conduit-0.2.0: Streaming data processing library.

Safe HaskellSafe-Infered




Allocate resources which are guaranteed to be released.

For more information, see

One point to note: all register cleanup actions live in the base monad, not the main monad. This allows both more efficient code, and for monads to be transformed.


Data types

data ResourceT m a Source

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 with or withIO. The with functions correspond 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.

data ReleaseKey Source

A lookup key for a specific release action. This value is returned by register, with and withIO, and is passed to release.



runResourceT :: Resource m => ResourceT m a -> m aSource

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.

Resource allocation



:: Resource m 
=> Base m a


-> (a -> Base m ())

free resource

-> ResourceT m (ReleaseKey, a) 

Perform some allocation, and automatically register a cleanup action.

If you are performing an IO action, it will likely be easier to use the withIO function, which handles types more cleanly.



:: ResourceIO m 
=> IO a


-> (a -> IO ())

free resource

-> ResourceT m (ReleaseKey, a) 

Same as with, but explicitly uses IO as a base.

register :: Resource m => Base m () -> ResourceT m ReleaseKeySource

Register some action that will be called precisely once, either when runResourceT is called, or when the ReleaseKey is passed to release.

release :: Resource m => ReleaseKey -> ResourceT m ()Source

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

Use references

readRef :: Resource m => Ref (Base m) a -> ResourceT m aSource

Read a value from a reference.

writeRef :: Resource m => Ref (Base m) a -> a -> ResourceT m ()Source

Write a value to a reference.

newRef :: Resource m => a -> ResourceT m (Ref (Base m) a)Source

Create a new reference.

Special actions

resourceForkIO :: ResourceIO m => ResourceT m () -> ResourceT m ThreadIdSource

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.

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 resourceForkIO from there.

Monad transformation

transResourceT :: Base m ~ Base n => (m a -> n a) -> ResourceT m a -> ResourceT n aSource

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 the original and new monad must both have the same Base monad.

A specific Exception transformer

newtype ExceptionT m a Source

The express purpose of this transformer is to allow the ST monad to catch exceptions via the ResourceThrow typeclass.



runExceptionT_ :: Monad m => ExceptionT m a -> m aSource

Same as runExceptionT, but immediately throw any exception returned.

Type class/associated types

class (HasRef (Base m), Monad m) => Resource m whereSource

A Monad with a base that has mutable references, and allows some way to run base actions and clean up properly.

Associated Types

type Base m :: * -> *Source

The base monad for the current monad stack. This will usually be IO or ST.


resourceLiftBase :: Base m a -> m aSource

Run some action in the Base monad. This function corresponds to liftBase, but due to various type issues, we need to have our own version here.



:: Base m ()


-> Base m ()


-> m c


-> m c 

Guarantee that some initialization and cleanup code is called before and after some action. Note that the initialization and cleanup lives in the base monad, while the body is in the top monad.


class Resource m => ResourceUnsafeIO m whereSource

A Resource based on some monad which allows running of some IO actions, via unsafe calls. This applies to IO and ST, for instance.


unsafeFromIO :: IO a -> m aSource

class (ResourceBaseIO (Base m), ResourceUnsafeIO m, ResourceThrow m, MonadIO m, MonadBaseControl IO m) => ResourceIO m Source

A Resource which can safely run IO calls.


class ResourceBaseIO m whereSource

A helper class for ResourceIO, stating that the base monad provides IO actions.


safeFromIOBase :: IO a -> m aSource


class Resource m => ResourceThrow m whereSource

A Resource which can throw exceptions. Note that this does not work in a vanilla ST monad. Instead, you should use the ExceptionT transformer on top of ST.


resourceThrow :: Exception e => e -> m aSource


class Monad m => HasRef m whereSource

A base monad which provides mutable references and some exception-safe way of interacting with them. For monads which cannot handle exceptions (e.g., ST), exceptions may be ignored. However, in such cases, scarce resources should not be allocated in those monads, as exceptions may cause the cleanup functions to not run.

The instance for IO, however, is fully exception-safe.

Minimal complete definition: Ref, newRef', readRef' and writeRef'.

Associated Types

type Ref m :: * -> *Source


newRef' :: a -> m (Ref m a)Source

readRef' :: Ref m a -> m aSource

writeRef' :: Ref m a -> a -> m ()Source

atomicModifyRef' :: Ref m a -> (a -> (a, b)) -> m bSource

For monads supporting multi-threaded access (e.g., IO), this much be an atomic modification.

mask :: ((forall a. m a -> m a) -> m b) -> m bSource

mask_ :: m a -> m aSource

try :: m a -> m (Either SomeException a)Source


HasRef IO 
HasRef (ST s) 
HasRef (ST s)