A mutable channel to evaluate multiple streams concurrently and provide the combined results as output stream.

Stop the channel. Kill all running worker threads.

Send a stream to a given channel for concurrent evaluation.

Write a stream to an SVar in a non-blocking manner. The stream can then be read back from the SVar using fromSVar.

Generate a stream of results from concurrent evaluations from a channel. Evaluation of the channel does not start until this API is called. This API must not be called more than once on a channel. It kicks off evaluation of the channel by dispatching concurrent workers and ensures that as long there is work queued on the channel workers are dispatched proportional to the demand by the consumer.

Create a new async style concurrent stream evaluation channel. The monad state used to run the stream actions is taken from the call site of newAppendChannel.

Create a new async style concurrent stream evaluation channel. The monad state used to run the stream actions is taken from the call site of newInterleaveChannel.

Start the evaluation of the channel's work queue by kicking off a worker. Note: Work queue must not be empty otherwise the worker will exit without doing anything.

A mutable channel to evaluate multiple streams concurrently and provide the combined results as output stream.

Create a new concurrent stream evaluation channel. The monad state used to run the stream actions is captured from the call site of newChannel.

Allocate a channel and evaluate the stream using the channel and the supplied evaluator function. The evaluator is run in a worker thread.

Allocate a channel and evaluate the stream using the channel and the supplied evaluator function. The evaluator is run in a worker thread.

An abstract type for specifying the configuration parameters of a Channel. Use Config -> Config modifier functions to modify the default configuration. See the individual modifier documentation for default values.

The fields prefixed by an _ are not to be accessed or updated directly but via smart accessor APIs. Use get/set routines instead of directly accessing the Config fields

Specify the maximum number of threads that can be spawned by the channel. A value of 0 resets the thread limit to default, a negative value means there is no limit. The default value is 1500.

When the actions in a stream are IO bound, having blocking IO calls, this option can be used to control the maximum number of in-flight IO requests. When the actions are CPU bound this option can be used to control the amount of CPU used by the stream.

Specify the maximum size of the buffer for storing the results from concurrent computations. If the buffer becomes full we stop spawning more concurrent tasks until there is space in the buffer. A value of 0 resets the buffer size to default, a negative value means there is no limit. The default value is 1500.

CAUTION! using an unbounded maxBuffer value (i.e. a negative value) coupled with an unbounded maxThreads value is a recipe for disaster in presence of infinite streams, or very large streams. Especially, it must not be used when pure is used in ZipAsyncM streams as pure in applicative zip streams generates an infinite stream causing unbounded concurrent generation with no limit on the buffer or threads.

Specifies the stream yield rate in yields per second (Hertz). We keep accumulating yield credits at rateGoal. At any point of time we allow only as many yields as we have accumulated as per rateGoal since the start of time. If the consumer or the producer is slower or faster, the actual rate may fall behind or exceed rateGoal. We try to recover the gap between the two by increasing or decreasing the pull rate from the producer. However, if the gap becomes more than rateBuffer we try to recover only as much as rateBuffer.

rateLow puts a bound on how low the instantaneous rate can go when recovering the rate gap. In other words, it determines the maximum yield latency. Similarly, rateHigh puts a bound on how high the instantaneous rate can go when recovering the rate gap. In other words, it determines the minimum yield latency. We reduce the latency by increasing concurrency, therefore we can say that it puts an upper bound on concurrency.

If the rateGoal is 0 or negative the stream never yields a value. If the rateBuffer is 0 or negative we do not attempt to recover.

Specify the stream evaluation rate of a channel.

A Nothing value means there is no smart rate control, concurrent execution blocks only if maxThreads or maxBuffer is reached, or there are no more concurrent tasks to execute. This is the default.

When rate (throughput) is specified, concurrent production may be ramped up or down automatically to achieve the specified stream throughput. The specific behavior for different styles of Rate specifications is documented under Rate. The effective maximum production rate achieved by a channel is governed by:

• The maxThreads limit
• The maxBuffer limit
• The maximum rate that the stream producer can achieve
• The maximum rate that the stream consumer can achieve

Maximum production rate is given by:

\(rate = \frac{maxThreads}{latency}\)

If we know the average latency of the tasks we can set maxThreads accordingly.

Same as rate (Just $ Rate (r/2) r (2*r) maxBound)

Specifies the average production rate of a stream in number of yields per second (i.e. Hertz). Concurrent production is ramped up or down automatically to achieve the specified average yield rate. The rate can go down to half of the specified rate on the lower side and double of the specified rate on the higher side.

Same as rate (Just $ Rate r r (2*r) maxBound)

Specifies the minimum rate at which the stream should yield values. As far as possible the yield rate would never be allowed to go below the specified rate, even though it may possibly go above it at times, the upper limit is double of the specified rate.

Same as rate (Just $ Rate (r/2) r r maxBound)

Specifies the maximum rate at which the stream should yield values. As far as possible the yield rate would never be allowed to go above the specified rate, even though it may possibly go below it at times, the lower limit is half of the specified rate. This can be useful in applications where certain resource usage must not be allowed to go beyond certain limits.

Same as rate (Just $ Rate r r r 0)

Specifies a constant yield rate. If for some reason the actual rate goes above or below the specified rate we do not try to recover it by increasing or decreasing the rate in future. This can be useful in applications like graphics frame refresh where we need to maintain a constant refresh rate.

Specify when the Channel should stop.

Specify when the Channel should stop.

By default, processing of output from the worker threads is given priority over dispatching new workers. More workers are dispatched only when there is no output to process. When eager is set to True, workers are dispatched aggresively as long as there is more work to do irrespective of whether there is output pending to be processed by the stream consumer. However, dispatching may stop if maxThreads or maxBuffer is reached.

Note: This option has no effect when rate has been specified.

Note: Not supported with interleaved.

When enabled the streams may be evaluated cocnurrently but the results are produced in the same sequence as a serial evaluation would produce.

Note: Not supported with interleaved.

Interleave the streams fairly instead of prioritizing the left stream. This schedules all streams in a round robin fashion over limited number of threads.

Note: Can only be used on finite number of streams.

Note: Not supported with ordered.

Print debug information about the Channel when the stream ends.

A monad that can perform concurrent or parallel IO operations. Streams that can be composed concurrently require the underlying monad to be MonadAsync.

parEval evaluates a stream as a whole asynchronously with respect to the consumer of the stream. A worker thread evaluates multiple elements of the stream ahead of time and buffers the results; the consumer of the stream runs in another thread consuming the elements from the buffer, thus decoupling the production and consumption of the stream. parEval can be used to run different stages of a pipeline concurrently.

It is important to note that parEval does not evaluate individual actions in the stream concurrently with respect to each other, it merely evaluates the stream serially but in a different thread than the consumer thread, thus the consumer and producer can run concurrently. See parMapM and parSequence to evaluate actions in the stream concurrently.

The evaluation requires only one thread as only one stream needs to be evaluated. Therefore, the concurrency options that are relevant to multiple streams do not apply here e.g. maxThreads, eager, interleaved, ordered, stopWhen options do not have any effect on parEval.

Useful idioms:

>>> parUnfoldrM step = Stream.parEval id . Stream.unfoldrM step
>>> parIterateM step = Stream.parEval id . Stream.iterateM step

Definition:

>>> parRepeatM cfg = Stream.parSequence cfg . Stream.repeat

Generate a stream by repeatedly executing a monadic action forever.

Generate a stream by concurrently performing a monadic action n times.

Definition:

>>> parReplicateM cfg n = Stream.parSequence cfg . Stream.replicate n

Example, parReplicateM in the following example executes all the replicated actions concurrently, thus taking only 1 second:

>>> Stream.fold Fold.drain $ Stream.parReplicateM id 10 $ delay 1
...

Definition:

>>> parMapM modifier f = Stream.parConcatMap modifier (Stream.fromEffect . f)

For example, the following finishes in 3 seconds (as opposed to 6 seconds) because all actions run in parallel. Even though results are available out of order they are ordered due to the config option:

>>> f x = delay x >> return x
>>> Stream.fold Fold.toList $ Stream.parMapM (Stream.ordered True) f $ Stream.fromList [3,2,1]
1 sec
2 sec
3 sec
[3,2,1]

Definition:

>>> parSequence modifier = Stream.parMapM modifier id

Useful idioms:

>>> parFromListM = Stream.parSequence id . Stream.fromList
>>> parFromFoldableM = Stream.parSequence id . StreamK.toStream . StreamK.fromFoldable

Binary operation to evaluate two streams concurrently using a channel.

If you want to combine more than two streams you almost always want the parList or parConcat operation instead. The performance of this operation degrades rapidly when more streams are combined as each operation adds one more concurrent channel. On the other hand, parConcat uses a single channel for all streams. However, with this operation you can precisely control the scheduling by creating arbitrary shape expression trees.

Definition:

>>> parTwo cfg x y = Stream.parList cfg [x, y]

Example, the following code finishes in 4 seconds:

>>> async = Stream.parTwo id
>>> stream1 = Stream.fromEffect (delay 4)
>>> stream2 = Stream.fromEffect (delay 2)
>>> Stream.fold Fold.toList $ stream1 `async` stream2
2 sec
4 sec
[2,4]

Evaluates the streams being zipped in separate threads than the consumer. The zip function is evaluated in the consumer thread.

>>> parZipWithM cfg f m1 m2 = Stream.zipWithM f (Stream.parEval cfg m1) (Stream.parEval cfg m2)

Multi-stream concurrency options won't apply here, see the notes in parEval.

If you want to evaluate the zip function as well in a separate thread, you can use a parEval on parZipWithM.

>>> parZipWith cfg f = Stream.parZipWithM cfg (\a b -> return $ f a b)

>>> m1 = Stream.fromList [1,2,3]
>>> m2 = Stream.fromList [4,5,6]
>>> Stream.fold Fold.toList $ Stream.parZipWith id (,) m1 m2
[(1,4),(2,5),(3,6)]

Like mergeByM but evaluates both the streams concurrently.

Definition:

>>> parMergeByM cfg f m1 m2 = Stream.mergeByM f (Stream.parEval cfg m1) (Stream.parEval cfg m2)

Like mergeBy but evaluates both the streams concurrently.

Definition:

>>> parMergeBy cfg f = Stream.parMergeByM cfg (\a b -> return $ f a b)

Like concat but works on a list of streams.

>>> parListLazy = Stream.parList id

Like parListLazy but with ordered on.

>>> parListOrdered = Stream.parList (Stream.ordered True)

Like parListLazy but interleaves the streams fairly instead of prioritizing the left stream. This schedules all streams in a round robin fashion over limited number of threads.

>>> parListInterleaved = Stream.parList (Stream.interleaved True)

Like parListLazy but with eager on.

>>> parListEager = Stream.parList (Stream.eager True)

Like parListEager but stops the output as soon as the first stream stops.

>>> parListEagerFst = Stream.parList (Stream.eager True . Stream.stopWhen Stream.FirstStops)

Like parListEager but stops the output as soon as any of the two streams stops.

Definition:

>>> parListEagerMin = Stream.parList (Stream.eager True . Stream.stopWhen Stream.AnyStops)

Like parConcat but works on a list of streams.

>>> parList modifier = Stream.parConcat modifier . Stream.fromList

Apply an argument stream to a function stream concurrently. Uses a shared channel for all individual applications within a stream application.

Evaluate the streams in the input stream concurrently and combine them.

>>> parConcat modifier = Stream.parConcatMap modifier id

Map each element of the input to a stream and then concurrently evaluate and concatenate the resulting streams. Multiple streams may be evaluated concurrently but earlier streams are perferred. Output from the streams are used as they arrive.

Definition:

>>> parConcatMap modifier f stream = Stream.parConcat modifier $ fmap f stream

Examples:

>>> f cfg xs = Stream.fold Fold.toList $ Stream.parConcatMap cfg id $ Stream.fromList xs

The following streams finish in 4 seconds:

>>> stream1 = Stream.fromEffect (delay 4)
>>> stream2 = Stream.fromEffect (delay 2)
>>> stream3 = Stream.fromEffect (delay 1)
>>> f id [stream1, stream2, stream3]
1 sec
2 sec
4 sec
[1,2,4]

Limiting threads to 2 schedules the third stream only after one of the first two has finished, releasing a thread:

>>> f (Stream.maxThreads 2) [stream1, stream2, stream3]
...
[2,1,4]

When used with a Single thread it behaves like serial concatMap:

>>> f (Stream.maxThreads 1) [stream1, stream2, stream3]
...
[4,2,1]

>>> stream1 = Stream.fromList [1,2,3]
>>> stream2 = Stream.fromList [4,5,6]
>>> f (Stream.maxThreads 1) [stream1, stream2]
[1,2,3,4,5,6]

Schedule all streams in a round robin fashion over the available threads:

>>> f cfg xs = Stream.fold Fold.toList $ Stream.parConcatMap (Stream.interleaved True . cfg) id $ Stream.fromList xs

>>> stream1 = Stream.fromList [1,2,3]
>>> stream2 = Stream.fromList [4,5,6]
>>> f (Stream.maxThreads 1) [stream1, stream2]
[1,4,2,5,3,6]

Same as concatIterate but concurrent.

Pre-release

fromCallback f creates an entangled pair of a callback and a stream i.e. whenever the callback is called a value appears in the stream. The function f is invoked with the callback as argument, and the stream is returned. f would store the callback for calling it later for generating values in the stream.

The callback queues a value to a concurrent channel associated with the stream. The stream can be evaluated safely in any thread.

Pre-release

parTapCount predicate fold stream taps the count of those elements in the stream that pass the predicate. The resulting count stream is sent to a fold running concurrently in another thread.

For example, to print the count of elements processed every second:

>>> rate = Stream.rollingMap2 (flip (-)) . Stream.delayPost 1
>>> report = Stream.fold (Fold.drainMapM print) . rate
>>> tap = Stream.parTapCount (const True) report
>>> go = Stream.fold Fold.drain $ tap $ Stream.enumerateFrom 0

Note: This may not work correctly on 32-bit machines because of Int overflow.

Pre-release

Same as parTapCount. Deprecated.

Generate a stream by running an action periodically at the specified time interval.

Generate a tick stream consisting of () elements, each tick is generated after the specified time delay given in seconds.

>>> ticks = Stream.periodic (return ())

Generate a tick stream, ticks are generated at the specified Rate. The rate is adaptive, the tick generation speed can be increased or decreased at different times to achieve the specified rate. The specific behavior for different styles of Rate specifications is documented under Rate. The effective maximum rate achieved by a stream is governed by the processor speed.

>>> tickStream = Stream.repeatM (return ())
>>> ticksRate r = Stream.parEval (Stream.rate (Just r)) tickStream

Intersperse a monadic action into the input stream after every n seconds.

Definition:

>>> interject n f xs = Stream.parListEagerFst [xs, Stream.periodic f n]

Example:

>>> s = Stream.fromList "hello"
>>> input = Stream.mapM (\x -> threadDelay 1000000 >> putChar x) s
>>> Stream.fold Fold.drain $ Stream.interject (putChar ',') 1.05 input
h,e,l,l,o

takeInterval interval runs the stream only upto the specified time interval in seconds.

The interval starts when the stream is evaluated for the first time.

Take time interval i seconds at the end of the stream.

O(n) space, where n is the number elements taken.

Unimplemented

dropInterval interval drops all the stream elements that are generated before the specified interval in seconds has passed.

The interval begins when the stream is evaluated for the first time.

Drop time interval i seconds at the end of the stream.

O(n) space, where n is the number elements dropped.

Unimplemented

Group the input stream into windows of n second each and then fold each group using the provided fold function.

>>> twoPerSec = Stream.parEval (Stream.constRate 2) $ Stream.enumerateFrom 1
>>> intervals = Stream.intervalsOf 1 Fold.toList twoPerSec
>>> Stream.fold Fold.toList $ Stream.take 2 intervals
[...,...]

Like chunksOf but if the chunk is not completed within the specified time interval then emit whatever we have collected till now. The chunk timeout is reset whenever a chunk is emitted. The granularity of the clock is 100 ms.

>>> s = Stream.delayPost 0.3 $ Stream.fromList [1..1000]
>>> f = Stream.fold (Fold.drainMapM print) $ Stream.groupsOfTimeout 5 1 Fold.toList s

Pre-release

Continuously evaluate the input stream and sample the last event in each time window of n seconds.

This is also known as throttle in some libraries.

>>> sampleIntervalEnd n = Stream.catMaybes . Stream.intervalsOf n Fold.latest

Like sampleInterval but samples at the beginning of the time window.

>>> sampleIntervalStart n = Stream.catMaybes . Stream.intervalsOf n Fold.one

Sample one event at the end of each burst of events. A burst is a group of events close together in time, it ends when an event is spaced by more than the specified time interval (in seconds) from the previous event.

This is known as debounce in some libraries.

The clock granularity is 10 ms.

Like sampleBurstEnd but samples the event at the beginning of the burst instead of at the end of it.

classifySessionsBy tick keepalive predicate timeout fold stream classifies an input event stream consisting of (timestamp, (key, value)) into sessions based on the key, folding all the values corresponding to the same key into a session using the supplied fold.

When the fold terminates or a timeout occurs, a tuple consisting of the session key and the folded value is emitted in the output stream. The timeout is measured from the first event in the session. If the keepalive option is set to True the timeout is reset to 0 whenever an event is received.

The timestamp in the input stream is an absolute time from some epoch, characterizing the time when the input event was generated. The notion of current time is maintained by a monotonic event time clock using the timestamps seen in the input stream. The latest timestamp seen till now is used as the base for the current time. When no new events are seen, a timer is started with a clock resolution of tick seconds. This timer is used to detect session timeouts in the absence of new events.

To ensure an upper bound on the memory used the number of sessions can be limited to an upper bound. If the ejection predicate returns True, the oldest session is ejected before inserting a new session.

When the stream ends any buffered sessions are ejected immediately.

If a session key is received even after a session has finished, another session is created for that key.

>>> :{
Stream.fold (Fold.drainMapM print)
    $ Stream.classifySessionsBy 1 False (const (return False)) 3 (Fold.take 3 Fold.toList)
    $ Stream.timestamped
    $ Stream.delay 0.1
    $ Stream.fromList ((,) <$> [1,2,3] <*> ['a','b','c'])
:}
(1,"abc")
(2,"abc")
(3,"abc")

Pre-release

Same as classifySessionsBy with a timer tick of 1 second and keepalive option set to False.

>>> classifySessionsOf = Stream.classifySessionsBy 1 False

Pre-release

Same as classifySessionsBy with a timer tick of 1 second and keepalive option set to True.

classifyKeepAliveSessions = classifySessionsBy 1 True

Pre-release

Always produce the latest available element from the stream without any delay. The stream is continuously evaluated at the highest possible rate and only the latest element is retained for sampling.

Unimplemented

Evaluate the input stream continuously and keep only the latest n elements in a ring buffer, keep discarding the older ones to make space for the new ones. When the output stream is evaluated the buffer collected till now is streamed and it starts filling again.

Unimplemented

Evaluate the input stream continuously and keep only the oldest n elements in the buffer, discard the new ones when the buffer is full. When the output stream is evaluated the collected buffer is streamed and the buffer starts filling again.

Unimplemented

Run the action m b whenever the stream Stream m a stops normally, or if it is garbage collected after a partial lazy evaluation.

The semantics of the action m b are similar to the semantics of cleanup action in bracket.

See also after_

Run the alloc action IO b with async exceptions disabled but keeping blocking operations interruptible (see mask). Use the output b of the IO action as input to the function b -> Stream m a to generate an output stream.

b is usually a resource under the IO monad, e.g. a file handle, that requires a cleanup after use. The cleanup action b -> m c, runs whenever (1) the stream ends normally, (2) due to a sync or async exception or, (3) if it gets garbage collected after a partial lazy evaluation. The exception is not caught, it is rethrown.

bracket only guarantees that the cleanup action runs, and it runs with async exceptions enabled. The action must ensure that it can successfully cleanup the resource in the face of sync or async exceptions.

When the stream ends normally or on a sync exception, cleanup action runs immediately in the current thread context, whereas in other cases it runs in the GC context, therefore, cleanup may be delayed until the GC gets to run.

See also: bracket_

Inhibits stream fusion

Like bracket but can use 3 separate cleanup actions depending on the mode of termination:

1. When the stream stops normally
2. When the stream is garbage collected
3. When the stream encounters an exception

bracket3 before onStop onGC onException action runs action using the result of before. If the stream stops, onStop action is executed, if the stream is abandoned onGC is executed, if the stream encounters an exception onException is executed.

The exception is not caught, it is rethrown.

Pre-release

Run the action m b whenever the stream Stream m a stops normally, aborts due to an exception or if it is garbage collected after a partial lazy evaluation.

The semantics of running the action m b are similar to the cleanup action semantics described in bracket.

>>> finally action xs = Stream.bracket (return ()) (const action) (const xs)

See also finally_

Inhibits stream fusion

retry takes 3 arguments

1. A map m whose keys are exceptions and values are the number of times to retry the action given that the exception occurs.
2. A handler han that decides how to handle an exception when the exception cannot be retried.
3. The stream itself that we want to run this mechanism on.

When evaluating a stream if an exception occurs,

1. The stream evaluation aborts
2. The exception is looked up in m

a. If the exception exists and the mapped value is > 0 then,

i. The value is decreased by 1.

ii. The stream is resumed from where the exception was called, retrying the action.

b. If the exception exists and the mapped value is == 0 then the stream evaluation stops.

c. If the exception does not exist then we handle the exception using han.

Internal

See retry