zio.stream

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A SubscriptionRef[A] is a Ref that can be subscribed to in order to receive the current value as well as all changes to the value.

A Take[E, A] represents a single take from a queue modeling a stream of values. A Take may be a failure cause Cause[E], an chunk value A or an end-of-stream marker.

A ZChannel[Env, InErr, InElem, InDone, OutErr, OutElem, OutDone] is a nexus of I/O operations, which supports both reading and writing. A channel may read values of type InElem or process upstream failures of type InErr, while it may write values of type OutElem. When the channel finishes, it yields a value of type OutDone. A channel may fail with a value of type OutErr.

Channels are the foundation of ZIO Streams: both streams and sinks are built on channels. Most users shouldn't have to use channels directly, as streams and sinks are much more convenient and cover all common use cases. However, when adding new stream and sink operators, or doing something highly specialized, it may be useful to use channels directly.

Channels compose in a variety of ways:

• Piping. One channel can be piped to another channel, assuming the input type of the second is the same as the output type of the first.
• Sequencing. The terminal value of one channel can be used to create another channel, and both the first channel and the function that makes the second channel can be composed into a channel.
• Concating. The output of one channel can be used to create other channels, which are all concatenated together. The first channel and the function that makes the other channels can be composed into a channel.

A ZPipeline[Env, Err, In, Out] is a polymorphic stream transformer. Pipelines accept a stream as input, and return the transformed stream as output.

Pipelines can be thought of as a recipe for calling a bunch of methods on a source stream, to yield a new (transformed) stream. A nice mental model is the following type alias:

type ZPipeline[Env, Err, In, Out] = ZStream[Env, Err, In] => ZStream[Env, Err, Out]

This encoding of a pipeline with a type alias is not used because it does not infer well. In its place, this trait captures the polymorphism inherent to many pipelines, which can therefore be more flexible about the environment and error types of the streams they transform.

There is no fundamental requirement for pipelines to exist, because everything pipelines do can be done directly on a stream. However, because pipelines separate the stream transformation from the source stream itself, it becomes possible to abstract over stream transformations at the level of values, creating, storing, and passing around reusable transformation pipelines that can be applied to many different streams.

The most common way to create a pipeline is to convert a sink into a pipeline (in general, transforming elements of a stream requires the power of a sink). However, the companion object has lots of other pipeline constructors based on the methods of stream.