The main module, exporting types, utility functions, and fuse and connect operators.
 data SourceResult a
 data PreparedSource m a = PreparedSource {
 sourcePull :: ResourceT m (SourceResult a)
 sourceClose :: ResourceT m ()
 newtype Source m a = Source {
 prepareSource :: ResourceT m (PreparedSource m a)
 data BufferedSource m a = BufferedSource {
 bsourcePull :: ResourceT m (SourceResult a)
 bsourceUnpull :: a > ResourceT m ()
 bsourceClose :: ResourceT m ()
 data SourceInvariantException = PullAfterEOF String
 class BufferSource s where
 bufferSource :: Resource m => s m a > ResourceT m (BufferedSource m a)
 unbufferSource :: Monad m => BufferedSource m a > Source m a
 data SinkResult input output
 = Processing
  Done (Maybe input) output
 data PreparedSink input m output
 = SinkNoData output
  SinkData {
 sinkPush :: input > ResourceT m (SinkResult input output)
 sinkClose :: ResourceT m output
 newtype Sink input m output = Sink {
 prepareSink :: ResourceT m (PreparedSink input m output)
 data ConduitResult input output
 data PreparedConduit input m output = PreparedConduit {
 conduitPush :: input > ResourceT m (ConduitResult input output)
 conduitClose :: ResourceT m [output]
 newtype Conduit input m output = Conduit {
 prepareConduit :: ResourceT m (PreparedConduit input m output)
 ($$) :: (BufferSource bsrc, Resource m) => bsrc m a > Sink a m b > ResourceT m b
 ($=) :: (Resource m, BufferSource bsrc) => bsrc m a > Conduit a m b > Source m b
 (=$) :: Resource m => Conduit a m b > Sink b m c > Sink a m c
 (=$=) :: Resource m => Conduit a m b > Conduit b m c > Conduit a m c
 sourceState :: Resource m => state > (state > ResourceT m (state, SourceResult output)) > Source m output
 sourceIO :: ResourceIO m => IO state > (state > IO ()) > (state > m (SourceResult output)) > Source m output
 transSource :: (Base m ~ Base n, Monad m) => (forall a. m a > n a) > Source m output > Source n output
 sinkState :: Resource m => state > (state > input > ResourceT m (state, SinkResult input output)) > (state > ResourceT m output) > Sink input m output
 sinkIO :: ResourceIO m => IO state > (state > IO ()) > (state > input > m (SinkResult input output)) > (state > m output) > Sink input m output
 transSink :: (Base m ~ Base n, Monad m) => (forall a. m a > n a) > Sink input m output > Sink input n output
 conduitState :: Resource m => state > (state > input > ResourceT m (state, ConduitResult input output)) > (state > ResourceT m [output]) > Conduit input m output
 conduitIO :: ResourceIO m => IO state > (state > IO ()) > (state > input > m (ConduitResult input output)) > (state > m [output]) > Conduit input m output
 transConduit :: (Monad m, Base m ~ Base n) => (forall a. m a > n a) > Conduit input m output > Conduit input n output
 type SequencedSink state input m output = state > Sink input m (SequencedSinkResponse state input m output)
 sequenceSink :: Resource m => state > SequencedSink state input m output > Conduit input m output
 data SequencedSinkResponse state input m output
 = Emit state [output]
  Stop
  StartConduit (Conduit input m output)
 data ResourceT m a
 class (HasRef (Base m), Monad m) => Resource m where
 type Base m :: * > *
 resourceLiftBase :: Base m a > m a
 resourceBracket_ :: Base m () > Base m () > m c > m c
 class (ResourceBaseIO (Base m), ResourceUnsafeIO m, ResourceThrow m, MonadIO m, MonadBaseControl IO m) => ResourceIO m
 class Resource m => ResourceUnsafeIO m
 runResourceT :: Resource m => ResourceT m a > m a
 class Resource m => ResourceThrow m where
 resourceThrow :: Exception e => e > m a
Types
Source
data SourceResult a Source
Result of pulling from a source. Either a new piece of data (Open
), or
indicates that the source is now Closed
.
Functor SourceResult  
Eq a => Eq (SourceResult a)  
Ord a => Ord (SourceResult a)  
Show a => Show (SourceResult a) 
data PreparedSource m a Source
A PreparedSource
has two operations on it: pull some data, and close the
PreparedSource
. Since PreparedSource
is built on top of ResourceT
, all
acquired resources should be automatically released anyway. Closing a
PreparedSource
early
is merely an optimization to free scarce resources as soon as possible.
A PreparedSource
has three invariants:
 It is illegal to call
sourcePull
after a previous call returnsClosed
, or after a call tosourceClose
.  It is illegal to call
sourceClose
multiple times, or after a previoussourcePull
returns aClosed
.  A
PreparedSource
is responsible to free any resources when eithersourceClose
is called or aClosed
is returned. However, based on the usage ofResourceT
, this is simply an optimization.
PreparedSource  

BufferSource PreparedSource  
Monad m => Functor (PreparedSource m) 
All but the simplest of PreparedSource
s (e.g., repeat
) require some
type of state to track their current status. This may be in the form of a
mutable variable (e.g., IORef
), or via opening a resource like a Handle
.
While a PreparedSource
is given no opportunity to acquire such resources,
this type is.
A Source
is simply a monadic action that returns a PreparedSource
. One
nice consequence of this is the possibility of creating an efficient
Monoid
instance, which will only acquire one resource at a time, instead
of bulk acquiring all resources at the beginning of running the Source
.
Note that each time you "call" a Source
, it is started from scratch. If
you want a resumable source (e.g., one which can be passed to multiple
Sink
s), you likely want to use a BufferedSource
.
Source  

data BufferedSource m a Source
When actually interacting with Source
s, we usually want to be able to
buffer the output, in case any intermediate steps return leftover data. A
BufferedSource
allows for such buffering, via the bsourceUnpull
function.
A BufferedSource
, unlike a Source
, is resumable, meaning it can be passed to
multiple Sink
s without restarting.
Finally, a BufferedSource
relaxes one of the invariants of a Source
: calling
bsourcePull
after an EOF
will simply return another EOF
.
A BufferedSource
is also known as a resumable source, in that it can be
called multiple times, and each time will provide new data. One caveat:
while the types will allow you to use the buffered source in multiple
threads, there is no guarantee that all BufferedSource
s will handle this
correctly.
BufferedSource  

BufferSource BufferedSource  Note that this instance hides the bsrc < bufferSource $ sourceFile "myfile.txt" bsrc $$ drop 5 rest < bsrc $$ consume bsourceClose bsrc Note that the call to the 
class BufferSource s whereSource
This typeclass allows us to unify operators on Source
and BufferedSource
.
bufferSource :: Resource m => s m a > ResourceT m (BufferedSource m a)Source
BufferSource BufferedSource  Note that this instance hides the bsrc < bufferSource $ sourceFile "myfile.txt" bsrc $$ drop 5 rest < bsrc $$ consume bsourceClose bsrc Note that the call to the 
BufferSource Source  
BufferSource PreparedSource 
unbufferSource :: Monad m => BufferedSource m a > Source m aSource
Turn a BufferedSource
into a Source
. Note that in general this will
mean your original BufferedSource
will be closed. Additionally, all
leftover data from usage of the returned Source
will be discarded. In
other words: this is a nogoingback move.
Sink
data SinkResult input output Source
A Sink
ultimately returns a single output value. Each time data is
pushed to it, a Sink
may indicate that it is still processing data, or
that it is done, in which case it returns some optional leftover input and
an output value.
Processing  
Done (Maybe input) output 
Functor (SinkResult input) 
data PreparedSink input m output Source
In general, a sink will consume data and eventually produce an output when it has consumed "enough" data. There are two caveats to that statement:
 Some sinks do not actually require any data to produce an output. This is
included with a sink in order to allow for a
Monad
instance.  Some sinks will consume all available data and only produce a result at
the "end" of a data stream (e.g.,
sum
).
To allow for the first caveat, we have the SinkNoData
constructor. For the
second, the SinkData
constructor has two records: one for receiving more
input, and the other to indicate the end of a stream. Note that, at the end
of a stream, some output is required. If a specific Sink
implementation
cannot always produce output, this should be indicated in its return value,
using something like a Maybe
or Either
.
Invariants:
 After a
PreparedSink
produces a result (either viasinkPush
orsinkClose
), neither of those two functions may be called on theSink
again.  If a
Sink
needs to clean up any resources (e.g., close a file handle), it must do so whenever it returns a result, either viasinkPush
orsinkClose
. Note that, due to usage ofResourceT
, this is merely an optimization.
SinkNoData output  
SinkData  

Monad m => Functor (PreparedSink input m) 
newtype Sink input m output Source
Most PreparedSink
s require some type of state, similar to
PreparedSource
s. Like a Source
for a PreparedSource
, a Sink
is a
simple monadic wrapper around a PreparedSink
which allows initialization
of such state. See Source
for further caveats.
Note that this type provides a Monad
instance, allowing you to easily
compose Sink
s together.
Sink  

Conduit
data ConduitResult input output Source
When data is pushed to a Conduit
, it may either indicate that it is
still producing output and provide some, or indicate that it is finished
producing output, in which case it returns optional leftover input and some
final output.
Functor (ConduitResult input) 
data PreparedConduit input m output Source
A conduit has two operations: it can receive new input (a push), and can be closed.
Invariants:
 Neither a push nor close may be performed after a conduit returns a
Finished
from a push, or after a close is performed.
PreparedConduit  

Monad m => Functor (PreparedConduit input m) 
newtype Conduit input m output Source
A monadic action generating a PreparedConduit
. See Source
and Sink
for more motivation.
Conduit  

Connect/fuse operators
($$) :: (BufferSource bsrc, Resource m) => bsrc m a > Sink a m b > ResourceT m bSource
The connect operator, which pulls data from a source and pushes to a sink. There are three ways this process can terminate:
 In the case of a
SinkNoData
constructor, the source is not opened at all, and the output value is returned immediately.  The sink returns
Done
, in which case any leftover input is returned viabsourceUnpull
the source is closed.  The source return
Closed
, in which case the sink is closed.
Note that the input source is converted to a BufferedSource
via
bufferSource
. As such, if the input to this function is itself a
BufferedSource
, the call to bsourceClose
will have no effect, as
described in the comments on that instance.
($=) :: (Resource m, BufferSource bsrc) => bsrc m a > Conduit a m b > Source m bSource
Left fuse, combining a source and a conduit together into a new source.
(=$) :: Resource m => Conduit a m b > Sink b m c > Sink a m cSource
Right fuse, combining a conduit and a sink together into a new sink.
(=$=) :: Resource m => Conduit a m b > Conduit b m c > Conduit a m cSource
Middle fuse, combining two conduits together into a new conduit.
Utility functions
Source
:: Resource m  
=> state  Initial state 
> (state > ResourceT m (state, SourceResult output))  Pull function 
> Source m output 
Construct a Source
with some stateful functions. This function address
all mutable state for you.
:: ResourceIO m  
=> IO state  resource and/or state allocation 
> (state > IO ())  resource and/or state cleanup 
> (state > m (SourceResult output))  Pull function. Note that this need not explicitly perform any cleanup. 
> Source m output 
Construct a Source
based on some IO actions for alloc/release.
transSource :: (Base m ~ Base n, Monad m) => (forall a. m a > n a) > Source m output > Source n outputSource
Transform the monad a Source
lives in.
Sink
:: Resource m  
=> state  initial state 
> (state > input > ResourceT m (state, SinkResult input output))  push 
> (state > ResourceT m output)  Close. Note that the state is not returned, as it is not needed. 
> Sink input m output 
Construct a Sink
with some stateful functions. This function address
all mutable state for you.
:: ResourceIO m  
=> IO state  resource and/or state allocation 
> (state > IO ())  resource and/or state cleanup 
> (state > input > m (SinkResult input output))  push 
> (state > m output)  close 
> Sink input m output 
Construct a Sink
. Note that your push and close functions need not
explicitly perform any cleanup.
transSink :: (Base m ~ Base n, Monad m) => (forall a. m a > n a) > Sink input m output > Sink input n outputSource
Transform the monad a Sink
lives in.
Conduit
:: Resource m  
=> state  initial state 
> (state > input > ResourceT m (state, ConduitResult input output))  Push function. 
> (state > ResourceT m [output])  Close function. The state need not be returned, since it will not be used again. 
> Conduit input m output 
Construct a Conduit
with some stateful functions. This function address
all mutable state for you.
:: ResourceIO m  
=> IO state  resource and/or state allocation 
> (state > IO ())  resource and/or state cleanup 
> (state > input > m (ConduitResult input output))  Push function. Note that this need not explicitly perform any cleanup. 
> (state > m [output])  Close function. Note that this need not explicitly perform any cleanup. 
> Conduit input m output 
Construct a Conduit
.
transConduit :: (Monad m, Base m ~ Base n) => (forall a. m a > n a) > Conduit input m output > Conduit input n outputSource
Transform the monad a Conduit
lives in.
Sequencing
type SequencedSink state input m output = state > Sink input m (SequencedSinkResponse state input m output)Source
Helper type for constructing a Conduit
based on Sink
s. This allows you
to write higherlevel code that takes advantage of existing conduits and
sinks, and leverages a sink's monadic interface.
:: Resource m  
=> state  initial state 
> SequencedSink state input m output  
> Conduit input m output 
Convert a SequencedSink
into a Conduit
.
data SequencedSinkResponse state input m output Source
Return value from a SequencedSink
.
Emit state [output]  Set a new state, and emit some new output. 
Stop  End the conduit. 
StartConduit (Conduit input m output)  Pass control to a new conduit. 
Convenience reexports
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.
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.
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.
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 (ResourceBaseIO (Base m), ResourceUnsafeIO m, ResourceThrow m, MonadIO m, MonadBaseControl IO m) => ResourceIO m Source
ResourceIO IO  
(MonadTransControl t, ResourceIO m, Monad (t m), ResourceThrow (t m), MonadBaseControl IO (t m), MonadIO (t m)) => ResourceIO (t m) 
class Resource m => ResourceUnsafeIO m Source
A Resource
based on some monad which allows running of some IO
actions, via unsafe calls. This applies to IO
and ST
, for instance.
ResourceUnsafeIO IO  
(MonadTransControl t, ResourceUnsafeIO m, Monad (t m)) => ResourceUnsafeIO (t m)  
ResourceUnsafeIO (ST s)  
ResourceUnsafeIO (ST s) 
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.
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
ResourceThrow IO  
ResourceThrow m => ResourceThrow (MaybeT m)  
ResourceThrow m => ResourceThrow (ListT m)  
ResourceThrow m => ResourceThrow (IdentityT m)  
(Resource m, MonadBaseControl (Base m) m) => ResourceThrow (ExceptionT m)  
(Monoid w, ResourceThrow m) => ResourceThrow (WriterT w m)  
(Monoid w, ResourceThrow m) => ResourceThrow (WriterT w m)  
ResourceThrow m => ResourceThrow (StateT s m)  
ResourceThrow m => ResourceThrow (StateT s m)  
ResourceThrow m => ResourceThrow (ReaderT r m)  
(Error e, ResourceThrow m) => ResourceThrow (ErrorT e m)  
(Monoid w, ResourceThrow m) => ResourceThrow (RWST r w s m)  
(Monoid w, ResourceThrow m) => ResourceThrow (RWST r w s m) 