stream
A SubscriptionRef[A] contains a RefM with a value of type A and a
ZStream 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[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 ZStream[R, E, O] is a description of a program that, when evaluated, may
emit 0 or more values of type O, may fail with errors of type E and uses
an environment of type R.
A ZStream[R, E, O] is a description of a program that, when evaluated, may
emit 0 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.
Another analogue to ZStream is an imperative iterator:
trait Iterator[A] { def next: A }
This data type can emit multiple A values through multiple calls to next.
Similarly, embedded inside every ZStream is a ZIO program: ZIO[R,
Option[E], Chunk[O]]. This program will be repeatedly evaluated as part of
the stream execution. For every evaluation, it will emit a chunk of values or
end with an optional failure. A failure of type None signals the end of the
stream.
ZStream is a purely functional *pull* based stream. Pull based streams
offer inherent laziness and backpressure, relieving users of the need to
manage buffers between operators. As an optimization, ZStream does not emit
single values, but rather zio.Chunk values. This allows the cost of
effect evaluation to be amortized and most importantly, keeps primitives
unboxed. This allows ZStream to model network and file-based stream
processing extremely efficiently.
The last important attribute of ZStream is resource management: it makes
heavy use of ZManaged to manage resources that are acquired and released
during the stream's lifetime.
ZStream forms a monad on its O type parameter, and has error management
facilities for its E type parameter, modeled similarly to ZIO (with
some adjustments for the multiple-valued nature of ZStream). These aspects
allow for rich and expressive composition of streams.
The current encoding of ZStream is *not* safe for recursion. ZStream
programs that are defined in terms of themselves will leak memory. For
example, the following implementation of ZStream#forever is not
heap-safe:
def forever = self ++ foreverInstead, recursive operators must be defined explicitly. See the definition of ZStream#forever for an example. This limitation will be lifted in the future.
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