resourcet-1.2.2: Deterministic allocation and freeing of scarce resources.

Description

Allocate resources which are guaranteed to be released.

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.

Synopsis

# Data types

data ResourceT m a #

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

Instances

type ResIO = ResourceT IO #

Convenient alias for ResourceT IO.

data ReleaseKey #

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 :: MonadUnliftIO m => ResourceT m a -> m a #

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 1.2.0, this function will throw a ResourceCleanupException if any of the cleanup functions throw an exception.

Since: 0.3.0

## Check cleanup exceptions

runResourceTChecked :: MonadUnliftIO m => ResourceT m a -> m a #

Backwards compatible alias for runResourceT.

Since: 1.1.11

Thrown when one or more cleanup functions themselves throw an exception during cleanup.

Since: 1.1.11

Constructors

 ResourceCleanupException FieldsrceOriginalException :: !(Maybe SomeException)If the ResourceT block exited due to an exception, this is that exception.Since: 1.1.11rceFirstCleanupException :: !SomeExceptionThe first cleanup exception. We keep this separate from rceOtherCleanupExceptions to prove that there's at least one (i.e., a non-empty list).Since: 1.1.11rceOtherCleanupExceptions :: ![SomeException]All other exceptions in cleanups.Since: 1.1.11
Instances

# Special actions

resourceForkWith :: MonadUnliftIO m => (IO () -> IO a) -> ResourceT m () -> ResourceT m a #

Introduce a reference-counting 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 :: MonadUnliftIO m => ResourceT m () -> ResourceT m ThreadId #

Launch a new reference counted resource context using forkIO.

This is defined as resourceForkWith forkIO.

Since: 0.3.0

transResourceT :: (m a -> n b) -> ResourceT m a -> ResourceT n b #

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 #

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

Arguments

 :: 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 registering the release action, but this properly handles masking of asynchronous exceptions.

Since 0.3.0

register :: MonadResource m => IO () -> m ReleaseKey #

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 () #

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 ())) #

Unprotect resource from cleanup actions; this allows you to send resource into another resourcet process and reregister it there. It returns a 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 #

This is more general then Control.Exception.mask, yet more efficient than Control.Exception.Lifted.mask.

Since 0.3.0

# Type class/associated types

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 MonadUnliftIO 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

Methods

liftResourceT :: ResourceT IO a -> m a #

Lift a ResourceT IO action into the current Monad.

Since 0.4.0

Instances

Just use MonadUnliftIO directly now, legacy explanation continues:

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 of MonadUnliftIO.
• MonadResource requires an instance of MonadIO

Note that earlier versions of conduit had a typeclass ResourceIO. This fulfills much the same role.

Since 0.3.2

## Low-level

data InvalidAccess #

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

Constructors

 InvalidAccess FieldsfunctionName :: String
Instances
 # Instance detailsDefined in Control.Monad.Trans.Resource.Internal MethodsshowList :: [InvalidAccess] -> ShowS # # Instance detailsDefined in Control.Monad.Trans.Resource.Internal Methods

# Re-exports

Monads which allow their actions to be run in IO.

While MonadIO allows an IO action to be lifted into another monad, this class captures the opposite concept: allowing you to capture the monadic context. Note that, in order to meet the laws given below, the intuition is that a monad must have no monadic state, but may have monadic context. This essentially limits MonadUnliftIO to ReaderT and IdentityT transformers on top of IO.

Laws. For any value u returned by askUnliftIO, it must meet the monad transformer laws as reformulated for MonadUnliftIO:

• unliftIO u . return = return
• unliftIO u (m >>= f) = unliftIO u m >>= unliftIO u . f

The third is a currently nameless law which ensures that the current context is preserved.

• askUnliftIO >>= (u -> liftIO (unliftIO u m)) = m

If you have a name for this, please submit it in a pull request for great glory.

Since: unliftio-core-0.1.0.0

Minimal complete definition

Instances

# 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.

The internal state held by a ResourceT transformer.

Since 0.4.6

Get the internal state of the current ResourceT.

Since 0.4.6

runInternalState :: ResourceT m a -> InternalState -> m a #

Unwrap a ResourceT using the given InternalState.

Since 0.4.6

withInternalState :: (InternalState -> m a) -> ResourceT m a #

Run an action in the underlying monad, providing it the InternalState.

Since 0.4.6

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 :: MonadIO m => InternalState -> m () #

Close an internal state created by createInternalState.

Since 0.4.9

# Reexport

class Monad m => MonadThrow (m :: Type -> Type) 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.

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