pekko/akka-docs/src/main/paradox/stream/operators/Sink/fold.md

Sink.fold

Fold over emitted element with a function, where each invocation will get the new element and the result from the previous fold invocation.

@refSink operators

@@@div { .group-scala }

Signature

@@signature Sink.scala { #fold }

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Description

Fold over emitted element with a function, where each invocation will get the new element and the result from the previous fold invocation. The first invocation will be provided the zero value.

Materializes into a @scala[Future] @java[CompletionStage] that will complete with the last state when the stream has completed.

This operator allows combining values into a result without a global mutable state by instead passing the state along between invocations.

Reactive Streams semantics

@@@div { .callout }

cancels never

backpressures when the previous fold function invocation has not yet completed

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