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Version: 2.x

Introduction to ZStream

A ZStream[R, E, O] is a description of a program that, when evaluated, may emit zero or more values of type O, may fail with errors of type E, and uses an environment of type R.

One way to think of ZStream is as a ZIO program that could emit multiple values. As we know, a ZIO[R, E, A] data type, is a functional effect which is a description of a program that needs an environment of type R, it may end with an error of type E, and in case of success, it returns a value of type A. The important note about ZIO effects is that in the case of success they always end with exactly one value. There is no optionality here, no multiple infinite values, we always get exact value:

val failedEffect: ZIO[Any, String, Nothing]       = ZIO.fail("fail!")
val oneIntValue : ZIO[Any, Nothing, Int]          = ZIO.succeed(3)
val oneListValue: ZIO[Any, Nothing, List[Int]]    = ZIO.succeed(List(1, 2, 3))
val oneOption   : ZIO[Any, Nothing , Option[Int]] = ZIO.succeed(None)

A functional stream is pretty similar, it is a description of a program that requires an environment of type R and it may signal with errors of type E and it yields O, but the difference is that it will yield zero or more values.

So a ZStream represents one of the following cases in terms of its elements:

  • An Empty Stream — It might end up empty; which represent an empty stream, e.g. ZStream.empty.
  • One Element Stream — It can represent a stream with just one value, e.g. ZStream.succeed(3).
  • Multiple Finite Element Stream — It can represent a stream of finite values, e.g. ZStream.range(1, 10)
  • Multiple Infinite Element Stream — It can even represent a stream that never ends as an infinite stream, e.g. ZStream.iterate(1)(_ + 1).
import zio.stream.ZStream
val emptyStream         : ZStream[Any, Nothing, Nothing]   = ZStream.empty
val oneIntValueStream   : ZStream[Any, Nothing, Int]       = ZStream.succeed(4)
val oneListValueStream  : ZStream[Any, Nothing, List[Int]] = ZStream.succeed(List(1, 2, 3))
val finiteIntStream     : ZStream[Any, Nothing, Int]       = ZStream.range(1, 10)
val infiniteIntStream   : ZStream[Any, Nothing, Int]       = ZStream.iterate(1)(_ + 1)

Another example of a stream is when we're pulling a Kafka topic or reading from a socket. There is no inherent definition of an end there. Stream elements arrive at some point, or even they might never arrive at any point.

Based on type parameters of ZStream, there are 4 types of streams:

  1. ZStream[Any, Nothing, O] — A stream that emits O values and cannot fail.
  2. ZStream[Any, Throwable, O] — A stream that emits O values and can fail with Throwable.
  3. ZStream[Any, Nothing, Nothing] — A stream that emits no elements.
  4. ZStream[R, E, O] — A stream that requires access to the R service, can fail with error of type E and emits O values.