Safe Haskell  None 

 type Source m o = Pipe () () o () m ()
 type Conduit i m o = Pipe i i o () m ()
 type Sink i m r = Pipe i i Void () m r
 ($$) :: Monad m => Source m a > Sink a m b > m b
 ($=) :: Monad m => Source m a > Conduit a m b > Source m b
 (=$) :: Monad m => Conduit a m b > Sink b m c > Sink a m c
 (=$=) :: Monad m => Conduit a m b > Conduit b m c > Conduit a m c
 data Pipe l i o u m r
 (>+>) :: Monad m => Pipe l a b r0 m r1 > Pipe Void b c r1 m r2 > Pipe l a c r0 m r2
 (<+<) :: Monad m => Pipe Void b c r1 m r2 > Pipe l a b r0 m r1 > Pipe l a c r0 m r2
 runPipe :: Monad m => Pipe Void () Void () m r > m r
 injectLeftovers :: Monad m => Pipe i i o u m r > Pipe l i o u m r
 await :: Pipe l i o u m (Maybe i)
 awaitE :: Pipe l i o u m (Either u i)
 awaitForever :: Monad m => (i > Pipe l i o r m r') > Pipe l i o r m r
 yield :: Monad m => o > Pipe l i o u m ()
 yieldOr :: Monad m => o > m () > Pipe l i o u m ()
 leftover :: l > Pipe l i o u m ()
 bracketP :: MonadResource m => IO a > (a > IO ()) > (a > Pipe l i o u m r) > Pipe l i o u m r
 addCleanup :: Monad m => (Bool > m ()) > Pipe l i o u m r > Pipe l i o u m r
 data ResumableSource m o
 ($$+) :: Monad m => Source m a > Sink a m b > m (ResumableSource m a, b)
 ($$++) :: Monad m => ResumableSource m a > Sink a m b > m (ResumableSource m a, b)
 ($$+) :: Monad m => ResumableSource m a > Sink a m b > m b
 unwrapResumable :: MonadIO m => ResumableSource m o > m (Source m o, m ())
 transPipe :: Monad m => (forall a. m a > n a) > Pipe l i o u m r > Pipe l i o u n r
 mapOutput :: Monad m => (o1 > o2) > Pipe l i o1 u m r > Pipe l i o2 u m r
 mapOutputMaybe :: Monad m => (o1 > Maybe o2) > Pipe l i o1 u m r > Pipe l i o2 u m r
 mapInput :: Monad m => (i1 > i2) > (l2 > Maybe l1) > Pipe l2 i2 o u m r > Pipe l1 i1 o u m r
 withUpstream :: Monad m => Pipe l i o u m r > Pipe l i o u m (u, r)
 type GSource m o = forall l i u. Pipe l i o u m ()
 type GSink i m r = forall l o u. Pipe l i o u m r
 type GLSink i m r = forall o u. Pipe i i o u m r
 type GInfSink i m = forall l o r. Pipe l i o r m r
 type GLInfSink i m = forall o r. Pipe i i o r m r
 type GConduit i m o = forall l u. Pipe l i o u m ()
 type GLConduit i m o = forall u. Pipe i i o u m ()
 type GInfConduit i m o = forall l r. Pipe l i o r m r
 type GLInfConduit i m o = forall r. Pipe i i o r m r
 data Flush a
 data ResourceT m a
 class (MonadThrow m, MonadUnsafeIO m, MonadIO m, Applicative m) => MonadResource m
 class Monad m => MonadThrow m where
 monadThrow :: Exception e => e > m a
 class Monad m => MonadUnsafeIO m where
 unsafeLiftIO :: IO a > m a
 runResourceT :: MonadBaseControl IO m => ResourceT m a > m a
 newtype ExceptionT m a = ExceptionT {
 runExceptionT :: m (Either SomeException a)
 runExceptionT_ :: Monad m => ExceptionT m a > m a
 runException :: ExceptionT Identity a > Either SomeException a
 runException_ :: ExceptionT Identity a > a
 class MonadBase b m => MonadBaseControl b m  m > b
Conduit interface
Let's start off with a few simple examples of conduit
usage. First, a file
copy utility:
>>>
:load Data.Conduit.Binary
>>>
runResourceT $ sourceFile "input.txt" $$ sinkFile "output.txt"
runResourceT
is a function provided by the resourcet
package, and ensures
that resources are properly cleaned up, even in the presence of exceptions. The
type system will enforce that runResourceT
is called as needed. The remainder
of this tutorial will not discuss runResourceT
; please see the documentation
in resourcet
for more information.
Looking at the rest of our example, there are three components to understand:
sourceFile
, sinkFile
, and the $$
operator (called "connect"). These
represent the most basic building blocks in conduit
: a Source
produces a
stream of values, a Sink
consumes such a stream, and $$
will combine these
together.
In the case of file copying, there was no value produced by the Sink
.
However, often times a Sink
will produce some result value. For example:
>>>
:load Data.Conduit.List
>>>
:module +Prelude
>>>
sourceList [1..10] $$ fold (+) 0
55
sourceList
is a convenience function for turning a list into a Source
.
fold
implements a strict left fold for consuming the input stream.
The next major aspect to the conduit
library: the Conduit
type.
This type represents a stream transformer. In order to use a Conduit
, we
must fuse it with either a Source
or Sink
. For example:
>>>
:load Data.Conduit.List
>>>
:module +Prelude
>>>
sourceList [1..10] $= Data.Conduit.List.map (+1) $$ consume
[2,3,4,5,6,7,8,9,10,11]
Notice the addition of the $=
, or left fuse operator. This combines a
Source
and a Conduit
into a new Source
, which can then be connected to a
Sink
(in this case, consume
). We can similarly perform right fusion to
combine a Conduit
and Sink
, or middle fusion to combine two Conduit
s.
A number of very common functions are provided in the Data.Conduit.List module. Many of these functions correspond very closely to standard Haskell functions.
In addition to connecting and fusing components together, we can also build up
more sophisticated components through monadic composition. For example, to
create a Sink
that ignores the first 3 numbers and returns the sum of the
remaining numbers, we can use:
>>>
:load Data.Conduit.List
>>>
:module +Prelude
>>>
sourceList [1..10] $$ Data.Conduit.List.drop 3 >> fold (+) 0
49
In some cases, we might end up consuming more input than we needed, and want to
provide that input to the next component in our monadic chain. We refer to this
as leftovers. The simplest example of this is peek
.
>>>
:load Data.Conduit.List
>>>
:set XNoMonomorphismRestriction
>>>
:module +Prelude
>>>
let sink = do { first < peek; total < fold (+) 0; return (first, total) }
>>>
sourceList [1..10] $$ sink
(Just 1,55)
Notice that, although we "consumed" the first value from the stream via
peek
, it was still available to fold
. This idea becomes even more important
when dealing with chunked data such as ByteString
s or Text
.
Final note: Notice in the types below that Source
, Sink
, and Conduit
are just type aliases. This will be explained later. Another important aspect
is resource finalization, which will also be covered below.
type Source m o = Pipe () () o () m ()Source
Provides a stream of output values, without consuming any input or producing a final result.
Since 0.5.0
type Conduit i m o = Pipe i i o () m ()Source
Consumes a stream of input values and produces a stream of output values, without producing a final result.
Since 0.5.0
type Sink i m r = Pipe i i Void () m rSource
Consumes a stream of input values and produces a final result, without producing any output.
Since 0.5.0
Connect/fuse
It is important to understand the lifecycle of our components. Notice that we can connect or fuse two components together. When we do this, the component providing output is called upstream, and the component consuming this input is called downstream. We can have arbitrarily long chains of such fusion, so a single component can simultaneously function as upstream and downstream.
Each component can be in one of four states of operation at any given time:
 It hasn't yet started operating.
 It is providing output downstream.
 It is waiting for input from upstream.
 It has completed processing.
Let's use sourceFile
and sinkFile
as an example. When we run sourceFile
input $$ sinkFile output
, both components begin in the "not started"
state. Next, we start running sinkFile
(note: we always begin processing on
the downstream component). sinkFile
will open up the file, and then wait for
input from upstream.
Next, we'll start running sourceFile
, which will open the file, read some
data from it, and provide it as output downstream. This will be fed to
sinkFile
(which was already waiting). sinkFile
will write the data to a
file, then ask for more input. This process will continue until sourceFile
reaches the end of the input. It will close the file handle and switch to the
completed state. When this happens, sinkFile
is sent a signal that no more
input is available. It will then close its file and return a result.
Now let's change things up a bit. Suppose we were instead connecting
sourceFile
to take 1
. We start by running take 1
, which will wait for
some input. We'll then start sourceFile
, which will open the file, read a
chunk, and send it downstream. take 1
will take that single chunk and return
it as a result. Once it does this, it has transitioned to the complete state.
We don't want to pull any more data from sourceFile
, as we do not need it.
So instead, we call sourceFile
's finalizer. Each time upstream provides
output, it also provides a finalizer to be run if downstream finishes
processing.
One final case: suppose we connect sourceFile
to return ()
. The latter does
nothing: it immediately switches to the complete state. In this case, we never
even start running sourceFile
(it stays in the "not yet started" state),
and so no finalization occurs.
So here are the takeaways from the above discussion:
 When upstream completes before downstream, it cleans up all of its resources and sends some termination signal. We never think about upstream again. This can only occur while downstream is in the "waiting for input" state, since that is the only time that upstream is called.
 When downstream completes before upstream, we finalize upstream immediately. This can only occur when upstream produces output, because that's the only time when control is passed back to downstream.
 If downstream never awaits for input before it terminates, upstream was never started, and therefore it does not need to be finalized.
Note that all of the discussion above applies equally well to chains of components. If you have an upstream, middle, and downstream component, and downstream terminates, then the middle component will be finalized, which in turn will trigger upstream to be finalized. This setup ensures that we always have prompt resource finalization.
($$) :: Monad m => Source m a > Sink a m b > m bSource
The connect operator, which pulls data from a source and pushes to a sink.
When either side closes, the other side will immediately be closed as well.
If you would like to keep the Source
open to be used for another
operations, use the connectandresume operator $$+
.
Since 0.4.0
($=) :: Monad m => Source m a > Conduit a m b > Source m bSource
Left fuse, combining a source and a conduit together into a new source.
Both the Source
and Conduit
will be closed when the newlycreated
Source
is closed.
Leftover data from the Conduit
will be discarded.
Since 0.4.0
(=$) :: Monad m => Conduit a m b > Sink b m c > Sink a m cSource
Right fuse, combining a conduit and a sink together into a new sink.
Both the Conduit
and Sink
will be closed when the newlycreated Sink
is closed.
Leftover data returned from the Sink
will be discarded.
Since 0.4.0
(=$=) :: Monad m => Conduit a m b > Conduit b m c > Conduit a m cSource
Fusion operator, combining two Conduit
s together into a new Conduit
.
Both Conduit
s will be closed when the newlycreated Conduit
is closed.
Leftover data returned from the right Conduit
will be discarded.
Since 0.4.0
Pipe interface
We discussed three main types in the conduit
package: Source
, Sink
, and
Conduit
. In fact, these are all unified into a single type, Pipe
. This
greatly simplifies the internal workings of this package, and makes it much
easier to build more powerful components from simpler ones. For example, it is
easy to combine a number of simple Sink
s together to produce a more powerful
Conduit
. To create a Conduit
which drops 3 input elements and doubles the
rest, we could use:
>>>
:load Data.Conduit.List
>>>
:set XNoMonomorphismRestriction
>>>
let conduit = do { drop 3; map (Prelude.* 2) }
>>>
sourceList [1..10] $$ conduit =$ consume
[8,10,12,14,16,18,20]
If we look again at our examples from above, we'll see a few different aspects
to Pipe
s:

Sink
s andConduit
s can consume a stream of input values. Bothmap
andfold
took a stream ofInt
s, whilesinkFile
took a stream ofByteString
s. 
Source
s andConduit
s can produce a stream of output values.sourceFile
produced a stream ofByteString
s, which was then consumed bysinkFile
. This is an important point inconduit
: the output of the lefthand pipe (a.k.a., upstream) must match the input of the righthand pipe (a.k.a., downstream).  All
Pipe
s have some underlyingMonad
. ThesourceFile
andsinkFile
functions needed to useMonadResource
fromresourcet
to get exception handling, but our other functions could live in any monad. SincePipe
provides aMonadTrans
instance, you can actually lift any action from the underlyingMonad
into yourPipe
. 
Sink
s can provide a result type. Ourfold
returned a finalInt
, whilesinkFile
returned()
.
A Pipe
also exposes two other features as well, not covered by the above
three types:
 Each
Pipe
has some leftover value. Above, we described a situation where the leftover would be identical to the input type. However,Pipe
provides a type parameter for this instead, so that you can alternatively set the leftover type toVoid
, thereby ensuring that aPipe
does not provide any leftover values. This is important for ensuring that leftover values aren't accidentally discarded.  Above, we described a situation where only
Sink
s could return results. However, sometimes it's advantageous to allow stream producers to also produce a result type. We call this the upstream result.
Putting this all together, a Pipe
has six type parameters: Pipe l i o u m r
,
corresponding to each of the bullets above. Source
, Conduit
, and Sink
are
simply type aliases that restrict one or more of these type parameters to
specific types. For example, both Source
and Conduit
have r
restricted to
()
, since neither may return a result.
There are two ways that Pipe
s can be composed: via the Monad
instance, and
via fusion. (Note: connecting is just a special case of fusion, where the
Pipe
is then run. We'll discuss that more later on.) In the pipes
package,
these are referred to as vertical and horizontal composition, respectively.
Let's clarify the distinction between these two:
Monadic composition takes two Pipe
s with the same input and output types, and
combines them into a single Pipe
. These two Pipe
s will be run one after the
other, and they will share the same input and output streams. Essentially, the
second Pipe
will continue consuming input where the first left off, and the
output streams of each will be concatenated. Any leftover values from the first
Pipe
will be fed to the second Pipe
. Let's see a simple example:
>>>
:load Data.Conduit.List
>>>
sourceList [1..10] $$ do { x < take 3; y < take 2; return (x, y) }
([1,2,3],[4,5])
Fusion, on the other hand, will connect the output from an upstream Pipe
to
the input of a downstream Pipe
. The upstream Pipe
is required to have a
result type of ()
, since any results it produces are thrown out. This form of
composition produces a new Pipe
with the input parameter of the upstream
Pipe
and the output and result parameters of the downstream Pipe
. (For
examples, see the initial examples on this page. Every usage of the connect or
fusion operators is fusion composition.)
Note: If you are building a library of conduit
functions, it is best to
keep the type signatures as general as possible. For example, even though the
simplest type signature for the drop
function would be Int > Sink i m ()
,
this would prevent it from being used in construction of Conduit
s. Instead,
we give it a type signature of Int > Pipe l i o u m ()
.
The underlying datatype for all the types in this package. In has six type parameters:
 l is the type of values that may be left over from this
Pipe
. APipe
with no leftovers would useVoid
here, and one with leftovers would use the same type as the i parameter. Leftovers are automatically provided to the nextPipe
in the monadic chain.  i is the type of values for this
Pipe
's input stream.  o is the type of values for this
Pipe
's output stream.  u is the result type from the upstream
Pipe
.  m is the underlying monad.
 r is the result type.
A basic intuition is that every Pipe
produces a stream of output values
(o), and eventually indicates that this stream is terminated by sending a
result (r). On the receiving end of a Pipe
, these become the i and u
parameters.
Since 0.5.0
(Applicative base, Applicative (Pipe l i o u m), Monad base, Monad (Pipe l i o u m), MonadBase base m) => MonadBase base (Pipe l i o u m)  
MonadTrans (Pipe l i o u)  
Monad m => Monad (Pipe l i o u m)  
Monad m => Functor (Pipe l i o u m)  
(Functor (Pipe l i o u m), Monad m) => Applicative (Pipe l i o u m)  
(Monad (Pipe l i o u m), MonadIO m) => MonadIO (Pipe l i o u m)  
(Monad (Pipe l i o u m), MonadThrow m) => MonadThrow (Pipe l i o u m)  
(Monad (Pipe l i o u m), MonadUnsafeIO m) => MonadUnsafeIO (Pipe l i o u m)  
(Monad (Pipe l i o u m), MonadActive m) => MonadActive (Pipe l i o u m)  
Monad m => Monoid (Pipe l i o u m ()) 
(>+>) :: Monad m => Pipe l a b r0 m r1 > Pipe Void b c r1 m r2 > Pipe l a c r0 m r2Source
Fuse together two Pipe
s, connecting the output from the left to the
input of the right.
Notice that the leftover parameter for the Pipe
s must be Void
. This
ensures that there is no accidental data loss of leftovers during fusion. If
you have a Pipe
with leftovers, you must first call injectLeftovers
. For
example:
>>>
:load Data.Conduit.List
>>>
:set XNoMonomorphismRestriction
>>>
let pipe = peek >>= \x > fold (Prelude.+) 0 >>= \y > return (x, y)
>>>
runPipe $ sourceList [1..10] >+> injectLeftovers pipe
(Just 1,55)
Since 0.5.0
(<+<) :: Monad m => Pipe Void b c r1 m r2 > Pipe l a b r0 m r1 > Pipe l a c r0 m r2Source
Same as >+>
, but reverse the order of the arguments.
Since 0.5.0
runPipe :: Monad m => Pipe Void () Void () m r > m rSource
Run a pipeline until processing completes.
Since 0.5.0
injectLeftovers :: Monad m => Pipe i i o u m r > Pipe l i o u m rSource
Transforms a Pipe
that provides leftovers to one which does not,
allowing it to be composed.
This function will provide any leftover values within this Pipe
to any
calls to await
. If there are more leftover values than are demanded, the
remainder are discarded.
Since 0.5.0
Primitives
While conduit
provides a number of builtin Source
s, Sink
s, and
Conduit
s, you will almost certainly want to construct some of your own.
Previous versions recommended using the constructors directly. Beginning with
0.5, the recommended approach is to compose existing Pipe
s into larger ones.
It is certainly possible (and advisable!) to leverage existing Pipe
s like
those in Data.Conduit.List. However, you will often need to go to a lower
level set of Pipe
s to start your composition. The following few functions
should be sufficient for expressing all constructs besides finalization. Adding
in bracketP
and addCleanup
, you should be able to create any Pipe
you
need. (In fact, that's precisely how the remainder of this package is written.)
The three basic operations are awaiting, yielding, and leftovers.
Awaiting asks for a new value from upstream, or returns Nothing
if upstream
is done. For example:
>>>
:load Data.Conduit.List
>>>
sourceList [1..10] $$ await
Just 1
>>>
:load Data.Conduit.List
>>>
sourceList [] $$ await
Nothing
Similarly, we have a yield
function, which provides a value to the downstream
Pipe
. yield
features autotermination: if the downstream Pipe
has
already completed processing, the upstream Pipe
will stop processing when it
tries to yield
.
The upshot of this is that you can write code that appears to loop infinitely, and yet will terminate.
>>>
:set XNoMonomorphismRestriction
>>>
let infinite = yield () >> infinite
>>>
infinite $$ await
Just ()
Or for something a bit more sophisticated:
>>>
let enumFrom' i = yield i >> enumFrom' (succ i)
>>>
enumFrom' 1 $$ take 5
[1,2,3,4,5]
The final primitive Pipe
is leftover
. This allows you to return unused
input to be used by the next Pipe
in the monadic chain. A simple use case
would be implementing the peek
function:
>>>
let peek = await >>= maybe (return Nothing) (\x > leftover x >> return (Just x))
>>>
enumFrom' 1 $$ do { mx < peek; my < await; mz < await; return (mx, my, mz) }
(Just 1,Just 1,Just 2)
Note that you should only return leftovers that were previously yielded from upstream.
await :: Pipe l i o u m (Maybe i)Source
Wait for a single input value from upstream, terminating immediately if no data is available.
Since 0.5.0
awaitE :: Pipe l i o u m (Either u i)Source
This is similar to await
, but will return the upstream result value as
Left
if available.
Since 0.5.0
awaitForever :: Monad m => (i > Pipe l i o r m r') > Pipe l i o r m rSource
Wait for input forever, calling the given inner Pipe
for each piece of
new input. Returns the upstream result type.
Since 0.5.0
Send a single output value downstream. If the downstream Pipe
terminates, this Pipe
will terminate as well.
Since 0.5.0
Similar to yield
, but additionally takes a finalizer to be run if the
downstream Pipe
terminates.
Since 0.5.0
leftover :: l > Pipe l i o u m ()Source
Provide a single piece of leftover input to be consumed by the next pipe 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 > Pipe l i o u m r) > Pipe l i o u m rSource
Perform some allocation and run an inner Pipe
. Two guarantees are given
about resource finalization:
 It will be prompt. The finalization will be run as early as possible.
 It is exception safe. Due to usage of
resourcet
, the finalization will be run in the event of any exceptions.
Since 0.5.0
:: Monad m  
=> (Bool > m ()) 

> Pipe l i o u m r  
> Pipe l i o u m r 
Add some code to be run when the given Pipe
cleans up.
Since 0.4.1
Connectandresume
Sometimes, we do not want to force our entire application to live inside the
Pipe
monad. It can be convenient to keep normal control flow of our program,
and incrementally apply data from a Source
to various Sink
s. A strong
motivating example for this use case is interleaving multiple Source
s, such
as combining a conduit
powered HTTP server and client into an HTTP proxy.
Normally, when we run a Pipe
, we get a result and can never run it again.
Connectandresume allows us to connect a Source
to a Sink
until the latter
completes, and then return the current state of the Source
to be applied
later. To do so, we introduce three new operators. Let' start off by
demonstrating them:
>>>
:load Data.Conduit.List
>>>
(next, x) < sourceList [1..10] $$+ take 5
>>>
Prelude.print x
[1,2,3,4,5]>>>
(next, y) < next $$++ (isolate 4 =$ fold (Prelude.+) 0)
>>>
Prelude.print y
30>>>
next $$+ consume
[10]
data ResumableSource m o Source
A Source
which has been started, but has not yet completed.
This type contains both the current state of the Source
, and the finalizer
to be run to close it.
Since 0.5.0
($$+) :: Monad m => Source m a > Sink a m b > m (ResumableSource m a, b)Source
The connectandresume 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
($$++) :: Monad m => ResumableSource m a > Sink a m b > m (ResumableSource m a, b)Source
Continue processing after usage of $$+
.
Since 0.5.0
($$+) :: Monad m => ResumableSource m a > Sink a m b > m bSource
Complete processing of a ResumableSource
. This will run the finalizer
associated with the ResumableSource
. In order to guarantee process resource
finalization, you must use this operator after using $$+
and $$++
.
Since 0.5.0
unwrapResumable :: MonadIO m => ResumableSource m o > m (Source m o, m ())Source
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
Utility functions
transPipe :: Monad m => (forall a. m a > n a) > Pipe l i o u m r > Pipe l i o u n rSource
Transform the monad that a Pipe
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/Dealingwithmonadtransformers
Since 0.4.0
mapOutputMaybe :: Monad m => (o1 > Maybe o2) > Pipe l i o1 u m r > Pipe l i o2 u m rSource
Same as mapOutput
, but use a function that returns Maybe
values.
Since 0.5.0
:: Monad m  
=> (i1 > i2)  map initial input to new input 
> (l2 > Maybe l1)  map new leftovers to initial leftovers 
> Pipe l2 i2 o u m r  
> Pipe l1 i1 o u m r 
Apply a function to all the input values of a Pipe
.
Since 0.5.0
withUpstream :: Monad m => Pipe l i o u m r > Pipe l i o u m (u, r)Source
Returns a tuple of the upstream and downstream results. Note that this will force consumption of the entire input stream.
Since 0.5.0
Generalized conduit types
It's recommended to keep your type signatures as general as possible to
encourage reuse. For example, a theoretical signature for the head
function
would be:
head :: Sink a m (Maybe a)
However, doing so would prevent usage of head
from inside a Conduit
, since
a Sink
sets its output type parameter to Void
. The most general type
signature would instead be:
head :: Pipe l a o u m (Maybe a)
However, that signature is much more confusing. To bridge this gap, we also provide some generalized conduit types. They follow a simple naming convention:
 They have the same name as their nongeneralized types, with a
G
prepended.  If they have leftovers, we add an
L
.  If they consume the entirety of their input stream and return the upstream
result, we add
Inf
to indicate infinite consumption.
type GSink i m r = forall l o u. Pipe l i o u m rSource
Generalized Sink
without leftovers.
Since 0.5.0
type GInfSink i m = forall l o r. Pipe l i o r m rSource
Generalized Sink
without leftovers returning upstream result.
Since 0.5.0
type GLInfSink i m = forall o r. Pipe i i o r m rSource
Generalized Sink
with leftovers returning upstream result.
Since 0.5.0
type GConduit i m o = forall l u. Pipe l i o u m ()Source
Generalized conduit without leftovers.
Since 0.5.0
type GLConduit i m o = forall u. Pipe i i o u m ()Source
Generalized conduit with leftovers.
Since 0.5.0
type GInfConduit i m o = forall l r. Pipe l i o r m rSource
Generalized conduit without leftovers returning upstream result.
Since 0.5.0
type GLInfConduit i m o = forall r. Pipe i i o r m rSource
Generalized conduit with leftovers returning upstream result.
Since 0.5.0
Flushing
Provide for a stream of data that can be flushed.
A number of Conduit
s (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
Convenience reexports
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
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
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 nonIO
base monad.
Since 0.3.0
monadThrow :: Exception e => e > m a
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
unsafeLiftIO :: IO a > m a
MonadUnsafeIO IO  
(MonadTrans t, MonadUnsafeIO m, Monad (t m)) => MonadUnsafeIO (t m)  
Monad (ST s) => MonadUnsafeIO (ST s)  
Monad (ST s) => MonadUnsafeIO (ST s)  
(Monad (Pipe l i o u m), MonadUnsafeIO m) => MonadUnsafeIO (Pipe l i o u m) 
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 nonIO
based monad
stacks to catch exceptions via the MonadThrow
typeclass.
Since 0.3.0
ExceptionT  

MonadTrans ExceptionT  
MonadTransControl ExceptionT  
(Monoid w, MonadReader r (ExceptionT m), MonadWriter w (ExceptionT m), MonadState s (ExceptionT m), MonadRWS r w s m) => MonadRWS r w s (ExceptionT m)  
(Applicative b, Applicative (ExceptionT m), Monad b, Monad (ExceptionT m), MonadBase b m) => MonadBase b (ExceptionT m)  
(MonadBase b (ExceptionT m), MonadBaseControl b m) => MonadBaseControl b (ExceptionT m)  
(Monad (ExceptionT m), MonadError e m) => MonadError e (ExceptionT m)  
(Monad (ExceptionT m), MonadReader r m) => MonadReader r (ExceptionT m)  
(Monad (ExceptionT m), MonadState s m) => MonadState s (ExceptionT m)  
(Monoid w, Monad (ExceptionT m), MonadWriter w m) => MonadWriter w (ExceptionT m)  
Monad m => Monad (ExceptionT m)  
Monad m => Functor (ExceptionT m)  
(Functor (ExceptionT m), Monad m) => Applicative (ExceptionT m)  
(Monad (ExceptionT m), MonadCont m) => MonadCont (ExceptionT m)  
(Monad (ExceptionT m), Monad m) => MonadThrow (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