The Semigroup operation for SerialT behaves like a regular append operation. Therefore, when a <> b is evaluated, stream a is evaluated first until it exhausts and then stream b is evaluated. In other words, the elements of stream b are appended to the elements of stream a. This operation can be used to fold an infinite lazy container of streams.

import Streamly
import qualified Streamly.Prelude as S

main = (S.toList . serially $ (S.fromList [1,2]) <> (S.fromList [3,4])) >>= print

[1,2,3,4]

The Monad instance runs the monadic continuation for each element of the stream, serially.

main = S.drain . serially $ do
    x <- return 1 <> return 2
    S.yieldM $ print x

1
2

SerialT nests streams serially in a depth first manner.

main = S.drain . serially $ do
    x <- return 1 <> return 2
    y <- return 3 <> return 4
    S.yieldM $ print (x, y)

(1,3)
(1,4)
(2,3)
(2,4)

We call the monadic code being run for each element of the stream a monadic continuation. In imperative paradigm we can think of this composition as nested for loops and the monadic continuation is the body of the loop. The loop iterates for all elements of the stream.

Note that the behavior and semantics of SerialT, including Semigroup and Monad instances are exactly like Haskell lists except that SerialT can contain effectful actions while lists are pure.

In the code above, the serially combinator can be omitted as the default stream type is SerialT.

Since: 0.2.0

A serial IO stream of elements of type a. See SerialT documentation for more details.

Since: 0.2.0

Polymorphic version of the Semigroup operation <> of SerialT. Appends two streams sequentially, yielding all elements from the first stream, and then all elements from the second stream.

Since: 0.2.0

Fix the type of a polymorphic stream as SerialT.

Since: 0.1.0

The Semigroup operation for WSerialT interleaves the elements from the two streams. Therefore, when a <> b is evaluated, stream a is evaluated first to produce the first element of the combined stream and then stream b is evaluated to produce the next element of the combined stream, and then we go back to evaluating stream a and so on. In other words, the elements of stream a are interleaved with the elements of stream b.

Note that evaluation of a <> b <> c does not schedule a, b and c with equal priority. This expression is equivalent to a <> (b <> c), therefore, it fairly interleaves a with the result of b <> c. For example, S.fromList [1,2] <> S.fromList [3,4] <> S.fromList [5,6] :: WSerialT Identity Int would result in [1,3,2,5,4,6]. In other words, the leftmost stream gets the same scheduling priority as the rest of the streams taken together. The same is true for each subexpression on the right.

Note that this operation cannot be used to fold a container of infinite streams as the state that it needs to maintain is proportional to the number of streams.

The W in the name stands for wide or breadth wise scheduling in contrast to the depth wise scheduling behavior of SerialT.

import Streamly
import qualified Streamly.Prelude as S

main = (S.toList . wSerially $ (S.fromList [1,2]) <> (S.fromList [3,4])) >>= print

[1,3,2,4]

Similarly, the Monad instance interleaves the iterations of the inner and the outer loop, nesting loops in a breadth first manner.

main = S.drain . wSerially $ do
    x <- return 1 <> return 2
    y <- return 3 <> return 4
    S.yieldM $ print (x, y)

(1,3)
(2,3)
(1,4)
(2,4)

Since: 0.2.0

An interleaving serial IO stream of elements of type a. See WSerialT documentation for more details.

Since: 0.2.0

Polymorphic version of the Semigroup operation <> of WSerialT. Interleaves two streams, yielding one element from each stream alternately. When one stream stops the rest of the other stream is used in the output stream.

Since: 0.2.0

Like wSerial but stops interleaving as soon as the first stream stops.

Since: 0.7.0

Like wSerial but stops interleaving as soon as any of the two streams stops.

Since: 0.7.0

Fix the type of a polymorphic stream as WSerialT.

Since: 0.2.0

Build a stream by unfolding a monadic step function starting from a seed. The step function returns the next element in the stream and the next seed value. When it is done it returns Nothing and the stream ends. For example,

let f b =
        if b > 3
        then return Nothing
        else print b >> return (Just (b, b + 1))
in drain $ unfoldrM f 0

 0
 1
 2
 3

Internal

map = fmap

Same as fmap.

> S.toList $ S.map (+1) $ S.fromList [1,2,3]
[2,3,4]

Since: 0.4.0

Since: 0.1.0

Since: 0.1.0

Same as wSerial.

Since: 0.1.0

Same as wSerially.

Since: 0.1.0