Data.Conduit

If this is your first time with conduit, you should probably start with the tutorial: https://haskell.fpcomplete.com/user/snoyberg/library-documentation/conduit-overview.

Core interface

Types

type Source m o = ConduitM () o m ()

Provides a stream of output values, without consuming any input or producing a final result.

Since 0.5.0

type Conduit i m o = ConduitM i o m ()

Consumes a stream of input values and produces a stream of output values, without producing a final result.

Since 0.5.0

type Sink i = ConduitM i Void

Consumes a stream of input values and produces a final result, without producing any output.

 type Sink i m r = ConduitM i Void m r

Since 0.5.0

data ConduitM i o m r

Core datatype of the conduit package. This type represents a general component which can consume a stream of input values i, produce a stream of output values o, perform actions in the m monad, and produce a final result r. The type synonyms provided here are simply wrappers around this type.

Since 1.0.0

Instances

MonadRWS r w s m => MonadRWS r w s (ConduitM i o m)

MonadBase base m => MonadBase base (ConduitM i o m)

MonadError e m => MonadError e (ConduitM i o m)

MonadReader r m => MonadReader r (ConduitM i o m)

MonadWriter w m => MonadWriter w (ConduitM i o m)

MonadState s m => MonadState s (ConduitM i o m)

MFunctor (ConduitM i o)

MonadTrans (ConduitM i o)

Monad m => Monad (ConduitM i o m)

Monad m => Functor (ConduitM i o m)

Monad m => Applicative (ConduitM i o m)

MonadActive m => MonadActive (ConduitM i o m)

MonadResource m => MonadResource (ConduitM i o m)

MonadThrow m => MonadThrow (ConduitM i o m)

MonadIO m => MonadIO (ConduitM i o m)

Monad m => Monoid (ConduitM i o m ())

Connect/fuse operators

($$) :: Monad m => Source m a -> Sink a m b -> m b

The connect operator, which pulls data from a source and pushes to a sink. If you would like to keep the Source open to be used for other operations, use the connect-and-resume operator $$+.

Since 0.4.0

($=) :: Monad m => Source m a -> Conduit a m b -> Source m b

Left fuse, combining a source and a conduit together into a new source.

Both the Source and Conduit will be closed when the newly-created Source is closed.

Leftover data from the Conduit will be discarded.

Since 0.4.0

(=$) :: Monad m => Conduit a m b -> Sink b m c -> Sink a m c

Right fuse, combining a conduit and a sink together into a new sink.

Both the Conduit and Sink will be closed when the newly-created Sink is closed.

Leftover data returned from the Sink will be discarded.

Since 0.4.0

(=$=) :: Monad m => Conduit a m b -> ConduitM b c m r -> ConduitM a c m r

Fusion operator, combining two Conduits together into a new Conduit.

Both Conduits will be closed when the newly-created Conduit is closed.

Leftover data returned from the right Conduit will be discarded.

Since 0.4.0

Primitives

await :: Monad m => Consumer i m (Maybe i)

Wait for a single input value from upstream. If no data is available, returns Nothing.

Since 0.5.0

yield

Arguments

:: Monad m

=> o

output value

-> ConduitM i o m ()

Send a value downstream to the next component to consume. If the downstream component terminates, this call will never return control. If you would like to register a cleanup function, please use yieldOr instead.

Since 0.5.0

leftover :: i -> ConduitM i o m ()

Provide a single piece of leftover input to be consumed by the next component in the current monadic binding.

Note: it is highly encouraged to only return leftover values from input already consumed from upstream.

Since 0.5.0

Finalization

bracketP :: MonadResource m => IO a -> (a -> IO ()) -> (a -> ConduitM i o m r) -> ConduitM i o m r

Perform some allocation and run an inner component. Two guarantees are given about resource finalization:

1. It will be prompt. The finalization will be run as early as possible.
2. It is exception safe. Due to usage of resourcet, the finalization will be run in the event of any exceptions.

Since 0.5.0

addCleanup :: Monad m => (Bool -> m ()) -> ConduitM i o m r -> ConduitM i o m r

Add some code to be run when the given component cleans up.

The supplied cleanup function will be given a True if the component ran to completion, or False if it terminated early due to a downstream component terminating.

Note that this function is not exception safe. For that, please use bracketP.

Since 0.4.1

yieldOr

Arguments

:: Monad m

=> o

-> m ()

finalizer

-> ConduitM i o m ()

Similar to yield, but additionally takes a finalizer to be run if the downstream component terminates.

Since 0.5.0

Generalized conduit types

type Producer m o = forall i. ConduitM i o m ()

A component which produces a stream of output values, regardless of the input stream. A Producer is a generalization of a Source, and can be used as either a Source or a Conduit.

Since 1.0.0

type Consumer i m r = forall o. ConduitM i o m r

A component which consumes a stream of input values and produces a final result, regardless of the output stream. A Consumer is a generalization of a Sink, and can be used as either a Sink or a Conduit.

Since 1.0.0

toProducer :: Monad m => Source m a -> Producer m a

Generalize a Source to a Producer.

Since 1.0.0

toConsumer :: Monad m => Sink a m b -> Consumer a m b

Generalize a Sink to a Consumer.

Since 1.0.0

Utility functions

awaitForever :: Monad m => (i -> ConduitM i o m r) -> ConduitM i o m ()

Wait for input forever, calling the given inner component for each piece of new input. Returns the upstream result type.

This function is provided as a convenience for the common pattern of awaiting input, checking if it's Just and then looping.

Since 0.5.0

transPipe :: Monad m => (forall a. m a -> n a) -> ConduitM i o m r -> ConduitM i o n r

Transform the monad that a ConduitM lives in.

Note that the monad transforming function will be run multiple times, resulting in unintuitive behavior in some cases. For a fuller treatment, please see:

https://github.com/snoyberg/conduit/wiki/Dealing-with-monad-transformers

This function is just a synonym for hoist.

Since 0.4.0

mapOutput :: Monad m => (o1 -> o2) -> ConduitM i o1 m r -> ConduitM i o2 m r

Apply a function to all the output values of a ConduitM.

This mimics the behavior of fmap for a Source and Conduit in pre-0.4 days. It can also be simulated by fusing with the map conduit from Data.Conduit.List.

Since 0.4.1

mapOutputMaybe :: Monad m => (o1 -> Maybe o2) -> ConduitM i o1 m r -> ConduitM i o2 m r

Same as mapOutput, but use a function that returns Maybe values.

Since 0.5.0

mapInput

Arguments

:: Monad m

=> (i1 -> i2)

map initial input to new input

-> (i2 -> Maybe i1)

map new leftovers to initial leftovers

-> ConduitM i2 o m r

-> ConduitM i1 o m r

Apply a function to all the input values of a ConduitM.

Since 0.5.0

Connect-and-resume

data ResumableSource m o

A Source which has been started, but has not yet completed.

This type contains both the current state of the Source, and the finalizer to be run to close it.

Since 0.5.0

($$+) :: Monad m => Source m a -> Sink a m b -> m (ResumableSource m a, b)

The connect-and-resume operator. This does not close the Source, but instead returns it to be used again. This allows a Source to be used incrementally in a large program, without forcing the entire program to live in the Sink monad.

Mnemonic: connect + do more.

Since 0.5.0

($$++) :: Monad m => ResumableSource m a -> Sink a m b -> m (ResumableSource m a, b)

Continue processing after usage of $$+.

Since 0.5.0

($$+-) :: Monad m => ResumableSource m a -> Sink a m b -> m b

Complete processing of a ResumableSource. This will run the finalizer associated with the ResumableSource. In order to guarantee process resource finalization, you must use this operator after using $$+ and $$++.

Since 0.5.0

unwrapResumable :: MonadIO m => ResumableSource m o -> m (Source m o, m ())

Unwraps a ResumableSource into a Source and a finalizer.

A ResumableSource represents a Source which has already been run, and therefore has a finalizer registered. As a result, if we want to turn it into a regular Source, we need to ensure that the finalizer will be run appropriately. By appropriately, I mean:

• If a new finalizer is registered, the old one should not be called.
• If the old one is called, it should not be called again.

This function returns both a Source and a finalizer which ensures that the above two conditions hold. Once you call that finalizer, the Source is invalidated and cannot be used.

Since 0.5.2

Flushing

data Flush a

Provide for a stream of data that can be flushed.

A number of Conduits (e.g., zlib compression) need the ability to flush the stream at some point. This provides a single wrapper datatype to be used in all such circumstances.

Since 0.3.0

Constructors

Chunk a

Flush

Instances

Functor Flush

Eq a => Eq (Flush a)

Ord a => Ord (Flush a)

Show a => Show (Flush a)

Convenience re-exports

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

MFunctor ResourceT

Since 0.4.7

MMonad ResourceT

Since 0.4.7

MonadTrans ResourceT

MonadTransControl ResourceT

MonadRWS r w s m => MonadRWS r w s (ResourceT m)

MonadBase b m => MonadBase b (ResourceT m)

MonadBaseControl b m => MonadBaseControl b (ResourceT m)

MonadError e m => MonadError e (ResourceT m)

MonadReader r m => MonadReader r (ResourceT m)

MonadWriter w m => MonadWriter w (ResourceT m)

MonadState s m => MonadState s (ResourceT m)

Monad m => Monad (ResourceT m)

Functor m => Functor (ResourceT m)

Typeable1 m => Typeable1 (ResourceT m)

Applicative m => Applicative (ResourceT m)

(MonadIO m, MonadActive m) => MonadActive (ResourceT m)

(MonadThrow m, MonadUnsafeIO m, MonadIO m, Applicative m) => MonadResource (ResourceT m)

MonadThrow m => MonadThrow (ResourceT m)

MonadIO m => MonadIO (ResourceT m)

MonadCont m => MonadCont (ResourceT m)

class (MonadThrow m, MonadUnsafeIO m, MonadIO m, Applicative m) => MonadResource m

A Monad which allows for safe resource allocation. In theory, any monad transformer stack included 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

Instances

(MonadThrow m, MonadUnsafeIO m, MonadIO m, Applicative m) => MonadResource (ResourceT m)

MonadResource m => MonadResource (ExceptionT m)

MonadResource m => MonadResource (MaybeT m)

MonadResource m => MonadResource (ListT m)

MonadResource m => MonadResource (IdentityT m)

(Monoid w, MonadResource m) => MonadResource (WriterT w m)

(Monoid w, MonadResource m) => MonadResource (WriterT w m)

MonadResource m => MonadResource (StateT s m)

MonadResource m => MonadResource (StateT s m)

MonadResource m => MonadResource (ReaderT r m)

(Error e, MonadResource m) => MonadResource (ErrorT e m)

MonadResource m => MonadResource (ContT r m)

MonadResource m => MonadResource (ConduitM i o m)

(Monoid w, MonadResource m) => MonadResource (RWST r w s m)

(Monoid w, MonadResource m) => MonadResource (RWST r w s m)

MonadResource m => MonadResource (Pipe l i o u m)

class Monad m => MonadThrow m where

A Monad which can throw exceptions. Note that this does not work in a vanilla ST or Identity monad. Instead, you should use the ExceptionT transformer in your stack if you are dealing with a non-IO base monad.

Since 0.3.0

Methods

monadThrow :: Exception e => e -> m a

Instances

MonadThrow []

MonadThrow IO

MonadThrow Maybe

MonadThrow (Either SomeException)

MonadThrow m => MonadThrow (ResourceT m)

Monad m => MonadThrow (ExceptionT m)

MonadThrow m => MonadThrow (MaybeT m)

MonadThrow m => MonadThrow (ListT m)

MonadThrow m => MonadThrow (IdentityT m)

(Monoid w, MonadThrow m) => MonadThrow (WriterT w m)

(Monoid w, MonadThrow m) => MonadThrow (WriterT w m)

MonadThrow m => MonadThrow (StateT s m)

MonadThrow m => MonadThrow (StateT s m)

MonadThrow m => MonadThrow (ReaderT r m)

(Error e, MonadThrow m) => MonadThrow (ErrorT e m)

MonadThrow m => MonadThrow (ContT r m)

MonadThrow m => MonadThrow (ConduitM i o m)

(Monoid w, MonadThrow m) => MonadThrow (RWST r w s m)

(Monoid w, MonadThrow m) => MonadThrow (RWST r w s m)

MonadThrow m => MonadThrow (Pipe l i o u m)

class Monad m => MonadUnsafeIO m where

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

Since 0.3.0

Methods

unsafeLiftIO :: IO a -> m a

Instances

MonadUnsafeIO IO

(MonadTrans t, MonadUnsafeIO m, Monad (t m)) => MonadUnsafeIO (t m)

MonadUnsafeIO (ST s)

MonadUnsafeIO (ST s)

runResourceT :: MonadBaseControl IO 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.

Since 0.3.0

newtype ExceptionT m a

The express purpose of this transformer is to allow non-IO-based monad stacks to catch exceptions via the MonadThrow typeclass.

Since 0.3.0

Constructors

ExceptionT

Fields

runExceptionT :: m (Either SomeException a)

Instances

MonadTrans ExceptionT

MonadTransControl ExceptionT

MonadRWS r w s m => MonadRWS r w s (ExceptionT m)

MonadBase b m => MonadBase b (ExceptionT m)

MonadBaseControl b m => MonadBaseControl b (ExceptionT m)

MonadError e m => MonadError e (ExceptionT m)

MonadReader r m => MonadReader r (ExceptionT m)

MonadWriter w m => MonadWriter w (ExceptionT m)

MonadState s m => MonadState s (ExceptionT m)

Monad m => Monad (ExceptionT m)

Monad m => Functor (ExceptionT m)

Monad m => Applicative (ExceptionT m)

MonadResource m => MonadResource (ExceptionT m)

Monad m => MonadThrow (ExceptionT m)

MonadIO m => MonadIO (ExceptionT m)

MonadCont m => MonadCont (ExceptionT m)

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

Same as runExceptionT, but immediately throw any exception returned.

Since 0.3.0

runException :: ExceptionT Identity a -> Either SomeException a

Run an ExceptionT Identity stack.

Since 0.4.2

runException_ :: ExceptionT Identity a -> a

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

Since 0.4.2

class MonadBase b m => MonadBaseControl b m | m -> b

Instances

MonadBaseControl [] []

MonadBaseControl IO IO

MonadBaseControl STM STM

MonadBaseControl Maybe Maybe

MonadBaseControl Identity Identity

MonadBaseControl b m => MonadBaseControl b (MaybeT m)

MonadBaseControl b m => MonadBaseControl b (ListT m)

MonadBaseControl b m => MonadBaseControl b (IdentityT m)

MonadBaseControl b m => MonadBaseControl b (ResourceT m)

MonadBaseControl b m => MonadBaseControl b (ExceptionT m)

(Monoid w, MonadBaseControl b m) => MonadBaseControl b (WriterT w m)

(Monoid w, MonadBaseControl b m) => MonadBaseControl b (WriterT w m)

MonadBaseControl b m => MonadBaseControl b (StateT s m)

MonadBaseControl b m => MonadBaseControl b (StateT s m)

MonadBaseControl b m => MonadBaseControl b (ReaderT r m)

(Error e, MonadBaseControl b m) => MonadBaseControl b (ErrorT e m)

(Monoid w, MonadBaseControl b m) => MonadBaseControl b (RWST r w s m)

(Monoid w, MonadBaseControl b m) => MonadBaseControl b (RWST r w s m)

MonadBaseControl ((->) r) ((->) r)

MonadBaseControl (Either e) (Either e)

MonadBaseControl (ST s) (ST s)

MonadBaseControl (ST s) (ST s)