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

Convenient alias for ResourceT IO.

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 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.

Since 0.3.0

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

Launch a new reference counted resource context using forkIO.

This is defined as resourceForkWith forkIO.

Since: 0.3.0

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

This function mirrors join at the transformer level: it will collapse two levels of ResourceT into a single ResourceT.

Since 0.4.6

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 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

Call a release action early, and deregister it from the list of cleanup actions to be performed.

Since 0.3.0

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

Perform asynchronous exception masking.

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

Since 0.3.0

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

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 MonadBaseControl IO.
• MonadResource requires an instance of MonadThrow, MonadIO, and Applicative.

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

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

Writing instances

The usual way to write a MonadBaseControl instance for a transformer stack over a base monad B is to write an instance MonadBaseControl B B for the base monad, and MonadTransControl T instances for every transformer T. Instances for MonadBaseControl are then simply implemented using ComposeSt, defaultLiftBaseWith, defaultRestoreM.

The internal state held by a ResourceT transformer.

Since 0.4.6

Get the internal state of the current ResourceT.

Since 0.4.6

Unwrap a ResourceT using the given InternalState.

Since 0.4.6

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

Close an internal state created by createInternalState.

Since 0.4.9

For backwards compatibility.

For backwards compatibility.

Same as runExceptionT, but immediately throw any exception returned.

Since 0.3.0

Run an ExceptionT Identity stack.

Since 0.4.2

Run an ExceptionT Identity stack, but immediately throw any exception returned.

Since 0.4.2

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.

Backwards compatibility