource into another resourcet process and reregister it there. It returns an release action that should be run in order to clean resource or Nothing in case if resource is already freed.
Since 0.4.5
resourceMask :: MonadResource m => ((forall a. ResourceT IO a > ResourceT IO a) > ResourceT IO b) > m b Source #
Perform asynchronous exception masking.
This is more general then Control.Exception.mask
, yet more efficient
than Control.Exception.Lifted.mask
.
Since 0.3.0
Type class/associated types
class (MonadThrow m, MonadIO m, Applicative m, MonadBase IO m) => MonadResource m where Source #
A Monad
which allows for safe resource allocation. In theory, any monad
transformer stack which includes a ResourceT
can be an instance of
MonadResource
.
Note: runResourceT
has a requirement for a MonadBaseControl IO m
monad,
which allows control operations to be lifted. A MonadResource
does not
have this requirement. This means that transformers such as ContT
can be
an instance of MonadResource
. However, the ContT
wrapper will need to be
unwrapped before calling runResourceT
.
Since 0.3.0
liftResourceT :: ResourceT IO a > m a Source #
Lift a ResourceT IO
action into the current Monad
.
Since 0.4.0
MonadResource m => MonadResource (ListT m) Source #  
MonadResource m => MonadResource (MaybeT m) Source #  
(MonadThrow m, MonadBase IO m, MonadIO m, Applicative m) => MonadResource (ResourceT m) Source #  
(Monoid w, MonadResource m) => MonadResource (WriterT w m) Source #  
MonadResource m => MonadResource (StateT s m) Source #  
MonadResource m => MonadResource (ExceptT e m) Source #  
(Error e, MonadResource m) => MonadResource (ErrorT e m) Source #  
MonadResource m => MonadResource (IdentityT * m) Source #  
MonadResource m => MonadResource (StateT s m) Source #  
(Monoid w, MonadResource m) => MonadResource (WriterT w m) Source #  
MonadResource m => MonadResource (ReaderT * r m) Source #  
MonadResource m => MonadResource (ContT * r m) Source #  
(Monoid w, MonadResource m) => MonadResource (RWST r w s m) Source #  
(Monoid w, MonadResource m) => MonadResource (RWST r w s m) Source #  
type MonadResourceBase m = (MonadBaseControl IO m, MonadThrow m, MonadBase IO m, MonadIO m, Applicative m) Source #
A Monad
which can be used as a base for a ResourceT
.
A ResourceT
has some restrictions on its base monad:
runResourceT
requires an instance ofMonadBaseControl IO
.MonadResource
requires an instance ofMonadThrow
,MonadIO
, andApplicative
.
While any instance of MonadBaseControl IO
should be an instance of the
other classes, this is not guaranteed by the type system (e.g., you may have
a transformer in your stack with does not implement MonadThrow
). Ideally,
we would like to simply create an alias for the five type classes listed,
but this is not possible with GHC currently.
Instead, this typeclass acts as a proxy for the other five. Its only purpose is to make your type signatures shorter.
Note that earlier versions of conduit
had a typeclass ResourceIO
. This
fulfills much the same role.
Since 0.3.2
Lowlevel
data InvalidAccess Source #
Indicates either an error in the library, or misuse of it (e.g., a
ResourceT
's state is accessed after being released).
Since 0.3.0
Reexports
class MonadBase b m => MonadBaseControl (b :: * > *) (m :: * > *)  m > b #
Writing instances
The usual way to write a
instance for a transformer
stack over a base monad MonadBaseControl
B
is to write an instance MonadBaseControl B B
for the base monad, and MonadTransControl T
instances for every transformer
T
. Instances for
are then simply implemented using
MonadBaseControl
, ComposeSt
, defaultLiftBaseWith
.defaultRestoreM
Internal state
A ResourceT
internally is a modified ReaderT
monad transformer holding
onto a mutable reference to all of the release actions still remaining to be
performed. If you are building up a custom application monad, it may be more
efficient to embed this ReaderT
functionality directly in your own monad
instead of wrapping around ResourceT
itself. This section provides you the
means of doing so.
type InternalState = IORef ReleaseMap Source #
The internal state held by a ResourceT
transformer.
Since 0.4.6
getInternalState :: Monad m => ResourceT m InternalState Source #
Get the internal state of the current ResourceT
.
Since 0.4.6
runInternalState :: ResourceT m a > InternalState > m a Source #
Unwrap a ResourceT
using the given InternalState
.
Since 0.4.6
withInternalState :: (InternalState > m a) > ResourceT m a Source #
Run an action in the underlying monad, providing it the InternalState
.
Since 0.4.6
createInternalState :: MonadBase IO m => m InternalState Source #
Create a new internal state. This state must be closed with
closeInternalState
. It is your responsibility to ensure exception safety.
Caveat emptor!
Since 0.4.9
closeInternalState :: MonadBase IO m => InternalState > m () Source #
Close an internal state created by createInternalState
.
Since 0.4.9
Backwards compatibility
type ExceptionT = CatchT Source #
For backwards compatibility.
runExceptionT :: ExceptionT m a > m (Either SomeException a) Source #
For backwards compatibility.
runExceptionT_ :: Monad m => ExceptionT m a > m a Source #
Same as runExceptionT
, but immediately throw
any exception returned.
Since 0.3.0
runException :: ExceptionT Identity a > Either SomeException a Source #
Run an ExceptionT Identity
stack.
Since 0.4.2
runException_ :: ExceptionT Identity a > a Source #
Run an ExceptionT Identity
stack, but immediately throw
any exception returned.
Since 0.4.2
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 shortcircuits 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
MonadThrow []  
MonadThrow Maybe  
MonadThrow IO  
MonadThrow Q  
MonadThrow STM  
(~) * e SomeException => MonadThrow (Either e)  
Monad m => MonadThrow (CatchT m)  
MonadThrow m => MonadThrow (ListT m)  
MonadThrow m => MonadThrow (MaybeT m)  Throws exceptions into the base monad. 
MonadThrow m => MonadThrow (ResourceT m) #  
(MonadThrow m, Monoid w) => MonadThrow (WriterT w m)  
MonadThrow m => MonadThrow (StateT s m)  
MonadThrow m => MonadThrow (ExceptT e m)  Throws exceptions into the base monad. 
(Error e, MonadThrow m) => MonadThrow (ErrorT e m)  Throws exceptions into the base monad. 
MonadThrow m => MonadThrow (IdentityT * m)  
MonadThrow m => MonadThrow (StateT s m)  
(MonadThrow m, Monoid w) => MonadThrow (WriterT w m)  
MonadThrow m => MonadThrow (ReaderT * r m)  
MonadThrow m => MonadThrow (ContT * r m)  
(MonadThrow m, Monoid w) => MonadThrow (RWST r w s m)  
(MonadThrow m, Monoid w) => MonadThrow (RWST r w s m)  
monadThrow :: (Exception e, MonadThrow m) => e > m a Source #
Backwards compatibility